After studying this chapter, the student should be able to:
Distinguish the 3 components of the brainstem and their general functions.
Outline the cranial nerves and describe their overall motor, sensory and/or autonomic functions.
Diagram the connections from the forebrain to and from the cerebellum and describe the functions of the cerebellum.
Identify the gray and white matter regions of the spinal cord, explain their functions, and define the spinal cord segments.
Outline the spinal nerves and their dorsal and ventral roots, main branches, and peripheral nerves.
Delineate the meninges and describe its locations and 3 membrane components.
Define the ventricular system and the flow of cerebrospinal fluid.
Distinguish and describe the main central nervous system arteries and their origins and branches and venous drainage.
The forebrain is highly interconnected with the midbrain, hindbrain, and spinal cord. These regions also provide the connections to the peripheral nervous system (PNS); the PNS includes the cranial nerves, spinal nerves, peripheral nerves, and ganglia located outside of the brain and spinal cord. The midbrain, pons, and medulla oblongata (hereafter medulla) compose the brainstem, which contains nuclei that are critical for survival through automatic responses such as breathing and reflexes such as coughing. The brainstem also contains nuclei for cranial nerves (CNs) III to XII. Emerging from the brainstem, CNs III to XII serve somatic and autonomic motor and sensory functions in the face, head and neck, special senses, and autonomic innervation of the heart, lungs, and other organs. The other region of the hindbrain is the cerebellum, which is connected to the rest of the brain and spinal cord via the pons. The cerebellum functions in motor functions including coordination, precision, timing, and motor learning and has been implicated in cognition and emotion.
Caudal to the medulla is the spinal cord. The spinal cord contains both ascending and descending white matter tracts and neurons involved in motor output, sensory relay, integration, and spinal reflexes. Spinal cord gray matter contains the cell bodies of somatic motor neurons, preganglionic autonomic neurons, and sensory relay neurons, as well as spinal interneurons involved in integration and spinal reflexes. The spinal cord is the origin of the 31 pairs of spinal nerves and their branches, called peripheral nerves, which contain axons that innervate the skin, muscles, tendons, joints, and organs (viscera). The circuitry within the spinal cord generates spinal reflexes and contributes to central pattern generators. Another component of the PNS is the enteric nervous system (ENS); located in the gastrointestinal (GI) tract, the ENS functions in peristalsis and secretions.
The central nervous system (CNS) is protected on the outside by the skull and meninges, membranes that prevent the movement of the brain within the skull and contain cerebrospinal fluid (CSF) that helps cushion the CNS. Inside the CNS lies the ventricular system and central canal, with the main function of production and circulation of CSF. The CNS has a robust vascularization, with blood supplied from branches of the dorsal aorta and drainage via the jugular vein. The nerves are also vascularized and contain layers of protective connective tissue.
Considered one of the most primitive parts of the human brain, the brainstem is the structure most important to life (Figures 4–1 and 4–2). The brainstem contains white matter tracts involved in transmission of motor impulses that control the body and head and the largest majority of sensory tracts. In addition, 10 of the 12 pairs of CNs emerge directly from the brainstem, with nuclei involved in both somatic motor and sensory functions of the head, face, and neck and in autonomic parasympathetic functions. The brainstem also contains nuclei involved in essential automatic processes, including breathing, and encompasses the reticular formation, a group of nuclei that function in arousal, alertness, and consciousness. Anatomically, the brainstem rests on the base of the skull, known as the clivus, and ends at the foramen magnum, the large opening in the occipital bone.
Illustration of the divisions of the brainstem in a midsagittal plane. The major external divisions are the midbrain, pons, and medulla. The major internal longitudinal divisions are the tectum, tegmentum, and basis. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
Illustration of the brainstem from a dorsolateral view, with most of the cerebellum hidden. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
Rostrally, the midbrain adjoins the thalamus; caudally, the midbrain connects to the pons (Figure 4–3). During development, the dorsal surface of the mesencephalic vesicle becomes a structure called the tectum or roof, while the ventral region becomes the tegmentum or floor of the midbrain. The smaller tectum and larger tegmentum are separated at the midline by the narrow CSF-filled cerebral aqueduct, which connects rostrally with the third ventricle and caudally with the fourth ventricle. As the midbrain develops, large white matter regions called the cerebral peduncles form ventral to and laterally around the tegmentum.
Illustration of brainstem transverse sections—simplified. A. The midbrain at the level of the superior colliculus. B. The pons, showing massive crossover of descending fibers in the ventral pons, which are destined for the cerebellum. C. The medulla oblongata, showing the large medullary pyramids ventrally, which are the site of crossover of the descending corticotracts. Several cranial nerve (CN) nuclei are shown.
During development, the tectum differentiates into 2 structures: the superior colliculus and inferior colliculus. Together, the superior and inferior colliculi are called the corpora quadrigemina. The superior colliculus, which is called the optic tectum in lower vertebrates, receives direct input from the retina and visual cortex and is a primary integration center that provides key output to the thalamus and nearby nuclei of the oculomotor nerve (CN III) and the trochlear nerve (CN IV). One function of the superior colliculus is to control visual reflexes, such as eye movements, lens shape, and pupil diameter. The inferior colliculus is involved in processing auditory information. It receives input from several brainstem nuclei in the auditory pathway and the auditory cortex and provides a major output to the thalamus. Although the tectum is localized to the midbrain region, the tegmentum extends through the pons.
The midbrain tegmentum contains 4 regions distinguished by their pigmentation. The substantia nigra is a large highly pigmented nucleus that consists of the pars reticulata and pars compacta. The pars compacta contains dopaminergic neurons that project to the caudate nucleus and putamen, which form the striatum of the basal ganglia involved in mediating movement and motor coordination. The striatal neurons in turn project to neurons in the pars reticulata, which, by projecting fibers to the thalamus, are part of the output system of the striatum. Located near the midline, the ventral tegmental area is the origin of dopaminergic neurons of the mesocorticolimbic dopamine system that is widely implicated in the reward circuitry of the brain. The red nucleus is a large centrally located structure that receives inputs from the motor and sensory cortices via the corticorubral tract and from the cerebellum via the cerebellar peduncles; it gives rise to the rubrospinal tract, providing motor information to the spinal cord. The periaqueductal gray functions as a primary control center for descending pain modulation.
The midbrain contains a number of other important nuclei and white matter tracts, including the mesencephalic nucleus of the trigeminal nerve (CN V) involved in proprioception of the face and a portion of the reticular formation, a neural network that is involved in arousal and alertness (see later discussion). The cerebral peduncles house the crus cerebri, which contain the descending motor axons of the corticospinal, corticobulbar, and corticopontine tracts. The ascending sensory axons from the spinothalamic and dorsal column medial lemniscus (DCML) tracts are located more dorsally, closer to the tegmentum.
Caudal to the midbrain is the pons (Figure 4–4). The dorsal/posterior border of the pons is separated from the cerebellum by the aqueduct of Sylvius and, more inferiorly, by the fourth ventricle. The pons appears as a broad anterior bulge rostral to the medulla that consists of 2 pairs of thick stalks called cerebellar peduncles; the specific nuclei in the pons are located in the medial regions. An important function of the pons is to relay information between the cortex and the cerebellum. The pons contains a number of other important nuclei, including those involved in automatic functions and numerous CN nuclei.
Illustration of brainstem transverse sections—complex. A. Transverse section of the midbrain at the level of the IIIrd nerve. B. Transverse section of the pons at the level of the Vth nerve (rostral). C. Transverse section of the medulla oblongata. (Reproduced with permission from Biller J, Gruener G, Brazis P. DeMyer’s The Neurologic Examination: A Programmed Text, 7th ed. New York, NY: McGraw Hill; 2016.)
The pons can be broadly divided into 2 parts: the basilar part of the pons, located ventrally, and the pontine tegmentum, located dorsally. The ventral pons contains numerous pontine nuclei that relay signals between the cerebral cortex and the cerebellum. The pontine nuclei receive inputs from many parts of the cerebral cortex including the motor cortex and auditory, visual, somatosensory, and association regions, as well as subcortical areas. In turn, the pontine nuclei project, via the middle cerebellar peduncle, to the cerebellar cortex and interposed nucleus. Information relayed via the corticopontocerebellar tract represents a critical contribution from the cortex to the cerebellar regulation of motor function and movement. In addition, the cerebellum has been implicated in several cognitive processes (see later discussion).
The pons is involved in several automatic functions necessary for life. In the dorsal posterior pons lie nuclei that have critical functions in breathing (respiration), sleep, and swallowing. In addition, the pons contains nuclei involved in bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture. The pons contains the breathing pneumotaxic center, which regulates the change from inhalation to exhalation. The pons has been implicated in sleep paralysis and rapid eye movement (REM) sleep and may also play a role in sleep cycles and generating dreams. The pons contains a number of CN nuclei, including the pontine nucleus and motor nucleus for the trigeminal nerve (CN V), the abducens nucleus (CN VI), the facial nerve nucleus (CN VII), and the vestibulocochlear nuclei (vestibular nuclei and cochlear nuclei) (CN VIII). The functions of these 4 CNs (V to VIII) include sensory roles in hearing, equilibrium, and taste and in facial sensations such as touch and pain, as well as motor roles in eye movement, facial expressions, chewing, swallowing, and the secretion of saliva and tears.
Similar to the midbrain, the pons contains the descending motor and ascending sensory white matter tracts between the forebrain, medulla, and spinal cord. Other white matter regions include the superior cerebellar peduncles, which contain the main output route from the cerebellum and extend to the red nucleus in the midbrain or to the thalamus. The inferior cerebellar peduncle is a region of the medulla that contains tracts that connect the spinal cord and medulla with the cerebellum.
The medulla is the caudal-most region of the brainstem situated between the pons and the spinal cord and ventral/anterior to the cerebellum. The medulla merges with the spinal cord at the opening called the foramen magnum at the base of the skull. The lateral areas of the medulla form the inferior cerebellar peduncles. In the upper or superior part, the dorsal surface of the medulla is formed by the fourth ventricle. In the lower or inferior part, the fourth ventricle narrows at the obex in the caudal medulla and becomes the central canal.
Motor and sensory tracts passing between the brain and spinal cord pass through the medulla. The medulla contains the pyramids, a region where the crossing (decussation) of motor axons that form the lateral corticospinal tract occurs, and the medial lemniscus, where decussation of sensory axons of the DCML tract takes place. The medulla contains CN nuclei (Figure 4–5) and other nuclei and serves as a major processing center involved in automatic, autonomic, and somatic functions.
Organization of cranial nerve nuclei in the brainstem. The cranial nerve nuclei are organized in functional columns along the rostrocaudal axis of the brainstem. A. This dorsal view of the human brainstem shows the location of the cranial nerve sensory nuclei (right) and motor nuclei (left). B. A schematic view of the functional organization of the motor and sensory columns. C. The medial-lateral arrangement of the cranial nerve nuclei is shown in a cross section at the level of the medulla. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM, et al: Principles of Neural Science, 5th ed. New York, NY: McGraw Hill; 2013.)
Automatic functions and reflexes of the medulla include breathing, vomiting, sneezing, coughing, swallowing, and balance. Two of the major regions (the dorsal and ventral respiratory groups) of the respiratory center are located in the medulla, with the other regions located in the pons. The respiratory center receives input from chemoreceptors, mechanoreceptors, the cerebral cortex, and the hypothalamus and is responsible for generating and maintaining the rhythm and rate of respiration and adjusting those to physiologic needs. Some other reflexes may involve the raphe nucleus and the solitary nucleus (SN). The SN contains a series of sensory nuclei forming a vertical column of gray matter in the medulla. The nuclei are part of the medullary reticular formation and contain serotonergic neurons (see later discussion).
Four CNs originate from the medulla oblongata; these are the glossopharyngeal nerve (CN IX), vagus nerve (CN X), accessory nerve (CN XI), and hypoglossal nerve (CN XII), which serve both autonomic and somatic functions. Autonomic functions include maintaining blood pressure and regulation of heart rate and contraction force, with control by the cardiovascular centers. The medulla also contributes to autonomic control of the digestive system. Somatic motor and sensory functions include taste, hearing, and control of muscles of the face and neck. The SN has been implicated in regulation of autonomic functions since it receives input from the facial, glossopharyngeal, and vagus nerves. The SN projects to the reticular formation, parasympathetic preganglionic neurons, hypothalamus, and thalamus.
The medulla also contains important relay nuclei. Forming the olivary nucleus (or olive), the superior olivary nuclei form an important component of the ascending and descending auditory pathways of the auditory system, whereas the inferior olivary nuclei coordinate signals from the spinal cord to the cerebellum to regulate motor coordination and learning. The cuneate nucleus and gracile nucleus are part of DCML pathway, carrying fine touch and proprioceptive information from the upper body (cuneate) and the legs and trunk.
Phylogenetically one of the oldest parts of the brain, the reticular formation is a set of interconnected nuclei that are located throughout the brainstem, from the midbrain to the lower medulla. The reticular formation contains the raphe nuclei, which encompass the majority of serotonergic neurons in the CNS and have been implicated in mood, pain, alertness, and memory. One of its 2 components, the ascending reticular formation is called the reticular activating system (RAS), is responsible for the sleep–wake cycle, and controls various levels of alertness. The RAS projects to the thalamus, which transmits this information to the cortex. The other component, the descending reticular formation (DRF), is involved in posture and equilibrium, as well as motor movement and autonomic functions. The DRF receives information from the hypothalamus and some fibers from the corticobulbar tract that originate in the motor cortex and are involved in eye movement. Other corticobulbar fibers innervate CNs directly. Nuclei in the DRF have also been implicated in reflexes such as coughing, chewing, swallowing, and vomiting.
The CNs are a set of 12 paired nerves that emerge from the brain. The first 2 CNs arise from the cerebrum, whereas the remaining 10 emerge from the brainstem. The CNs provide motor and sensory innervation to structures within the head, face, and neck and autonomic regulation of organs in the body. Assigned Roman numerals I to XII based on the order from which they emerge from the cerebrum or brainstem, the CNs are also named based on their function or appearance. The CNs serve functions such as smell, sight, eye movement, feeling in the face, balance, hearing, movement of the mouth and tongue for speech, chewing and swallowing, and parasympathetic control of heart rate, GI peristalsis, and sweating.
The olfactory nerve (CN I) and optic nerve (CN II) are considered CNS nerves because they emerge from the cerebrum and the tracts travel within the CNS. CNs III to XII are considered part of the PNS. The nuclei for CNs III to XII lie in the brainstem, and their axons exit or enter the skull bone and relay information between the brain and the body, primarily to and from regions of the head and neck. A thirteenth CN designated 0 in lower vertebrates does not appear to have a function in humans.
The CNs provide sensory innervation that includes general somatic sensation such as touch, temperature, and pain; visceral somatic sensation from internal regions; and special sense innervation such as smell, vision, taste, hearing, and balance. Motor function can involve somatic motor or autonomic motor innervation. Hence, CNs can include fibers for somatic sensory, visceral sensory, special sensory, somatic motor, or autonomic motor functions. Some CNs contain only 1 type of fiber. Many of the CNs contain mixtures of fiber types.
The PNS CNs (III to XII) have nuclei in the brainstem, and their axons exit or enter the skull bone via foramina and innervate regions of the head, face, neck, and body on the same side from which they originate. CNs and the axons inside travel in regions within the skull called intracranial paths; they travel outside the skull via extracranial paths. The CN nuclei in the brainstem contain the cell bodies of the neurons that send or receive the axons that form the CNs. The midbrain of the brainstem has the nuclei of CN III and IV. The pons contains the nuclei of CNs V to VIII. The medulla houses the nuclei of CNs IX to XII.
All the CNs, except CNs I, II, and IV, have associated ganglia, which are groups of neuronal cell bodies that are located outside the brainstem. The sensory ganglia are directly correspondent to dorsal root ganglia of spinal nerves and are known as cranial sensory ganglia. Sensory ganglia exist for nerves with somatic sensory components: CNs V, VII, VIII, IX, and X. Parasympathetic ganglia are present for the autonomic components of CNs III, VII, IX, and X that innervate the glands in the head and neck and include the ciliary, pterygopalatine, submandibular, and otic ganglion. In addition, the vagus nerve (CN X) supplies the output to parasympathetic ganglia that lie in the body, close to the organs they innervate in the chest and abdomen.
Considered part of the ANS, the general visceral afferent (GVA) fibers transmit sensory information about local changes in chemical and mechanical environments, including pain and reflex sensations, from the internal organs, glands, and blood vessels to the CNS. Unlike the efferent motor fibers of the ANS, the GVA fibers are not classified as sympathetic or parasympathetic. The CNs that contain GVA fibers include the facial nerve, glossopharyngeal nerve, and vagus nerve. CN GVA fibers are important in complex automatic motor acts such as swallowing, vomiting, and coughing.
The olfactory nerve conveys the sense of smell. Containing only special sensory fibers, the olfactory nerve is a purely sensory nerve in charge of transmitting olfactory sensation from the nose to the brain. The olfactory nerve originates in the olfactory epithelium and contains sensory axons (afferents) that extend to the olfactory bulb. Derived from the nasal (otic) placodes, neurons in the olfactory epithelium can be replaced and extend new axons within the nerve. Hence, the olfactory nerve is unique in that it is capable of some regeneration if damaged. The olfactory bulb extends axons via the olfactory tract, which transmits olfactory information to the primary olfactory cortex in the temporal lobe. The olfactory nerve is unmyelinated but covered with meningeal-like membranes. It is the shortest CN and does not exit the brain.
The optic nerve is dedicated to vision. Composed of axons from retinal ganglion cells, the optic nerve transsensory visual information from the retina of the eye to the brain. Emerging from the retina, the optic nerve travels through the optic canal, partially decussates in the optic chiasm and becomes the optic tract. The majority of optic tract axons terminate in the lateral geniculate nucleus of the thalamus, while a few project to nuclei in the midbrain and hypothalamus. The thalamus transmits visual perception information to the visual cortex in the occipital lobe. The few axons that terminate in the superior colliculus are responsible for reflexive eye movements and those that terminate in the pretectum control the pupillary light reflex. Axons that project to the suprachiasmatic nucleus in the hypothalamus are involved in circadian rhythms. The optic nerve is heavily myelinated by oligodendrocytes and is encased in the 3 meningeal layers. Since retinal ganglion neurons and axons do not regenerate, damage to the optic nerve can produce irreversible blindness.
The oculomotor nerve controls eye movements and pupil size. The oculomotor nerve originates in 2 nuclei in the midbrain called the oculomotor nucleus and Edinger-Westphal nucleus. The nerve enters the orbit via the superior orbital fissure to innervate muscles that enable most movements of the eye and that raise the eyelid. Located in the midbrain, the oculomotor nucleus controls striated muscle in levator palpebrae superioris and extraocular muscles except for the superior oblique muscle and the lateral rectus muscle. The Edinger-Westphal nucleus supplies parasympathetic nerve fibers via the ciliary ganglion to the eye to control the pupillae muscle for pupil constriction and the ciliary muscle for the accommodation reflex, the ability to focus on near objects as in reading. Sympathetic postganglionic fibers join the oculomotor nerve to innervate the superior tarsal muscle, a smooth muscle. CNs IV and VI also participate in control of eye movement.
The trochlear nerve is a somatic motor nerve that enters the orbit through the superior orbital fissure and innervates a single muscle, the superior oblique muscle of the eye. This eye muscle ends in a tendon, which passes through a fibrous loop called the trochlea that functions through a pulley-like mechanism to make the eyeballs move and rotate. The nucleus of the trochlear nerve originates in the midbrain immediately below the oculomotor nucleus. The trochlear nerve is the smallest nerve in terms of its axon number but has the longest intracranial path and is the only CN that exits from the dorsal aspect of the brainstem.
The trigeminal nerve contains both sensory and motor fibers and is responsible for sensation in the skin of the face and mouth and motor functions such as biting, chewing, and swallowing. The largest of the CNs, its name derives from the fact that it contains 3 branches, the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). The ophthalmic and maxillary nerves are purely somatic sensory, whereas the mandibular nerve supplies both somatic motor and some sensory functions.
The sensory functions of the trigeminal nerve are to provide the tactile, motion, position, temperature, and pain sensations of the top and front of the head, the face, and the mouth. Each of the 3 nerves innervates specific regions of the front of the head and face, with V1 innervating the approximate dorsal third of the face and head, V2 innervating the middle third, and V3 providing information from the approximate ventral third of the face.
The 3 trigeminal branches converge on the trigeminal ganglion, a sensory ganglion. It contains the cell bodies of incoming sensory nerve fibers from the face. From the trigeminal ganglion, a single large sensory root enters the brainstem at the level of the pons and synapses on the sensory nuclei. Immediately adjacent to the sensory root, a smaller motor root emerges from the pons at the same level. The trigeminal motor nuclei are located in the pons, and the motor fibers pass through the trigeminal ganglion en route to their muscle targets.
The abducens nerve is also known as the abducent nerve. It is a responsible for somatic motor output to control the lateral rectus muscle of the ipsilateral eye. In humans, this allows the eyes to move horizontally and is responsible for outward, lateral gaze. The abducens nerve nucleus is present in the pons, and the nerve exits the brainstem at the junction of the pons and the medulla and ascends upward to the eye. The exact control of eye movements requires input from integration centers in the brain that coordinate the output from the oculomotor, trochlear, and abducens nuclei, which control the 6 extraocular muscles.
The facial nerve is a mixed nerve that has 4 components with distinct functions. The facial nerve is responsible for producing facial expressions by voluntary movements, including brow wrinkling, teeth showing, frowning, eyes closing, lip pursing, and cheek puffing. It also controls the stapedius muscle of the ear. The facial nerve also functions in the transmission of taste sensations from the anterior two-thirds of the tongue and oral cavity. A somatic sensory component innervates the external ear. It also supplies preganglionic parasympathetic fibers to several head and neck ganglia and signals to the salivary, mucous, and lacrimal glands. The facial nerve’s motor component originates in the facial nerve nucleus in the pons, and the sensory nuclei are located near the pons–medulla junction. The section of the facial nerve that arises from the sensory root is also known as the intermediate nerve. The geniculate ganglion contains the cell bodies of the sensory component of the intermediate nerve. The nerve traverses the facial canal, through the parotid gland, and divides into 5 branches.
The vestibulocochlear nerve transmits hearing (sound) information and balance (equilibrium) information from the inner ear to the brain. It contains 2 functionally separate nerves, the vestibular nerve and the cochlear nerve, which are both sensory nerves that combine in the pons. The vestibular nuclei complex, situated in the pons and medulla, receives input from 5 sensory organs in the vestibules and semicircular canal of the inner ear. The vestibular ganglion includes the cell bodies of the sensory neurons. The vestibular nerve transmits information about balance and movement and is an important component of the vestibulo-ocular reflex, which keeps the head stable and allows the eyes to track moving objects.
The ventral and dorsal cochlear nuclei, located in the inferior cerebellar peduncle, receive information from the cochlear nerve, also known as the auditory nerve, which transmits sound information for the sensation of hearing. Sound waves are detected by hair cells in the cochlea that send information to the spiral ganglia, which house the cell bodies of neurons of the cochlear nerve, and information is then transmitted to the cochlear nuclei.
The glossopharyngeal nerve is a mixed nerve, consisting of both sensory and motor nerve fibers, and is responsible for swallowing, the gag reflex, taste sensation from the back of the tongue, and saliva production. It contains somatic sensory fibers that originate in the tonsils, pharynx, middle ear, and posterior third of the tongue. It also contains special sense fibers from the posterior third of the tongue, and GVA fibers from the carotid sinus and bodies. These various sensory fibers terminate in nuclei in the medulla. The motor fibers originate in nuclei in the medulla. Parasympathetic fibers innervate the parotid salivary gland and glands of the posterior tongue. Somatic motor fibers innervate the stylopharyngeus muscle, which provides voluntary muscle control that elevates the pharynx during swallowing and speech. GVAs synapse on neurons of the nucleus of the solitary tract.
The vagus nerve is also known as the pneumogastric nerve. The vagus nerve supplies motor parasympathetic fibers to all the organs, except the adrenal glands, from the neck down to the second segment of the transverse colon. The vagus nerve sends somatic and visceral sensory information about the body’s organs to the brain, with estimates that 80% to 90% of the nerve fibers in the vagus nerve are afferent (sensory) nerves. Vagal GVAs carry information from aortic, cardiac, pulmonary, and GI receptors and synapse on neurons of the nucleus of the solitary tract.
The vagus nerve is responsible for such varied tasks as regulation of heart rate, GI peristalsis, sweating, the gag reflex, and satiation following food consumption. It also controls a few skeletal muscles, including muscles of the mouth and larynx involved in speech, swallowing, and keeping the larynx open for breathing. The vagus nerve also has a minor sympathetic function via peripheral chemoreceptors. Upon leaving the medulla between the medullary pyramid and the inferior cerebellar peduncle, it extends through the jugular foramen, then passes into the carotid sheath between the internal carotid artery and the internal jugular vein below the head, to the neck, chest, and abdomen, where it contributes to the innervation of the viscera.
The accessory nerve is also called the spinal accessory nerve. The accessory nerve provides motor innervation from the CNS to 2 neck muscles, the sternocleidomastoid, which tilts and rotates the head, and the trapezius muscle, which has several actions on the scapula, including shoulder elevation and adduction of the scapula. Thus, the accessory nerve governs movements of the head and shoulders. Most of the fibers of the accessory nerve originate in neurons situated in the upper spinal cord. These fibers enter the skull through the foramen magnum and proceed to exit the jugular foramen with CNs IX and X. Due to its unusual course, the accessory nerve is the only nerve that enters and exits the skull. Traditional descriptions of the accessory nerve divide it into 2 components: a spinal component and a cranial component. However, contemporary characterizations of the nerve regard the cranial component as separate and part of the vagus nerve.
The hypoglossal nerve is a somatic motor nerve that, similar to the glossopharyngeal and vagus nerves, is also involved in tongue muscles, swallowing, and speech. A nerve with solely motor function, the hypoglossal nerve provides motor control of the extrinsic muscles of the tongue (genioglossus, hyoglossus, and styloglossus) and the intrinsic muscles of the tongue. These represent all muscles of the tongue except the palatoglossus muscle, which is innervated by the vagus nerve. These muscles are involved in moving and manipulating the tongue. The hypoglossal nerve also supplies movements including clearing the mouth of saliva and other involuntary activities. The hypoglossal nucleus interacts with the reticular formation, which is involved in the control of several reflexive or automatic motions, and several corticonuclear originating fibers supply innervation, aiding in unconscious movements relating to speech and articulation. The hypoglossal nerve (CN XII) is unique in that it is innervated from the motor cortex of both hemispheres of the brain. See Table 4–1 for a summary of the CNs.
TABLE 4–1Summary of the cranial nerves. ||Download (.pdf) TABLE 4–1 Summary of the cranial nerves.
|Number of Nerve ||Name of Nerve ||Type ||Major Function(s) |
|I ||Olfactory ||Sensory || |
|II ||Optic ||Sensory || |
|III ||Occulomotor ||Motor || |
|IV ||Trochlear ||Motor || |
|V || |
Cornea and skin of the forehead, scalp, nose, and eyelid
Maxillary facial skin, upper teeth, maxillary sinus, and palate
Sensory to skin over mandible, lower teeth, inside of mouth, and anterior part of the tongue
Motor to muscles of mastication
|VI ||Abducens ||Motor || |
|VII ||Facial ||Mixed || |
|VIII || |
|IX ||Glossopharyngeal ||Mixed || |
Sensory to the pharynx and posterior third of the tongue; carotid sinus baroreceptor and carotid body chemoreceptor
Motor to muscles of swallowing
Parasympathetic secretomotor to salivary gland
|X ||Vagus ||Mixed || |
|XI ||Accessory ||Motor || |
Spinal root: sternomastoid and trapezius muscles
Cranial root: muscles of palate, pharynx, and larynx
|XII ||Hypoglossal ||Motor || |
Lying underneath the forebrain and adjacent to the brainstem is the cerebellum (Figure 4–6). The cerebellum receives substantial motor and sensory inputs via the cerebellar peduncles, directly from the brainstem and spinal cord and indirectly from the cerebral cortex. Primarily a movement control center, cerebellar functions include control of posture and balance, coordination of voluntary movements that result in smooth and balanced muscular activity of parts of the body, and learning motor behaviors. The cerebellum has also been implicated in mood and cognitive functions including language, attention, and mental imagery. Receiving extensive inputs, the cerebellar intrinsic circuits are thought to have substantial computational capacity in producing their outputs.
Gross features and functional anatomy of the cerebellum. A. Part of the right hemisphere has been cut away to reveal the underlying cerebellar peduncles. B. The cerebellum is shown detached from the brain stem. C. A midsagittal section through the brainstem and cerebellum shows the branching structure of the cerebellum. The cerebellar lobules are labeled with their latin names and Larsell Roman numerals. D. Functional regions of the cerebellum.
The cerebellum rests at the back of the cranial cavity, with the appearance of being a separate structure lying beneath the cerebral lobes, and is separated from the occipital lobes by a sheet of fibers called the cerebellar tentorium. However, the cerebellum is robustly connected with other regions of the CNS via the cerebellar peduncles. The cerebellum has fissures that divide it into 3 lobes. The primary fissure divides the anterior lobe from the posterior lobe, which are connected in the midline by the vermis. The posterolateral fissure divides the posterior lobe from the medial flocculonodular lobe. Each of the 3 lobes has a left and right half, and each lobe consists of an inner medulla of white matter and a richly folded thin outer layer of cortical gray matter.
The continuous cortical surface contains finely spaced parallel grooves with the appearance of an accordion. Although it represents only 10% of the brain mass, the cerebellum contains as many neurons as the entire cerebrum, but many fewer types of neurons. The outer cortical layer consists of a regular 3-layer arrangement that contains Purkinje neurons and granule neurons. The majority of inputs reach the cerebellar cortex, where Purkinje neurons integrate incoming signals and send outputs to the deep cerebellar nuclei located in the white matter interior and the vestibular nuclei. Hence, the majority of outputs from the cerebellum to the brainstem and thalamus occur via the deep cerebellar nuclei.
The cerebellum has 3 functional subdivisions and is involved in control of both conscious and unconscious movement. The flocculonodular lobe and adjacent vermis is called the vestibulocerebellum (archicerebellum) and is involved in balance and eye movements. The spinocerebellum (paleocerebellum) includes the vermis and paravermis in the anterior lobe and paramedian lobules and functions in posture and proprioception. The cerebrocerebellum (neocerebellum or pontocerebellum) makes up the lateral hemispheres of the anterior and posterior lobes and functions in control of voluntary movement.
The cerebral cortex provides the largest source of inputs to the cerebellum, although indirectly via the pons. Corticopontine fibers originate in the frontal lobe motor cortex and the parietal lobe somatosensory cortex and visual association areas, which project to the pontine nuclei in the pons. Neurons in the pons send outputs to the cerebellum by the middle cerebellar peduncles. The target of the pontine output is the cerebrocerebellum, which functions in coordination and smoothing of complex motor movements, evaluation of sensory information for action, and some cognitive functions. The cerebrocerebellum sends output via the dentate nucleus and fibers in the superior cerebellar peduncles to both the red nucleus and the ventral lateral nucleus of the thalamus. Hence, the thalamus provides the feedback to the motor cortex for the adjustment of movement.
The vestibulocerebellum is involved in maintenance of balance and coordinating eye movements. It receives direct input from the vestibular nerve and the vestibular nuclei and sends output to the medial and lateral vestibular nuclei by 2 pathways. One output pathway originates directly from the cerebellar cortex and sends fibers to the vestibular nuclear complex in the brainstem. The other output occurs via nuclei in the flocculonodular lobe. The vestibulocerebellum also receives visual input from the superior colliculus via the superior peduncles and is involved in coordinating eye movements and speech.
The spinocerebellum regulates body and limb movements involved in proprioception and posture. It receives proprioception input from the spinocerebellar tract, other dorsal columns of the spinal cord, and the trigeminal nerve nuclei. It also receives vestibular input and sensory information from visual and auditory systems. The spinocerebellum sends fibers to deep cerebellar nuclei (including the fastigial and interposed nuclei), for modulation of descending motor systems. The fastigial and interposed nuclei project to the cerebral cortex, via the thalamus, and to the brainstem, via the red nucleus, reticular formation in the pons, and vestibular nuclei in the medulla.
Numerous studies support the conclusion that the cerebellum plays an important role in some types of motor learning, in particular those voluntary motor actions that require fine adjustments for performance. In addition, a well-studied cerebellar learning task involves involuntary muscle contractions that underlie the eye blink conditioning response. Lesions or pharmacologic disruption of circuits to specific deep cerebellar nuclei or cerebellar cortical regions abolish learning in the conditioned eye blink response. Studies have identified candidate learning circuits involving Purkinje cells and their synapses, which undergo long-term plasticity.
The spinal cord is a long, thin, tubular bundle of nervous tissue that is continuous with the medulla and emerges from the foramen magnum, where it enters the spinal (vertebral) canal at the beginning of the cervical vertebrae (Figures 4–7 and 4–8). It extends down to the first or second lumbar region, terminating in the conus medullaris. Between 40 and 50 cm long, the human spinal cord diameter is variable along its length, between 0.6 and 1.3 cm. Two prominent grooves, or sulci, run along its length. The posterior median sulcus is the groove in the dorsal side, and the anterior median fissure is the groove in the ventral side. The spinal cord is protected by the meninges and the bony vertebral column. In the median, the central canal is an extension of the fourth ventricle and contains CSF.
The central and peripheral nervous systems. A. Location of the central and peripheral nervous system in the body. Major cranial, spinal, and peripheral nerves are shown in yellow. B. The brain and spinal cord, viewed laterally. C. The seven major divisions of the central nervous system; the spinal cord is continuous with the brainstem. (Reproduced with permission from Martin JH. Neuroanatomy: Text and Atlas, 4th ed. New York, NY: McGraw Hill; 2012.)
Anatomy of the spinal cord and spinal nerves. A. Schematic illustration of the relationships between the vertebral column, the spinal cord, and the spinal nerves. Note the mismatch between the location of spinal cord segments and of vertebral level where roots exit from the vertebral column. Note also the termination of the spinal cord at the level of the L1 or L2 vertebral body. B. Transverse sections of the spinal cord at the levels shown. C. Schematic dorsal view of isolated spinal cord and spinal nerves. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
Along its length, the spinal cord connects with the spinal nerves of the PNS. The human spinal cord contains 31 segments, consisting of cervical, thoracic, lumbar, sacral, and coccygeal regions, with each region connecting to a spinal nerve on either side. The spinal nerves enter and exit the cord via roots, which then merge into bilaterally symmetrical pairs of spinal nerves. The spinal roots, nerves, and associated ganglia functionally connect the spinal cord to the skin, muscles, joints, and viscera.
Functionally, the spinal cord transmits both somatic and autonomic motor and sensory information between the brain and the body and accomplishes important integration tasks. The spinal cord white matter is found in the more lateral regions and contains ascending sensory and descending motor tracts that are often described as columns. Located more medially, spinal cord gray matter contains motor neurons, secondary sensory neurons, and interneurons (Figure 4–9). Ten regions of gray matter are named the Rexed laminae and are numbered from dorsal to ventral. The dorsal (posterior) horns are dedicated to sensory functions (laminae I to VI). The ventral (anterior) horns are involved in motor functions (laminae VIII and IX). Between the horns is the intermediate gray region (laminae VII and X).
Illustration of the laminae of the spinal cord gray matter in the dorsal and ventral horns. The Rexed laminae of the gray matter from one half of the spinal cord are labeled. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
The spinal cord plays a major role in transmission of both somatic and visceral sensory information to the brain. Axons from somatosensory neurons and GVAs enter the spinal cord via the dorsal root from the dorsal root ganglia. Depending on the sensory modality relayed, the entering axons either ascend in the cord, forming some of the sensory tracts, or synapse with sensory relay neurons, which are located in the dorsal horn and extend axons to form other sensory tracts. The 3 main ascending white matter tracts are the DCML tract, the spinothalamic tracts, and the spinocerebellar tracts.
The DCML transmits sensory information about fine touch, vibration, 2-point discrimination, and conscious proprioception. In the DCML, the primary somatosensory axons enter the cord and form the dorsal column. If an axon enters below T6, it travels in the fasciculus gracilis of the DCML. If the axon enters above T6, it travels in the fasciculus cuneatus of the DCML. For both, the primary sensory axons ascend in the dorsal column to the lower medulla, where they synapse on sensory relay neurons (called secondary sensory neurons) located in the nucleus gracilis or nucleus cuneatus. Axons from the medullary neurons form the internal arcuate fibers, which cross and ascend to become the medial lemniscus. These axons connect to neurons in the thalamus, and the thalamic neurons project to the primary somatosensory cortex.
The spinothalamic tract (also called the anterolateral system) transmits crude touch, pain, and temperature sensation. Axons from the primary somatosensory neurons enter the spinal cord and synapse on secondary sensory neurons located in the dorsal horn. The axons of the secondary sensory neurons cross over to the contralateral side of the spinal cord and form the spinothalamic tract, which ascends in the anterolateral region of the white matter and extend all the way to the thalamus. The thalamic neurons project to the primary somatosensory cortex, cingulate cortex, and insular cortex. Traveling along side the spinothalamic tract, the spinoreticular tract and spinotectal tracts also transmit information to the brainstem.
The spinocerebellar tracts transmit unconscious proprioception from the muscles and joints to the cerebellum. The axons from primary somatosensory neurons enter the spinal cord via the dorsal root and synapse on secondary sensory neurons in or near the dorsal horn. Depending on where the sensory information originates and enters the spinal cord, the ascending axons sort into 1 of 4 tracts called the dorsal (posterior), ventral (anterior), and rostral spinocerebellar tracts and the cuneocerebellar tract. After traversing the medulla, the axons travel via the inferior cerebellar peduncle to innervate the ipsilateral cerebellum.
Part of the ANS, GVA fibers transmit visceral sensory information about local changes in the chemical and mechanical environments from the internal organs, glands, and blood vessels to the CNS. The GVAs transmit conscious sensations, such as gut distention and cardiac ischemia, and unconscious sensations, such as blood pressure and chemical composition of the blood. Some of the key functions of GVAs are to initiate autonomic reflexes at the local, ganglion, spinal, and supraspinal levels.
Visceral sensory neuronal cell bodies are located in the dorsal root ganglia of the spinal nerves. After they enter the spinal cord, the axons branch extensively and synapse on viscerosomatic neurons in the dorsal horn and intermediate gray matter. Visceral sensation is carried primarily by the spinothalamic and spinoreticular pathways, which transmit visceral pain and sexual sensations. The DCML tract may relay sensations related to micturition, defecation, and gastric distention. After relay in the thalamic nuclei, viscerosensory inputs project to the insula and other cortical autonomic areas. Viscerosomatic neurons in the dorsal gray regions that receive convergent visceral and somatic inputs have been implicated in referred pain.
The spinal cord white matter contains descending somatic motor tracts involved in both voluntary and involuntary control of muscle contraction (Figure 4–10). The axons in these tracts synapse on lower motor neurons (LMNs) and spinal interneurons (SIs) in the ventral horn. The 2 corticospinal tracts (CSTs) originate in the motor cortex with upper motor neurons. About 85% to 90% of the axons cross to the contralateral side at the pyramids of the medulla, forming the lateral CST. The remaining 10% to 15% of uncrossed axons descend on the ipsilateral side, forming the anterior CST, with most axons crossing to the contralateral side of the cord right before synapsing. The lateral CST axons synapse on dorsolateral (DL) LMNs in the ventral horn and control distal limb movement; these DL LMNs are located in the cervical and lumbosacral enlargements within the spinal cord. Anterior CST axons synapse on ventromedial (VM) LMNs in the ventral horn. The VM LMNs control the large, postural muscles of the axial skeleton and are located along the length of the spinal cord.
Somatotopic organization of several ascending sensory and descending motor tracts in the spinal cord white matter. One major descending motor tract, the lateral corticospinal tract is shown. Two major sensory tracts, the DCML composed of the gracile and cuneate fasciculus, and the spinothalamic tracts are depicted. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
Four additional motor tracts (called extrapyramidal tracts) extend axons down the spinal cord to LMNs and SIs. These tracts originate with upper motor neurons located in specific nuclei in the brainstem. Their tracts include the rubrospinal, vestibulospinal, tectospinal, and reticulospinal tracts. The rubrospinal tract descends with the lateral CST and synapses on cervical DL LMNs involved in voluntary control of arm muscles. The remaining 3 tracts descend with the anterior CST, synapse on VM LMNs, and are involved in involuntary muscle control for reflexes, locomotion, complex movements, and postural control.
LMNs in the dorsal horn receive descending inputs directly from the upper motor neurons. They also receive inputs from SIs, including a type of SI called propriospinal neurons that interconnect multiple spinal cord segments. LMNs also receive direct sensory signals from 1a somatosensory neurons and both excitatory and inhibitory SIs that relay sensory information. The axons of the LMNs exit through the ventral roots of the spinal cord and merge with the sensory components to form the spinal nerves, eventually emerging from the nerve to innervate its specific skeletal muscle.
For the autonomic motor system, preganglionic sympathetic neurons originate from the thoracolumbar region of the spinal cord, specifically at T1 to L2-L3 (Figure 4–11). A gray matter region in the intermediolateral nucleus of the lateral horn contains these neurons. These are analogous to somatic LMNs, with axons that leave the cord via the ventral root, but instead travel to either the paravertebral or prevertebral ganglia, where they synapse with the postganglionic sympathetic neurons, which extend their axons to their targets. Although the majority of parasympathetic motor neurons emerge from the brainstem via CNs, 3 spinal nerves in the sacral region (S2 to S4), commonly referred to as the pelvic splanchnic nerves, include parasympathetic preganglionic neurons located in the spinal cord. The targets of the splanchnic nerves include the bladder, colon, and genital organs.
Overview of the sympathetic (thoracolumbar) and parasympathetic (craniosacral) divisions of the autonomic nervous system. The spinal cord and brainstem contain the preganglionic neuron cell bodies, while the spinal and cranial nerves contain the axons of these neurons, which synapse on postganglionic neurons in the autonomic ganglia shown. Inf., inferior; Sup., superior. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
In addition to motor and sensory relaying functions, the spinal cord contains neuronal circuits that generate spinal reflexes, such as the stretch reflexes involved in balance and the withdrawal reflex involved in removing a limb from a noxious stimulus. The circuits within the spine also contribute to more complex movements involving central pattern generators. For example, networks responsible for locomotion have been shown to be distributed throughout the lower thoracic and lumbar regions of the mammalian spinal cord.
The spinal nerves are components of the PNS that transmit motor and sensory signals between the spinal cord and the body. As mixed nerves, the spinal nerves include both somatic motor axons that control the actions of striated muscles and somatic sensory fibers that receive sensory information from the skin, muscles, and joints. Many spinal nerves also contain autonomic motor and visceral sensory components that provide information to and from organs, smooth muscles, and glands in the body. Humans have 31 pairs of spinal nerves, 1 on each side of the vertebral column, which arise from the spinal cord and are named for the spinal cord segment where they originate, with 8 cervical pairs, 12 thoracic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 coccygeal pair. The spinal nerves branch and reorganize to form what are called peripheral nerves.
Sensory axons enter the spinal cord via small rootlets that combine to form the dorsal root (Figure 4–12). Sensory neuron cell bodies form the dorsal root ganglia that lie just outside the spinal cord. The sensory axons are called the general somatic afferents, which transmit information from the skin, muscles, and tendons, and the GVAs, which send information from the visceral organs. Motor axons leave the spinal cord via small rootlets that combine to form the ventral root. Motor neuron cell bodies lie in the ventral or lateral horn gray matter in the spinal cord itself. The motor axons include the general somatic efferents to striated muscle and the general visceral efferents, which are autonomic (sympathetic and a few parasympathetic) axons that transmit impulses to smooth muscle, cardiac muscle, and glands. The ventral and dorsal roots also provide the anchorage and fixation of the spinal cord to the vertebral cauda.
The spinal cord and spinal nerve roots. The cell bodies of neurons that transmit sensory information from the skin, muscles, joints, and viscera lie in the dorsal root ganglia adjacent to the spinal cord. The ventral roots contain axons from lower motor neurons. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM, et al: Principles of Neural Science, 5th ed. New York, NY: McGraw Hill; 2013.)
Spinal nerves initially form by the union of sensory axons from the dorsal root and motor axons from the ventral root. After joining, the spinal nerves exit the spinal canal and vertebral column. All spinal nerves, except the first spinal nerve C1 pair, emerge from the vertebral column through the intervertebral foramina between adjacent vertebrae. C1 emerges between the occipital bone and the first vertebra called the atlas. After it exits, each nerve then divides into branches called the dorsal ramus, the ventral ramus, and the rami communicates (Figure 4–13).
Schematic illustration of a spinal cord segment with its roots, ganglia, and branches. The axons from the dorsal root and ventral root merge to form a spinal nerve. Dorsal roots contain axons from somatic sensory neurons and many contain visceral sensory axons as well; ventral roots contain axons from somatic motor neurons and many contain axons from autonomic neurons as well. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
The dorsal ramus contains nerves that supply the dorsal portion of the trunk, with somatic and visceral motor and somatic sensory information relayed to and from the back muscles, dorsal muscles, and skin (Figure 4–14). The ventral ramus supplies somatic and visceral motor and sensory information to and from the ventrolateral body surface, structures in the body wall, trunk, and limbs. The rami communicantes contain autonomic axons that transmit sympathetic motor and visceral sensory information to and from the visceral organs. Additional small branches from the spinal nerves, called meningeal branches, supply nerve function to the vertebrae themselves, including the ligaments, dura, blood vessels, intervertebral disks, facet joints, and periosteum.
Many spinal nerves contain both somatic and autonomic components. Both somatic and autonomic components of spinal nerves branch, with branches called peripheral nerves and identified with specific names. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM, et al: Principles of Neural Science, 5th ed. New York, NY: McGraw Hill; 2013.)
In the thoracic region, the ventral rami, which form the intercostal nerves, remain distinct from each other, and each innervates a narrow strip of muscle and skin along the sides, chest, ribs, and abdominal wall (Figure 4–15). In other regions, ventral rami converge with each other to form networks of nerves called nerve plexuses. Within each nerve plexus, fibers from various ventral rami branch and become redistributed such that each nerve exiting the plexus has fibers from several different spinal nerves, which travel together to their target location, mainly to the limbs. The major nerve plexuses are the cervical, brachial, lumbar, and sacral plexuses.
Dermatomes—the peripheral nerve cutaneous fields. (Reproduced with permission from Haymaker W, Woodhall B. Peripheral Nerve Injuries, 2nd ed. Philadelphia, PA: Saunders/Elsevier; 1953.)
The cervical plexus is formed by the ventral rami from spinal nerves C1 to C4. The cervical plexus innervates muscles of the neck and diaphragm and the skin of the neck and upper chest. One branch forms the phrenic nerve, which provides motor innervation of the diaphragm. The brachial plexus is formed by ventral rami from spinal nerves C5 to T1. The brachial plexus provides almost all the innervation of the upper limb. The lumbar plexus contains ventral rami from spinal nerves L1 to L4. The sacral plexus contains ventral rami from spinal nerves L4 to S4. Together, the lumbar and sacral plexuses innervate the pelvic girdle and lower limbs.
Each spinal nerve receives a branch called a gray ramus communicans from the adjacent paravertebral ganglion of the sympathetic trunk. The gray rami contain postganglionic nerve fibers of the sympathetic neurons. The white rami communicantes exist only at the levels of the spinal cord where the intermediolateral cell column is present (T1-L2), which contain the sympathetic motor neuron cell bodies and are responsible for carrying preganglionic nerve fibers from the spinal cord to the adjoining paravertebral sympathetic ganglia. The 3 sacral parasympathetic spinal nerves in S2-S4 emerge from the spinal cord and form nerve plexuses with sympathetic fibers called the pelvic splanchnic nerves (Figure 4–16). Table 4–2 lists the peripheral nerves and their functions.
The nerve plexuses and peripheral nerves. A. Side view showing the spinal cord segments and two spinal cord enlargements. B. Dorsal view showing many of nerve plexus and peripheral nerves.
TABLE 4–2The peripheral nerves and their functions. ||Download (.pdf) TABLE 4–2 The peripheral nerves and their functions.
|Name ||Spinal Nerves Involved ||Function |
|Musculocutaneous nerves ||C5-T1 ||Supply muscles of the arms on the anterior sides, and skin of the forearms |
|Radial nerves ||C5-T1 ||Supply muscles of the arms on the posterior sides, and skin of the forearms and hands |
|Median nerves ||C5-T1 ||Supply muscles of the forearms, and muscles and skin of the hands |
|Ulnar nerves ||C5-T1 ||Supply muscles of the forearms and hands, and skin of the hands |
|Phrenic nerves ||C3-C5 ||Supply the diaphragm |
|Intercostal nerves ||T2-T12 ||Supply intercostal muscles, abdominal muscles, and skin of the trunk |
|Femoral nerves ||L2-L4 ||Supply muscles and skin of the thighs and legs |
|Sciatic nerves ||L4-S3 ||Supply muscles and skin of the thighs, legs and feet |
THE ENTERIC NERVOUS SYSTEM
Formerly thought to be part of the ANS, the ENS is now considered a separate component of the PNS. The ENS consists of a system of approximately 200 to 600 million neurons and glial cells distributed in many thousands of small ganglia in the lining of the GI system. Two types of ENS ganglia have been identified, called the myenteric and submucosal plexuses. The myenteric plexus forms a continuous network that extends from the upper esophagus to the internal anal sphincter. Submucosal ganglia and connecting fiber bundles form plexuses in the small and large intestines, but not in the stomach and esophagus. The ENS forms a neuronal circuitry that controls or modulates most aspects of GI function, including motility and GI transit, secretion and adsorption, water and electrolyte balance, chemical sensing, and communication between intestinal segments and the CNS.
The ENS is derived from the neural crest. A variety of different subtypes of enteric neurons and glia differentiate from neural crest progenitors that undergo proliferation and migration into the GI tract during embryonic and postnatal development. The ENS includes motor (efferent) neurons, sensory (afferent) neurons, and interneurons, all of which make the ENS capable of generating reflexes and acting as an integration center in the absence of input from the CNS or ANS. The ENS controls various types of primary GI effector cells, including epithelial cells and smooth muscle cells, which mediate GI functions. ENS sensory neurons report on mechanical and chemical conditions. Motor neurons control peristalsis, the coordinated contraction and relaxation of intestinal smooth muscles, and churning of intestinal contents. Contact with the GI epithelium can modulate nutrient uptake. Alteration of regional blood flow can allow responses to metabolic needs. Other neurons control the secretion of enzymes. ENS interneurons coordinate the various activities.
In addition to containing the ENS, the digestive system is innervated and controlled by both the CNS and ANS. For example, movements of the striated muscles of the esophagus are determined by neural pattern generators in the CNS. Sympathetic stimulation causes inhibition of GI secretion and motor activity and contraction of GI sphincters and blood vessels. Conversely, parasympathetic stimuli typically stimulate these digestive activities. Thus, although the ENS can function as an independent system, it works in concert with CNS reflex and ANS command centers to control digestive function. Moreover, there is bidirectional information flow between the ENS and CNS and between the ENS and ANS, via the pelvic nerves and ANS pathways. Neurons also project from the ENS to prevertebral ganglia and the gallbladder, pancreas, and trachea. Approximately 30 different types of neurotransmitters have been identified in the ENS, including acetylcholine and glutamate. Interestingly more than 90% of the body’s serotonin and about 50% of the body’s dopamine are found in the intestine.
THE MENINGES & VENTRICULAR SYSTEM
Both the brain and spinal cord are covered and protected by the meninges (Figure 4–17), which is composed of 3 layers: the dura mater, arachnoid mater, and pia mater. The dura mater is the outermost layer, which is a thick, tough double membrane composed of collagenous connective tissue. The middle layer is called the arachnoid mater because of spider web–like processes called arachnoid trabeculae. Composed of a fine collagenous layer, regions of the arachnoid mater extend toward the third layer, the pia mater, and also contain the arachnoid granulations and villi that resorb CSF. The pia mater is a thin, delicate translucent layer of connective tissue that attaches to the outermost region of neural tissue, called the glia limitans, a thin barrier of astrocyte foot processes associated with the parenchymal basal lamina.
The meninges surround the CNS. A. Schematic illustration of a coronal section through the brain and coverings, and an enlargement of the area at the top showing the meninges, scull and scalp. B. The coverings around the cerebral cortex showing the three layers of the meninges and their spaces. (Part A, reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017; part B, reproduced with permission from McKinley MP, O’Loughlin VD, Bidle TS. Anatomy and Physiology: An Integrative Approach. New York, NY: McGraw Hill; 2013.)
The arachnoid mater is attached to the dura mater, whereas the pia mater is attached to the CNS tissue. Between the arachnoid mater and the pia mater is the subarachnoid space, which contains CSF and blood vessels. The CSF serves to support the CNS and to cushion and protect it from physical shock and trauma. The falx cerebri is a double-fold of dura mater that descends through the interhemispheric fissure in the midline of the brain to separate the cerebral hemispheres and is attached to the cerebellar tentorium. The dural venous sinuses are venous channels found between the endosteal and meningeal layers of dura mater in the brain. The transverse and superior sagittal sinuses are the largest dural venous sinuses. The space between the bone and the dura mater is known as the epidural space, which is prominent in the spinal canal where it contains spinal nerve roots, connective tissue, and blood vessels.
The ventricular system is a series of interconnected, CSF-filled spaces, called ventricles, that lie in the core of the forebrain and brainstem (Figure 4–18). The ventricles produce CSF and circulate CSF. The ventricles originate as the inside or lumen of the neural tube during fetal development. The lumen of the telencephalon gives rise to the left and right lateral ventricles (formerly called the first and second ventricles). The largest of the ventricles, each lateral ventricle has a C shape that mirrors the cerebral hemisphere where it is located, and each contains a posterior horn that extends into the occipital lobe and an anterior horn that extends into the frontal lobe. The third ventricle originates from the diencephalon and is located in the midline between the left and right halves of the thalamus. CSF flows from the lateral ventricles through 2 small openings (called the interventricular foramen or foramen of Monro) into the third ventricle.
The brain ventricular system. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
The third ventricle is continuous caudally with the cerebral aqueduct (also called the aqueduct of Sylvius). The cerebral aqueduct, which forms from the lumen of the mesencephalon, proceeds though the midbrain and then opens into the fourth ventricle. The fourth ventricle arises from the lumen of the metencephalon and myelencephalon and is located in the dorsal or roof of the pons and medulla. At the posterior region of the medulla, the fourth ventricle narrows to form the central canal of the spinal cord. As it flows from the fourth ventricle, some CSF is diverted to the subarachnoid space and some continues through the central canal.
CSF is a clear extracellular fluid that is constantly synthesized and resorbed. CSF is produced by a modified vascular structure called the choroid plexus, which is present in each of the 4 ventricles. The ventricles are lined by glial cells called ependymal cells. The choroid plexus consists of a layer of specialized choroid ependymal cells (CECs) surrounding a core of capillaries and loose connective tissue. CSF is formed as plasma is filtered from the blood through the CECs. The CECs use active transport mechanisms to transport ions and glucose into the ventricles, and water follows the resulting osmotic gradient. Hence, the ependymal cells form the blood–CSF barrier in the choroid plexus, which serves the same purpose as the blood–brain barrier in the rest of the brain. In addition to the production of CSF, the choroid plexus also acts as a filter to remove metabolic waste and excess neurotransmitters from the CSF. CSF is produced at a rate of about 600 mL/d, replacing the entire volume about once every 5 to 6 hours.
As it is produced, CSF flows from the lateral ventricles to the third and then the fourth ventricle. From the fourth ventricle, CSF can flow through the median aperture (foramina of Magendie) and 2 lateral apertures (foramina of Luschka) to the cisterna magna. From there, CSF flows to the subarachnoid space around the brain and spinal cord, where it resorbed by specialized structures called arachnoid villi and granulations and returned to the venous circulation (Figure 4–19).
Schematic illustration, in coronal projection, of the circulation (arrows) of CSF. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
In other parts of the body, circulation in the lymphatic system participates in the clearing of extracellular waste products and damaged cells from tissues and the movement of immune cells, such as white blood cells. For many years, a similar system was thought to be absent from the brain. However, recent studies have demonstrated the presence of lymphatic vessels that run parallel with blood vessels in the meninges. A potential role for brain lymphatic vessels may be to provide a route for transport of fluid and immune cells such as T cells from the CSF.
The CNS is highly vascularized. The adult cerebral blood flow is approximately 750 mL/min, consuming about 15% to 20% of the cardiac output. Similar to other organs and tissues, the arteries deliver oxygen, glucose, and other nutrients to the brain, and the veins carry deoxygenated blood back to the heart, removing carbon dioxide and other metabolic products. The larger arteries and veins branch to smaller arterioles and venules, and then to smaller capillaries, which supply and remove blood from the nervous tissue. Brain capillaries are lined by specialized vascular endothelial cells joined by tight junctions and covered by pericytes and astrocytic end feet, so hydrophilic molecules are unable to diffuse directly into the brain, thereby creating the blood–brain barrier. Astrocytic end feet transport specific nutrients from the blood into the brain.
The entire blood supply to the brain arises from 2 paired arteries, the internal carotid arteries and vertebral arteries, which form from branches of the dorsal aorta and ascend to the cranium (Figure 4–20). The internal carotid arteries arise from the bifurcation of the left and right common carotid arteries, on each side of the head and neck, and supply blood to the front and middle regions of the brain. The vertebral arteries emerge as branches of the left and right subclavian arteries, ascend separately, and converge near the base of the pons to form the unpaired basilar artery and supply blood to the back of the brain. The blood supply to the spinal cord is via the vertebral arteries, which branch to form the anterior and posterior spinal arteries, and the medullary arteries.
Major cerebral arteries. The arterial blood supply to the brain arises from the internal carotid arteries and vertebral arteries. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
The internal carotid arteries enter the cranium through the carotid canal of the temporal bone, travel through the cavernous sinus, and penetrate the dura just ventral to the optic nerve. At the level just lateral to the optic chiasm, the internal carotid artery branches to form the anterior cerebral artery and continues to form the middle cerebral artery (Figure 4–21). The left and right anterior cerebral arteries supply blood to most medial portions of the frontal lobes and anterior parietal lobes. The anterior cerebral arteries travel along the sphenoid bone of the eye socket, then upward through the insula cortex, where final branches arise. The middle cerebral artery is the larger branch of the internal carotid and continues into the lateral sulcus, where it then branches and projects to lateral portions of the frontal, temporal, and parietal lobes. It also supplies blood to deeper structures of the basal forebrain and the insular cortices.
Arterial blood supply to different regions of the brain. A. Circle of Willis and principal arteries of the brainstem. B. Anterior view of the brain showing the arteries (a.) with the cerebral hemispheres separated. C. Lateral view of the brain showing the arteries (a.). (Part A, reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017; parts B and C, reproduced with permission from Morton DA, Foreman KB, Albertine KH. The Big Picture: Gross Anatomy. New York, NY: McGraw Hill; 2011.)
The vertebral arteries ascend upward through foramen transversarium of the cervical spine and enter the cranial cavity via the foramen magnum, emerging as 2 vessels, 1 on the left and 1 on the right of the medulla. Within the cranial vault, each vertebral artery gives off 3 branches that form the posterior inferior cerebellar artery and 2 spinal arteries. The vertebral arteries then join in front of the middle part of the medulla to form the larger basilar artery, which sends multiple branches to supply the medulla and pons, and the anterior inferior cerebellar artery. Finally, the basilar artery terminates by bifurcating into the posterior cerebral arteries and superior cerebellar artery. The posterior cerebral arteries supply the midbrain, thalamus, and subthalamic nucleus. They also travel outward, around the superior cerebellar peduncles and top of the cerebellar tentorium, where they send branches to supply the temporal and occipital lobes. The blood supply to the medulla includes the anterior spinal artery, the posterior inferior cerebellar artery, and the vertebral artery’s direct branches.
The carotid and vertebral systems join together to form the cerebral arterial circle (circle of Willis), a ring of connected arteries that lies in the interpeduncular cistern between the midbrain and pons. The circle is formed by the posterior cerebral arteries, the posterior communicating arteries, and the internal carotids (from a region immediately proximal to the origin of the middle cerebral arteries, the anterior cerebral arteries, and the anterior communicating artery). Importantly, the circle of Willis provides alternative inputs to the internal carotid and posterior cerebral arteries, which is crucial in the event of stroke.
The brain is drained by a system of veins that empty into the dural sinuses, which eventually empty into the internal jugular veins (Figure 4–22). The brain has 2 main networks of veins: an exterior 3-branch network on the surface of the cerebrum and an interior network. The exterior and interior networks communicate via anastomosing (joining) veins. In the brain, the veins drain into larger cavities called the dural venous sinuses, which are typically located between the dura mater and the covering of the skull.
Organization of veins and sinuses involved in venous drainage of blood from the brain. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
The great cerebral vein drains blood from the cerebellum and midbrain. The spinal veins or adjacent cerebral veins drain blood from the medulla and pons. Blood in the deep parts of the brain drains into region-specific sinuses. For the exterior network, the superior sagittal sinus, which is located at the midline at the top of the brain, receives blood from the outer portion of the brain and combines its drainage with the outflow from the straight sinus, at the confluence of sinuses, which drains into the transverse sinuses. Next, these drain into the sigmoid sinuses, which also receives blood from the cavernous sinus and superior and inferior petrosal sinuses. The sigmoid sinuses then drain into the left and right internal jugular veins.
The spinal cord is primarily supplied by 3 longitudinal arteries, the anterior spinal artery and 2 posterior spinal arteries, and the medullary arteries (Figure 4–23). In the cranium, at the medullary level, each vertebral artery branches to produce the anterior spinal artery and the posterior inferior cerebellar artery, which in about 75% of people gives rise to the posterior spinal artery. The anterior spinal artery travels along the midline of the ventral surface of the spinal cord and gives rise to the sulcal arteries, which enter the spinal cord and supply blood to the anterior two-thirds of the spinal cord. The paired posterior spinal arteries descend on the dorsolateral surface of the spinal cord slightly medial to the dorsal roots and supply blood to the posterior third of the cord.
Vascularization of the spinal cord (ventral view). (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 28th ed. New York, NY: McGraw Hill; 2017.)
Additional arterial supply occurs via the anterior and posterior segmental medullary arteries, which arise from the vertebral and other arteries and join with the anterior and posterior spinal arteries along the length of the cord. Venous drainage of the cord occurs via 3 anterior and 3 posterior spinal veins, which in turn empty into the systemic segmental veins. The internal vertebral plexus also empties into the dural venous sinuses. Radicular veins also contribute to venous drainage of the cord.
The brainstem and cerebellum develop from the mesencephalon and rhombencephalon vesicles of the neural tube.
The midbrain, pons, and medulla form the brainstem, which contains nuclei for essential automatic, reflex, and autonomic functions that are critical for survival, and white matter tracks that connect the forebrain with the cerebellum and spinal cord.
The brainstem houses the reticular formation, a set of interconnected nuclei located from the upper midbrain to the lower medulla involved in sleep and wakefulness, arousal and alertness, consciousness, and other motor and sensory functions.
The midbrain encompasses nuclei in the superior and inferior colliculi involved in vision and hearing, nuclei for cranial nerves III and IV, and white matter regions including the cerebral peduncles.
Also located in the midbrain are the substantia nigra and ventral tegmental area, 2 regions that contain dopaminergic neurons that contribute to the basal ganglia and are involved in motor control and the reward pathway.
The pons contains nuclei that relay signals between the forebrain and cerebellum and nuclei involved in sleep, respiration, swallowing, and bladder control.
The pons also houses nuclei for cranial nerves V to VIII, which are involved in hearing and taste, equilibrium, eye movement, facial expressions, and facial sensations.
The pons includes the cerebellar peduncles, which are large white matter areas that provide the routes for all information transmitted to and from the cerebellum.
The medulla includes nuclei involved in the automatic reflex of breathing and the autonomic regulation of heart rate and blood pressure, as well as control of several somatic functions.
The medulla contains the nuclei for cranial nerves IX to XII and a white matter region called the pyramids, where the corticospinal tract and dorsal column medial lemniscus tract travel and decussate.
The cranial nerves emerge from either the cerebrum or brainstem and transmit both special sense and somatic motor and sensory information between the brain and the head, face, and neck.
The cranial nerves also supply parasympathetic motor control to and receive visceral sensory information from the head, neck, and organs of the body.
The cerebellum functions in motor control, including coordination, precision and timing of movements, and motor learning, and has been implicated in cognitive functions and emotion.
A component of the CNS, the spinal cord is a thin tube of nervous tissue that is enclosed in the vertebral canal, protected by meninges and the vertebral column, and divided into cervical, thoracic, lumbar, sacral, and coccygeal regions along its length.
The spinal cord contains white matter tracts and gray matter regions that transmit and relay motor signals from the CNS to the body and sends sensory signals from the PNS to the CNS.
Motor axons exit the spinal cord via the ventral roots, whereas sensory axons enter the spinal cord via the dorsal roots, forming the spinal nerves with 31 pairs along the length of the cord.
The peripheral nerves assemble from branches of the spinal nerves and innervate the body and limbs.
The enteric nervous system involves neurons and glia located in the gastrointestinal tract that function in regulation of peristalsis and secretions.
The meninges include the dura mater, arachnoid mater, and pia mater, which cover, protect, and cushion the brain and spinal cord.
The brain ventricular system unites the brain ventricles, which house the choroid plexuses that synthesize cerebrospinal fluid and circulate it within and around the brain and spinal cord.
The CNS arterial system arises from 2 branches from the dorsal aorta: the internal carotid arteries, which supply the anterior and middle regions of the brain, and the vertebral arteries, which supply the posterior brain, brainstem, and spinal cord.
The CNS venous system involves numerous venous sinuses that drain into the jugular vein.