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Motor Pathways
by Annie Burke-Doe, PT, MPT, PhD
Practicing physical therapist and associate professor at the University of St. Augustine for Health Sciences in San Diego, California

Slide 1: Motor Pathways

Welcome to Neuroanatomy in Physical Therapy. I'm Dr. Annie Burke-Doe, a practicing physical therapist and an associate professor at the University of St. Augustine for Health Sciences in San Diego, California. In this lecture, we will be looking at the organization of the motor system, specifically, the corticospinal tract and other motor pathways. At the end of this section, you should be able to: describe the anatomy and functions of the spinal cord including nuclei and laminae; discuss spinal cord blood supply; identify the site of origin, decussation, and levels of termination for the corticospinal tract and other motor pathways; describe the autonomic nervous system, division, fibers, neurotransmitters, and regulation; differentiate upper and lower motor neurons in the nervous system; and describe common gait disorders in neurology.

Slide 2: Primary Sensory and Motor Areas

Here, in slide 2, the primary sensory and motor areas are shown. Remember that these areas are located on either side of the central sulcus, which divides the frontal lobe from the parietal lobe. The primary motor cortex in red is in the precentral gyrus while the primary sensory cortex is in the post-central gyrus. There are several important areas of motor association cortex that lie just anterior to the primary motor cortex including the supplemental motor area in green and the premotor cortex in orange. These regions are involved in higher order motor planning and project to the primary motor cortex. Similarly, somatosensory association cortex in the parietal lobe receives inputs from the primary somatosensory cortex and is also important in higher order sensory processing.

Slide 3: Somatotopic Organization

Functional mapping and lesion studies have demonstrated that the primary motor and somatosensory cortices are what is described as somatotopically organized. Somatotopic organization ensures that information from a specific area of the body is represented in a specific area of the cortex. Fibers in a pathway are arranged so that information from the lower part of the body travels in a particular pathway or part of the central nervous system that is different from information from the upper parts of the body. For example, information related to the lower part of the body travels in fibers located medially in the white matter of the spinal cord while those from the upper part of the body travel in fibers located at the lateral parts. Thus, the pathway is said to have somatotopic or spatial orientation. This spatial orientation ensures that the point of origin of the signal is maintained all the way to the final point of termination of the pathway. Let's look at this more closely.

Slide 4: Sensory & Motor Homunculus

The cortical maps depicted here—on the left, the sensory homunculus (homunculus meaning "little man" in Latin), and, on the right, the motor homunculus—were developed to assist us in understanding broadly that regions of the cortex correspond to regions in the body. These depictions are used to assist us for clinical localization of cortical function. The resulting image is a grotesquely disfigured human with disproportionately huge hands, lips, and face in comparison to the rest of the body. Because of the fine motor skills and sense nerves found in these particular parts of the body, they are represented as being larger on the homunculus. A part of the body with fewer sensory and/or motor connections to the brain is represented to appear smaller.

Slide 5: Spinal Cord

When looking at spinal cord anatomy, the cord is divided into gray matter, depicted here on the left, and surrounding white matter. The spinal cord contains an "H" or butterfly-shaped central gray matter surrounded by ascending and descending white matter columns, also called funiculi. Sensory neurons in the dorsal root ganglia have axons that bifurcate. One branch conveys the sensory information from the periphery, and the other carries the information through the dorsal nerve root filaments into the dorsal aspect of the spinal cord. The central matter has a dorsal posterior horn that is involved mainly in sensory processing; an intermediate zone that contains interneurons and certain specialized nuclei; and a ventral horn, or anterior horn, that contains motor neurons. Motor neurons send their axons out of the spinal cord via the ventral nerve root filaments. The spinal gray matter can also be divided into nuclei or—using a different nomenclature—intralaminar, named by Bror Rexed, with different functions that will be discussed later. The spinal cord white matter consists of dorsal posterior columns, lateral columns, and ventral anterior columns.

Slide 6: Spinal Cord

Along its length, the spinal cord varies in size and shape. The white matter, made up of longitudinal tracts, is thickest in the cervical level here at the top where most ascending fibers have already entered the cord and most descending fibers have not yet terminated on their targets, while the sacral cord is mostly gray matter. In addition, the spinal cord has two enlargements: the cervical enlargement and the lumbosacral enlargement, which give rise to the nerve plexus for the arms and legs, respectively. The spinal cord has more gray matter at the cervical and lumbosacral levels than at the thoracic levels, particularly in the ventral horns. In the thoracic cord, a lateral horn is presented that contains the intermediolateral cell columns. The intermediolateral cell columns exist at vertebral levels T1 to L2 and mediate the entire sympathetic innervation of the body.

Slide 7: Blood Supply to the Spinal Cord

The blood supply to the spinal cord arises from branches of the vertebral arteries and the spinal radicular arteries. These vertebral arteries give rise to the anterior spinal artery that runs along the ventral surface of the spinal cord. In addition, two posterior spinal arteries arise from the vertebral or posterior inferior cerebellar arteries and supply the dorsal surface of the cord. The anterior and posterior spinal arteries are variable in prominence at different levels and form a spinal arterial plexus that surrounds the spinal cord. Thirty-one segmental arterial branches enter the spinal canal along its length. Most of the branches arise from the aorta and supply the meninges. Only 6 to 10 of these reach the spinal cord as radicular arteries, arising again at variable levels, depending on the person. The great radicular artery of Adamkiewicz, typically a prominent artery arising from the left side anywhere from T5 to L3 but typically seen between T9 and T12, provides the major blood supply to the lumbar and sacral cord. The mid-thoracic region, approximately T4 to T8, lies between the lumbar and vertebral artery supply and is a vulnerable zone of relatively decreased perfusion. This region is very susceptible to infarction during thoracic surgery or other conditions causing decreased aortic pressure. The anterior spinal artery supplies approximately the anterior two-thirds of the cord including the anterior horns and the anterior lateral white matter columns. The posterior spinal arteries supply the posterior dorsal column and part of the posterior horns.

Slide 8: Motor Systems

When we think about the complexity of movement required for athletic performance, playing an instrument, or other skilled movement, we can understand that motor systems involve elaborate networks or neuronal circuits that communicate through many hierarchical feedback loops. A summary of motor systems are presented here with only the principal motor loops depicted and the important sensory inputs omitted. Remember that the cerebellum and the basal ganglia, pictured, participate in these feedback loops and project back to the cerebral cortex via the thalamus. They do not themselves project to lower motor neurons. Previously, we discussed that within the cerebral cortex itself are numerous motor circuits for motor control, and they project to the basal ganglia pictured here in blue. The basal ganglia are a massive gray matter that lies within each hemisphere deep to the floor of the lateral ventricles. They are embedded in white matter that projects through the thalamus back to the cortex. The basal ganglia are involved with subconscious control of skeletal muscle tone and the coordination of learned movement patterns. Under normal conditions, these nuclei do not initiate particular movements, but once the movement has started, they are going to provide general pattern and rhythm, especially for the trunk and the proximal limb muscles. Cerebral areas, including association cortex regions such as the supplementary motor area, premotor cortex, and parietal association cortex, are crucial to planning and formulation of motor activities. Lesions of these regions of association cortex can lead to what we call apraxia, in which there is a deficit of higher order motor planning and execution, despite normal strength. Cerebellar feedback loops assist with programming and fine-tuning of movements controlled at the conscious and subconscious levels. The cerebellum refines learned movement patterns by comparing motor commands with proprioceptive information and performs adjustments needed to make movements smooth. Damage to these areas can cause what we call ataxia, or lack of order, a disturbance of muscle coordination.

Slide 9: Upper Motor and Lower Motor Neurons

Recall that somatic motor pathways involve at least two motor neurons: an upper motor neuron, whose cell body lies in a central nervous system processing center, and a lower motor neuron, depicted here in blue, whose cell body lies in a nucleus of the brain stem as it relates to cranial nerves or in the spinal cord as it relates to peripheral nerves. The upper motor neuron synapses on the lower motor neuron and innervates a single motor unit in a skeletal muscle. Activity in the upper motor neuron may facilitate or inhibit the lower motor neuron. Activation of the lower motor neuron triggers a contraction in the innervated muscle. Only the axon of the lower motor neuron extends outside the CNS. Destruction or damage to a lower motor neuron will eliminate voluntary and reflex control over the innervated motor unit.

Slide 10: Primary Descending (Motor) Pathways

This table describes the primary descending motor pathways by location of upper motor neurons, destination, side of crossover or decussation, and their role in motor function. Descending motor pathways can be divided into lateral and medial motor systems based on their location in the spinal cord. The two lateral motor systems are the lateral corticospinal tract and the rubrospinal tract, which control movements of the extremities. The lateral corticospinal tract in particular is essential for rapid dexterous movement at individual digits or joints. Most of these pathways cross over from their site of origin and descend in the contralateral spinal cord to control contralateral extremities. That is why the left side of the brain controls the right side of the body. The four medial motor systems are the anterior corticospinal tract, the vestibulospinal tract, the reticulospinal tract, and the tectospinal tract. These pathways control proximal axial and girdle muscles involved in postural tone, balance, orienting movements of the head and neck, and automatic gait-related movements. The medial motor systems descend ipsilateral or bilaterally. Some descend only to the upper few cervical segments, as we will see depicted in the following slides. The medial motor systems tend to terminate on interneurons that project to both sides of the spinal cord, controlling movements that involve multiple bilateral spinal segments. Because of this, unilateral lesions of the medial motor systems produce no obvious deficits. The rubrospinal tract in humans is small, and its function is not well understood. It is thought to play a role in motor function after corticospinal tract injury and in flexor or decorticate posturing of the upper extremities, which is seen with lesions above the level of the red nuclei.

Slide 11: Somatotopic Organization of Medial and Lateral Motor System Projections to Anterior Horn Cells

When we look at the somatotopic organization of the spinal cord, here we are seeing the medial and lateral motor systems projecting to anterior horn cells. Our lateral motor systems—corticospinal and rubrospinal tracts in red—project to the lateral anterior horn cells, while the medial motor systems—anterior corticospinal, vestibulospinal, reticulospinal, and tectospinal in blue—project to the medial anterior horn cells. Lateral anterior horn cells control distal muscles of the extremity while medial anterior horn cells control proximal trunk musculature.

Slide 12: Corticospinal Tract

Let's now take a look at the descending motor tracts. The corticospinal tract is a clinically important descending motor pathway that consists of a lateral and an anterior portion. This pathway is sometimes called the pyramidal system because of its relationship to the medullary pyramids. The corticospinal tract provides voluntary control over skeletal muscles. This system begins in the pyramidal cells of the primary motor cortex, and the axons of these upper motor neurons descend into the brain stem and spinal cord to synapse on lower motor neurons, depicted in red, that control skeletal muscles.

Slide 13: Corticospinal Tract

When looking at the pathway of the corticospinal tract, over half of the fibers originate in the primary motor cortex, Brodmann area 4, of the precentral gyrus. The remainder arise from the premotor and supplemental motor area 6, or from the parietal lobe areas 3, 1, 2, 5, and 7. The primary motor cortex neurons contributing to the corticospinal tract are located mostly in cortical layer 5. Layer 5 pyramidal cell projection synapses directly onto motor neurons in the ventral or anterior horn of the spinal cord, as well as onto spinal interneurons. About 3% of corticospinal neurons are giant pyramidal cells called Betz cells, which are the largest neurons in the human nervous system. Axons from the cortex enter the upper portions of the cerebral white matter or the corona radiata and descend toward the internal capsule. Remember that in addition to the corticospinal tract, the cerebral white matter conveys bidirectional information between different cortical areas and between cortex and deep structures, such as the basal ganglia, the thalamus, and the brain stem. These white-matter pathways form a fan-like structure as they enter the internal capsule, which condenses down to fewer and fewer fibers as connections to different subcortical structures are made. Let's continue to follow the tract over the next few slides.

Slide 14: Internal Capsule

Again, the corticospinal tract begins in the motor cortex and descends into the internal capsule. The internal capsule, or projection fibers, link the cortex to the diencephalon, the brain stem, the cerebellum, and the spinal cord. All projection fibers must pass through the diencephalon where axons ascending to sensory areas of the cortex pass among the axons descending from motor areas of the cortex. The internal capsule is best appreciated in this horizontal brain section, in which the right and left internal capsules look like arrowheads, here in red, or two letter "V"s with their points facing each other. Note, that the thalamus and caudate nucleus are always medial to the internal capsule, while the globus pallidus and the putamen are always lateral to the internal capsule.

Slide 15: Internal Capsule

Here is a schematic representation of the horizontal view showing the three parts of the internal capsule: the anterior limb on top, the posterior limb on the bottom, and the genu. Note that the anterior limb of the internal capsule separates the head of the caudate from the globus pallidus and putamen while the posterior limb separates the thalamus from the globus pallidus and the putamen. The genu, or "knee" in Latin, is the transition between anterior and posterior limbs at the level of the foramen of Monro. The corticospinal tract lies in the posterior limb of the internal capsule, and you can see that the somatotopic map is preserved so fibers of the face, "F," are more anterior, and those of the arm, letter "A," and leg, letter "L," are progressively more posterior. Fibers projecting from the cortex to the brain stem, including motor fibers for the face, are called corticobulbar instead of corticospinal because they project from the cortex to the brain stem or bulb. Despite this somatotopic arrangement, the fibers of the internal capsule are compact enough that lesions at this level generally produce weakness of the entire contralateral body—face, arm, and leg.

Slide 16: Corticospinal Tract

As we continue descending from the internal capsule, the corticospinal tracts meet the cerebral peduncles, or feet of the brain. The white matter is located in the ventral portion of the cerebral peduncles and is called the basis pedunculi. The corticospinal tract fibers' necks descend through the ventral pons. These form the medullary pyramids. Again, for this reason, the corticospinal tract is sometimes referred to as the pyramidal tract. The transition from the medulla to the spinal cord is called the cervicomedullary junction and occurs at the level of the foramen magnum. At this point, about 85% of the pyramidal tract fibers cross over in the pyramidal decussation to enter the lateral white matter columns of the spinal cord forming the lateral corticospinal tract. A somatotopic representation is present in the lateral corticospinal tract, with fibers controlling the upper extremity located medially to those controlling the lower extremity. Finally, the axons in the lateral corticospinal tract enter the spinal cord central gray matter to synapse onto anterior motor horn cells. The remaining approximately 15% of the corticospinal fibers continue into the spinal cord ipsilateral without crossing and enter the anterior white matter columns to form the anterior corticospinal tract.

Slide 17: Descending Motor Pathways

Again, here we can see the lateral corticospinal tract pathway, as well as the rubrospinal tract, which are responsible for contralateral motor function. Note, on the right, the rubrospinal tract is small in humans and terminates in the cervical cord. It is thought to be responsible for taking over functions after corticospinal tract injury and may also play a role in flexor or decorticate posturing in the upper extremities with lesions above the level of the red nucleus in which the rubrospinal tract is preserved.

Slide 18: Descending Motor Pathways

On this slide, we have on the left the anterior corticospinal tract terminating at the cervical and upper thoracic cord, and it has control of the bilateral axilla and girdle muscle, and on the right, the vestibulospinal tract with medial fibers terminating in the cervical and upper thoracic cord and lateral portions, which are still considered part of the medial system, terminating in the entire cord. Vestibulospinal tracts are responsible for positioning of the head and neck (the medial fibers) and balance (the lateral fibers).

Slide 19: Descending Motor Pathways

The final two descending motor pathways are the tectospinal tract, terminating in the cervical cord, and the reticulospinal tract, terminating along the entire cord. The tectospinal tract is responsible for coordination of head and eye movements, and this is poorly understood in humans. The reticulospinal tract is responsible for automatic posture and gait-related movements.

Slide 20: The Autonomic Nervous System (ANS)

In contrast to the somatic motor pathways described in the preceding slides, the autonomic nervous system generally controls more automatic and visceral body functions. The autonomic efferents are unique anatomically. There is a peripheral synapse located in a ganglion that's between the central nervous system and the effector gland, or smooth muscle. There are sensory (afferent) inputs to the autonomic nervous system, both centrally and in the periphery. However, the autonomic nervous system itself consists of only efferent pathways.

Slide 21: ANS Divisions

The autonomic nervous system has two main divisions: the sympathetic, also called thoracolumbar, which arises from thoracic level 1 to lumbar level 2 or 3. The sympathetic division is pictured here on the left side of the slide and is involved mainly in "fight or flight" functions, such as increasing heart rate and blood pressure, bronchodilation, and increasing pupil size. The other main division, the parasympathetic division, also called the craniosacral division, in contrast arises from cranial nerve nuclei and from sacral levels 2 through 4, and it is involved in the "rest and digest" functions, such as increasing gastric secretions and peristalsis, slowing the heart rate, and decreasing the pupil size. The enteric nervous system is considered a third division of the autonomic nervous system consisting of a neural plexus lying in the walls of the gut that is involved in controlling peristalsis and gastrointestinal secretions. Preganglionic neurons of the sympathetic division form a chain called the sympathetic trunk, pictured in red. The sympathetic trunk runs all the way from cervical to sacral levels on each side of the spinal cord. The sympathetic trunk allows sympathetic efferents, which exit only at thoracolumbar levels, to reach other parts of the body as well. Parasympathetic preganglionic fibers arise from cranial nerve parasympathetic nuclei and from sacral nerve parasympathetic nuclei and are located in gray matter of S2, 3, and 4.

Slide 22: Autonomic Motor Fibers

Here we can see the somatic motor system in pink compared with the autonomic sympathetic visceral motor efferents in blue. Preganglionic neurons of the sympathetic division are located in an intermediolateral cell column in lamina 7 of the spinal cord at levels T1 to L2 or 3. With sympathetic stimulation, efferent motor commands will travel to the sympathetic ganglia that innervate post-ganglionic fibers to smooth muscles, glands, and organs.

Slide 23: ANS Neurotransmitters

When looking at autonomic nervous system neurotransmitters, synaptic neurotransmission in cholinergic preganglionic neurons in both the sympathetic and parasympathetic ganglia is mediated by the neurotransmitter acetylcholine at nicotinic receptors. The sympathetic and parasympathetic systems differ in terms of their post-ganglionic neurotransmitters. In the sympathetic post-ganglion here at the top, neurons release predominantly norepinephrine onto end organs—our target tissue—and activate adrenergic alpha or beta receptors. Parasympathetic post-ganglionic neurons, here at the bottom, release predominantly acetylcholine, and activate muscarinic cholinergic receptors at end organs. Noradrenergic or adrenergic alpha 1, alpha 2, beta 1, beta 2, and beta 3 subtypes, and cholinergic muscarinic 1, muscarinic 2, and muscarinic 3 receptor subtypes mediate different actions of these neurotransmitters on end organs. One notable exception to the norepinephrines sympathetic/acetylcholine parasympathetic rule is for post-ganglionic neurotransmitters at sweat glands, which are innervated by sympathetic post-ganglionic neurons that release acetylcholine.

Slide 24: ANS Regulation

When looking at the autonomic nervous system and its regulation, sympathetic and parasympathetic outflow are controlled both directly and indirectly by higher centers including the hypothalamus, brain stem nuclei such as the nucleus solitarius, the amygdala of the limbic system, and several other regions of limbic cortex. Autonomic responses are also regulated by afferent sensory information including signals from internal receptors such as chemoreceptors, osmoreceptors, thermoreceptors, and baroreceptors. This regulation will adjust and coordinate the activities of the autonomic centers that are located in the pons and the medulla that regulate heart rate, blood pressure, respiration, and digestion. It should also be noted that parasympathetic and sympathetic functions each have unique locations in the hypothalamus.

Slide 25: Upper Motor Neuron vs. Lower Motor Neuron

The concept of upper motor neuron versus lower motor neuron is very useful clinically. Specific signs are associated with upper motor neuron and lower motor neuron that assist the localization of lesions. Remember that upper motor neuron of the corticospinal tract project from the cerebral cortex to lower motor neuron located in the anterior horn of the spinal cord. Lower motor neurons in turn project via peripheral nerves to skeletal muscles. An identical concept applies to the corticobulbar tract and cranial nerve motor nuclei. Signs of lower motor neuron lesion include muscle weakness, atrophy, fasciculations, and hyporeflexia. Fasciculations are abnormal muscle twitches caused by spontaneous activity in groups of muscle cells. An example of a benign fasciculation, not associated with motor neuron damage, is the eyelid twitching often experienced after periods of fatigue, excess caffeine, and eye strain such as reading for long periods of time. Signs of upper motor neuron lesions include muscle weakness and a combination of increased tone and hyperreflexia, sometimes referred to as spasticity. Spasticity is an involuntary velocity-dependent increase in tone that results in muscle resistance to movement.

Slide 26: Terms Commonly Used to Describe Weakness

Weakness is one of the most important functional consequences of both upper and lower motor neuron lesions. Various terms are used and may be used interchangeably in clinical practice to describe both the severity and distribution of the weakness. Paresis is defined as weakness or partial paralysis. An example would be hemiparesis, and clinically you would see weakness on one side of the body. Plegia is defined as no movement. An example would be hemiplegia, and clinically you would see no movement on one side of the body. Paralysis is also defined as no movement. An example would be arm paralysis with no movement of the arm. Palsy is a less precise term for weakness or no movement. An example would be a facial palsy with weakness or paralysis of the face muscles. Terms denoting location include hemi-, meaning half or one side of the body; para- meaning both legs; mono- meaning one limb; di- meaning both sides of the body equally affected; and quadri- or tetra- meaning four limbs involved.

Slide 27: Localization of Common Gait Disorders

Gait disorders can be caused by abnormal function in almost any part of the nervous system, as well as by some orthopedic conditions. Careful examination of gait is therefore one of the most sensitive tests of subtle neurologic dysfunction. Characteristic disorders of gait can be seen with lesions in specific systems. Spastic gait can be unilateral or bilateral corticospinal tract involvement and appears as stiff-legged circumduction, sometimes scissoring of the legs and toe-walking, a decreased arm swing, unsteady falling toward one side, and can be seen in cortical, subcortical, brain stem infarcts affecting upper motor neuron pathways; cerebral palsy; degenerative conditions; multiple sclerosis; and spinal cord injury. Ataxic gait can be localized to the cerebellar vermis or other midline cerebellar structures appearing as wide-based gait, unsteadiness, staggering side to side, falling toward the worse side of the pathology. A subtle deficit may be detected with the tandem Romberg test or drunk walk. Vertiginous gait is localized in the vestibular nuclei, vestibular nerve, or the semicircular canals. Looking similar to an ataxic gait, wide-based and unsteady, patients will sway and fall when asked to stand with their feet together and their eyes closed, which is called the Romberg sign. Frontal gait involves lesions localized in the frontal lobes or frontal subcortical white matter and presents as slow, shuffling neuro- or wide-based "magnetic"—barely raising the feet off the floor—and then steady, sometimes resembling Parkinsonian gait. Parkinsonian gait can be localized in the substantia nigra or other regions of the basal ganglia and presents as slow, shuffling, and narrow-based. The client will have difficulty in initiation of movement and often will be stooped forward with decreased arm swing and en bloc turning. They are unsteady with retropulsion, which is taking several steps backward to regain balance when pushed.

Slide 28: Localization of Common Gait Disorders

Dyskinetic gait can often be localized in the subthalamic nucleus or other regions of the basal ganglia. It can present as unilateral or bilateral dance-like (choreic), flinging (ballistic), or writhing (athetoid) movements that occur during walking and may be accompanied by unsteadiness. Common causes are Huntington's disease, infarcts of the subthalamic nucleus or striatum, as a side effect of levodopa or other familial or drug-induced dyskinesias. Tabetic gait can be localized in the posterior columns of sensory nerve fibers and presents as high-stepping, foot-flapping gait with particular difficulty walking in the dark or on uneven surfaces. The patients sway and fall with attempts to perform the Romberg sign. Posterior cord syndrome and severe sensory neuropathy may be causative.

Slide 29: Clinical Case 1

The following clinical cases have been developed for your review. They contain subject matter that is clinically related and will reinforce the lecture content in each slide series. The questions for the case follow the introductory case slide, and the discussion for the case is in the slide notes. I recommend not looking for the answer in the discussion section until you have attempted to answer the question on your own. Good luck, and I will see you in the next topic.

Slide 30: Questions

Slide 31: References