Chapter 16. Brain

Big Picture

The brain contains millions of neurons arranged in a vast array of synaptic connections that provide seemingly unfathomable circuitry. Through that circuitry, the brain integrates and processes sensory information and provides motor output.

Divisions of the Brain

The brain is divided into the cerebrum, diencephalon, brainstem, and cerebellum (Figure 16-1A and B).

Cerebrum

The cerebrum is the organ of thought and serves as the control site of the nervous system, enabling us to possess the qualities associated with consciousness such as perception, communication, understanding, and memory (Figure 16-1A). The cerebral hemispheres consist of elevations (gyri) and valleys (sulci), with a longitudinal cerebral fissure separating the hemispheres. Each cerebral hemisphere is divided into lobes, which correspond roughly to the overlying bones of the skull.

• Frontal lobe. The frontal lobe is located in the anterior cranial fossa. The central sulcus divides the frontal lobe from the parietal lobe in a coronal plane. The gyrus anterior to the central sulcus is called the precentral sulcus and serves as the primary motor area of the brain. The remainder of the frontal lobe is used in modifying motor actions.
• Parietal lobe. The parietal lobe interprets sensations from the body. The gyrus posterior to the central sulcus, the postcentral sulcus, is the primary area for receipt of these sensations.
• Occipital lobe. The occipital lobe is located superior to the tentorium cerebelli, in the posterior cranial fossa, and is primarily concerned with vision.
• Temporal lobe. The temporal lobe is located in the middle cranial fossa and is primarily concerned with hearing.

Diencephalon

The diencephalon consists of the thalamus, hypothalamus, epithalamus, and subthalamus, and is situated between the cerebrum and the brainstem (Figure 16-1B). The diencephalon serves as the main processing center for information destined to reach the cerebral cortex from the ascending pathways.

Brainstem

The brainstem consists of the midbrain, pons, and medulla oblongata (Figure 16-1B).

• Midbrain. The midbrain (mesencephalon) contains the nuclei for the oculomotor nerve and the trochlear nerve, cranial nerves (CNN) III and IV, respectively. The cerebral aqueduct is a portion of the ventricular system and courses through the center of the midbrain to connect the third and fourth ventricles.
• Pons. The pons is situated against the clivus and the dorsum sellae and contains the nuclei for the trigeminal, abducens, facial, and vestibulocochlear nerves (CNN V, VI, VII, and VIII, respectively).
• Medulla oblongata. The medulla oblongata, commonly called the medulla, is located at the level of the foramen magnum. It serves as the major autonomic reflex center that relays visceral motor control to the heart, blood vessels, respiratory system, and gastrointestinal tract. It possesses the nuclei for the glossopharyngeal, vagal, accessory, and hypoglossal nerves (CNN IX, X, XI, and XII, respectively).

Cerebellum

The cerebellum lies in the posterior cranial fossa and assists in the coordination of skeletal muscle contraction (Figure 16-1A and B). It functions at a subconscious level and provides skeletal muscles with precise timing and appropriate patterns of contraction needed for smooth, coordinated movements.

Big Picture

The ventricular system of the brain is a set of four chambers within the brain and is continuous with the central canal of the spinal cord. Cerebrospinal fluid (CSF) flows within these chambers and serves as a liquid cushion, providing buoyancy to the brain and spinal cord.

Ventricles

The four connected ventricles form chambers within the brain and are filled with CSF (Figure 16-2A and B).

• Lateral ventricles. These paired, C-shaped chambers are located deep within each cerebral hemisphere. Each lateral ventricle communicates via the interventricular foramen (of Monro) with the third ventricle.
• Third ventricle. The third ventricle is narrow and is located midline and inferior to the lateral ventricles. The third ventricle is positioned between the left and right diencephalon. The third ventricle communicates with the fourth ventricle via the cerebral aqueduct (of Sylvius).
• Fourth ventricle. The fourth ventricle is located superior to the pons and the medulla oblongata.

Cerebrospinal Fluid

CSF is produced by the choroid plexuses, located within each of the ventricles, and flows from the lateral and third ventricles to the fourth ventricle via the cerebral aqueduct. From the fourth ventricle, CSF enters an enlarged part of the subarachnoid space (cisterna magna) via the central median aperture (of Magendie) and the lateral apertures (of Luschka). The CSF circulates around the spinal cord and brain in the subarachnoid space to empty into the superior sagittal sinus via the arachnoid granulations. Arachnoid granulations are projections of the arachnoid mater along the superior sagittal sinus. The brain and spinal cord are very delicate and, as such, are susceptible to damage. Therefore, the brain and spinal cord are well protected through the bony encasement (skull and vertebral column) as well as by a second protective layer of connective tissue coverings (the meninges). The third protective mechanism is CSF, which supports the brain and spinal cord (Figure 16-2C). Despite all of these protections, trauma to the brain and spinal cord can still occur and can result in devastating injuries and deficits.

Big Picture

The brain receives its arterial supply from two sources, the internal carotid and the vertebral arteries.

Internal Carotid Artery

The common carotid arteries originate from the aortic arch on the left and the brachiocephalic trunk on the right. The common carotid arteries bifurcate at the level of the thyroid cartilage into the external and internal carotid arteries. The external carotid artery sends branches to the neck and face, whereas the internal carotid artery ascends to the base of the skull, entering the carotid canal. Upon exiting the carotid canal, the internal carotid artery courses horizontally over the foramen lacerum and enters the cavernous sinus and, after turning superiorly, divides into its terminal branches. The terminal branches of the internal carotid are as follows (Figure 16-3A–C):

• Ophthalmic artery. Courses through the optic canal to supply the retina, orbit and part of the scalp.
• Posterior communicating artery. Joins the posterior cerebral artery with the internal carotid artery.
• Anterior cerebral artery. Courses superior to the optic chiasma and enters the longitudinal cerebral fissures. The anterior cerebral artery courses superiorly and then posteriorly along the corpus callosum, providing blood supply to the medial sides of both cerebral hemispheres.
  • Anterior communicating artery. Is a very short artery that connects the two anterior cerebral arteries.
• Middle cerebral artery. Courses into the lateral fissure between the parietal and temporal lobes. It sends many branches to the lateral sides of the cerebral hemispheres and central branches into the brain.

Vertebral Arteries

Each vertebral artery arises from the subclavian artery and ascends through the transverse foramina of C1–C6. The vertebral artery courses horizontally across C1 through the suboccipital triangle before entering the skull via the foramen magnum (Figure 16-3A–C). After penetrating the dura mater, the vertebral arteries then course along the inferior aspect of the medulla oblongata before converging into the basilar artery on the pons. Branches of the vertebral arteries travel to the spinal cord, the meninges, and the brainstem. The major branches are as follows:

• Posterior inferior cerebellar arteries. Course between the origins of cranial nerve (CN) X and CN XI en route to the inferior surface of the cerebellum. Often referred to by the acronym PICA.
• Posterior cerebral arteries. The terminal branches of the basilar artery provide vascular supply to that part of the brain base that is superior to the tentorium cerebelli. CN III and CN IV exit the brain between the superior cerebellar and the posterior cerebral arteries.

Basilar Artery

The basilar artery ascends along the ventral surface of the pons and gives rise to the following branches:

• Anterior inferior cerebellar artery. Courses along the inferior surface of the cerebellum.
• Superior cerebellar artery. Courses along the superior surface of the cerebellum.

Circle of Willis

The anterior communicating artery connects the two anterior cerebral arteries, and the posterior communicating arteries connect the internal carotid and posterior cerebral arteries. As a result of these connections, an arterial circle, known as the cerebral arterial circle (of Willis), is formed around the infundibulum (stalk connecting the pituitary gland to the hypothalamus).