Chapter 3: Overview of the Anatomy of the Nervous System

The nervous system serves three main functions: perception, cognition, and action. Perception is the translation of the outer world into electrochemical signals that can be interpreted by the brain. For example, light information is converted by the retina and then sent to the brain by the optic nerves (cranial nerve 2); sound is transformed by the inner ear apparatus and transmitted to the brain via the auditory nerves (cranial nerve 8). Action is the brain’s way of allowing the organism to interact with the environment by moving the body (and in the case of humans and some other animals, by using movements of the vocal apparatus to communicate). Cognition includes all of the operations that interpret perceptual input to understand the external environment, and plan the interaction with the environment through action.

In neuroanatomic terms, perception is carried out by the input to the nervous system (afferent pathways), action is the output (efferent pathways), and cognition arises from interconnections within and between perceptual modalities, as well as between perception and action. Perception begins with the sense organs (skin, eyes, ears, nose, mouth) and travels in peripheral nerves (including cranial nerves for the structures of the head), ultimately transmitting information to the sensory cortices of the cerebral hemispheres (e.g., somatosensory cortex, visual cortex, auditory cortex). Motor output is controlled by the motor cortex, whose signals travel by way of the motor pathways to ultimately reach the peripheral nerves that will command muscles to move (see Ch. 4). The motor cortex collaborates with adjacent structures (premotor and supplementary motor cortices) and participates in circuits involving the basal ganglia (see Ch. 7) and cerebellum (see Ch. 8), all of which work to coordinate and execute movements.

The central nervous system (CNS) consists of the brain, brainstem, cerebellum, and spinal cord (Fig. 3–1). The peripheral nervous system (PNS) includes the dorsal and ventral roots that enter (dorsal roots) and exit (ventral roots) the spinal cord and continue into the peripheral nerves. Before devoting the rest of this chapter to the internal and external structures of the brain, a brief orientation to the brainstem, spinal cord, and cerebellum is provided here. The brainstem is divided into three levels from superior to inferior: midbrain, pons, and medulla (see Ch. 9). The medulla transitions seamlessly into the spinal cord, which itself is divided into four levels: cervical, thoracic, lumbar, and sacral (see Ch. 5). The cerebellum lies posterior to the brainstem and inferior to the posterior aspects of the cerebral hemispheres, and is connected to the brainstem by way of the three cerebellar peduncles (see Ch. 8).

The brain is divided into two hemispheres (left and right), each of which is divided into four lobes (frontal, temporal, parietal, and occipital) (Fig. 3–2). In order to maximize surface to volume ratio of the brain in the skull, the cortical surface folds during development. The folds are called gyri, and the spaces between them are called sulci. Large divisions between the hemispheres or lobes are referred to as fissures.

The left and right hemispheres are separated by the longitudinal (interhemispheric) fissure. The frontal lobe is in the front, the occipital lobe is at the back (at the occiput), and the parietal lobe is between the occipital and frontal lobes. The temporal lobe is inferior to these three lobes. The frontal lobe is separated from the parietal lobe by the central sulcus. The frontal lobe is separated from the temporal lobe by the Sylvian fissure. The parietal lobe and occipital lobe have no clear boundary on the lateral surface of the brain, but are separated by the parieto-occipital sulcus on the medial surface of the brain.

The nervous system is composed of neurons and supporting cells (glia) (Fig. 3–3). The cell bodies of neurons comprise the gray matter, and their myelinated axons form the white matter. In the brain, cell bodies are found in the cortex on the outer surface of the brain, and the axons from those cell bodies travel in the subcortical white matter. There are also “islands” of subcortical gray matter structures within the subcortical white matter such as the basal ganglia and thalamus, which form nodes in a variety of networks, most of which begin and/or end in the cortex (see Ch. 7).

In contrast to the brain, in the spinal cord, the gray matter is on the inside, and the white matter is on the outside.

The brain and spinal cord are covered by several protective layers called the meninges (Fig. 3–4). The meninges have three layers. From external to internal these are the dura mater, arachnoid, and pia mater. The dura mater (literally “tough mother”) is a tough outer “skin” that covers the entire brain, with folds that extend between the hemispheres (falx cerebri) and between the cerebral hemispheres and cerebellum (tentorium cerebelli). The dura surrounds the brain but does not invaginate into the sulci. The outer surface of the dura mater is tightly affixed to the skull.

The arachnoid is a thin membrane beneath the dura mater. Like the dura mater, the arachnoid also does not invaginate into the sulci. The dura mater can be thought of as being similar to the thick peel of an orange, and the arachnoid can be thought of as similar to the thin skin below the peel of an orange that can also be peeled off.

The pia mater is the only one of the three meningeal layers that invaginates into the sulci and therefore contacts the entire surface area of the brain.

The pia and arachnoid are collectively referred to as the leptomeninges, and the dura mater is referred to as the pachymeninges (pachy means thick, as used in the term pachyderm meaning elephant). In general, infectious meningitis predominantly affects the leptomeninges and inflammatory meningitis predominantly affects the pachymeninges, although there are exceptions (e.g., neurosarcoidosis can affect the leptomeninges, and tuberculosis and fungal infections can affect the pachymeninges). Metastatic cancer can affect the leptomeninges or the pachymeninges.

The space between the bone and dura is the epidural space, the space between the dura and arachnoid is the subdural space, and the space between the arachnoid and pia is the subarachnoid space. Pathologic processes can lead to blood or infection in any of these spaces: epidural hematoma, subdural hematoma, and subarachnoid hemorrhage (see Ch. 19); epidural abscess, subdural empyema, and meningitis (infectious meningitis predominantly involves the leptomeninges and subarachnoid space) (see Ch. 20).

Although arterial blood supply to the CNS will be discussed in more detail in relation to specific brain and brainstem regions (Ch. 7), the venous drainage of the brain is discussed here since it is anatomically related to the meninges (Fig. 3–5). The venous drainage from the brain ultimately ends up in the internal jugular veins that travel to the superior vena cava by way of the subclavian veins. To arrive in the jugular veins, the blood travels in venous sinuses formed by folds of dura.

The superior sagittal sinus travels along the superior surface of the brain at the midline and descends posteriorly to empty into the bilateral transverse sinuses. The transverse sinuses travel laterally through folds in the tentorium cerebelli and pass into the sigmoid sinuses, which empty into the jugular veins. The deep structures of the brain are drained by the internal cerebral veins and the basal veins of Rosenthal. These empty into the great vein of Galen, which empties into the straight sinus. The straight sinus joins the transverse sinuses and superior sagittal sinus at the confluence of sinuses (also called the torcula).

The lateral hemispheres are drained by superficial cortical veins. The largest of these are the veins of Trolard and Labbé. Mnemonic: Trolard is on top and Labbé is lower and more lateral.

When venous drainage is impaired (e.g., due to venous sinus thrombosis), intracranial pressure rises, which can cause headache, visual disturbances, and ultimately coma. If pressure in the venous system rises to a sufficient level, intracerebral or subarachnoid hemorrhage can occur, causing focal deficits (see “Cerebral Venous Sinus Thrombosis and Cortical Vein Thrombosis” in Ch. 19).

The brain and spinal cord are bathed in cerebrospinal fluid (CSF) in order to provide buoyancy so as to create a shock absorber in the case of trauma (Fig. 3–6 and Fig. 3–7). The CSF is produced in the choroid plexus, which lines the ventricles. The two lateral ventricles drain into the third ventricle via the foramina of Monro (one for each lateral ventricle). The CSF then flows from the third ventricle to the fourth ventricle (between the brainstem and cerebellum) by way of the cerebral aqueduct in the midbrain. The fourth ventricle is continuous with the central canal of the spinal cord. From the fourth ventricle, CSF can exit the ventricular system into the subarachnoid space via the foramen of Magendie (midline) and the foramina of Luschka (lateral) to bathe the outer surface of the brain and spinal cord. CSF is then reabsorbed via the arachnoid granulations into the venous sinuses. CSF is produced at a rate of approximately half a liter per day (approximately 20 milliliters per hour).

Disruption of CSF flow due to obstruction anywhere along this pathway can lead to hydrocephalus. Hydrocephalus can be classified as communicating or noncommunicating. The “communication” in question refers to the ability of the ventricles to communicate with one another. Noncommunicating hydrocephalus signifies a blockage in the ventricular system itself (e.g., by a tumor, mass effect from a large stroke with edema, intraventricular hemorrhage) such that the ventricles proximal to the obstruction dilate. Communicating hydrocephalus occurs when there is failure of CSF reabsorption in the arachnoid granulations such that the ventricles can still communicate with one another but hydrocephalus develops since CSF cannot be absorbed. In communicating hydrocephalus, all of the ventricles will dilate.

The distinction between communicating and noncommunicating hydrocephalus is important because in chronic communicating hydrocephalus, lumbar puncture can relieve pressure from the ventricular system (e.g., in cryptococcal meningitis [Ch. 20] and normal pressure hydrocephalus [Ch. 22]), whereas in noncommunicating hydrocephalus, the elevated intracranial pressure proximal to the obstruction can lead to herniation if fluid is removed from below by lumbar puncture (see Ch. 25).