Chapter 3: The nervous system
The nervous system comprises the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system, consisting of the cranial, spinal, and peripheral nerves, together with their motor and sensory endings.
Central nervous system
The central nervous system is composed of millions of nerve and glial cells, together with blood vessels and a little connective tissue. The nerve cells, or neurons, are characterized by many processes and are specialized for reception and transmission of signals. The glial cells, termed neuroglia, are characterized by short processes that have special relationships to neurons, blood vessels, and connective tissue.
Brain
The brain is the enlarged, head end of the central nervous system; it occupies the cranium, or brain case. The term cerebrum (L., brain; adjective cerebral) generally means brain, but sometimes is used for the forebrain and midbrain only. Encephalon, of Greek origin, is found in such terms as encephalitis, which means inflammation of the brain.
The brain presents three main divisions: forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) (fig. 3-1). The forebrain in turn has two subdivisions, telencephalon (endbrain) and diencephalon (interbrain). The hindbrain likewise has two subdivisions, the metencephalon (afterbrain) and the myelencephalon (marrowbrain). The bulk of the brain is formed by two cerebral hemispheres, which are derived from the telencephalon. The hemispheres are distinguished by convolutions, or gyri, which are separated by sulci. The diencephalon lies between the hemispheres. It forms the upper part of the brain stem, an unpaired stalk that descends from the base of the brain. The brain stem is formed by the diencephalon, midbrain, pons, and myelencephalon, or medulla oblongata. The last is continuous with the spinal cord at the foramen magnum. The cerebellum is a fissured mass of gray matter that occupies the posterior cranial fossa and is attached to the brain stem by three pairs of peduncles. Twelve pairs of cranial nerves issue from the base of the brain and brain stem.
The cerebral cortex, which is the most superficial part of the hemispheres and is only a few millimeters in thickness, is composed of gray matter, in contrast to the interior of the hemispheres, which is composed partly of white matter.
Gray matter consists largely of the bodies of nerve and glial cells, whereas white matter consists largely of the processes or fibers of nerve and glial cells.
The interior of the cerebral hemispheres, including the diencephalon, contains not only white matter but also large masses of gray matter known collectively as basal ganglia. This term is a misnomer since the term "ganglion" should be reserved for collections of nerve cell bodies outside the central nervous system and nuclei should be used for collections of neurons inside. Therefore, it would be more appropriate to call these "basal nuclei" however, that term is reserved for another structure.
The cerebellar cortex, like the cerebral, is composed of a thin rind of gray matter. The interior of the cerebellum is composed mainly of white matter, but also contains nuclei of gray matter. The brain stem, by contrast, contains nuclei and diffuse masses of gray matter in its interior.
The interior of the brain also contains cavities termed ventricles, which are filled with cerebrospinal fluid.
Functions
The highest mental and behavioral activities characteristic of humans are mediated by the cerebral hemispheres, in particular by the cerebral cortex. Important aspects of these functions are learning and language. In addition, there are association mechanisms for the integration of motor and sensory functions.
Some areas of the cerebral hemispheres control muscular activity, and their nerve cells send processes to the brain stem and spinal cord, where they are connected with motor neurons, the processes of which leave by way of cranial nerves or ventral roots in the spinal cord. Other areas are sensory and receive impulses that have reached the spinal cord by way of peripheral nerves and dorsal roots, and have ascended in the spinal cord and brain stem by pathways that consist of a succession of nerve cells and their processes. Fibers that ascend and descend in the brain and spinal cord often segregate into bundles having similar courses and functions, known as "tracts"are generally grouped into tracts. The tracts are usually named according to their origin and destination, e.g., corticospinal.
The brain stem contains, in addition to tracts that descend and ascend through it, collections of cells that (1) comprise major integrating centers for motor and sensory functions, (2) form the nuclei of most cranial nerves (all of the cranial nerves except the first are attached to the brain stem), (3) form centers concerned with the regulation of a variety of visceral, endocrinological, behavioral, and other activities, (4) are functionally associated with most of the special senses, (5) control muscular activity in the head and part of the neck, (6) supply pharyngeal arch structures, and (7) are connected with the cerebellum.
The cerebellum is concerned with the automatic regulation of movement and posture, and the learning of new motor patterns. It functions closely with the cerebral cortex and the brain stem.
Spinal cord
The spinal cord is a long, cylindrical mass of nervous tissue, oval or rounded in transverse section. It occupies the upper two-thirds of the vertebral canal. In contrast to the cerebral hemispheres, gray matter is found in the interior, surrounded by white matter (fig. 3-2).
The neurons of the spinal cord include (1) somatic motor cells, the axons of which leave by way of ventral roots and supply skeletal muscles; (2) autonomic motor cells, the axons of which leave by way of ventral roots and go to autonomic ganglia; and (3) transmission neurons that give rise to ascending projections to the brain and to connections with other spinal cord levels; and (4) interneurons, which connect with other neurons at the spinal level and are concerned with sensory and reflex mechanisms. The white matter contains ascending and descending tracts. Some ascend to or descend from the brain, whereas others connect cells at various levels of the cord.
Attached to the spinal cord on each side is a series of spinal roots, termed dorsal and ventral according to their position. Generally there are 31 pairs, which comprise 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Corresponding dorsal and ventral roots join to form a spinal nerve. Each spinal nerve divides into a dorsal and a ventral ramus, and these are distributed to various parts of the body.
The spinal cord carries out sensory, integrative, and motor functions, which can be categorized as reflex, reciprocal activity (as one activity starts, another stops), monitoring and modulation of sensory and motor mechanisms, and transmission of impulses to the brain.
Meninges and cerebrospinal fluid
The brain and spinal cord are surrounded and protected by layers of non-nervous tissue, collectively termed meninges. These layers, from without inward, are the dura mater, arachnoid, and pia mater, and are described in more detail elsewhere. The space between the arachnoid and the pia mater, the subarachnoid space, contains cerebrospinal fluid (C.S.F.).
The ventricles of the brain contain vascular choroid plexuses, from which C.S.F., an almost protein-free liquid, is formed. This fluid circulates through the ventricles, enters the subarachnoid space, and eventually filters into the venous system. CSF protects the brain which basically floats. It serves to minimize damage from blows to the head and neck.
Blood supply
The brain is supplied by the cerebral branches of the vertebral and internal carotid arteries, the meninges mainly by the middle meningeal branch of the maxillary artery. The spinal cord and spinal roots are supplied by the vertebral arteries and by segmental arteries. Peripheral nerves are supplied by a number of small branches along the course of the nerves.
Peripheral nervous system
A nerve is a collection of nerve fibers that is visible to the naked eye. The constituent fibers are bound together by connective tissue. Each fiber is microscopic in size and is surrounded by a sheath formed by a neurilemmal cell (comparable to the glial cells of the central nervous system). Hundreds or thou sands of fibers are present in each nerve. Thus, according to the number of constituent fibers, a nerve may be barely visible, or it may be quite thick. A nerve as a whole is surrounded by a connective tissue sheath, the epineurium. Connective tissue fibers run inward from the sheath and enclose bundles of nerve fibers. Such bundles are termed fasciculi (funiculi); the connective tissue that encloses them is called perineurium. Very small nerves may consist of only one fasciculus derived from the parent nerve. Finally, each nerve fiber and its neurilemmal sheath are enclosed by a connective tissue sheath termed endoneurium.
Peripheral nerve fibers may be classified according to the structures they supply, that is, according to function. A fiber that stimulates or activates skeletal muscle is termed a motor (efferent) fiber. A fiber that carries impulses from a sensory ending is termed a sensory (afferent) fiber. Fibers that activate glands and smooth muscle are also motor fibers, and various kinds of sensory fibers arise from endings in viscera. Consequently, a more detailed classification of functional components is sometimes required.
Spinal nerves
The spinal roots, which are anchored to the spinal cord, consist of a dorsal root, attached to the dorsal aspect of the spinal cord, and a ventral root, attached to the ventral aspect of the cord. Each dorsal root (which contains sensory fibers from skin, subcutaneous and deep tissues, and often from viscera also) is formed by neuronal processes that carry afferent impulses into the spinal cord and which arise from neurons that are collected together to form an enlargement termed a spinal (dorsal root) ganglion (fig. 3-2). The peripheral processes from the dorsal root ganglion neurons arise directly within the organ or structure from which they are conveying sensation. Each of the ventral roots (which contain motor fibers to skeletal muscle, and of which many contain preganglionic autonomic fibers) is formed by processes of neurons in the gray matter of the spinal cord. While the projections from the motor neurons to skeletal muscle go directly to their termination in the muscle, the autonomic motor axons synapse on neurons in a ganglion (hence the term preganglionic). The neurons in the ganglion (postganglionic neurons) have axons that reach their target on glands or smooth muscles. Basically, dorsal roots are afferent, ventral roots efferent. The corresponding dorsal and ventral roots join to form a spinal nerve. Each spinal nerve then divides into a dorsal and a ventral primary ramus.
Distribution of spinal and peripheral nerves
The dorsal primary rami (or just dorsal rami) of spinal nerves supply the skin and muscles of the back. The ventral primary rami (ventral rami) supply the limbs and the rest of the trunk. The ventral rami that supply the thoracic and abdominal wall remain relatively separate throughout their course. In the cervical and lumbosacral regions, however, the ventral rami intermingle to form plexuses, from which the major peripheral nerves emerge.
When the ventral ramus of a spinal nerve enters a plexus and joins other such rami, its component funiculi or bundles ultimately enter several of the nerves emerging from the plexus. Thus, as a general principle, each spinal nerve entering a plexus contributes to several peripheral nerves, and each peripheral nerve contains fibers derived from several spinal nerves. This arrangement leads to two fundamental and important types of distribution (fig. 3-3). Each spinal nerve has a segmental, or dermatomal, distribution. A dermatome is the area of skin supplied by the sensory fibers of a single dorsal root through the dorsal and ventral rami of its spinal nerve. Dermatomes based largely on Foerster are shown in this figure.
The mixture of nerve fibers in plexuses is such that it is difficult if not impossible to trace their course by dissection; hence, dermatomal distribution has been determined by physiological experimentation and by studies of disorders of spinal nerves. Methods have included stimulation of spinal roots, study of residual sensation when a root is left intact after section of the roots above and below it, study of the diminution of sensation after section of a single root, and study of the distribution of the vessicles that follow inflammation of roots and spinal ganglia in herpes zoster (shingles). Such studies have yielded complex maps, chiefly because of variation, overlap, and differences in method. Variation results from intersegmental rootlet anastomoses adjacent to the cervical and lumbosacral spinal cord and from individual differences in plexus formation and peripheral nerve distribution. Overlap is such that section of a single root does not produce complete anesthesia in the area supplied by that root: at most, some degree of hypalgesia may result, particularly in the distal extremities, where overlap is less complete. By contrast, when a peripheral nerve is cut, the result is a central area of total loss of sensation surrounded by an area of diminished sensation.
There is little specific correspondence between dermatomes and underlying muscles. The general arrangement is that the more rostral segments of the cervical and lumbosacral enlargements of the spinal cord supply the more proximal muscles of the limbs, and that the more caudal segments supply the more distal muscles. A muscle usually receives fibers from each of the spinal nerves that contribute to the peripheral nerve supplying it (although one spinal nerve may be its chief supply). Section of a single spinal nerve weakens several muscles but usually does not paralyze them. Section of a peripheral nerve results in severe weakness or total paralysis of the muscles it supplies. Moreover, autonomic dysfunction occurs in the area of its distribution.
Cranial nerves
The 12 pairs of cranial nerves are special nerves associated with the brain. The fibers in cranial nerves are of diverse functional types. Some cranial nerves are composed of only one type, others of several.
Cranial nerves differ significantly from spinal nerves, especially in their development and their relation to the special senses and because some cranial nerves supply pharyngeal arch structures. They are attached to the brain at irregular rather than regular intervals; they are not formed of dorsal and ventral roots; some have more than one ganglion, whereas others have none; and the optic nerve is actually a tract of the central nervous system rather than a peripheral nerve.
Characteristic features of peripheral nerves
The branches of major peripheral nerves are usually muscular, cutaneous (or mucosal), articular, vascular (to adjacent blood vessels), and terminal (one, several, or all of the foregoing types). Muscular branches are the most important: section of even a small muscular branch results in complete paralysis of all muscle fibers supplied by that branch and may be seriously disabling. The importance of sensory loss varies, but such loss is most disabling in the hand, head, and face.
Peripheral nerves vary in their course and distribution, but not as much as blood vessels do. Adjacent nerves may communicate with each other. Such communications sometimes account for residual sensation or movement after damage to a nerve above the level of a communication.
Autonomic nervous system
The autonomic nervous system regulates the activity of cardiac muscle, smooth muscle, and glands.
The autonomic system can be considered as a series of heirarchical levels, with the higher levels producing more widespread and general functions. The highest level is the cerebral cortex, certain areas of which control or regulate visceral functions. These areas send fibers to the next lower level, the hypothalamus, located at the base of the brain. The hypothalamus is a coordinating center for the motor control of visceral activity. One of its many functions, for example, is the regulation of body temperature. The hypothalamus has nervous and vascular connections with the pititary gland (hypophysis), by virtue of which it influences the pituitary and, through the pititary gland, the other endocrine glands. The hypothalamus also sends nerve fibers to lower centers in the brain stem that are concerned with still more specific functions, for example, the reflex regulation of respiration, heart rate, and circulation. These centers function through connections with still lower centers, which are collections of nerve cells in the brain stem and spinal cord that send their axons into certain cranial and spinal nerves. It is characteristic of these axons that, unlike motor fibers to skeletal muscle, they synapse with multipolar neurons located in ganglia outside the central nervous system before they reach the structure to be supplied. The axons that pass from the central nervous system to these ganglion cells are termed preganglionic fibers. The axons of ganglion cells are called postganglionic fibers.
Sympathetic system
The sympathetic, or thoracolumbar, part of the autonomic system comprises the preganglionic fibers that issue from the thoracic and upper lumbar levels of the spinal cord. These fibers reach spinal nerves by way of ventral roots and then leave the spinal nerves, reaching adjacent ganglia by way of rami communicantes (see fig. 3-2). These ganglia are contained in long nerve strands, the sympathetic trunks, one on each side of the vertebral column, extending from the base of the skull to the coccyx. Some preganglionic fibers synapse in ganglia that are studded along this nerve trunk, others continue to ganglia located anterior to the vertebrae, along the aorta (prevertebral or aortic plexuses), and still others synapse with cells in the medulla of the suprarenal (adrenal) glands. The postganglionic fibers either go directly to adjacent viscera and blood vessels or return to spinal nerves by way of other rami communicantes and, in the area of distribution of these nerves, supply the skin with (1) secretory fibers to sweat glands, (2) motor fibers to smooth muscle attached to hair follicles (arrectores pilorum), and (3) vasomotor fibers to the blood vessels of the limbs.
Parasympathetic system
The parasympathetic, or craniosacral, part of the autonomic system comprises the preganglionic fibers that issue from the brain stem (cranial nerves III, VII, IX, X, XI) and sacral part of the spinal cord (segments S2,3 or S3,4). The ganglion cells with which these fibers synapse are in or near the organs innervated. The postganglionic fibers are very short: apparently none go to blood vessels, smooth muscle, or glands of the limbs or body wall. Most viscera, however, have a double motor supply, sympathetic and parasympathetic, often with opposing roles.
Functions of the autonomic nervous system
By its role in central integrating mechanisms, the autonomic system is involved in behavioral and neuroendocrinological mechanisms, and in the processes whereby the body keeps its internal environment constant, that is, maintains temperature, fluid balance, and ionic composition of the blood. The parasympathetic system is concerned with many specific functions, such as digestion, intermediary metabolism, and excretion. The sympathetic system is an important part of the mechanism of reaction to stress.
Additional reading
Bossy, J., Atlas du systeme nerveux, Editions Offiduc, Paris, 1971. Beautiful photographs.
De Armond, S. J., Fusco, M. M., and Dewey, M. M., Structure of the Human Brain, 2nd ed., Oxford University Press, New York, 1976. A good example of several atlases that are now available.
Gardner, E., Fundamentals of Neurology, 6th ed., W. B. Saunders Company, Philadelphia, 1975. A concise account of the nervous system. Excellent for orientation and review.
Questions
3-1 Look up the origin of the word encephalon.
3-2 Is the diencephalon a part of the brain stem?
3-3 How are the 31 pairs of spinal nerves subdivided?
3-4 What is the basic functional distinction between ventral and dorsal roots of spinal nerves?
3-5 What are the two fundamental types of cutaneous distribution of sensory fibers in spinal nerves?
3-6 What is a dermatome?
3-7 In which disease may a dermatome be outlined?
3-8 Why do the cranial nerves have that name?
3-9 What are the main differences between cranial and spinal nerves?
3-10 Is the autonomic system purely motor?
Figure legends
Figure 3-1 Diagram of the major subdivisions of the brain. The syllables followed by hyphens are prefixes to the word encephalon (e.g., prosencephalon). The shaded areas constitute the brain stem, in which the diencephalon is generally included. Much of the brain stem and part of the cerebellum are under cover of the telencephalon.
Figure 3-2 A horizontal section of the spinal cord, and dorsal and ventral roots and a spinal nerve. The arrangement of the rami communicantes is usually much more complicated.
Figure 3-3 Spinal and peripheral nerve distributions. Of the two sensory fibers of spinal nerve A, one joins peripheral nerve X and the other joins Y. A plexus allows both fibers to enter spinal nerve A. Two fibers of spinal nerve B also join the two peripheral nerves. Thus, the areas (dermatomes) supplied by the two spinal nerves differ from those supplied by the two peripheral nerves, as is shown in the subdivided rectangle. Overlap is omitted.