The human nervous system is a complex of two subdivisions.
The CNS, comprising the brain and spinal cord, is enclosed in bone and wrapped in protective coverings (meninges) and fluid-filled spaces.
2. Peripheral nervous system (PNS)
The PNS is formed by the cranial and spinal nerves (Fig 1–1).
The structure of the central nervous system and the peripheral nervous system, showing the relationship between the central nervous system and its bony coverings.
Functionally, the nervous system is divided into two systems.
1. Somatic nervous system
This innervates the structures of the body wall (muscles, skin, and mucous membranes).
2. Autonomic (visceral) nervous system (ANS)
The ANS contains portions of the central and peripheral systems. It controls the activities of the smooth muscles and glands of the internal organs (viscera) and the blood vessels and returns sensory information to the brain.
Structural Units and Overall Organization
The central portion of the nervous system consists of the brain and the elongated spinal cord (Fig 1–2 and Table 1–1). The brain has a tiered structure and, from a gross point of view, can be subdivided into the cerebrum, the brain stem, and the cerebellum.
The two major divisions of the central nervous system, the brain, and the spinal cord, as seen in the midsagittal plane.
TABLE 1–1Major Divisions of the Central Nervous System.
The most rostral part of the nervous system (cerebrum, or forebrain) is the most phylogenetically advanced and is responsible for the most complex functions (eg, cognition). The brain stem, medulla, and spinal cord serve less advanced, but essential, functions.
The cerebrum (forebrain) consists of the telencephalon and the diencephalon; the telencephalon includes the cerebral cortex (the most highly evolved part of the brain, sometimes called "gray matter"), subcortical white matter, and the basal ganglia, which are gray masses deep within the cerebral hemispheres. The white matter carries that name because, in a freshly sectioned brain, it has a glistening appearance as a result of its high lipid-rich myelin content; the white matter consists of myelinated fibers and does not contain neuronal cell bodies or synapses (Fig 1–3). The major subdivisions of the diencephalon are the thalamus and hypothalamus. The brain stem consists of the midbrain (mesencephalon), pons, and medulla oblongata. The cerebellum includes the vermis and two lateral lobes. The brain, which is hollow, contains a system of spaces called ventricles; the spinal cord has a narrow central canal that is largely obliterated in adulthood. These spaces are filled with cerebrospinal fluid (CSF) (Figs 1–4 and 1–5; see also Chapter 11).
Cross section through the spinal cord, showing gray matter (which contains neuronal and glial cell bodies, axons, dendrites, and synapses) and white matter (which contains myelinated axons and associated glial cells). (Reproduced, with permission, from Junqueira LC, Carneiro J, Kelley RO: Basic Histology: Text & Atlas. 11th ed. McGraw-Hill, 2005.)
Photograph of a midsagittal section through the head and upper neck, showing the major divisions of the central nervous system. (Reproduced, with permission, from deGroot J: Correlative Neuroanatomy of Computed Tomography and Magnetic Resonance Imagery. 21st ed. Appleton & Lange, 1991.)
Magnetic resonance image of a midsagittal section through the head (short time sequence; see Chapter 22). Compare with Figure 1–2.
The brain, which accounts for about 2% of the body's weight, contains many billions (perhaps even a trillion) of neurons and glial cells (see Chapter 2). Neurons, or nerve cells, are specialized cells that receive and send signals to other cells through their extensions (nerve fibers, or axons). The information is processed and encoded in a sequence of electrical or chemical steps that occur, in most cases, very rapidly (in milliseconds). Many neurons have relatively large cell bodies and long axons that transmit impulses quickly over a considerable distance. Interneurons, on the other hand, have small cell bodies and short axons and transmit impulses locally. Nerve cells serving a common function, often with a common target, are frequently grouped together into nuclei. Nerve cells with common form, function, and connections that are grouped together outside the CNS are called ganglia.
Other cellular elements that support the activity of the neurons are the glial cells, of which there are several types. Glial cells within the brain and spinal cord outnumber neurons 10:1.
Computation in the Nervous System
Nerve cells convey signals to one another at synapses (see Chapters 2 and 3). Chemical transmitters are associated with the function of the synapse: excitation or inhibition. A neuron may receive thousands of synapses, which bring it information from many sources. By integrating the excitatory and inhibitory inputs from these diverse sources and producing its own message, each neuron acts as an information-processing device.
Some very primitive behaviors (eg, the reflex and unconscious contraction of the muscles around the knee in response to percussion of the patellar tendon) are mediated by a simple monosynaptic chain of two neurons connected by a synapse. More complex behaviors, however, require larger polysynaptic neural circuits in which many neurons, interconnected by synapses, are involved.
The connections, or pathways, between groups of neurons in the CNS are in the form of fiber bundles, or tracts (fasciculi). Aggregates of tracts, as seen in the spinal cord, are referred to as columns (funiculi). Tracts may descend (eg, from the cerebrum to the brain stem or spinal cord) or ascend (eg, from the spinal cord to the cerebrum). These pathways are vertical connections that in their course may cross (decussate) from one side of the CNS to the other. Horizontal (lateral) connections are called commissures.
Symmetry of the Nervous System
A general theme in neuroanatomy is that, to a first approximation, the nervous system is constructed with bilateral symmetry. This is most apparent in the cerebrum and cerebellum, which are organized into right and left hemispheres. Some higher cortical functions such as language are represented more strongly in one hemisphere than in the other, but to gross inspection, the hemispheres have a similar structure. Even in more caudal structures, such as the brain stem and spinal cord, which are not organized into hemispheres, there is bilateral symmetry.
Another general theme in the construction of the nervous system is decussation and crossed representation: Neuroanatomists use the term "decussation" to describe the crossing of a fiber tract from one side of the nervous system (right or left) to the other. The right side of the brain receives information about, and controls motor function pertaining to, the left side of the world and vice versa. Visual information about the right side of the world is processed in the visual cortex on the left. Similarly, sensation of touch, sensation of heat or cold, and joint position sense from the body's right side are processed in the somatosensory cortex in the left cerebral hemisphere. In terms of motor control, the motor cortex in the left cerebral hemisphere controls body movements that pertain to the right side of the external world. This includes, of course, control of the muscles of the right arm and leg, such as the biceps, triceps, hand muscles, and gastrocnemius. There are occasional exceptions to this pattern of "crossed innervation": For example, the left sternocleidomastoid muscle is controlled by the left cerebral cortex. However, even this exception makes functional sense: As a result of its unusual biomechanics, contraction of the left sternocleidomastoid rotates the neck to the right. Even for the anomalous muscle, then, control of movements relevant to the right side of the world originates in the contralateral left cerebral hemisphere, as predicted by the principle of crossed representation.
There is one major exception to the rule of crossed motor control: As a result of the organization of cerebellar inputs and outputs, each cerebellar hemisphere controls coordination and muscle tone on the ipsilateral side of the body (see Chapter 7).
Maps of the World Within the Brain
At each of many levels, the brain maps (contain a representation of) various aspects of the outside world. For example, consider the dorsal columns (which carry sensory information, particularly with respect to touch and vibration, from sensory endings on the body surface upward within the spinal cord). Axons within the dorsal columns are arranged in an orderly manner, with fibers from the arm, trunk, and leg forming a map that preserves the spatial relationship of these body parts. Within the cerebral cortex, there is also a sensory map (which has the form of a small man and is, therefore, called a homunculus) within the sensory cortex. There are multiple maps of the visual world within the occipital lobes and within the temporal and parietal lobes. These maps are called retinotopic because they preserve the geometrical relationships between objects imaged on the retina and thus provide spatial representations of the visual environment within the brain.
The existence of these maps within the brain is important to clinicians. Focal lesions of the brain may interfere with function of only part of the map, thus producing signs and symptoms (such as loss of vision in only part of the visual world) that can help to localize the lesions.
The earliest tracts of nerve fibers appear at about the second month of fetal life; major descending motor tracts appear at about the fifth month. Myelination (sheathing with myelin) of the spinal cord's nerve fibers begins about the middle of fetal life; some tracts are not completely myelinated for 20 years. The oldest tracts (those common to all animals) myelinate first; the corticospinal tracts myelinate largely during the first and second years after birth.
Growing axons are guided to the correct targets during development of the nervous system by extracellular guidance molecules (including the netrins and semaphorins). Some of these act as attractants for growing axons, guiding them toward a particular target. Others act as repellants. There are many types of guidance molecules, probably each specific for a particular type of axon, and they are laid down in gradients of varying concentration. In many parts of the developing nervous system, there is initially an overabundance of young axons, and those that do not reach the correct targets are subsequently lost by a process of pruning.