The nervous system is composed of excitable cells, called neurons, which are specialized for information processing and transmission. Neurons make contact with each other at junctions called synapses, at which information is transferred from one neuron to the next by means of chemical messenger substances called neurotransmitters. Neurons can be divided into two classes according to their function: excitatory and inhibitory. They are further categorized according to the neurotransmitters that they secrete.
For practical purposes, the nervous system as a whole can be considered as consisting of two components, the central nervous system (CNS) and the peripheral nervous system (PNS). The autonomic nervous system consists of the components of the CNS and PNS that control visceral function; “vegetative nervous system” is an earlier term for it that is still occasionally heard. The autonomic nervous system is distinct from those parts of the nervous system that subserve conscious perception and control voluntary movement (by way of the striated muscles).
Information flow in the nervous system can be broken down schematically into three steps (▶Fig. 1.1): an external or internal stimulus impinging on the sense organs induces the generation of nerve impulses that travel toward the CNS (afferent impulses); complex processing occurs within the CNS (information processing); and, as the product of this processing, the CNS generates impulses that travel toward the periphery (efferent impulses) and effect the (motor) response of the organism to the stimulus. Thus, when a pedestrian sees a green traffic light, afferent impulses are generated in the optic nerves and visual system that convey information about the specific color present. Then, at higher levels in the CNS, the stimulus is interpreted and assigned a meaning (green light = go). Efferent impulses to the legs then affect the motor response (crossing the street).
In the simplest case, information can be transferred directly from the afferent to the efferent arm, without any intervening complex processing in the CNS; this is what happens, for example, in an intrinsic muscle reflex such as the knee-jerk (patellar) reflex.
The neurons and their processes (see below) and the synapses (see ▶Synapses) are responsible for the flow of information in the nervous system. At the synapses, information is transferred from one neuron to the next by means of chemical substances called neurotransmitters.
Dendrites and axons. Neurons transfer information in one direction only because they are bipolar: they receive information from other neurons at one end, and transmit information to other neurons at the other end.
The receptive structures of a neuron, called dendrites, are branched processes attached to the cell body. Neurons vary considerably with regard to the number and branching pattern of their dendrites. The forward conducting structure is the axon, which in humans can be up to a meter in length. In contrast to the variable number of dendrites, each neuron possesses only a single axon. At its distal end, the axon splits into a number of terminal branches, each of which ends in a so-called terminal bouton that makes contact with the next neuron (▶Fig. 1.2).
The long peripheral processes of the pseudounipolar neurons of the spinal ganglia are an important special case. These are the fibers that relay information regarding touch, pain, and temperature from the body surface to the CNS. Although they are receptive structures, they nonetheless possess the structural characteristics of axons and are designated as such.
Axonal transport. The neurotransmitters, or the enzymes catalyzing their biosynthesis, are synthesized in the perikaryon and then carried down axonal microtubules to the end of the axon in a process known as axoplasmic transport. The neurotransmitter molecules are stored in synaptic vesicles inside the terminal boutons (each bouton contains many synaptic vesicles). Axoplasmic transport, generally speaking, can be in either direction—from the cell body toward the end of the axon (anterograde transport), or in the reverse direction (retrograde transport). Rapid axoplasmic transport proceeds at a speed of 200 to 400 mm/day. This is distinct from axoplasmic flow, whose speed is 1 to 5 mm/day. Axoplasmic transport is exploited in the research laboratory by anterograde and retrograde tracer techniques for the anatomical demonstration of neural projections (▶Fig. 1.3). The functional significance of distinct neural projections can now be investigated in living animals with a technique called optogenetics, in which genetic manipulation is used to induce the expression of ion channels whose activation can be controlled with an applied laser-light stimulus.
Fig. 1.3 Tracing of neuronal projections with retrograde and anterograde tracer substances. Tracer substances, such as fluorescent dyes, are injected either at the site of origin or at the destination of the neuronal pathway in question. The tracer substances are then transported along the neurons, either from the cell bodies to the axon terminals (anterograde transport) or in the reverse direction (retrograde transport). It is thus possible to trace the entire projection from one end to the other. (a) Retrograde transport. (b) Retrograde transport from multiple projection areas of a single neuron. (c) Anterograde transport from a single cell body into multiple projection areas. (Reproduced with permission from Kahle W, Frotscher M. Color Atlas of Human Anatomy. Vol. 3. 6th ed. Stuttgart: Thieme; 2010.)
Axon myelination. Axons are surrounded by a sheath of myelin (▶Fig. 1.4). The myelin sheath, which is formed by oligodendrocytes (a special class of glial cells) in the CNS and by Schwann cells in the PNS, is a sheetlike continuation of the oligodendrocyte or Schwann cell membrane that wraps itself around the axon multiple times, providing electrical insulation. Many oligodendrocytes or Schwann cells form the myelin surrounding a single axon. The segments of myelin sheath formed by two adjacent cells are separated by an area of uncovered axonal membrane called a node of Ranvier. Because of the insulating property of myelin, an action potential causes depolarization only at the nodes of Ranvier; thus, neural excitation jumps from one node of Ranvier to the next, a process known as saltatory conduction. It follows that neural conduction is fastest in neurons that have thick insulating myelin with nodes of Ranvier spaced widely apart. On the other hand, in axons that lack a myelin covering, excitation must travel relatively slowly down the entire axonal membrane. Between these two extremes, there are axons with myelin of intermediate thickness. Thus, axons are divided into thickly myelinated, thinly myelinated, and unmyelinated axons (nerve fibers); these classes are also designated by the letters A, B, and C. The Erlanger and Gasser classification of nerve fibers by their diameter and conduction velocity is shown in ▶Table 1.1.