The neuron is made up of three basic parts. The soma, or cell body, is the point at which the cell nucleus resides along with other cellular organelles (Fig. 5-1). Extending from the soma are two types of processes. The dendrite, which receives signaling information from the local environment, contains, or is associated with, receptors that are designed to sense specific stimuli or react to specific chemical transmitters. Neurons may have hundreds or even thousands of dendrites, depending on the size and complexity of their receptive fields. The second type of cellular process is the axon, along which a signal travels as an action potential. Action potentials are typically generated in an axon via the summation of signals received from the dendrites, which can be either stimulatory or inhibitory. When the action potential reaches the end of the axon, it causes the release of one or more neurotransmitters, which signal either other neurons or specific target tissues. Axons can branch extensively, allowing a single neuron to communicate with and send signals to many other neurons. This phenomenon is known as divergence and is significant when the spread of information is desirable for the organism (Fig. 5-2). In this case, the signal is said to be amplified. Conversely, when signals from multiple neuronal sources terminate on a single neuron, convergence is said to exist (Fig. 5-3). When convergence exists, some signals may be excitatory; others may be inhibitory. As with all neurons, the summation of signals may (or may not) reach the threshold for the stimulated neuron to generate an action potential.

Neurons of the PNS are associated with Schwann cells (myelin). These cells are responsible for supporting and insulating axons, allowing for more efficient propagation of action potentials. Neuronsare said to be myelinated or unmyelinated, depending on the number of times a Schwann cell wraps around a given axon (often referred to as a fiber) (Fig. 5-4).

Action potentials are propagated along axons with varying speeds. The speed of an action potential depends primarily on two factors: the diameter of the axon and the degree of myelination. Larger diameter and heavily myelinated axons (A fibers) convey impulses considerably faster than smaller diameter unmyelinated axons (C fibers). The difference in propagation speeds becomes important when discussing the transmission of pain (see later discussion).

A cross section of the spinal cord reveals two basic components: gray matter and white matter (Fig. 5-5). The gray matter is a butterfly-shaped column of nerve cell bodies, axons, and dendrites. Because of the metabolic needs of the neuronal cell bodies, a robust blood supply is contained in the gray matter, providing its characteristic appearance. The butterfly is subdivided into a pair of dorsal (or posterior) horns, or columns (when seen three dimensionally), and a pair of ventral (or anterior) horns, or columns. The dorsal horns are responsible for processing sensory information, which enters the cord where the dorsal root of the spinal nerve joins it. Axons from the dorsal root will penetrate the dorsal horn to varying depths before terminating and synapsing with other neurons, or they will leave the gray matter to form a tract.

The white matter, in contrast, consists primarily of myelinated axons forming ascending and descending tracts that convey information toother areas of the CNS. Specialized cells called oligodendrocytes are responsible for myelination in the CNS. The white matter is subdivided into columns or funiculi, whose names are derived from their relationship with the gray matter: anterior, posterior, and lateral. The fatty myelin sheaths are responsible for the coloration of the white matter.

The sensory neurons contained in the spinal nerves are of the pseudounipolar variety (Fig. 5-6). The cell bodies of these sensory neurons are located in the dorsal root ganglion (DRG).

These nerves convey a wide range of sensations. Each specific sensation is referred to as a modality. Some examples are light touch, proprioceptive sensations (giving a sense of position to the body), and pain. These and other modalities are discussed later in this chapter. Specific receptors exist for each modality (Fig. 5-7). Sensory receptors are divided into three basic types: exteroceptors, which carry information that originates outside or on the surface of the body, interoceptors, which convey information originating from inside the body, and proprioceptors, which provide information on body position and movement.

Subclasses of the exteroceptors include:

Most sensory receptors can be classified as slowly adapting (SA) or rapidly adapting (RA). In either case, the receptors are responding to some type of change in the local environment. Adaptation of receptors occurs when a constant stimulus is applied. As the stimulus is applied over time, the rate of impulse response to the stimulus will gradually decrease to a very low level (i.e., The individual “gets used to it.”). Therefore SA receptors will adapt more slowly to a stimulus, and it will be perceived for a longer period. SA receptors are also referred to as tonic receptors. In contrast, RA receptors will respond more efficiently to changes in stimulus strength, such as occurs with vibration, and are referred to as phasic, rate, or movement receptors. Nociceptors, or receptors that perceive painful stimuli, are referred to as nonadapting, implying that there is no getting used to pain. (See Chapter 9 for an extensive discussion of nociceptors.)

The neurons contained in the DRGs of spinal nerves generally convey either somatic information from the body surface, walls of body cavities, and extremities or visceral information from the internal organs and contents of body cavities. The special senses (smell, vision, hearing, and taste) are conveyed by the cranial nerves. An important point to remember is that each neuron has a specific receptor or set of receptors on its peripheral end that are consistent with the type of information that the neuron conveys (i.e., neurons are modality specific). Also worthy of note is that some DRG neurons are quite long. For example, a single neuron carrying pain information from the foot might be 3 to 4 feet or longer, depending on the height of the individual.

The peripheral distribution of the sensory neuron conforms to its distribution in the embryo and fetus (see Chapter 4). In addition, these neurons can branch or collateralize quite extensively on their peripheral ends. This branching can result in an extensive sensory field for a single neuron. Thus the sensory component (along with the motor component) of the spinal nerve is distributed to the dermatome (for touch, temperature, and pain), the myotome, and the sclerotome (for proprioception and pain). The dermatomes of the spinal nerves follow a specific pattern, which is routinely exploited by clinicians (Fig. 5-8). For example, if a spinal nerve is damaged, the patient may experience numbness or tingling along the nerve’s dermatome. By contrast, if a spinal nerve is impinged by another structure such as an intervertebral disk, the patient may experience radiating pain along the entire length of the dermatome, in spite of the site of impingement being quite small and proximal.

The central process of the DRG neuron projects through the dorsal root of the spinal nerve into the dorsal horn of the spinal cord. As these nerves collateralize (branch) peripherally, they can also collateralize centrally, resulting in a specific field of distribution for the incoming signal. This property means that a single DRG neuron can communicate with many neurons in the spinal cord, producing a variety of effects. From the dorsal horn of the spinal cord, modality-specific neurons relay to or directly enter the nerve tracts that convey specific types of information upward to the brainstem and cerebral cortex (discussed later). This specificity in conveying a specific type of information from the periphery to a specific tract and subsequently to a specific area of the CNS is known as a labeled line.