Figure 3-9 Autonomic neuromuscular junction between a nonmyelinated axon and a smooth muscle fiber.
Figure 3-10 Photomicrograph of a longitudinal section of a peripheral nerve stained with hematoxylin and eosin (÷400).
Figure 3-11 Photomicrograph of a transverse section of a peripheral nerve stained with hematoxylin and eosin (÷275).
Spinal Nerves and Spinal Nerve Roots
There are 31 pairs of spinal nerves, which leave the spinal cord and pass through intervertebral foramina in the vertebral column. (For details, see p. 12.) Each spinal nerve is connected to the spinal cord by two roots: the anterior root and the posterior root (Fig. 3-13).
The anterior root consists of bundles of nerve fibers carrying nerve impulses away from the central nervous system; these nerve fibers are called efferent fibers. The posterior root consists of bundles of nerve fibers carrying nerve impulses to the central nervous system; these nerve fibers are called afferent fibers. Because these fibers are concerned with conveying information to the central nervous system, they are called
sensory fibers. The cell bodies of these nerve fibers are situated in a swelling on the posterior root called the posterior root ganglion.
Figure 3-12 Structure of a peripheral nerve.
Table 3-2 Classification of Nerve Fibers by Speed of Conduction and Size
Fiber Type Co nductio n Velo city (m /s)
Fiber
Diam eter (µ m ) Functio ns M yelin Sensitiv ity to Lo cal
A nesthetics
A Fibers
Alpha 70–120 12–20 M otor, skeletal muscle Yes Least
Beta 40–70 5–12 Sensory, touch, pressure, vibration Yes
P.83
Gamma 10–50 3–6 M uscle spindle Yes
Delta 6–30 2–5 Pain (sharp, localized), temperature, touch Yes
B Fibers 3–15 <3 Preganglionic autonomic Yes
C Fibers 0.5–2.0 0.4–1.2 Pain (diffuse, deep), temperature,
postganglionic autonomic No M ost
Cranial Nerves
There are 12 pairs of cranial nerves (Fig. 3-13), which leave the brain and pass through foramina in the skull. Some of these nerves are composed entirely of afferent nerve fibers bringing sensations to the brain (olfactory, optic, and vestibulocochlear), others are composed entirely of efferent fibers (oculomotor, trochlear, abducent, accessory, and hypoglossal), while the remainder possess both afferent and efferent fibers (trigeminal, facial, glossopharyngeal, and vagus). The cranial nerves are described in detail in Chapter 11.
Sensory Ganglia
The sensory ganglia of the posterior spinal nerve roots and of the trunks of the trigeminal, facial, glossopharyngeal, and vagal cranial nerves have the same structure. Each ganglion is surrounded by a layer of connective tissue that is continuous with the epineurium and
perineurium of the peripheral nerve. The neurons are unipolar, possessing cell bodies that are rounded or oval in shape (Fig. 3-14). The cell bodies tend to be aggregated and separated by bundles of nerve fibers. A single nonmyelinated process leaves each cell body and, after a convoluted course, bifurcates at a T junction into peripheral and central branches. The former axon terminates in a series of dendrites in a peripheral sensory ending, and the latter axon enters the central nervous system. The nerve impulse, on reaching the T junction, passes directly from the peripheral axon to the central axon, thus bypassing the nerve cell body.
Each nerve cell body is closely surrounded by a layer of flattened cells called capsular cells or satellite cells (Fig. 3-14). The capsular cells are similar in structure to Schwann
cells and are continuous with these cells as they envelop the peripheral and central processes of each neuron.
Figure 3-13 A: Transverse section of the thoracic region of the spinal cord showing the formation of a spinal nerve from the union of an anterior and a posterior nerve root. B: Transverse section of the pons showing the sensory and motor roots of the trigeminal nerve.
Figure 3-14 Photomicrograph of a longitudinal section of a posterior root ganglion of a spinal nerve stained with hematoxylin and eosin (÷400).
Autonomic Ganglia
The autonomic ganglia (sympathetic and parasympathetic ganglia) are situated at a distance from the brain and spinal cord. They are found in the sympathetic trunks, in prevertebral autonomic plexuses (e.g., in the cardiac, celiac, and mesenteric plexuses), and as ganglia in or close to viscera. Each ganglion is surrounded by a layer of connective tissue that is continuous with the epineurium and perineurium of the peripheral nerve. The neurons are multipolar and possess cell bodies that are irregular in shape (Fig. 3-15). The dendrites of the neurons make synaptic connections with the myelinated axons of preganglionic neurons. The axons of the neurons are of small diameter (C fibers) and unmyelinated, and they pass to viscera, blood vessels, and sweat glands.
Each nerve cell body is closely surrounded by a layer of flattened cells called capsular cells or satellite cells. The capsular cells, like those of sensory ganglia, are similar in structure to Schwann cells and are continuous with them as they envelop the peripheral and central processes of each neuron.
Peripheral Nerve Plexuses
Peripheral nerves are composed of bundles of nerve fibers. In their course, peripheral nerves sometimes divide into branches that join neighboring peripheral nerves. If this occurs frequently, a network of nerves, called a nerve plexus, is formed. It should be emphasized that the formation of a nerve plexus allows individual nerve fibers to pass from one peripheral nerve to another, and in most instances, branching of nerve fibers does not take place. A plexus thus permits a redistribution of the nerve fibers within the different peripheral nerves.
Figure 3-15 Photomicrograph of a longitudinal section of a ganglion of the sympathetic trunk stained with hematoxylin and eosin (÷300).
Figure 3-16 Brachial plexus.
At the root of the limbs, the anterior rami of the spinal nerves form complicated plexuses. The cervical and brachial plexuses are at the root of the upper limbs (Fig. 3-16), and the lumbar and sacral plexuses are at the
root of the lower limbs. This allows the nerve fibers derived from different segments of the spinal cord to be arranged and distributed efficiently in different nerve trunks to the various parts of the upper and lower limbs.
Cutaneous nerves, as they approach their final destination, commonly form fine plexuses that again permit a rearrangement of nerve fibers before they reach their terminal sensory endings.
The autonomic nervous system also possesses numerous nerve plexuses that consist of preganglionic and postganglionic nerve fibers and ganglia.