Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Part
Peripheral Nervous System, systema nervosum periphericum – General Overview
General structural principles of peripheral nerves

Based on their origin in the Central Nervous system, peripheral nerves are divided into spinal and Cranial Nerves.

There are 31 pairs of Spinal Nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal (see Fig. 124). They are characterized by segmental emergence from the Spinal Cord. Each spinal cord segment, along with its 2 pairs of rootlets and a pair of spinal nerves (Fig. 177) known as a neurotome, corresponds to a specific Skin area (dermatome), bone (sclerotome), and Muscles (myotome). It should be noted that each metamere (sclerotome, myotome, dermatome) receives fibers not only from its corresponding neurotome, but also from the adjacent superior and inferior ones (overlapping innervation zones).

Class="center">

Fig. 177. Structure of spinal nerves, nn. spinales (schematic)

Fig. 178. Diagram of The formation of a spinal nerve, n. spinalis (after I. Ye. Kefeli, with modifications)

Spinal nerves, nervi spinales, are mixed nerves, meaning they contain sensory, motor, and autonomic nerve fibers (Fig. 178).

Sensory fibers represent the peripheral processes of pseudounipolar Neurons located in the spinal ganglia, ganglia spinalia. These are situated within the intervertebral foramina. The central processes from the pseudounipolar neurons of the spinal ganglia travel via the posterior (sensory) roots to the nuclei of the posterior horns or the posterior funiculi of the spinal cord. The motor and autonomic fibers of the anterior roots of the spinal nerves are the processes of neurons from the motor nuclei of the anterior horns or the corresponding autonomic nuclei of the lateral horns of the spinal cord.

Thus, a spinal nerve, n. spinalis, is formed by the combination of dendrites from pseudounipolar neurons of the spinal ganglion and axons of motor neurons from the anterior horns of the spinal cord, as well as fibers of the Autonomic nervous system at the C_VIII-L_II and S_II-S_IV segment levels.

Upon exiting the intervertebral foramen, the spinal nerve branches into 3-4 branches (Fig. 177):

1. Anterior branches, rami anteriores — mixed nerves that extend to the anterior PARTS OF THE trunk and limbs, where they innervate the skin and muscles.

2. Posterior branches, rami posteriores — mixed nerves (with some exceptions — see the section "Spinal Nerves") that extend to the posterior parts of the trunk. They innervate the skin and autochthonous Muscles of the posterior surface of the trunk.

3. Meningeal branches, rami meningei — sensory branches that extend to the Meninges of the spinal cord.

4. Communicating branches, rami communicantes — autonomic branches that lead to the Sympathetic trunk ganglia for the innervation of Internal Organs and Blood Vessels (see the section

"Autonomic Nervous System").

Cranial nerves, nervi craniales, can be mixed, motor, or sensory. Their sensory fibers are the processes of pseudounipolar (bipolar) neurons of the sensory ganglia of the cranial nerves, ganglia sensoria nn. cranialium, while their motor and autonomic fibers are the processes of Cells from the corresponding nuclei.

Each nerve consists of bundles of myelinated and Cytology/practical/64.html">Unmyelinated nerve fibers, between which lies loose Fibrous Connective Tissue known as the endoneurium, endoneurium. Each bundle of nerve fibers is covered by a lamellar sheath called the perineurium, perineurium. All the bundles of a nerve trunk are surrounded by an external sheath composed of loose fibrous connective tissue, the epineurium, epineurium.

The number of bundles varies in different nerves. Even identically named nerves at the same level in different individuals may have different fascicular structures. Furthermore, the number of bundles varies at different levels of the same nerve. Along the course of the nerve, fibers pass from one bundle into another, and a very large number of inter-fascicular connections are frequently observed. This led O. M. Maksymenkov (1939) to view nerve trunks as plexuses extended into the periphery.

It should be noted that in cross-sections of nerve trunks, the area occupied by nerve fibers does not exceed 50%. The rest is occupied by sheaths (epi-, peri-, and endoneurium) and blood vessels.

The Blood supply to nerves originates from the Arteries closest to them. These arteries approach the nerve trunks segmentally, dividing within the epineurium into ascending and descending branches that anastomose with similar branches from superior and inferior arteries. As a result of their further branching within the epineurium, a network of epineurial arterioles is formed, which serves as the source of the microcirculatory bed of the epi-, peri-, and endoneurium. Venules of the epi-, peri-, and endoneurium drain into epineurial Veins, which in turn empty into veins running alongside the nerve.

The Lymphatic vessels of nerves are located exclusively in their epineurium. They ensure the outflow of Lymph to the nearest lymphatic collectors and subsequently to regional Lymph Nodes. In the perineurium, between its layers, and in the endoneurium, a system of clefts is formed through which peri- and endoneurial fluids circulate. Infection can spread along these clefts.

The nerve sheaths are innervated by fibers from the nerve itself, which form quite dense plexuses within the epi- and perineurium.

The intraneurial vascular-perineurial microcirculatory System of the nerve sheaths, together with their intrinsic nerve plexus, integrates mechanical, trophic, metabolic, and homeostatic Functions to ensure the proper functioning of nerve fibers within the bundles—namely, the normal conduction of nerve impulses.

Detailed studies of the Peripheral Nervous System have shown that in different individuals, identically named nerves may have varying levels of formation and branching patterns. Connections are observed between nerves through which they exchange nerve fibers; however, such connections are formed exclusively between genetically related nerves.

It should also be noted that nerve fibers from specific segments may be distributed differently among individuals across the various nerves originating from those segments. Consequently, identically named nerves in different people may innervate different areas.

A system of nerve trunks that share a genetic affinity and segmental unity is referred to as a nerve complex. For example, the following nerve complexes are distinguished in the limbs: the musculocutaneous and median nerves in the arm region; the median and ulnar nerves in the forearm and hand region; the femoral, obturator, and lateral femoral cutaneous nerves in the anterior thigh region; the sciatic and posterior femoral cutaneous nerves in the posterior thigh region; and the superficial and deep fibular nerves in the leg region.