Peripheral nerve

General structural plan of a nerve trunk

On a histological cross-section, any nerve trunk appears as a collection of rounded bundles wedged between layers of connective tissue. Structurally, three main components are distinguished:

• Nerve fibers. These are directly the processes of nerve cells acting as electrical signal conductors.
• Connective tissue elements. A system of protective sheaths that bind fibers together, organize them into bundles, and provide mechanical strength.
• Blood vessels. Essential for adequate trophism (nutrition) of the nerve tissue.

Connective tissue framework

The angioarchitectonics of the sheaths follows the "Matryoshka doll" principle. Moving from the outer edge deep into the structure, three levels of organization are distinguished:

1. Epineurium (Epineurium). The outermost and strongest sheath, forming the overall external contour of the nerve. It is formed by fibrous connective tissue. Its thickness contains the largest nourishing blood vessels and clusters of adipocytes (fat cells).
2. Perineurium (Perineurium). Internal layers of connective tissue that divide the general trunk mass into separate isolated bundles (fascicles) of nerve fibers. Vessels and fat cells are also present here.
3. Endoneurium (Endoneurium). The deepest and thinnest layer, visible only under high magnification within the bundles. The endoneurium is located between individual nerve fibers. Morphologically, these are microscopic layers that stain oxyphilic (pink) due to the presence of collagen fibers. The main cellular element of the endoneurium is fibroblasts, which possess characteristic spindle-shaped nuclei.

Morphology of nerve fibers under a microscope

When examining bundles in a hematoxylin and eosin stained preparation, it is evident that the main type of fibers in a mixed nerve is myelinated. In the center of each such fiber lies the axon (directly the neuronal process), which appears as a darker structure.

Around the axon, a light, optically empty halo is visualized—this is the myelin sheath. Such a pale appearance is due to the specifics of histological sample preparation: lipids, which form the basis of myelin, dissolve in alcohols during tissue fixation, leaving an empty space behind.

Biological nature of processes

Axons in a mixed nerve have different functional origins. They can represent one of four variant processes:

• The dendrite of a receptor (sensory) neuron transmitting information from the periphery.
• The axon of an effector neuron of the somatic nervous system (motor pathway).
• The axon of an associative neuron of the autonomic nervous system (preganglionic fiber).
• The axon of an effector neuron of the sympathetic nervous system (postganglionic fiber).

By type of myelination, the first three types of processes are covered by a full-fledged sheath and form myelinated fibers. The fourth type (postganglionic sympathetic axons) belongs to unmyelinated fibers. Their key feature is that a single Schwann cell encloses several axons at once.

Spinal nerve roots and nerve formation

The formation of a spinal nerve is closely linked to the roots emerging in pairs from each segment of the spinal cord. Both roots contain myelinated nerve fibers.

• Anterior root (Radix anterior). Includes axons of somatic motor neurons and axons of associative autonomic cells. The direction of impulse transmission here is efferent—from the center to the periphery.
• Posterior root (Radix posterior). Represents an afferent pathway formed by processes of sensory neurons (carrying signals from peripheral receptors to the spinal cord or brainstem). A crucial topographical detail: the sensory spinal ganglion is located precisely on the posterior root.

Upon the fusion of the anterior and posterior roots, the spinal nerve trunk is formed. The site of their union is often referred to as the "bifurcation of the spinal nerve". As a result of the fusion, the nerve becomes functionally mixed, as it begins to contain both sensory and motor fibers.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Morphology and functions of Schwann cells in a nerve fiber
• Structure and physiological role of nodes of Ranvier
• Histological structure of the spinal ganglion
• Mechanisms of unmyelinated nerve fiber formation
• Features of synapses in the conduction part of the peripheral nervous system

Other topics in "Nervous system: nerves, ganglia, spinal cord"

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