Peripheral nervous system

Classification of the nervous system

Anatomically, the unified nervous system is generally divided into two major divisions that are closely interrelated. The central nervous system (CNS) includes the brain (traditionally divided into the brainstem, cerebellum, diencephalon, and cerebral hemispheres) and the spinal cord, which consists of exactly 31 segments. The peripheral nervous system (PNS) is an extensive network acting as a link between the central apparatuses and all body organs. It comprises 12 pairs of cranial nerves and 31 pairs of spinal nerves. The peripheral division also includes spinal and cranial roots, nerve ganglia, nerve plexuses, regional nerve trunks, their numerous branches, and terminal nerve endings.

In addition to the classical anatomical classification, there is an important functional division of the nervous system:

• Somatic (animal) nervous system — systema nervosum somaticum — is responsible for the body's connection with the external environment and conscious control of musculature.
• Vegetative (autonomic) nervous system — systema nervosum autonomicum — regulates internal processes. It, in turn, is strictly subdivided into sympathetic and parasympathetic parts.

Development in embryogenesis

The rudiments of nervous structures begin to form at the very early stages of embryonic development. The initial material for formation is a thickening of the dorsal ectoderm, which initially forms the neural plate. In the process of further growth, this plate folds inward, forming the neural groove. Soon, the edges of the groove gradually close, turning into a closed neural tube.

It is from the neural tube that both central structures (brain and spinal cord) and elements of the peripheral network—nerve trunks, ganglia, nerve plexuses, peripheral nerves, and their endings—subsequently develop. At the same time, it is important to remember that auxiliary elements form from a completely different source: the mesenchyme. Mesenchymal derivatives include blood vessels supplying the nerve tissue, connective tissue elements of the meninges, and microglial cells (specific brain macrophages performing a protective function).

Structure of a spinal nerve

Each of the 31 pairs of spinal nerves is formed by the fusion of two roots emerging from the spinal cord. The posterior (dorsal) root contains sensory fibers (both somatic and vegetative) that transmit information from peripheral receptors to the center. The anterior (ventral) root carries motor somatic fibers, and in segments from the eighth cervical (C8) to the third lumbar (L3), it also carries vegetative efferent fibers.

After emerging from the intervertebral foramen, the short trunk of the spinal nerve divides into several branches:

1. Anterior branch — runs toward the anterior parts of the trunk and limbs.
2. Posterior branch — goes dorsally, innervating the deep muscles and skin of the back.
3. Meningeal branch — returns back into the vertebral canal to innervate the meninges of the spinal cord.
4. White communicating branch — is present exclusively in segments C8–L3 and connects the spinal nerve to the ganglia of the sympathetic trunk.

Nerve plexuses

Unlike the posterior branches, which retain a strict segmental structure throughout, anterior branches of the spinal nerves intertwine with each other, forming complex nerve plexuses. The only exception is the thoracic segments (Th2–Th11)—their anterior branches run metamerically in the intercostal spaces and are called intercostal nerves.

Main nerve plexuses formed by the anterior branches:

• Cervical plexus: formed by branches of the upper cervical segments (C1–C4).
• Brachial plexus: formed by the lower cervical segments (C5–C8) and partially the first thoracic segment (Th1).
• Lumbar plexus: includes branches of the lumbar segments (L1–L4) and part of the twelfth thoracic segment (Th12).
• Sacral plexus: the largest in the human body, consisting of part of the L4–L5 fibers and the sacral segments S1–S4.
• Coccygeal plexus: formed by the lowest segments S5 and Co.

Class and exam questions

What else to study on this topic

• Topography and branches of the cervical plexus
• Nerves of the brachial plexus and their innervation zones
• Intercostal nerves: course and areas of innervation
• Lumbar and sacral plexuses: formation and main trunks
• Autonomic nervous system: sympathetic and parasympathetic divisions

Other topics in the "Additional Topics" section

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