Human Anatomy - Kotsan I. Ya. 2009

Peripheral Nervous System

The Peripheral Nervous system (systema nervosum periphericum) is a topographically distinct part of The Nervous System located outside the Brain AND SPINAL cord. Through the peripheral nervous system, the Central Nervous System regulates the Functions of all Organs, systems, and Tissues of the body.

The peripheral nervous system includes Cranial and Spinal Nerves, sensory ganglia of the cranial and spinal nerves, ganglia and NERVES OF THE Autonomic nervous system, somatic and autonomic nerve plexuses, as well as nerve endings—receptors that perceive stimuli from the external and internal environment, and effectors that transmit nerve impulses to effector organs.

Nerves consist of bundles of nerve fibers (processes of Nerve Cells). Externally, nerves and their branches are covered by a loose Connective Tissue sheath—the epineurium (epinevrium)—formed by Collagen and elastic fibers. Blood and Lymphatic vessels (vasa nervosum) and nerves (nervi nervosum) run within the epineurium. Beneath the epineurium lie bundles of nerve fibers covered by a thin sheath, the perineurium (perinevrium). Furthermore, each individual nerve fiber has its own connective tissue sheath, the endoneurium (endonevrium).

Nerves vary in length and thickness. Longer nerves are located in the tissues of the limbs, especially the lower extremities. The longest cranial nerve is the Vagus nerve. Large-diameter nerves are referred to as nerve trunks (trunci), and their branches as rami.

The number of nerve fibers within nerves varies, depending on the thickness of the nerve and the size of the area it innervates. In large nerves, nerve fibers may pass from one bundle to another along their course. Consequently, the thickness of the bundles and the number of nerve fibers within them are not uniform throughout the entire length of the nerve.

The nerve fibers forming a nerve do not always run in a straight line. They frequently follow a zigzag course, which protects them from overstretching during Movements of the trunk and limbs.

Nerve fibers within nerves can be myelinated (ranging from 1 to 22 µm in thickness) and unmyelinated (1–4 µm in thickness). Myelinated fibers are subdivided into thick (3–22 µm), medium, and thin (1–3 µm). The ratio of myelinated to Cytology/practical/64.html">Unmyelinated nerve fibers varies among different nerves. For instance, the proportion of medium and thin myelinated fibers in Muscle nerves is 18–40%, whereas in cutaneous nerves it reaches 60–80%.

Nerves are well vascularized by an abundant network of vessels that extensively anastomose with one another. Arterial branches supplying a nerve originate from vessels accompanying the nerve or running adjacent to it.

The nerve sheaths are innervated by branches originating from the nerve itself.

Nerve fibers forming the nerves of the peripheral nervous system can be divided into afferent and efferent. Afferent fibers transmit nerve impulses from receptors to the central nervous system; these are sensory nerve fibers. Efferent fibers conduct impulses from the central nervous system to the innervated organ; these are efferent nerve fibers. Depending on The Structure of the organs they innervate, efferent fibers are further subdivided into motor (innervating Muscle tissue), secretory (innervating glands), and trophic fibers, which regulate metabolic processes in tissues.

According to their site of origin from the central nervous system, nerves are classified into Cranial Nerves, emerging from the brain, and spinal nerves, originating from the Spinal Cord.

Based on their Location, a distinction is made between cutaneous (superficial) and muscular (deep) nerves. The former are located in the subcutaneous adipose tissue, upon the superficial Fascia of the body, whereas the latter lie beneath this fascia, between Muscles or muscle groups. As a rule, cutaneous nerves are not accompanied by blood and Lymphatic vessels and contain sensory (afferent) nerve fibers intended for Skin innervation, as well as autonomic fibers supplying Skin glands, arrector pili muscles, and Blood Vessels. Muscular nerves typically form part of neurovascular bundles and contain motor (efferent), sensory (afferent), and autonomic (visceral) nerve fibers that innervate muscles, joints, bones, and blood vessels.

Depending on their function, nerves are classified as sensory, motor, and mixed. Strictly speaking, purely sensory or purely motor nerves hardly exist, as all nerves contain postganglionic sympathetic fibers through which autonomic innervation of organs and tissues is mediated.

Sensory nerves are formed by the processes of nerve cells belonging to the sensory ganglia of cranial nerves or the sensory ganglia of spinal nerves.

Motor nerves are formed by the processes of nerve cells located in the anterior horns of the spinal cord or the motor nuclei of cranial nerves.

Mixed nerves, which contain both sensory and motor nerve fibers, predominate in the human peripheral nervous system.

Autonomic nerves are formed by the processes of cells originating from the autonomic nuclei of cranial nerves or the lateral horns of their spinal cord segments. Their course will be described in the chapter "Autonomic Nervous System".

The STRUCTURE OF THE peripheral nervous system exhibits several regularities. Let us outline the principal ones:

1. Nerves are paired and diverge symmetrically from the brain and spinal cord, which are situated along the body's axial line.

2. Like Arteries, nerves travel to organs via the shortest path. If an organ shifts its position during embryonic development, the corresponding nerve elongates and follows it accordingly.

3. Major nerves (nerve trunks) typically form part of neurovascular bundles enclosed within a common connective tissue sheath. A neurovascular bundle comprises arteries, Veins, lymphatic vessels, and a nerve.

4. Nerves supplying muscles originate from the spinal cord segments corresponding to the specific myotomes from which those muscles develop. During subsequent displacement, the source of innervation remains close to the original site of formation. Muscles originating from multiple myotomes are innervated by nerves whose fibers correspond to all the contributing myotomes.