Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Functional Anatomy of the Peripheral Nervous System
Concept of the Peripheral Nervous System

The Peripheral Nervous System is a collection of neural structures located outside the Brain AND SPINAL cord (Fig. 15.1). Peripheral nerves serve to conduct impulses from Sensory Organs to the Central Nervous System, and from the brain and Spinal Cord to effector organs (such as Muscles and glands). As a rule, nerves are mixed in terms of fiber composition, meaning they contain varying proportions of sensory, motor, and autonomic pathways.

Topographically, the peripheral nervous system is divided into the cranial and spinal divisions.

The cranial division is represented by neural structures connected to the Brainstem (Cranial Nerves, cranial sensory ganglia, nerve plexuses, organ nerves, and nerve endings). The spinal division is represented by neural structures connected to the spinal cord (Spinal Nerves, spinal nerve sensory ganglia, branches of spinal nerves, plexuses, organ nerves, and nerve endings).

Functionally, it is divided into the somatic division (innervating the soma, or body) and the autonomic division (innervating Internal Organs).

Nerves are formed by processes of Nerve Cells that group together into bundles of nerve fibers. These bundles are externally covered by a loose Connective Tissue sheath called the perineurium. Extensions of the perineurium penetrate between individual nerve fibers, forming an inner connective tissue sheath known as the endoneurium. A nerve comprising multiple fascicles is also surrounded externally by connective tissue called the epineurium, through which the Blood and Lymphatic Vessels of the nerve run.

Based on fiber composition, a distinction is made between motor, sensory, mixed, and autonomic nerves.

A motor nerve consists of nerve fibers formed by the axons of nerve cells located in the motor nuclei of the anterior horns of the spinal cord or in the motor nuclei of cranial nerves. In addition, they contain a small number of proprioceptive and sympathetic fibers.

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Fig. 15.1. Peripheral nervous system (diagram):

I — Cervical plexus; II — Brachial Plexus; III — intercostal nerves; IV — Lumbar plexus; V — Sacral Plexus; 1 — ophthalmic nerve; 2 — maxillary nerve; 3 — mandibular nerve; 4 — Facial Nerve; 5 — Vagus nerve; 6 — intercostal nerve; 7 — musculocutaneous nerve; 8 — radial nerve; 9 — median nerve; 10 — ilioinguinal nerve; 11 — iliohypogastric nerve; 12 — ulnar nerve; 13 — femoral nerve; 14 — sciatic nerve; 15 — obturator nerve; 16 — common fibular nerve; 17 — superficial fibular nerve; 18 — deep fibular nerve; 19 — tibial nerve; 20 — lateral femoral cutaneous nerve; 21 — Sympathetic trunk; 22 — celiac plexus

A sensory nerve consists of afferent nerve fibers, which are the peripheral processes of pseudounipolar or bipolar cells located within the sensory ganglia of spinal nerves or cranial nerves. Additionally, these nerves contain a small number of sympathetic nerve fibers.

A mixed nerve may include various combinations and percentages of sensory (afferent), motor (efferent), sympathetic, or parasympathetic fibers.

Autonomic nerves are formed by preganglionic or postganglionic fibers. Preganglionic fibers run from the Cells of the autonomic nuclei in the central nervous system to the autonomic ganglia. Postganglionic fibers extend from the cells of the autonomic ganglia to the organs and Tissues they innervate.

The conduction of impulses along nerve fibers is a complex physiological process. Running down the center of a myelinated nerve fiber is a nerve Cell process (the axon, or axis cylinder). Wrapped around it in multiple layers is the glial sheath, between the layers of which lies myelin—a protein-lipid compound with dielectric (insulating) properties. The myelin sheath does not cover the axon continuously, but rather at intervals known as nodes of Ranvier, where the fiber lacks a myelin coating.

At rest, a specific charge (potential) difference is maintained between the outer and inner sides of the nerve cell membrane. This is due to the differing concentrations of ions inside and outside the axon. When the charges of all ions inside and outside the axolemma are summed up, the inner side of the membrane is found to be negatively charged relative to the outer side. This state is called the resting Membrane Potential.

Embedded in the axolemma are specialized protein channels that allow ions to pass in the direction of their lower concentration. However, these channels remain closed at rest. When The Cell is stimulated, these channels open and ions cross to the opposite side of the membrane. This creates a state where the inner membrane becomes positively charged relative to the outer membrane. This shift is known as the membrane Action Potential.

The electrical field generated by this shift in charge difference propagates along the nerve fiber, activating Ion Channels in adjacent regions and allowing the excitation to spread further. In Cytology/practical/65.html">Myelinated nerve fibers, action potentials occur exclusively at the nodes of Ranvier, where neuronal processes are in contact with the Extracellular matrix. The impulse jumps from one node to the next thanks to the generated electrical field. The process of generating an action potential takes fractions of a second. The conduction velocity in myelinated fibers ranges from 10 to 120 m/s. Once the impulse has passed, the channels close and specialized protein pumps restore ion concentrations to the resting state—a process that requires the expenditure of ATP energy.

Unmyelinated fibers conduct nerve impulses at a significantly lower speed (about 1–2 m/s) due to the "diffusion" or dissipation of the impulse into the surrounding tissues.

Thus, the transmission of a Nerve Impulse is not a purely electrical phenomenon, but rather a combination of complex physiological processes involving the redistribution of ions across the nerve cell membrane. As such, electrical currents per se are not observed within nerves.