Peripheral Nervous System: Structure, Function, and Organization

Peripheral Nervous System (PNS)

Overview of the Peripheral Nervous System

The Peripheral Nervous System (PNS) is a major division of the nervous system that connects the central nervous system (CNS) to limbs and organs. It serves as a communication relay between the brain, spinal cord, and the rest of the body.

• Components: Includes all neural structures outside the CNS.

• Main Divisions: Spinal nerves and cranial nerves.

• Function: Conducts sensory (afferent) and motor (efferent) impulses to and from the CNS.

Example: Touching a hot surface activates sensory nerves in the PNS, which send signals to the CNS for processing and then relay motor commands back to muscles to withdraw the hand.

What is a Nerve?

Structure and Connective Tissue Layers

A nerve is a bundle of axons (nerve fibers) in the PNS, surrounded by connective tissue layers that provide protection and support.

• Epineurium: The outermost layer that encloses the entire nerve.

• Perineurium: Surrounds bundles of axons called fascicles.

• Endoneurium: Encloses each individual axon within a fascicle.

Key Point: These connective tissue layers help maintain the structural integrity of nerves and facilitate the transmission of electrical impulses.

Example: Damage to the perineurium can disrupt the function of an entire fascicle, affecting sensation or movement in a specific body region.

Anatomy of a Nerve

Detailed Structure of a Spinal Nerve

Spinal nerves are complex structures composed of multiple fascicles, blood vessels, and supporting cells.

• Fascicle: A bundle of axons within the nerve, each surrounded by perineurium.

• Myelinated Axon: An axon covered by a myelin sheath (produced by Schwann cells), which increases the speed of nerve impulse conduction.

• Arteries and Veins: Supply blood to the nerve, ensuring nutrient and oxygen delivery.

Example: The sciatic nerve, the largest nerve in the body, contains thousands of fascicles and axons, allowing it to control and sense the lower limb.

Connective Tissue Layer	Location	Function
Epineurium	Surrounds entire nerve	Protection and structural support
Perineurium	Surrounds each fascicle	Maintains blood-nerve barrier
Endoneurium	Surrounds individual axons	Insulates and supports axons