Chapter 1: The Anatomy and Physiology of the Peripheral Nerve

Objectives

On completion of this chapter, the student/practitioner will be able to:

• Describe the basic anatomical structure of a peripheral nerve.

• Relate the structural and functional anatomy of a peripheral nerve.

• Define the process of wallerian degeneration.

• Compare and contrast the various common nerve classifications.

• Discuss the structural and functional impact of aberrant tensile and compressive forces on peripheral nerves.

Key Terms

• Classification of peripheral nerve injury

• Compression

• Endoneurium

• Epineurium

• Perineurium

• Tension

• Wallerian degeneration

Introduction

Homeostasis (Greek: homoios, similar; histēmi, to cause to stand still) is the property of an open or closed system that allows regulation of its internal environment.¹ In other words, homeostasis is the physiological and anatomical capacity of an organism to regulate itself by rapidly analyzing and, if aberrant, restoring environmental conditions following a sudden perturbation in the internal or external environment. Such internal or external environmental conditions or “stimuli” initiate electrical impulses in peripheral and central sensory receptors. The impulses travel afferently from the receptors via nerves to the spinal cord and brain where they are analyzed, compared, learned, and coordinated by a process called “integration.” Once the afferent information is received and deciphered, the spinal cord and brain convey efferent impulses through nerves to muscles and glands. In an effort to maintain homeostasis, muscles either contract or relax, and glands either secrete or stop secreting their products.

The nervous and endocrine systems are the two major regulatory systems of the body, and both are specialized (and defined) by making appropriate and timely responses to internal or external stimuli. The nervous system, using a combination of electrical potentials and neurotransmitters to communicate messages and tasks, is the faster of the two; the endocrine system depends on a slower transmission system, a chemical system using hormones. Typically, long-term organism growth, metabolic activity, and the reproduction system are controlled by the endocrine system. Faster and immediate tasks such as movement and autonomic regulation are controlled by the neurological system. Although considered two distinct corporal systems, the endocrine and neurological systems are considered a singular regulatory system.

Albeit a unified system peripherally and centrally, the nervous system is typically anatomically defined as having central and peripheral components—the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists of all neurological tissue outside of the spinal cord and brain.

Failure of the PNS to transmit afferent, efferent, or autonomic data completely or in a timely manner because of illness or trauma is called peripheral neuropathy. Peripheral neuropathy results in possible loss of homeostasis in all corporal systems. Specific neurological illness or trauma leads to impairment of particular functional units ...