Components and nerve structure

The peripheral nervous system is a collection of nerves that connect the brain and spinal cord to other body parts (the peripheral system).

Definitions

Somatic nervous system

Innervates the skin and muscle under voluntary control.

Autonomic nervous system

Innervates the visceral organs, such as the heart, stomach, and liver; control is involuntary.

Lower motor neuron lesion

Nerve damage to the fibers connecting the spinal cord to peripheral muscles; common symptoms include muscle weakness, muscle atrophy, fasciculations, hypotonia, and hyporeflexia.

Fasciculations

Involuntary muscle twitching that occurs when a peripheral nerve is overactive and damaged.

The contrasting upper motor neuron (UMN) presentation - weakness with spasticity, hyperreflexia, and a positive Babinski sign (without significant atrophy or fasciculations) rather than hypotonia and hyporeflexia - is covered in the upcoming central nervous system pathology chapters.

Components of the peripheral nervous system

There are four major components of the peripheral nervous system: cranial nerves, spinal nerves, peripheral nerves, and the nerves that comprise the autonomic nervous system.

Cranial nerves

Twelve pairs of nerves arise from the brainstem, providing sensory and motor innervation to the head and neck.

Olfactory nerve - I

• Sensory only
• Function: smell

Optic nerve - II

• Sensory only
• Function: visual acuity

Oculomotor nerve - III

• Motor only
• Function: moves the eye up, down, and in (medially); elevation of the eyelid; constriction of the pupil

Trochlear nerve - IV

• Motor only
• Function: moves the eye down and in (innervates the superior oblique muscle)

Trigeminal nerve - V

• Sensory and motor
• Sensory: touch, pain, and temperature to the face; ophthalmic, maxillary, and mandibular regions
• Motor: muscles of mastication

Abducens nerve - VI

• Motor only
• Function: moves the eye out (laterally)

Facial nerve - VII

• Sensory and motor
• Sensory: taste to the anterior two-thirds of the tongue
• Motor: facial expressions, closing eyes tightly

Vestibulocochlear nerve - VIII

• Sensory only
• Function: balance and hearing acuity

Glossopharyngeal nerve - IX

• Sensory and motor
• Sensory: taste to the posterior one-third of the tongue
• Motor: gag reflex, pharynx, and larynx control

Vagus nerve - X

• Sensory and motor
• Sensory: small area of taste on the back of the tongue; primary parasympathetic autonomic nervous system supply to internal organs, assisting with digestion, slowing heart rate, and bronchial constriction
• Motor: pharynx and larynx control (swallowing and speech)

Accessory nerve - XI

• Motor only
• Function: controls the trapezius and sternocleidomastoid muscles

Hypoglossal nerve - XII

• Motor only
• Function: controls tongue movements

Spinal nerves

• Thirty-one pairs of nerves exiting the varying vertebral areas, divided into dorsal (sensory) and ventral (motor) roots
  • Each spinal nerve corresponds with a segment of the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)

Structure of peripheral nerves

Definitions

Nerve fibers

Axons, which conduct electrical signals away from the nerve cell body toward their targets.

Connective tissue

The endoneurium, perineurium, and epineurium surrounding and binding the nerves, protecting them from injury.

Blood vessels

Vascularize the nerve.

Myelin

Fatty tissue surrounding and protecting the nerve.

Definitions

Axon

A projection that conducts electrical impulses away from the nerve cell body.

Dendrite

The receiving connection of a nerve cell.

Nerve fascicle

A bundle of nerve fibers targeting the same anatomical location, surrounded by the perineurium.

Perineurium

The middle layer of connective tissue that surrounds a nerve fascicle.

Endoneurium

The innermost layer of connective tissue, composed of collagen, fibroblasts, macrophages, and mast cells, that surrounds and protects individual nerve fibers within a fascicle.

Epineurium

The outermost layer of connective tissue that surrounds the entire nerve (the full bundle of fascicles).

Autonomic nervous system

The autonomic nervous system provides motor innervation to smooth muscle, cardiac muscle, and glands, giving unconscious control over these systems. It’s further divided into the sympathetic, parasympathetic, and enteric divisions, each of which works to maintain homeostasis in the body.

Definitions

Sympathetic nervous system

Manages the body’s response to stress and anxiety.

Parasympathetic nervous system

Manages the body at rest.

Enteric nervous system

Manages the gastrointestinal system.