Components and nerve structure

The peripheral nervous system (PNS) is all neural tissue outside the brain and spinal cord; it carries sensory input to the central nervous system and motor output to muscles, organs, and glands.

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.

Here’s a side-by-side comparison of the two presentations:

Sign	LMN lesion	UMN lesion
Tone	Hypotonia	Spasticity
Reflexes	Hyporeflexia	Hyperreflexia
Atrophy	Marked	Minimal
Fasciculations	Present	Absent
Babinski sign	Absent	Present

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. A PTA screens each of these systems directly during data collection, so knowing their structure explains what a given exam finding means.

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
  • The lingual nerve, a branch of the mandibular division (CN V3), carries general sensation (touch, pain, temperature) from the anterior two-thirds of the tongue, while taste from that area travels with the chorda tympani of the facial nerve
• Motor: muscles of mastication

Abducens nerve - VI

• Motor only
• Function: moves the eye out (laterally); the nerve begins at a nucleus in the dorsal pons and controls the lateral rectus muscle, which abducts the eye
• Palsy of the abducens nerve weakens the lateral rectus, so the affected eye turns inward and cannot abduct, causing horizontal double vision (diplopia)
• Palsy of the trochlear nerve (CN IV), which supplies the superior oblique, causes vertical and/or torsional double vision that is usually worse on downgaze

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; sensation from the pharynx and thoracic and abdominal organs
• Motor: pharynx and larynx control (swallowing and speech); primary parasympathetic supply to internal organs, slowing heart rate, constricting the bronchi, and assisting digestion

Accessory nerve - XI

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

Hypoglossal nerve - XII

• Motor only
• Function: controls tongue movements

A PTA screens cranial nerve function as part of data collection:

Cranial nerve	Quick screen
I - olfactory	Identify a familiar scent with the eyes closed
II - optic	Visual acuity and visual fields by confrontation
III, IV, VI - eye movement nerves	Follow a finger through an “H” pattern; pupillary light reflex
V - trigeminal	Light touch to the face; resisted jaw clench
VII - facial	Raise eyebrows, close eyes tightly, smile
VIII - vestibulocochlear	Finger-rub hearing test; observe balance
IX, X - glossopharyngeal, vagus	Gag reflex; say “ah” and watch for symmetric palate elevation
XI - accessory	Resisted shoulder shrug and head turn
XII - hypoglossal	Stick out the tongue and check for deviation

A new or worsening finding is documented and reported to the supervising PT, who decides whether a formal re-evaluation is needed - the PTA’s role is to collect and report the finding, not to diagnose it.

Spinal nerves

Thirty-one pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) exit the spinal cord, one per cord segment. Each forms from a dorsal (sensory) root and a ventral (motor) root.

Structure of peripheral nerves

Definitions

Axon

Also called a nerve fiber, an axon conducts electrical signals away from the nerve cell body toward its targets.

Dendrite

The receiving connection of a nerve cell.

Myelin

A fatty sheath, formed by Schwann cells in the peripheral nervous system, wrapped around individual axons. It insulates the axon and increases conduction velocity.

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.

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.

Epineurium

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

Blood vessels

Vascularize the nerve.

Peripheral nerves also carry sensory receptors - specialized endings such as mechanoreceptors, thermoreceptors, and nociceptors - that convert a specific stimulus (pressure, temperature, pain) into the electrical signal the nerve fiber carries to the CNS, as shown below.

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.

Pitfall: autonomic dysreflexia. In a patient with a spinal cord injury at T6 or above, a noxious stimulus below the level of injury (such as a full bladder or skin irritation) can trigger a sudden spike in blood pressure, bradycardia, and headache. Sit the patient upright, look for and remove the triggering stimulus, and notify the supervising PT and nursing staff immediately.