The Nervous and Sensory Systems

The Nervous and Sensory Systems in Veterinary Practice

Understanding the nervous system allows veterinary assistants and technicians to recognize normal patient behavior, assist with neurologic examinations, and quickly identify signs that require immediate veterinary attention. Neurologic assessments are a routine part of patient care, so knowing the anatomy behind each test makes you a more effective and safer member of the clinical team.

Major Divisions of the Nervous System

The nervous system is divided into two major parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

The CNS consists of the brain and spinal cord. The brain is housed inside the skull and has three major regions:

• Cerebrum – The largest region, responsible for voluntary movement, learning, memory, and behavior. Changes here can appear as personality shifts, seizures, or loss of learned behaviors.
• Cerebellum – Located at the back of the skull, this structure coordinates balance and smooth muscle movement. A patient with cerebellar disease often shows a wide-based stance and exaggerated, bouncy steps (ataxia) without weakness.
• Brainstem – Connects the brain to the spinal cord and controls vital functions such as breathing, heart rate, and consciousness. It also houses the nuclei of most cranial nerves. Brainstem injury is life-threatening and may alter pupil size, swallowing, or the gag reflex.

The spinal cord runs through the vertebral column and is divided into cervical, thoracic, lumbar, sacral, and coccygeal segments. Each segment gives rise to spinal nerves. Knowing which segment controls which body region helps localize neurologic lesions.

The PNS includes all nervous tissue outside the CNS: the cranial nerves (which arise from the brain) and the spinal nerves (which arise from the spinal cord). The PNS is further divided into the somatic nervous system (voluntary control of skeletal muscle and sensory input from the skin and special senses) and the autonomic nervous system (involuntary control of heart, smooth muscle, and glands, further split into sympathetic and parasympathetic divisions).

Cranial Nerves Relevant to Veterinary Neurologic Examination

There are 12 pairs of cranial nerves. Veterinary practice most commonly assesses the following:

• CN II – Optic nerve: Tests vision and the pupillary light reflex (PLR). Shine a light in one eye and observe for pupil constriction in both eyes.
• CN V – Trigeminal nerve: Tests facial sensation and the muscles of mastication. Touching the cornea or nasal mucosa should produce a blink or head withdrawal. Loss of jaw tone may indicate CN V dysfunction.
• CN VII – Facial nerve: Controls the muscles of facial expression. Test by observing symmetry of the face, ability to blink, and the menace response (wave a hand toward the eye — a normal patient blinks).
• CN IX – Glossopharyngeal nerve and CN X – Vagus nerve: Work together to control swallowing and the gag reflex. Stimulate the pharynx gently and observe for a normal swallow or gag response.
• CN XII – Hypoglossal nerve: Controls tongue movement. Observe the tongue for symmetry and ability to retract; unilateral tongue deviation or atrophy suggests a lesion.

Always note asymmetry — differences between left and right sides are often the most clinically meaningful finding.

Reflex Arcs

A reflex arc is the simplest functional unit of the nervous system. It produces an automatic, involuntary response to a stimulus without requiring input from the brain. Understanding reflex arcs allows you to help assess lower motor neuron (LMN) versus upper motor neuron (UMN) lesions, which guides diagnosis and treatment urgency.

A complete reflex arc has five components:

1. Sensory receptor – Detects the stimulus (e.g., stretch receptor in the patellar tendon).

2. Afferent (sensory) neuron – Carries the impulse toward the spinal cord.

3. Integration center – Usually one or more interneurons within the spinal cord gray matter that process the signal.

4. Efferent (motor) neuron – Carries the impulse away from the spinal cord to the muscle.

5. Effector – The muscle that produces the response.

Patellar reflex: Strike the patellar tendon; the quadriceps muscle contracts, extending the stifle. This tests the femoral nerve and the lumbar spinal cord segments (approximately L4–L6 in dogs).

Withdrawal (flexor) reflex: Pinch the toe; the limb flexes and withdraws. This tests multiple nerve roots and assesses deep pain pathways. Absence of the withdrawal reflex indicates a serious LMN problem; presence of withdrawal without conscious pain perception points toward a more severe, higher-level lesion.

LMN vs. UMN signs: A LMN lesion (in the spinal cord segment or peripheral nerve) causes absent or reduced reflexes, flaccid paralysis, and rapid muscle atrophy. A UMN lesion (above the relevant spinal cord segment, in the brain or upper cord) causes exaggerated reflexes and spastic paralysis. Always report your findings to the supervising veterinarian; do not interpret or diagnose independently.

Safety and Scope of Practice

Neurologic patients may be disoriented, painful, or have unpredictable movements. Use appropriate restraint and never place your face or unprotected hands near an animal that may snap unpredictably. Spinal patients must be handled with great care — improper movement can worsen a spinal cord injury. Follow the veterinarian's instructions precisely regarding positioning and mobility restrictions. Document and report neurologic changes promptly; deterioration can be rapid and life-threatening.