Spinal reflexes (myotactic reflexes) test the integrity of sensory and motor components of the reflex arc and the influence of descending motor pathways on the reflex. An absent or depressed reflex indicates complete or partial loss of either the sensory or motor nerves responsible for the reflex (lower motor neuron [LMN]) A normal reflex indices that sensory and motor nerves are intact. An exaggerated reflex indicates an abnormality in the descending pathways from the brain and spinal cord that normally inhibit the reflex (upper motor neuron [UMN]). In general, forelimb reflexes are not as reliable for localizing lesions as rear limb reflexes.

The perineal or anal sphincter reflex is elicited by gentle perineal stimulation with a needle or forceps.

A normal response is contraction of the anal sphincter muscle.

Sensory and motor innervation for the anal sphincter reflex occurs through the pudendal nerve (perineal nerve is sensory; caudal rectal nerve is motor) and spinal cord segments S1-S3. Absence or depression

of the reflex (failure of the anus to contract) indicates a sacral spinal cord or pudendal nerve lesion (LMN). An exaggerated response indicates a lesion above the S1 spinal cord segment.

The two general components of urinary bladder innervation are autonomic (hypogastric and pelvic) and somatic (pudendal) nerves. Simplistically, clinical observations of bladder dysfunction can be attributed to spinal cord injury based on the pudendal nerve (S1 and S2). The pudendal nerve innervates urethral striated muscle and helps maintain urinary continence. A lesion above the sacral spinal cord segments causes detrusor spasticity, making the bladder difficult to express (UMN). A lesion involving sacral spinal cord segments causes lack of sphincter tone and an easily expressible bladder (LMN).

The crossed extensor reflex may be observed when withdrawal reflexes are elicited. With the animal in lateral recumbency and legs relaxed, the toes of the uppermost limb (thoracic or pelvic) are gently pinched with fingers, eliciting a withdrawal reflex. An abnormal response is flexion of the upper limb and simultaneous extension of the lower limb. The stimulus must be gentle; excessive stimulus causes the animal to attempt to right itself, negating any findings. The crossed extensor reflex results from a lesion that affects descending inhibitory pathways of the spinal cord (UMN). This reflex is commonly associated with chronicity, but does not constitute a poor prognosis.

The significance of tail wagging in patients with spinal cord injuries is often misinterpreted. Animals with a completely transected spinal cord above the sacral and caudal spinal cord segments can wag their tails. This reflex wag is often observed when expressing the bladder or eliciting the anal sphincter reflex. Tail wagging may also be a conscious response to pleasurable stimuli, such as petting the head, calling the animal’s name, or seeing the owner. This conscious response implies that some spinal cord pathways are intact. It is important to distinguish between a spontaneous (reflex) and conscious tail wagging.

Panniculus reflex (cutaneous trunci reflex) is elicited by pin-prick stimulus to the skin over the back, beginning at the lumbosacral region and continuing cranially. Normal response is twitching of the cutaneous trunci muscle on both sides of the dorsal midline, at the point of stimulation and cranially. Absence of a response occurs one or two segments caudal to the spinal cord lesion. This reflex must be interpreted with some caution; it may be unreliable with the exception of brachial plexus avulsion injuries, in which it is consistently absent only on the side of the avulsion.

Clonus refers to a sustained after-contraction or quivering that may be seen or felt when performing spinal reflexes, especially patellar and crossed extensor reflexes. The hand supporting the extremity being tested may feel this reaction; this reflex is often not visual. Presence of clonus implies a chronic condition.