Other Spinal Reflexes
PERINEAL REFLEX. The perineal reflex is elicited by pinching the mucocutaneous junction of the anus. The normal reflex includes tightening of the sphincter muscle and contraction of the ventral tail muscles.
Conscious perception of the stimulus produces avoidance or protective responses that may range from a slight movement of the pelvic limbs and pelvis to a violent kick. The reflex is mediated by the pudendal nerve and spinal cord segments S1 to S5. Lesions in the nerve or in the sacral spinal cord result in a dilated, atonic rectal sphincter that fails to respond to noxious stimuli. Additional signs of sacral spinal cord injury may include fecal impaction in the rectum and a dilated urinary bladder. The bladder is full of urine and dribbles whenever digital pressure is applied through the rectum or vagina. The perineum remains wet and may become irritated from constant contact with urine.CUTANEOUS TRUNCI REFLEX. The cutaneous trunci reflex (also known incorrectly as the “panniculus reflex”) is a shrugging or flinching of the skin over the thorax when it is stimulated by light touch or by pinching. Tap the skin with an extended thumb (in adult horses) or with a hemostat. The normal reflex response is a skin twitch often followed by a conscious avoidance maneuver such as moving away from the stimulus. The afferent part of the cutaneous trunci reflex is mediated by the segmental spinal nerves and dorsal nerve roots in the territory of the reflex. The tracts ascend in the dorsal funiculi of the spinal cord and synapse on efferent neurons in spinal segments C8 to T1 in ruminants and C8 to T2 in horses. The efferent axons exit in the ventral roots and form the lateral thoracic nerve, which innervates the cutaneous trunci muscle. The degree of reflex responsiveness varies among the large animal species. Sheep, goats, and many cattle possess a poor cutaneous trunci reflex.
Horses and Zebu cattle have a well-developed reflex.CERVICAL REFLEXES. Two reflexes have been described in the cervical area of the horse.47 The cervical reflex is tested by tapping or pinching the skin of the cervical region over the brachiocephalicus muscle and behind the second cervical vertebra. The expected response is contraction of the brachiocephalicus muscle and twitching of the skin. The pathway is believed to involve the cervical segmental spinal nerves and the local spinal cord segments. The cervicofacial reflex is elicited in horses by tapping the skin as described for the cervical reflex. As the skin is stimulated in normal horses, the ear reflexively twitches rostrally and ventrally, and the commissure of the lip retracts. This test measures the integrity of the dorsal funiculi of cervical spinal cord segments and the facial nerve in the medulla oblongata. The diagnostic usefulness of this test in ruminants is unknown.
Both reflexes may be abnormal in animals with lesions affecting the cervical spinal cord, such as cervical vertebral stenotic myelopathy in horses. The cervicofacial reflex also may be diminished or absent in animals with caudal brainstem lesions involving the facial nerve or with peripheral facial nerve lesions.
“SLAP” TEST (LARYNGEAL ADDUCTOR REFLEX). A sharp slap applied in the saddle region on one side of a horse's thorax results in adduction of the vocal folds of the larynx on the opposite side.48 Standing on one side of the animal, the examiner curls his or her fingers around the dorsolateral aspect of the larynx on the opposite side. A slap is applied to the saddle region on the side on which the examiner is standing. The response is palpated as a small movement of the arytenoid cartilage felt under the fingertips on the opposite side of the larynx. The laryngeal adduction elicited in this test can also be observed endoscopically. The pathway for the reflex is only partially understood. Sensory information from the skin and/ or pleural stretch receptors is relayed to the spinal cord in the segmental spinal nerves. The ascending pathways cross the spinal cord and run cranially in the spinal cord to the origin of the vagus nerve in the medulla oblongata. The efferent part of the reflex runs in the cervical vagosympathetic trunk and thence to the recurrent laryngeal nerve, which branches from the vagus in the cranial thorax. The recurrent laryngeal nerve courses cranially alongside the trachea to the larynx. Reduced or absent laryngeal adductor reflexes are found in animals that have lesions of the cervical spinal cord but also in those with caudal brainstem lesions, vagus nerve lesions, and lesions of the left recurrent laryngeal nerve (some of which are clinical “roarers”). The test's accuracy may be limited. In one study there was poor correlation when predicting the presence or absence of cervical spinal cord or brainstem disease.49