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An Urban Experience
to the lower motor neuron for motor function, resulting in a rapid correcting foot placement. Ascending sensory pathways are located in the outermost regions of the spinal cord and are very sensitive
to compression. With minor spinal cord injury, proprioceptive deficits may be present because of disrupted sensory pathways, while motor function persists because the deeper motor tracts
are unaffected. Both visual and tactile placing reactions require
an intact motor cortex and intact motor pathways to the involved limb. A cortical lesion may produce deficits in the contralateral limb, whereas lower lesion produces deficits in the ipsilateral limb.
Spinal reflexes
It is rare to have any reflex abnormalities if the animal
has no evidence of gait abnormality, muscle mass loss
or conscious proprioceptive deficits. In these cases,
a complete reflex examination is unlikely to be helpful. Completion of a reflex requires an intact sensory nerve that provides transmission to the spinal cord and an intact motor nerve that elicits function from the innervated muscle. The reflex arc itself does not involve the brain or the remainder of the spinal cord. Lesions in the motor arm of the reflex arc, termed lower motor neuron (LMN), may cause a decreased or absent reflex (hyporeflexia or areflexia). An exaggerated response (hyperreflexia) results from an interruption in proximal motor pathways that modulate the reflex, termed upper motor neuron (UMN); however, stress or anxiety may cause an apparent increased reflex response, so it should not be considered too important without other evidence of neurological disease. Lower motor neuron signs indicate damage to one or more components of the reflex arc. Upper motor neuron signs indicate damage anywhere between the reflex arc and the brain. The most reliable reflex is the flexor withdrawal in the thoracic and pelvic limbs. The other reflexes can appear to be present in small dogs just because the limbs will move when struck with a reflex hammer irrespective of reflex function.
The anal sphincter reflex
How to perform – pinch the anal sphincter with haemostats and watch for a wink-like contraction of the external sphincter muscles and tail flexion.
How to interpret – this reflex reveals information regarding the pudendal nerve and caudal segments of the spinal cord. A flaccid unresponsive anus indicates LMN damage to the pudendal nerve or its spinal roots.
A hypertonic, hyperresponsive anal sphincter indicates UMN damage at any point cranial to the pudendal nerve.
The pedal flexor reflex
How to perform – apply a pinch stimulus to each foot and evaluate the response of the ipsilateral and contralateral limb.
How to interpret – this is a withdrawal reflex in which stimulation of sensory receptors in the toes elicits contraction of flexor muscle groups in the leg. Presence of a withdrawal reflex requires an intact sciatic nerve (sensory and motor) and an intact spinal segment at the
lumbosacral plexus, but does not require transmission along the spinal cord to the brain. Absence of the withdrawal reflex in the pelvic limb denotes extensive lower motor neuron damage involving the lumbosacral spinal cord segements (L6-S2) as well as the nerve roots and the lumbosacral plexus; in the thoracic limb it denotes damage to the cervical spinal cord segments (C6-T2), the spinal nerve roots and the brachial plexus.
The patella reflex
How to perform – a tap stimulus should be applied
to the straight patella tendon and the response of the limb should be evaluated. Reflex hammer size must be adapted to patient size for improved accuracy.
How to interpret – this is a myotactic (stretch) reflex that effectively stretches the quadriceps muscle. This stretch stimulates the femoral nerve (L4-L5), which generates muscular contraction to extend the stifle. Upper motor neuron lesions cause hyperreflexia and should be accompanied by weakness and poor weight bearing. Disease in the L4-L5 spinal cord segments or nerves causes hyporeflexia.

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