Relationship of the patellar tendon reflex to the ventral branch of the fifth lumbar spinal nerve in the dog.

The lumbosacral plexuses of dogs were exposed, using a ventral abdominal approach. In 4 dogs, the ventral branches (VB) of the 4th, 5th, 6th, or 7th lumbar spinal nerves were severed bilaterally. In 4 other dogs, the VB of combinations of 3 of these lumbar spinal nerves were severed so that the branch of only 1 nerve was kept intact. Among many neurologic deficits seen, the reflex of the patellar tendon was absent if the VB of the 5th lumbar spinal nerve was severed and was present if the branch was left intact. This finding was confirmed in another 12 dogs in which the VG of the 5th lumbar spinal nerve was severed on 1 side and the VB of the 4th and 6th lumbar spinal nerves were severed on the opposite side. In 4 additional dogs, the dorsal and ventral roots of the 5th lumbar spinal cord segment were isolated by dorsal laminectomy. Severing the dorsal root caused loss of the patellar tendon reflex, whereas severing the ventral root resulted in hyporeflexia. These findings would suggest that the major afferent impulse elicited by tapping the patellar tendon reaches the spinal cord by way of the dorsal root of the 5th lumbar spinal nerve.