Final answer:
The patellar tendon stretch reflex involves the muscle spindle, a rapid signal through the dorsal root to the spinal cord, and a motor response causing leg extension. This reflex may still occur with cervical level spinal cord damage if the reflex pathway within the spinal cord is intact.
Step-by-step explanation:
When the patellar tendon is struck by a rubber mallet, it stretches the quadriceps muscle at the front of the thigh. The stretching activates muscle spindle receptors within the muscle, sending a signal via sensory nerve fibers into the spinal cord through the dorsal root. This signal synapses directly on the motor neurons in the ventral horn of the spinal cord, leading to contraction of the quadriceps muscle, which results in extending the leg.
In the event of spinal cord damage at the cervical level, whether this stretch reflex occurs depends on the extent and location of the injury. Since the reflex circuit is located within the spinal cord and does not require input from the brain, a reflex might still occur if the damage is above the level of the reflex arc. However, if the nerve pathways within the spinal cord that are involved in the reflex are damaged, the reflex would not occur.