Final answer:
The quick stretch of the patellar tendon activates the muscle spindle, causing the knee to extend in a healthy individual. This response is known as the knee-j__erk or stretch reflex and is a standard test in neurological exams.
Step-by-step explanation:
When a reflex hammer strikes the patellar tendon in a healthy individual, the quick stretch of the tendon activates the muscle spindle, and as a result, the knee extends. This type of reflex is known as a stretch reflex, specifically the knee-j__erk reflex, which is a common example demonstrated during a neurological exam. The stretch reflex serves to maintain muscle length by causing a reflexive contraction when the muscle spindle is stretched. Activation of the muscle spindle sends a signal through the dorsal root into the spinal cord, synapses directly onto the ventral horn motor neuron, and results in the contraction of the muscle.