The nervous system performs many reflexes, which are responses to stimuli that do not require conscious involvement. One of the simplest and most clinically useful reflexes is the muscle stretch reflex. If a skeletal muscle is rapidly stretched, such as by percussing (tapping) its tendon with a rubber hammer, the muscle will contract involuntarily, presumably as a protective response. Muscle spindles are a type of mechanoreceptor in skeletal muscles that detect stretch, and somatosensory neurons transmit this information into the spinal cord or brainstem. In the central nervous system, these somatosensory neurons synapse directly on lower motor neurons that innervate the stretched muscle, which cause it to contract. A diminished muscle stretch reflex may occur with dysfunction of either the somatosensory axons returning from muscle spindles, or the lower motor neuron axons travelling to the stretched muscle. The somatosensory axons also usually synapse on interneurons in the central nervous system that inhibit lower motor neurons to muscles that are antagonist to the stretched muscle, causing the antagonist muscles to relax, increasing the amount of the reflex response.
Muscle stretch reflexes are usually tested by percussing five tendons bilaterally for muscle groups of the limbs: the brachioradialis tendon in the distal arm; the biceps tendon at the elbow; the triceps tendon at the elbow; the patellar tendon at the knee; and the Achilles tendon at the ankle. These reflexes are quantified on a scale from zero to four compared to a population normal: zero is no response; one is a response that is present but diminished; two is normal; three is increased; and four is increased plus reflex spread or clonus. Reflex spread refers to activation of the stretch reflex of muscles neighboring the stretched muscle. Clonus refers to involuntary alternating contraction of antagonist muscles triggered by muscle stretch, where the antagonist muscles alternately contract and probably activate each other’s stretch reflex. Normal muscle stretch reflexes vary by age and sex, as younger people and women tend to have brisker normal responses than older people and men.