The muscle stretch reflexes are relayed over structures of both the CNS and PNS. Since the tendons are not the primary structures involved, the term muscle stretch reflexes is more precise than the commonly used deep tendon reflexes. Recall that a reflex is an involuntary stereotypical response that may involve as few as two neurons, one afferent (sensory) and one efferent (motor), across a single synapse. The muscle stretch reflexes in the arms and legs are such monosynaptic reflexes. They illustrate the simplest unit of sensory and motor function. Other reflexes are polysynaptic, involving interneurons interposed between sensory and motor neurons.
To elicit a muscle stretch reflex, briskly tap the tendon of a partially stretched muscle. For the reflex to occur, all components of the reflex arc must be intact: sensory nerve fibers, spinal cord synapse, motor nerve fibers, neuromuscular junction, and muscle fibers. Tapping the tendon activates special sensory fibers in the partially stretched muscle, triggering a sensory impulse that travels to the spinal cord via a peripheral nerve. The stimulated sensory fiber synapses directly with the anterior horn cell innervating the same muscle. When the impulse crosses the neuromuscular junction, the muscle suddenly contracts, completing
the reflex arc.
Because each muscle stretch reflex involves specific spinal segments, together with their sensory and motor fibers, an abnormal reflex helps you locate a pathologic lesion. Learn the segmental levels of the muscle stretch reflexes. You can remember them easily by their numerical sequence in ascending order from ankle to triceps: S1, L2–L4, C5–C6, C6–C7.
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