The deep tendon reflex exam represents one of medicine's most durable diagnostic tools. It requires minimal equipment: a percussor hammer, a relaxed patient, and an observant clinician.
When a physician taps your tendon, they strike a specialized sensory structure called a muscle spindle. This tap slightly stretches the muscle, firing an electrical impulse through an afferent sensory nerve fiber directly into the spinal cord. Inside the gray matter of the spinal cord, that sensory neuron synapses directly with an efferent alpha motor neuron. The motor neuron instantly sends a command back to the original muscle, forcing it to contract.
This loop is called a reflex arc. Because the signal processes entirely within the spinal cord without traveling up to the brain first, the resulting kick or twitch happens automatically in milliseconds.
During a standard neurological screening, clinicians evaluate several key tendon sites:
- Patellar Reflex (L2, L4): The classic knee-tap test that causes the lower leg to kick outward.
- Achilles Reflex (S1, S2): A tap to the back of the ankle that pushes the foot downward (plantar flexion).
- Biceps Reflex (C5, C6): A strike against the inner elbow that flexes the forearm.
- Brachioradialis Reflex (C5, C6): A tap just above the wrist along the thumb side of the forearm.
- Triceps Reflex (C7, C8): A strike to the tendon behind the elbow that straightens the arm.
Tension interferes with the signal. If you grip the exam table or anticipate the blow, your brain sends descending inhibitory signals that mute the reflex. When reflexes seem absent, doctors use distraction maneuvers, such as the Jendrassik maneuver, asking you to lock your fingers together and pull apart vigorously while they tap your knee.
