Spinal level of the Achilles reflex?

• A. L4
• B. L5
• C. S2
• D. S1

Answer: (D)

The Achilles reflex primarily tests the S1 spinal level. When the Achilles tendon is tapped, the gastrocnemius-soleus complex is stretched, and its muscle spindles send Ia afferent signals into the spinal cord. These impulses synapse on alpha motor neurons that exit the spinal cord at the ventral horn and travel back through the tibial nerve to cause plantarflexion of the foot. The motor neurons driving this reflex are located mainly in the S1 segment (with some contribution from nearby S2), so the reflex is most sensitive to integrity at S1. Clinically, a diminished or absent ankle jerk points to S1 (or tibial nerve) involvement. This contrasts with the knee-jerk reflex, which evaluates L4, helping distinguish the level of a lesion.