Also known as the ankle jerk reflex, the Achilles reflex is a response to a strike to the back of the ankle. It can be demonstrated by hitting the Achilles tendon when the foot is flexed upward at the ankle joint so that the top of the foot is brought toward the shin. This reaction is seen as a sudden downward jerk of the foot from the dorsiflexed position as caused by a contraction of the calf muscles. The reflex may be tested in a medical setting to determine whether the sciatic nerve, which crosses the back of the ankle just deep to the Achilles or calcaneal tendon, is functioning properly.
All reflexes are simply a reaction to a physical stimulus by the nervous system. It is a type of reflex known as a stretch reflex, in which a muscle is placed into a stretched position before the stimulus causes it to contract, thereby returning to a shortened position. A stretch reflex can be observed by tapping the anatomical structure in question, then noting the reaction time. A reflex should be both involuntary, meaning that it happens without conscious thought, and immediate. Therefore, if no response occurs, or if that response is delayed, it may indicate a nervous system problem.
The nerve being stimulated when testing the Achilles reflex, the sciatic nerve, finds its origins or roots in spinal nerves branching from the sacral region of the spine. Specifically, when testing this reflex, one is determining the function of two particular nerve roots, S1 and S2, which exit the spinal column from the first and second segments of the sacrum bone. These spinal nerves converge to form the sciatic nerve, which runs down the back of the leg and serves the large gastrocnemius muscle, the major muscle of the calf that inserts into the Achilles tendon and attaches to the heel bone. The sciatic nerve runs just beneath and along the inside border of the gastrocnemius and Achilles tendon.
During a reflex test, the communication between the nerve and muscle is assessed. The nerve innervates the muscle via motor neurons, nerve cells that penetrate muscle fibers. When the gastrocnemius is stretched and then tapped where it forms the tendon, structures inside of each muscle cell called muscle spindles cause the muscle to contract in response to the stimulus. If everything is functioning properly, the contraction should cause the foot to quickly point downward, a movement known as plantarflexion.
Circumstances under which the Achilles reflex may be examined include tests of sciatic nerve function. If the roots of S1 and S2 are damaged in any way, such as by pinching where they exit the spine or by disk herniation, the reaction may be delayed or diminished. Certain diseases may also create a latent or absent response, including hypothyroidism, alcoholism, and Type 1 diabetes.