A quick tap on a tendon, such as the patellar tendon below the knee cap, will elicit a reflexive contraction of the quadriceps muscle. The contraction occurs because the quick tapping on the tendon results in a stretch of the muscle spindles running parallel to the muscle fibers. In specific terms, the quick stretch activates neural connections between the activated muscle spindles and the alpha motor neurons supplying the same muscle. Because the Ia afferent fibers synapse directly onto the proximal dendrites and soma of the motor neuron in the spinal cord, the tendon reflex is monosynaptic and may be referred to as a phasic stretch reflex, muscle stretch reflex, or deep tendon reflex.
Reciprocal inhibition
While deep tendon reflexes involve a single connection between afferent and efferent neurons (i.e., a monosynaptic reflex), the expression of a deep tendon reflex requires additional spinal connections. These additional connections are typically described in terms of reciprocal inhibition and, from a neurological perspective, ensure that antagonistic muscles are inhibited during contraction of the agonistic muscle. For example, a quick tap on the biceps brachii tendon will elicit a deep tendon reflex in the biceps, which will abruptly shorten. This results in a quick stretch of the triceps (i.e., the antagonist to the biceps). Reciprocal inhibition will act to inhibit contraction of the triceps that would occur otherwise. Activity in collateral branches of the Ia afferent fibers facilitates interneurons in the spinal cord to inhibit the alpha efferents to the antagonist. Please see the simplified animation of a phasic stretch reflex on the next page.
