The sensory input (afferent) of the reflex is from two structures in the muscle called spindles and Golgi tendon organs. The output (efferent) is the alpha motor neuron that contracts (tightens) the muscle. (The muscle fibers are sometimes referred to as extrafusal fibers.)
A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord.
Afferent nerve fibers carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem.
