<Li> Effector muscle innervated by the efferent nerve fiber carries out the response . </Li> <P> A reflex arc, then, is the pathway followed by nerves which (a .) carry sensory information from the receptor to the spinal cord, and then (b) carry the response generated by the spinal cord to effector organ (s) during a reflex action . </P> <P> When a reflex arc in an animal consists of only one sensory neuron and one motor neuron, it is defined as monosynaptic, referring to the presence of a single chemical synapse . In the case of peripheral muscle reflexes (patellar reflex, achilles reflex), brief stimulation to the muscle spindle results in contraction of the agonist or effector muscle . By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferent (sensory) and efferent (motor) signals . All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the brain . </P> <P> When the patellar tendon is tapped just below the knee, the tap initiates an action potential in a specialized structure known as a muscle spindle located within the quadriceps . This action potential travels to the L3 and L4 nerve roots of the spinal cord, via a sensory axon which chemically communicates by releasing glutamate onto a motor nerve . The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee (i.e. the lower leg kicks forward). Ultimately, an improper patellar reflex may indicate cerebellar injury . </P>

Classes of nervous system involved in reflex arc