Reflex arc

A reflex arc is the neural pathway that mediates a reflex action. In higher animals, these pathways do not pass through the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly.

If a reflex arc consists of only two neurons (one sensory neuron and one motor neuron), it is defined as monosynaptic. By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferent (sensory) and efferent (motor) signals. While the reflex generation of these pathways may be initiated by nociceptive input, extensive processing of polysynaptic reflexes can take place within the spinal cord.

In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish.

Classic Example: The Patellar Reflex
Patellar reflex: when the patellar tendon is tapped just below the knee, the patellar reflex is initiated and the lower leg kicks forward. The tap initiates an impulse in the sensory neuron in the leg, which travels to the spinal cord, through the relay, or interneuron, and into the motor neuron where it stimulates the effector organ, the leg muscle to contract.