A reflex is an involuntary reaction in response to a stimulus applied to the periphery and transmitted to the CNS. Reflexes allow a person to react to stimuli more quickly than is possible if conscious thought is involved. A reflex arc is the neuronal pathway by which a reflex occurs (figure 8.13). The reflex arc is the basic functional unit of the nervous system because it is the smallest, simplest pathway capable of receiving a stimulus and yielding a response. A reflex arc generally has five basic components:(1) a sensory receptor; (2) a sensory neuron; (3) in some reflexes, interneurons, which are neurons located between and communi-cating with two other neurons; (4) a motor neuron; and (5) an effector organ (muscles or glands). The simplest reflex arcs donot involve interneurons. Most reflexes occur in the spinal cord or brainstem rather than in the higher brain centers.
One example of a reflex occurs when a person’s finger touches a hot stove. The heat stimulates pain receptors in the skin, and action potentials are produced. Sensory neurons conduct the action potentials to the spinal cord, where they synapse with interneurons. The interneurons, in turn, synapse with motor neurons in the spi-nal cord that conduct action potentials along their axons to flexor muscles in the upper limb. These muscles contract and pull the finger away from the stove. No conscious thought is required for this reflex, and withdrawal of the finger from the stimulus begins before the person is consciously aware of any pain.