Reflexes are involuntary stimulus-response behaviors of the human nervous system. The sequence of neural events that gives rise to reflexive behaviors is called the reflex arc:

1. A receptor organ (skin, eye, mouth) receives a stimulus from the external environment (hot touch, bright light, sour taste).

2. Information about the stimulus is transmitted along a sensory—that is, afferent—neuron in the peripheral nervous system (PNS).

3. The information reaches a specialized region of the central nervous system (CNS) where the information is integrated and synapse occurs.

4. The CNS relays the synaptic impulse to a motor neuron in the PNS.

5. The PNS transmits the synaptic impulse to an effector organ (muscles, glands) that produces the observable behavior called a reflex.

The conduction of all of this information is facilitated by the myelin sheaths that insulate the nerve cells along the reflex arc (Figure 21-1). As Rose (2005) notes, the previously unused infant reflexes now must be brought into play and under control. Popular examples include the eye blink and the knee jerk. All of the activities needed to sustain life’s functions are present at birth (breathing, sucking, swallowing, elimination). These reflexes serve a definite purpose: The gag reflex enables infants to spit up mucus; the eye blink protects the eyes from excessive light; the anti-smothering reflex facilitates breathing.

The specialized regions that give rise to reflexes are found in the more primitive areas of the CNS, such as the spinal cord, brain stem, and cerebellum. Evolutionary biologists consider reflexes to be builtin mechanisms that sustain life and allow humans to react to threat without thinking. As reviewed in Chapter 25, the CNS is not fully developed at birth, but rather continues to grow, reorganize, and mature. For example, myelination of the nerve cells in the cerebellum, which coordinates sensory-motor information, continues for several years.