reflexes

Meaning: A reflex is a rapid, stereotyped, unlearned response to a stimulus. In a reflex arc, a sensory neuron carries the signal to the spinal cord, where it synapses on a motor neuron, either directly (as in the monosynaptic knee-jerk reflex) or through an interneuron. The short loop makes the response fast, and the brain is informed afterward. Some reflexes, such as the pupillary reflex, are routed through the brainstem instead.

Newborns show primitive reflexes: rooting (head turns toward a touched cheek), sucking, grasping, Moro (startle with arm extension), and Babinski (toes fan out when the sole is stroked). They typically disappear as the cortex matures and voluntary control develops.

On the MCAT: Expect a development passage asking which behavior is a reflex and what its persistence or absence suggests about neural maturation. In learning questions, the unconditioned response in classical conditioning (salivating to food) is reflexive. In nervous system questions, you may be asked to order the neurons in an arc.

Common confusion: A reflex is not a conditioned response, because a reflex is inborn and a conditioned response is learned. It is also not the same as an instinct (fixed action pattern), which is a more complex, species-typical behavior sequence. Involuntary does not mean the brain is never involved.