reflex arc

Meaning: A reflex is a fast, involuntary response to a stimulus, and the reflex arc is its pathway: receptor, sensory (afferent) neuron, integration center, motor (efferent) neuron, effector. The sensory neuron enters the spinal cord by the dorsal root; the motor neuron exits by the ventral root. In a monosynaptic arc, such as the knee-jerk reflex, the sensory neuron synapses directly on the motor neuron. In a polysynaptic arc, such as withdrawal from a hot object, interneurons lie between them.

On the MCAT: Expect questions that order the components, label neurons as afferent or efferent, or predict the effect of cutting a dorsal or ventral root (lost sensation versus lost motor output).

Common confusion: Students assume the signal must reach the brain before the muscle responds. A spinal reflex is integrated in the cord, so it can occur even when the cord is cut off from the brain; the brain is informed in parallel and perceives the stimulus afterward.