Reflex Action and Reflex Arc

A reflex action is the body's way of handling situations where a response has to be immediate — faster than waiting for the brain to consciously decide would allow. When dust falls in the eye, the eyelids close before we will them to; when a hand touches something hot, it withdraws before we've 'thought' about it. In both cases the spinal cord itself, rather than the brain, initiates the motor response, even though a parallel message about the event is still sent up to the brain for conscious awareness. This makes a reflex action fast, involuntary and unplanned — a fixed sequence triggered directly by a stimulus rather than a decision.

The entire nervous pathway involved in carrying out one reflex is called a reflex arc, and it has five functional components working in sequence: a sensory receptor that responds to the specific stimulus; a sensory neuron that carries the resulting impulse into the spinal cord's grey matter via the dorsal root; one or more interneurons that relay the impulse across, from the sensory side to the motor side; a motor neuron that carries the outgoing impulse from the CNS to the effector; and the effector organ itself — a muscle or gland — that actually produces the response. Reflexes themselves come in two kinds. An unconditional (inborn) reflex needs no prior experience or training — blinking when dust approaches the eye, or sneezing and coughing when a foreign particle irritates the nose or larynx, are both built in from birth. A conditioned reflex, by contrast, is learned rather than innate — the classic example is a dog's mouth …