How Does the Nervous System Respond to Stimuli?

The nervous system detects a stimulus when a receptor responds to a change in the internal or external environment. It can then send nerve impulses to an effector, such as a muscle, which produces a response. A reflex is a rapid, automatic response that does not require a conscious decision.

Detecting a stimulus

Receptors are sensitive to particular types of stimulus. For example, a Pacinian corpuscle detects pressure. Pressure deforms its membrane, opening stretch-mediated sodium ion channels. Sodium ions enter, producing a generator potential: a change in electrical potential across the receptor membrane. If this reaches the threshold, it triggers an action potential, or nerve impulse, in a sensory neurone.

Producing a reflex

In a simple reflex, the impulse travels along a sensory neurone to the central nervous system. It passes across synapses to a relay neurone and then a motor neurone. At a synapse, a chemical messenger called a neurotransmitter carries the signal between neurones. The motor neurone carries the impulse to an effector, which responds.

This pathway, from receptor through neurones to effector, is called a reflex arc. Synaptic transmission is one-way, helping impulses travel through the arc in the correct order. The response can begin before you become consciously aware of the stimulus, which is useful when a quick reaction prevents harm.

For example, if you touch a hot surface, receptors sensitive to the harmful stimulus initiate impulses. In a withdrawal reflex, these impulses pass through the spinal cord to motor neurones. The relevant muscles contract, pulling your hand away. You can become aware of the heat, but conscious awareness is not needed to start the reflex.

Exam mistake: do not call the muscle a receptor. The receptor detects the stimulus; the muscle is an effector that responds. Also, the Pacinian corpuscle is the named example for pressure, not heat.