Structure & Function of Reflex Arcs

Why a reflex needs its own pathway

Touching something hot can make you pull your hand away before you consciously decide to move it. Heat is the stimulus—a change detected by receptors in the skin. The movement is a reflex action: it is rapid and automatic. Withdrawal can reduce the time your hand remains in contact with the hot object, helping to limit injury.

A reflex arc is the pathway that produces this response. It uses the same kinds of electrical impulse as other nervous responses, but it does not need the conscious part of the brain to decide what to do. In a typical withdrawal reflex, the spinal cord, which is part of the central nervous system (CNS), links the incoming signal to an outgoing one.

Following the impulse

A receptor in the skin detects the stimulus and generates an electrical impulse. A sensory neurone carries the impulse towards the spinal cord. There, it passes to a relay neurone, which links the incoming sensory pathway to the outgoing motor pathway. A motor neurone then carries an impulse away from the CNS to an effector. In this example, the effector is an arm muscle: it contracts, pulling the hand away. Muscles produce a response by contracting; glands are also effectors and respond by secreting substances.

The neurones do not form one continuous cell. At each synapse, an arriving electrical impulse causes a chemical messenger, called a neurotransmitter, to cross the small gap between neurones. This starts a new electrical impulse in the next neurone. A typical withdrawal arc has a synapse between the sensory and relay neurones and another between the relay and motor neurones.

The pathway is therefore stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response. The brain can receive information about what happened, but the withdrawal does not wait for a conscious decision. That is why a reflex is useful when a quick protective response is needed.