reflex arc

The reflex arc. When the leg is tapped just below the knee, the knee-jerk reflex is effected and the foot kicks forward. The tap stimulates an impulse in the sensory neuron in the leg, which travels to the spinal cord, through the relay neuron, and into the motor neuron where it stimulates the leg muscle to contract.

In animals, the passage of a nerve impulse from a receptor to a muscle that results in an automatic response to the stimulus. It is the simplest kind of animal behaviour. Examples include the removal of the hand when it touches a hot object, blinking when an object comes close to the eye, and the narrowing of the pupil of the eye in bright light. A reflex arc differs from other responses by being automatic. This is achieved by not allowing the brain to ‘consider’ whether to take action in response to the stimulus or not. In a reflex arc, a particular stimulus virtually always results in the same response.

Reflex arcs involve actions that protect the body. By cutting out having to ‘consider’ whether to take action or not, they do not involve conscious thought. Impulses travel along fewer neurones and so produce a response in the shortest possible time. The slowest steps in the pathway taken by nerve impulses are at the junctions between neurones - the synapses. In many reflex arcs the number of synapses to cross may be as low as three.