Reflex Responses

A reflex is a rapid, automatic (involuntary) response to a stimulus. The response is the same each time and does not need conscious thought. Reflexes protect the body from harm, for example pulling a hand away from a hot object, blinking when something touches the cornea, or the pupil narrowing in bright light.The pathway of neurones in a reflex is the reflex arc:stimulus → receptor → sensory neurone → relay neurone (in CNS) → motor neurone → effector → responseIn the hand-withdrawal reflex, pain receptors in the skin are stimulated. Impulses pass along a sensory neurone through the dorsal root into the spinal cord. There the sensory neurone synapses with a relay neurone in the grey matter, which synapses with a motor neurone. The motor neurone leaves through the ventral root and carries impulses to the effector, the biceps muscle, which contracts and pulls the hand away.Reflexes are fast because the pathway is short with few synapses, and the brain does not have to process the information first. The relay neurone also synapses with neurones running up the spinal cord, so the brain becomes aware of the pain, but only after the response has happened. The brain can sometimes override a reflex, for example if you choose to keep hold of a hot plate.Some reflexes, such as the knee-jerk reflex, have no relay neurone: the sensory neurone synapses directly with the motor neurone, making the response even faster.