A withdrawal reflex is an involuntary process that causes a part of the body to automatically pull away from something that is causing pain. Also called a flexion reflex, it is a three-stage process that involves nerves sending a message to the spinal cord to tell the muscles in that part of the body to flex and pull away from the source of the pain. The primary purpose of this involuntary reaction is to help prevent or minimize injury. In some cases, such as when an arm or leg is affected, a withdrawal reflex may be aided by a cross extension reflex to help better protect the body.
While it may seem simple at first, a withdrawal reflex actually involves three separate neurological steps. In the first step, pain receptors in the affected area a message to the spinal cord. When the message reaches the spinal cord, an interneuron sends a message along to the nerves that control flexor muscles near the affected area. In the third step, these motor neurons tell the appropriate muscles to flex, which results in pulling the body part away from whatever is causing the pain. Due to the fact that spinal cord is the main area of control in this process, rather than the brain, a withdrawal reflex is known as a type of spinal reflex.
The main purpose of this type of reflex is to avoid injury or lessen the severity of one. For example, if a person touched a prickly bush with his or her hand, a withdrawal reflex would typically kick in to quickly pull the hand away to help prevent a cut or deep puncture wound. Or, if a person leaned against a hot object, the body would pull away from the object to prevent or minimize a burn.
In some cases, such as when an arm or a leg is being subjected to pain, a withdrawal reflex may be accompanied by another one of the body’s spinal reflexes: the cross extension reflex. This reaction involves a similar three-stage process, but instead of the interneurons in the spinal cord sending the pain message to the motor neurons that control the flexors in the area of the body experiencing pain, they send it to the neurons that control the extensor muscles in the opposite limb. That limb then pushes out to assist the reflex in preventing injury, either by pushing away the source of the pain or providing extra support to the body to compensate for the pulling away of the other limb.
For example, if a person's left hand was being bitten by a dog, the two reflexes could work together to simultaneously pull the left hand back and use the right hand to push the dog away. Or, if a person stepped on a nail with his or her right foot, the opposite leg would push down so the person didn't fall over when his or her left foot and leg pulled up due to a withdrawal reflex.