What is a Withdrawal Reflex?

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.

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Post 3

@irontoenail - I think that people with leprosy end up losing the pain sensors in their limbs as well. That's why they have the reputation for "rotting away" even though they don't actually lose anything from the leprosy itself. I've heard that they have to spend a lot of time double checking all their extremities to ensure that none of them are being injured, because there is no other way to tell.

They end up getting so many injuries in the areas that go numb that eventually they get permanent damage. The body has fewer defenses for other diseases as well.

It's a really terrible disease and we are lucky to live in a world where it can basically be cured now.

Post 2

There was an interesting documentary on people with CIPA recently where they explored what it was like to live in a world where you don't feel pain. These people wouldn't have this kind of flexor withdrawal reflex and would simply not notice that they had put their hand onto the stove.

It's particularly dangerous when they are toddlers, because it's almost impossible to explain to a toddler why they shouldn't touch something hot when there is no pain to back you up.

I'll admit there have been times when I've wished that I didn't feel pain, but I can definitely see the point of it, particularly when it works in conjunction with something like this reflex.

Post 1

The thing I found interesting when we studied this in class is that you will often pull your hand away before you actually feel the pain. It's not a conscious decision and, in fact, it is extremely difficult to keep your hand on something that causes this reflex, even if you know it's not actually causing you damage.

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