The withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. It is polysynaptic, causing stimulation of sensory, association, and motor neurons.

Example

When a person touches a hot object and withdraws their hand from it without thinking about it, the heat stimulates temperature

Thermoreceptor

A thermoreceptor is a sensory receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range...

 and danger receptors

Nociceptor

A nociceptor is a sensory receptor that responds to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain.-History:...

 in the skin, triggering a sensory impulse that travels to the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

. The sensory neuron then synapse

Synapse

In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

s with interneuron

Interneuron

An interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...

s that connect to motor neuron

Neuron

A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s. Some of these send motor impulses to the flexors to allow withdrawal; some motor neurons send inhibitory impulses to the extensors so flexion

Flexion

In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal and muscular systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder...

 is not inhibited - this is referred to as reciprocal innervation

Reciprocal innervation

René Descartes was one of the first to conceive a model of reciprocal innervation as the principle that provides for the control of agonist and antagonist muscles. Reciprocal innervation describes skeletal muscles as existing in antagonistic pairs, with contraction of one muscle producing forces...

. While all of this occurs, other interneurons relay the sensory information up to the brain

Human brain

The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

so that the brain evaluates the danger message and, often, pain results.

Crossed extension reflex following withdrawal reflex

Once a danger receptor (called 'nociceptor') has been stimulated, the signal travels via the sensory nerve to the posterior horn of the spinal cord. The nerve synapses with ipsilateral motor neurons that exit the anterior horn of the spinal cord and work to pull the injured body part away from danger within 0.5 seconds. At the same time the sensory neuron synapses with the ipsilateral motor neuron, it also synapses with the motor neuron in the contralateral anterior horn. This motor neuron stabilizes the uninjured side of the body (for instance, preparing the opposite leg to support the entire body weight when the other foot has stepped on a tack). At the same time as these two synapses, the sensory neuron also sends signals up the spinal cord to get motor neurons to contract muscles that shift the center of gravity of the body to maintain balance. This contralateral stimulation of motor neurons to stabilize the body is called the crossed extension reflex, and is a result of the withdrawal reflex (usually in the lower extremities).

External links

• Ole K. Andersen, SMI, Aalborg University - Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex