The withdrawal reflex (nociceptive flexion reflex or flexor withdrawal reflex) is a

spinal reflex The stretch reflex (myotatic reflex), or more accurately ''muscle stretch reflex'', is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining the muscle at a constant length but t ...

intended to protect the body from damaging stimuli. The reflex rapidly coordinates the contractions of all the flexor muscles and the relaxations of the extensors in that limb causing sudden withdrawal from the potentially damaging stimulus. Spinal reflexes are often monosynaptic and are mediated by a simple

reflex arc A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons synapse in the spinal cord and the signal then travels through it into the brain. This allows for faster reflex actions to occur by activating spinal mo ...

. A withdrawal reflex is mediated by a polysynaptic reflex resulting in the stimulation of many motor neurons in order to give a quick response.

Example

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

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

and pain receptors in the skin, triggering a sensory impulse that travels to the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

. The sensory neuron then

synapse In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...

s with

interneuron Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are neurons that are not specifically motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enab ...

s that connect to motor

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

s. Some of these send motor impulses to the

flexor In anatomy, flexor is a muscle that contracts to perform flexion (from the Latin verb ''flectere'', to bend), a movement that decreases the angle between the bones converging at a joint. For example, one's elbow joint flexes when one brin ...

s that lead to the muscles in the arm to contract, while some motor neurons send inhibitory impulses to the extensors so

flexion Motion, the process of movement, is described using specific anatomical terminology, anatomical terms. Motion includes movement of Organ (anatomy), organs, joints, Limb (anatomy), limbs, and specific sections of the body. The terminology used de ...

is not inhibited. This is referred to as

reciprocal innervation René Descartes (1596–1650) was one of the first to conceive a model of reciprocal innervation (in 1626) as the principle that provides for the control of agonist and antagonist muscles. Reciprocal innervation describes skeletal muscles as ...

. The withdrawal reflex in the leg can be examined and measured, using an

electromyogram Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...

to monitor the muscle activity in the upper leg ( biceps femoris) while applying increasing electrical stimulation to the lower leg (

sural nerve The sural nerve ''(L4-S1)'' is generally considered a pure cutaneous nerve of the posterolateral leg to the lateral ankle. The sural nerve originates from a combination of either the sural communicating branch and medial sural cutaneous nerve, ...

) on the same side of the body. The stimulus intensity at which the reflex is evoked is often the intensity at which the subject reports the onset of pain, and the strength of the withdrawal reflex is correlated with the strength of the pain experienced.

Crossed extension reflex following withdrawal reflex

Once a danger receptor (called "nociceptor") has been stimulated, the signal travels via the sensory nerve to the dorsal (posterior) horn of the spinal cord. The nerve synapses with ipsilateral motor neurons that exit the ventral (anterior) horn of the spinal cord and work to pull the soon-to-be injured body part away from danger within 0.5 seconds. At the same time, the sensory neuron synapses with the ipsilateral motor neuron, as well as the motor neuron in the contralateral anterior horn. This motor neuron stabilizes the uninjured side of the body (for instance; preparing the other 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 along 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).

References

Other references

External links

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

Example

Crossed extension reflex following withdrawal reflex

See also

References

Other references

External links