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 within the muscle. The reflex functions to maintain the muscle at a constant length. The term deep tendon reflex is ...
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 withdrawal
reflex
In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.
Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...
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 and
pain receptors
Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, co ...
in the skin, triggering a sensory impulse that travels to the
central nervous system. The sensory neuron then
synapse
In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell.
Synapses are essential to the transmission of nervous impulses from ...
s with
interneuron
Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...
s that connect to motor
neurons.
Some of these send motor impulses to the
flexors that lead to the muscles in the arm to contract, while some motor neurons send inhibitory impulses to the extensors so
flexion 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 exi ...
.
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 electromyog ...
to monitor the muscle activity in the upper leg (
biceps femoris
The biceps femoris () is a muscle of the thigh located to the posterior, or back. As its name implies, it has two parts, one of which (the long head) forms part of the hamstrings muscle group.
Structure
It has two heads of origin:
*the ''long h ...
) while applying increasing electrical stimulation to the lower leg (
sural 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).
See also
*
Interneuron
Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...
*
Nociceptor
*
Reflex
In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.
Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...
References
Other references
*
External links
Ole K. Andersen, SMI, Aalborg University - Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex
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Reflexes