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The flexion reflex or nociceptive withdrawal reflex (NWR) is a spinal reflex intended to protect the body from damaging stimuli. The classic example is when a person touches something hot and withdraws their hand from the hot object. The heat stimulates temperature and pain receptors in the skin, triggering a sensory impulse that travels to the central nervous system. The sensory neuron then synapses with interneurons that connect to motor neurons. Some of these send motor impulses to the flexors to allow withdrawal; some motor neurons send inhibitory impulses to the extensors so flexion is not inhibited - this is referred to as reciprocal innervation. While all of this occurs, other interneurons relay the sensory information up to the brain so that the person becomes aware of the pain and what happened. The NWR is also known as the flexion reflex.

Crossed extension reflex following withdrawal reflex

Once a pain receptor 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. 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 stablizes 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 stablize the body is called the crossed extension reflex, and is a result of the withdrawal reflex (usually in the lower extremeties). Reference- Anatomy and Physiology, The Unity of Form and Function by Ken Saladin. Pg 505-506.

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