THE FLEXOR WITHDRAWAL REFLEX and CROSSED EXTENSOR REFLEX
When an injurious stimulus is applied to the skin, there is a reflex withdrawal of the limb away from the stimulus. The mechanism of withdrawal involves a polysynaptic pathway that initiates and maintains contraction of flexor muscles.
The flexor withdrawal reflex has a polysynaptic pathway that initiates withdrawal of a limb from a noxious stimulus and maintains the withdrawn position.
Nociceptors in the skin initiate activity in polysynaptic circuits within the spinal cord that :
cause flexor muscles to withdraw the limb and
maintain that new position and
extend the contralateral limb to support the body
Although pain normally accompanies injury, the withdrawal of the limb does not depend on the perception of pain. Pain perception depends on higher levels of the CNS, and the flexor reflex persists after spinal transection.
The Crossed Extensor Reflex is part of the flexor withdrawal reflex and ensures that extensors in the opposite limb contract so as to maintain posture and support the weight of the body while the opposite limb is flexed. The pathways that accomplish this are polysynaptic and cross the midline of the spinal cord.
The diagram shows the presence of an injurious stimulus (the flame) activating nociceptive unmyelinated afferents in the finger; the afferents exciting interneurones in the spinal grey matter (only one interneurone is shown for the sake of simplicity). The interneurones excite flexor motoneurones that move the finger away from the flame.
Flexor Reflex and Nociception
In the flexor reflex ,unmyelinated sensory endings that respond to injurious stimuli excite networks of interneurones in the spinal cord that excite flexor motoneurones, causing withdrawal from the noxious stimulus.
This movement is independent of the perception of pain, which requires communication with higher levels of the CNS.
The flexor reflex in elicited by noxious (injurious or potentially injurious) stimuli and involves many synapses within the spinal cord; the effector pathways are the motoneurones that control the flexor muscles which withdraw the limb from the noxious stimulus.
The afferent pathway for the flexor reflex is nociceptive afferents: these are slowly conducting A-delta and C fibres that sense the presence of a noxious, potentially injurious, stimulus to the skin; noxious stimuli include high temperatures (>45 degrees C, the temperature at which protein start to become denatured), intense forces (including objects that threaten to penetrate the skin) and strong chemical solutions that injure the skin.
The central pathway involves many synapses and interneurones in the spinal cord that maintain activation of flexor motoneurones.
The efferent pathway is the flexor motoneurones that cause flexor muscles to contract and withdraw the limb from the noxious stimulus.
The Crossed Extensor Reflex
The diagram shows the pathways involved in contractions of the contralateral extensor muscles when a limb is withdrawn from a potentially damaging stimulus.
The Crossed Extensor Reflex.
The crossed extensor reflex is an additional component to the flexor reflex.
When the flexor reflex is active and a limb is withdrawn from a noxious stimulus, the opposite limb becomes extended so as to bear the weight of the body.
The network of interneurones that are active in the flexor reflex project to, and excite, the contra-lateral extensor motoneurones, resulting in extension of the contra-lateral limb.
Clearly the neurones active in this reflex cross the midline of the cord.
Interneuronal networks in the spinal cord.
Image source: docstoc.com
The diagram shows circuits of interconnected interneurones. The simplest idea of the flexor reflex involves a single excitatory interneurone linking the nociceptive afferents and the flexor motoneurones. But branches of that interneurone and the nociceptive afferents excite many other interneurones that form circuits that reinforce the process and maintain flexor excitation. These self re-exciting circuits are known as reverberating circuits.
The spinal grey matter contains many interneurones, some of which have excitatory and others inhibitory actions on the next neurones in the pathway.
Excitatory interneurones may form circuits that re-excite other interneurones so as to maintain the activity in the system.
These circuits are known as reverberating circuits, and are of some important in maintaining the withdrawal of a limb from a noxious stimulus, and maintaining crossed extension.
Other interneurones release inhibitory transmitters, and some of these inhibit extensor motoneurones during the flexor reflex. This is reciprocal innervation operating in the context of the flexor reflex.
Other groups of interneurones are active in the superficial layers (the substantia gelatinosa) of the dorsal horn during noxious stimulation. These are involved in modulating transmission in ascending pathways concerned with pain. More on pain pathways.