Flexor reflexes

A range of afferents, involving group II and III muscle afferents, skin mechanoreceptor and nociceptor afferents, and joint afferents, elicit flexor reflexes in the ipsilateral limb and therefore are termed as flexor reflex afferents (FRAs). These similar afferents trigger an extension of the contralateral limb, the crossed extensor reflex or another reflex pathway. Different kinds of FRAs are connected to particular subsets of interneurons therefore the reflexes they excite vary in form and timing. Most of the normal limb movements consist of either flexion, or alternate flexion and extension, and are brought around by the actions of supraspinal motor systems on the interneurons targeted by the flexor reflex afferents. In single model to account for this these interneurons are ordered into sets known as half centers, among which reciprocal inhibitory connections exist. A flexor half center acquires inputs from ipsilateral FRAs and excites flexor motor neurons, whereas an extensor half center is activated by contralateral FRAs driving extensor motor neurons as shown in figure below. These half centers are also components of central pattern generators as shown below:

Figure: Organization of flexor reflex afferent (FRA) interneurons. The excitatory neuron cell bodies are open circles, and inhibitory interneuron cell bodies are filled circles.

For the execution of a specific movement, descending motor axons are activated that project to the particular set of FRA interneurons which bring about the movement. The reciprocal connections assure that the alternative set is reserved. As the movements proceed, FRA input from joints, muscles, and skin reinforces the movement and permits it to be fine-tuned.

However flexor reflexes are generally recruited as elements of common movement, the withdrawal flexor reflex, triggered by Aδ or C fiber (group IV) nociceptors is quite different in character. It overrides ongoing movement, includes flexor muscles throughout the limb therefore the response is dramatic, long lasting and protective.