Reciprocal inhibition

Reciprocal inhibition describes the process of muscles on one side of a joint relaxing to accommodate contraction on the other side of that joint. Joints are controlled by two opposing sets of muscles, extensors and flexors, which must work in synchrony for smooth movement. When a muscle spindle is stretched and the stretch reflex is activated, the opposing muscle group must be inhibited to prevent it from working against the resulting contraction of the homonymous muscle. This inhibition is accomplished by the actions of an inhibitory interneuron in the spinal cord.

The afferent of the muscle spindle bifurcates in the spinal cord. One branch innervates the alpha motor neuron that causes the homonymous muscle to contract, producing the reflex. The other branch innervates the inhibitory interneuron, which in turn innervates the alpha motor neuron that synapses onto the opposing muscle. Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle. Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other.

Reciprocal Inhibition can backfire by both muscles attempting to contract at the same time. Thus a common tear can occur. ^{[citation needed]} The body handles this pretty well during physical activities like running, where muscles that oppose each other are engaged and disengaged sequentially to produce coordinated movement. This facilitates ease of movement and is a safeguard against injury. Sometimes, for example, a football running back can experience a "misfiring" of motor units and end up simultaneously contracting the quads and hamstrings during a hard sprint. If these muscles, which act opposite to each other are fired at the same time, at a high intensity, a tear can result. The stronger muscle, usually the quadriceps in this case, overpowers the hamstrings. This sometimes results in a common injury known as a pulled hamstring.

The term reciprocal inhibition has also been used in psychology by Joseph Wolpe et al.

In the literature on parallel processing and lateralization, it was used to describe the reduced activation in the less dominant hemisphere in response to lateralized stimuli during processing. This term was used in Hirnstein, Hausmann, Gunturkun's (2008) article on Functional Cerebral Asymmetries. ^{[citation needed]}

“ When the central nervous system sends a message to the agonist muscle (muscle causing movement) to contract, the tension in the antagonist muscle (muscle opposing movement) is inhibited by impulses from motor neurons, and thus must simultaneously relax. This neural phenomenon is called reciprocal inhibition.

Taken from Massage Therapy Principles & Practices by Susan Salvo1999,pg 161

References

1. Reciprocal inhibition in man. http://www.ncbi.nlm.nih.gov/pubmed/8299401