Lec:2 The Golgi Tendon Reflex (Inverse Stretch Reflex)
When muscle tension is increased (by active muscle contraction), the Golgi tendon organs are stimulated and signals are sent to spinal cord to synapse with inhibitory inter-neurons that in turn inhibit the anterior alpha motor neurons innervated the same muscle from which same signals were originated, this reflex:
• Prevents tearing of the muscle or avulsion of the tendon from its attachment to the bone.
• Equalizes the contractile forces of the separate muscle fibers. When the tension becomes too little, impulses from the tendon organ cease, and the loss of inhibition leads to activation of alpha motor neurons again.
Control of Golgi Tendon Reflex
The brain sends:
1. Signals to the target muscle through alpha motor neurons to cause muscle contraction at a required tension.
2. Signals to inhibitory inter-neurons of the spinal cord to inform them of the tension required as detected by feedback from the Golgi tendon organs, the inhibitory inter-neurons automatically inhibit the muscle contraction to prevent additional tension. In this way the muscle tension is adjusted to set-point dictated by the brain.
Clinical Application
In a hypertonic muscle, if passive and sustained muscle stretch is applied this leads to:
1. Clasp-knife reflex: in a hypertonic muscle, moderate passive stretch will lead to muscle contraction, however, stronger passive stretch will lead to relaxation due to resistance followed by relaxation.
2. Clonus: regular, rhythmic contraction of a hypertonic muscle that is subjected to sudden sustained passive stretch. It is due to increased gamma efferent discharge. Best example in Ankle Clonus, when dorsiflex the foot there will be rhythmic plantar flexion at the ankle (the stretch reflex – inverse stretch reflex sequence may contribute ti this response).
The Withdrawal (Flexor) Reflex
It is a polysynaptic reflex, the painful (other sensory) stimulus passes into a group of inter-neurons and then to the anterior motor neurons to elicit muscle contraction and usually withdrawal of the affected limb. In the inter-neurons, the signals will stimulate the necessary muscles for withdrawal and inhibit the antagonistic side muscles (reciprocal inhibition circuits) and cross to the other side of the spinal cord to cause opposite reflex in the opposite limb (crossed extensor reflex) i.e. extend the opposite limb and push the entire body away from the object causing the painful stimulus.
Clinical Application
1. Abdominal Reflex.
2. Cremastric Reflex