Golgi tendon reflex
The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. In a Golgi tendon reflex, skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex. Though muscle tension is increasing during the contraction, alpha motor neurons in the spinal cord supplying the muscle are inhibited. However, antagonistic muscles are activated.
The Golgi tendon reflex operates as a protective feedback mechanism to control the tension of an active muscle by causing relaxation before the tendon tension becomes high enough to cause damage. First, as a load is placed on the muscle, the afferent neuron from the Golgi tendon organ fires into the central nervous system. Second, the motor neuron from the spinal cord is inhibited via an IPSP and muscle relaxes.
Contrast to stretch reflex
The stretch reflex operates as a feedback mechanism to control muscle length by causing muscle contraction. In contrast, the tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn. Although the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example. Like the stretch reflex, the tendon reflex is ipsilateral. The sensory receptors for this reflex are called tendon Golgi receptors, which lie within a tendon near its junction with a muscle. In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by muscular contraction, but not passive stretch.
A Golgi tendon reflex operates as follows:
- As the tension applied to a tendon increases, the Golgi tendon organ (sensor) is stimulated (depolarized to threshold)
- Nerve impulses (action potentials) arise and propagate along the 1b sensory neuron into the spinal cord
- Within the spinal cord (integrating center), the 1b sensory neuron synapses with and activates (via glutamate) an inhibitory interneuron that synapses with the alpha (α) motor neuron
- The inhibitory interneuron releases the neurotransmitter glycine that inhibits (hyperpolarizes) the alpha (α) motor neuron
- As a consequence fewer nerve impulses are generated in the alpha (α) motor neuron
- The muscle relaxes and excess tension is relieved
The clasp-knife response is a stretch reflex with a rapid decrease in resistance when attempting to flex a joint. However, it is actually thought to be caused by the tendon reflex of the antagonistic muscle of that joint, which gets extended. It is one of the characteristic responses of an upper motor neuron lesion.