Golgi tendon reflex

GTOs' inhibitory effects come from their reflex arcs: the Ib sensory fibers that are sent through the dorsal root into the spinal cord to synapse on Ib inhibitory interneurons that in turn terminate directly on the motor neurons that innervate the same muscle. The fibers also make direct excitatory synapses onto motoneurons that innervate the antagonist muscle. Note that the disynaptic reflex pathway does not always have inhibitory effects: under certain conditions, GTO stimulation can result in motoneuron excitation.

Besides protecting against too much tension on the muscle and tendon, the tendon reflex may help spread muscle load throughout the muscle fibers, thereby preventing damage to isolated fibers. Whereas the stretch reflex regulates muscle length, the tendon reflex helps regulate muscle force. It helps maintain steady levels of tension and stable joints to counteract effects that reduce muscle force (such as fatigue). Because the Ib inhibitory interneurons receive convergent multisensory inputs and descending pathways, they may allow fine control of muscle forces, and may be better at protective functions. Also, because the Ib fibers connect widely with the motoneurons innervating muscles working on different joints, the Golgi tendon reflex forms part of reflex networks that control movements of the whole limb.

Protective function, autogenic inhibition, and others

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.

Because the Ib inhibitory interneurons receive convergent descending pathways and multisensory inputs—including cutaneous receptors, muscle spindles, and joint receptors, they can provide better protection, such as when the joint receptors are signaling joint hyperextension or hyperflexion.

Autogenic inhibition refers to a reduction in excitability of a contracting or stretched muscle, that in the past has been solely attributed to the increased inhibitory input arising from Golgi tendon organs (GTOs) within the same muscle. It was first thought GTOs only had protective function which was to prevent muscles from damages because of the assumptions that they always inhibited motoneurons and that they fired only under high tension. But it is now known that GTOs signal muscle tension continuously providing precise information about muscle force, that the reflex pathway has multisensory inputs that may allow precise control of muscle forces for fine activities, and that Ib fibers connect widely with motoneurons innervating muscles acting on different joints, which when complemented with their reflex pathways, are part of reflex networks that control movements of the whole limbs.

The reduced efferent (motor) drive to the muscle by way of autogenic inhibition is a factor historically believed to assist target muscle elongation, although current literature casts doubt on this hypothesis.

Protective steps

With muscle tension, a Golgi tendon reflex operates as follows:

1. As tension is applied to a tendon, the Golgi tendon organ (sensor) is stimulated (depolarized)
2. Nerve impulses (action potentials) arise and propagate along sensory fiber Ib into the spinal cord
3. Within the spinal cord (integrating center), sensory fiber Ib synapses with and activates (via glutamate) an inhibitory interneuron
4. The inhibitory interneuron releases the neurotransmitter glycine that inhibits (hyperpolarizes) the α motor neuron
5. As a consequence fewer nerve impulses are generated in the α motor neuron
6. The muscle relaxes and excess tension is relieved