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Tendon reflex (or T-reflex) is an involuntary muscle contraction after one of its tendons is hit.
The sensory receptors responsible for the reflex are actually located not in the tendon but rather in the muscle itself. These are muscle spindles which are sensitive to muscle stretch. Afferent discharges from muscle spindles enter the spinal cord through the sensory nerve root and monosynaptically excite motoneurons, which in turn activate the muscle. The knee-jerk reflex and the Achilles reflex are examples of tendon 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 be passive stretch or muscular contraction. A tendon reflex operates as follows:
- As the tension applied to a tendon increases, the Golgi tendon organ (sensory receptor) is stimulated (depolarized to threshold).
- Nerve impulses (action potentials) arise and propagate into the spinal cord along a sensory neuron.
- Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that makes a synapse with a motor neuron.
- The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses.
- The muscle relaxes and relieves excess tension.
See also
"Tendon reflex"