Up to a point, the harder a muscle is stretched, the stronger is the reflex contraction. However, when the tension becomes great enough, contraction suddenly ceases and the muscle relaxes. This relaxation in response to strong stretch is called the inverse stretch reflex. The receptor for the inverse stretch reflex is in the Golgi tendon organ (Figure 1). This organ consists of a netlike collection of knobby nerve endings among the fascicles of a tendon. There are 3–25 muscle fibers per ten don organ. The fibers from the Golgi tendon organs make up the Ib group of myelinated, rapidly conducting sensory nerve fibers. Stimulation of these Ib fibers leads to the production of IPSPs on the motor neurons that supply the muscle from which the fibers arise. The Ib fibers end in the spinal cord on inhibitory interneurons that in turn terminate directly on the motor neurons (Figure 2). They also make excitatory connections with motor neurons supplying antagonists to the muscle.
Fig1. Golgi tendon organ. This organ is the receptor for the inverse stretch reflex and consists of a netlike collection of knobby nerve endings among the fascicles of a tendon. The innervation is the Ib group of myelinated, rapidly conducting sensory nerve fibers. (Reproduced with permission from Gray H: Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 40th ed. St. Louis, MO: Churchill Livingstone/Elsevier; 2009.)
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Fig2. Diagram illustrating the pathways responsible for the stretch reflex and the inverse stretch reflex. Stretch stimulates the muscle spindle, which activates Ia fibers that excite the motor neuron. Stretch also stimulates the Golgi tendon organ, which activates Ib fibers that excite an interneuron that releases the inhibitory mediator glycine. With strong stretch, the resulting hyperpolarization of the motor neuron is so great that it stops discharging.
Because the Golgi tendon organs, unlike the spindles, are in series with the muscle fibers, they are stimulated by both passive stretch and active contraction of the muscle. The threshold of the Golgi tendon organs is low. The degree of stimulation by passive stretch is not great because the more elastic muscle fibers take up much of the stretch, and this is why it takes a strong stretch to produce relaxation. However, discharge is regularly produced by contraction of the muscle, and the Golgi tendon organ thus functions as a transducer in a feedback circuit that regulates muscle force in a manner analogous to the spindle feedback circuit that regulates muscle length.
The importance of the primary endings in the spindles and the Golgi tendon organs in regulating the velocity of the muscle contraction, muscle length, and muscle force is illustrated by the fact that section of the afferent nerves to an arm causes the limb to hang loosely in a semiparalyzed state. The interaction of spindle discharge, tendon organ discharge, and reciprocal innervation determines the rate of discharge of α-motor neurons (Clinical Box 1).
Clinical Box 1.
MUSCLE TONE
The resistance of a muscle to stretch is often referred to as its tone or tonus. If the motor nerve to a muscle is severed, the muscle offers very little resistance and is said to be flaccid. A hypertonic (spastic) muscle is one in which the resistance to stretch is high because of hyperactive stretch reflexes. Some where between the states of flaccidity and spasticity is the ill-defined area of normal tone. The muscles are generally hypotonic when the rate of γ-motor neuron discharge is low and hypertonic when it is high.
When the muscles are hypertonic, the sequence of moder ate stretch → muscle contraction, strong stretch → muscle relaxation is clearly seen. Passive flexion of the elbow, for example, meets immediate resistance as a result of the stretch reflex in the triceps muscle. Further stretch activates the inverse stretch reflex. The resistance to flexion suddenly collapses, and the arm flexes. Continued passive flexion stretches the muscle again, and the sequence may be repeated. This sequence of resistance followed by a sudden decrease in resistance when a limb is moved passively is known as the clasp-knife effect because of its resemblance to the closing of a pocket knife. It is also known as the lengthening reaction because it is the response of a spastic muscle to lengthening.
