“Whenever a muscle is stretched, excitation of the spindles causes reflex contraction of the large skeletal muscle (extrafusal) fibers that lie around the spindles. In the basic circuit of the muscle spindle stretch reflex, a type Ia fiber (annulospiral nerve), originates in the muscle spindle and enters the dorsal root of the spinal cord. Then, in contrast to most other nerve fibers entering the cord, one branch of it passes directly to the anterior horn of the cord gray matter and synapses directly with anterior motor neurons that send nerve fibers back to the same muscle from whence the muscle spindle fiber originated. Thus, this is a monosynaptic pathway that allows a reflex signal to return with the shortest possible delay back to the same muscle after excitation of the spindle.
To emphasize the importance of the gamma efferent system, one needs to recognize that 31 percent of all the motor nerve fibers to the muscle are gamma efferent fibers rather than large, type A alpha motor fibers. Whenever signals are transmitted from the motor neurons, almost always, the gamma motor neurons are stimulated simultaneously, an effect called co-activation of the alpha and gamma motor neurons. This causes both the extrafusal and the intrafusal muscle fibers to contract at the same time.” Guyton’s Textbook of Medical Physiology, 8th Edition
The above quote from Guyton emphasizes the complete involvement of the muscle spindle in muscle physiology. The stretch reflex is a response to an outside force, something happening to the muscle. It is referred to as ‘dynamic’, meaning force, power, change, movement, or ‘myotatic’, which specifically refers to the stretch reflex in muscles, and therefore means the same thing. It delivers a powerful, short-term action which will override and nullify the constant low-level signal of muscle tone, providing the skeletal muscles are relatively healthy and not in hypertonic spasm.
The stretch reflex is generated as a protective mechanism when the spindle and its skeletal muscles are being stretched. During stretch, an action potential is transmitted to the central nervous system (CNS) at the spinal cord (by the annulospiral endings on the bag 1 fibers); the frequency of the action potential is proportional to the degree of stretching. As the spindle is stretched becomes longer the impulse frequency in the gamma afferent nerve increases, firing into the alpha motor nerves in the spinal cord, causing them to fire into the skeletal muscle to cause it to contract (reflex arc). As the skeletal muscles contract, the intrafusal muscles shorten; this relaxes the tension on the muscle spindles, and reduces the discharge rate of the gamma afferents on the annulospiral endings.
Most textbooks state that the muscle spindle signals the brain regarding the length of the skeletal muscle by the frequency of its nerve pulse to the spinal cord. However, the intrafusal fibers of the muscle spindle do not themselves have a fixed length, any more than the extrafusal fibers do. Therefore there is no way to determine the length of the extrafusal fibers. The spindle cannot give an absolute length value to an extrafusal muscle, it can only give the comparative lengths to a floating intrafusal length. Therefore the cerebellum does not determine the length of the extrafusal fibers from the spindle organ itself, but instead operates on information received from joint receptors, which tell the cerebellum the angle of the joint and the rate of change of angle.