The Muscle Spindle Conveys Information About Muscle Length to the Central Nervous System
As noted, stretching a muscle can stretch the intrafusal fibers of the muscle spindle that lie in parallel to the extrafusal muscle fibers. Stretching (lengthening) the equatorial segment of the intrafusal muscle fiber generates action potentials along the spindle sensory neurons.
As the equatorial segment is lengthened, it is believed that stretch-sensitive ion channels open on the sensory neurons, leading to membrane depolarization and action potential generation. Action potentials are generated along the sensory neuron output of the muscle spindle in proportion to the amount of lengthening of the middle of the intrafusal muscle fibers. There are actually subclasses of intrafusal fibers (dynamic nuclear bag, static nuclear bag, nuclear chain) and spindle sensory neurons (Ia, II). This variety allows the sensory neuron output of the spindle to detect not only a change in length during the dynamic phase of muscle lengthening, but also the rate of lengthening, as well as the steady-state length of the muscle as the animal holds the joint still !Figure 8-4).When a muscle is held at a constant length, such as when a joint is held still, there is usually enough stretch of the muscle’s spindles to produce a steady frequency of action potential discharge in the sensory neuron output of the muscle spindle. This steady-stale discharge gives the muscle spindle organ the ability to inform the CNS not only about a subsequent lengthening of the muscle, which would produce a proportional increase in action potential discharge, but about a shortening of the muscle as well, which would produce a proportional decrease in action potential discharge from the steady state (see Figure 8-4).