The neural innervation of an intrafusal fiber differs significantly from that of an extrafusal fiber, which is innervated by a single motor neuron. Intrafusal fibers are multiply innervated and receive both sensory and motor innervation. The sensory innervation typically includes a single group Ia afferent fiber and a variable number of group II afferent fibers (see
Fig. 9.2B). Group Ia fibers belong to the class of sensory nerve fibers with the largest diameters and conduct at 80 to 120 m/sec; group II fibers are intermediate in size and conduct at 35 to 75 m/sec. A group Ia afferent fiber forms a spiral-shaped termination, referred to as a primary ending, on each of the intrafusal muscle fibers in the spindle. Thus primary endings are found on both types of nuclear bag fibers and on nuclear chain fibers. The group II afferent fiber forms a secondary type ending on nuclear chain and bag2 fibers, but not on bag1 fibers. The primary and secondary endings have mechanosensitive channels that are sensitive to the level of tension on the intrafusal muscle fiber.
The motor supply to a muscle spindle consists of two types of γ motor axons (see
Fig. 9.2B ). Dynamic γ motor axons end on nuclear bag1 fibers, and static γ motor axons end on nuclear chain and bag2 fibers.
Muscle spindles respond to changes in muscle length because they lie in parallel with the extrafusal fibers and therefore are also stretched or shortened along with the extrafusal fibers. Because intrafusal fibers, like all muscle fibers, display spring-like properties, a change in their length changes the tension that they are under, and this change is sensed by mechanoreceptors of the group Ia and group II spindle afferent fibers. The nonselective cation channel Piezo2 has been identified as the principal transduction channel that allows spindle sensory afferent fibers to sense changes in mechanical stress that occur when a muscle changes length.
Fig. 9.3 shows the changes in activity of the afferent fibers of a muscle spindle when the muscle is stretched. It is clear that group Ia and group II fibers respond differently to stretch. Group Ia fibers are sensitive both to the amount of muscle stretch and to its rate, whereas group II fibers respond chiefly to the amount of stretch. Thus when a muscle is stretched to a new longer length, group II firing increases in proportion to the amount of stretch (see
Fig.
9.3 , left), and when the muscle is allowed to shorten, its firing rate decreases proportionately (see
Fig. 9.3 , right). Group Ia fibers show this same static-type response, and thus under steady-state conditions (i.e., constant muscle length), their firing rate reflects the amount of muscle stretch, similar to that of group II fibers.
While muscle length is changing, however, group Ia firing also reflects the rate of stretch or shortening that the muscle is undergoing. Its activity overshoots during muscle stretch and undershoots (and possibly ceases) during muscle shortening. These are called dynamic responses. This dynamic sensitivity also means that the activity of group Ia fibers is much more sensitive to transient and oscillatory stretches, such as shown in the middle diagrams of
Fig. 9.3 . In particular, the tap profile is what occurs when a reflex hammer is used to hit the muscle tendon and thereby cause a brief stretching of the attached muscle. The change in muscle length is too brief for significant changes in group II firing to occur, but because the magnitude of the rate of change (slopes of the tap profile) is so high with this stimulus, large dynamic responses are elicited in the group Ia fibers. Thus the functionality of reflex arcs involving group Ia afferent fibers is what is being assessed when a reflex hammer is used to tap on tendons.
γ Motor Neurons Adjust the Sensitivity of the Spindle