The innervation of intrafusal fibers in the neural system is distinct from that of extrafusal fibers. While extrafusal fibers are innervated by a single motor neuron, intrafusal fibers receive multiple innervations, including sensory and motor innervation. Typically, the sensory innervation consists of one group Ia afferent fiber and a variable number of group II afferent fibers. Group Ia fibers are the largest in diameter and conduct at a speed of 80 to 120 m/sec, while group II fibers are intermediate in size and conduct at a speed of 35 to 75 m/sec. The group Ia fiber forms a primary ending, which is a spiral-shaped termination, on each intrafusal muscle fiber in the spindle. These primary endings can be found on both types of nuclear bag fibers and nuclear chain fibers. On the other hand, the group II fiber forms a secondary type ending on nuclear chain and bag2 fibers, but not on bag1 fibers. The primary and secondary endings contain mechanosensitive channels that are sensitive to the tension on the intrafusal muscle fiber.
The motor supply to a muscle spindle consists of two types of γ motor axons. Dynamic γ motor axons end on nuclear bag1 fibers, while static γ motor axons end on nuclear chain and bag2 fibers.
Muscle spindles respond to changes in muscle length because they are in parallel with the extrafusal fibers and thus are stretched or shortened along with them. Since intrafusal fibers have spring-like properties, any change in their length alters the tension they experience, and this change is sensed by mechanoreceptors in the group Ia and group II spindle afferent fibers. The principal transduction channel responsible for allowing spindle sensory afferent fibers to detect mechanical stress changes caused by muscle length changes is the nonselective cation channel Piezo2.
When a muscle is stretched, the activity of the afferent fibers in a muscle spindle changes. Group Ia and group II fibers respond differently to stretch. Group Ia fibers are sensitive to both the amount and rate of muscle stretch, while group II fibers primarily respond to the amount of stretch. Therefore, when a muscle is stretched to a longer length, group II firing increases in proportion to the stretch, and when the muscle is allowed to shorten, their firing rate decreases accordingly. Group Ia fibers exhibit a similar static-type response, reflecting the amount of muscle stretch under steady-state conditions.
However, during changes in muscle length, group Ia firing also reflects the rate of stretch or shortening that the muscle undergoes. Group Ia fibers show dynamic responses, where their activity overshoots during muscle stretch and undershoots (possibly ceases) during muscle shortening. This dynamic sensitivity means that group Ia fibers are more sensitive to transient and oscillatory stretches. For instance, when a reflex hammer hits the muscle tendon, causing a brief stretching of the attached muscle (known as the tap profile), the rate of change is high, eliciting large dynamic responses in the group Ia fibers. The functionality of reflex arcs involving group Ia afferent fibers is assessed when a reflex hammer taps on tendons.
γ motor neurons play a role in adjusting the sensitivity of the spindle.
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