Nuclear bag fibers

The intrafusal fibers are positioned in parallel to the extrafusal fibers (the regular contractile units of the muscle) with the ends of the spindle capsule attached to the tendons at either end of the muscle. Intrafusal fibers do not contribute to the overall contractile force of the muscle, but rather serve a pure sensory function. There are two types of intrafusal fibers in mammalian muscle spindles. The first type contains many nuclei in a dilated central area and is called a nuclear bag fiber (Figure 9-2B). There are two subtypes of nuclear bag fibers, dynamic and static. Typically, there are two or three nuclear bag fibers per spindle. The second intrafusal fiber type, the nuclear chain fiber, is thinner and shoer and lacks a definite bag. Each spindle has about five of these fibers. There are two kinds of sensory endings in each spindle, a single primary (group Ia) ending and up to eight secondary (group II) endings. The Ia afferent fiber wraps around the center of the dynamic and static nuclear bag fibers and nuclear chain fibers. Group II sensory fibers are located adjacent to the centers of the static nuclear bag and nuclear chain fibers; these fibers do not innervate the dynamic nuclear bag fibers. Ia afferents are very sensitive to the velocity of the change in muscle length during a stretch (dynamic response); thus they provide information about the speed of movements and allow for quick corrective movements. The steady-state (tonic) activity of group Ia and II afferents provide information on steady-state length of the muscle (static response). The top trace in Figure 9-2C shows the dynamic and static components of activity in an Ia afferent during muscle stretch. Note that they discharge most rapidly while the muscle is being stretched (shaded)Ref: Ganong&;s review of medical physiology 23rd edition Page no: 158