Static Static<unk> s innervate static nuclear bag fibers (bag2 fibers), a type of nuclear bag fiber and nuclear chain fibers. Both of these fiber types are part of the intrafusal muscle spindle fibers, where the static<unk> s innervate onto. Nuclear chain fibers' nuclei are organized in longitudinal columns, which is where it gets its name from, whereas the nuclear bag fibers' nuclei are clumped in the midsection of the muscle spindle. There is approximately a 2:1 ratio of nuclear chain fibers to nuclear bag fibers. The static<unk> s increase their firing, in response to an increase in magnitude of change in length and controls the static sensitivity of the stretch reflex. For this reason, this type of<unk>  is mostly used in the maintenance of postures and slower movements such as lifting a box, rather than activities requiring rapid changes due to rapid change in muscle length. Dynamic Dynamic<unk> s innervate the dynamic nuclear bag fibers (bag1 fibers), another type of nuclear bag fiber smaller than the static nuclear bag fibers. This type of<unk>  can enhance the sensitivities of Ia sensory<unk> s. It is done so because the dynamic nuclear bag fibers, which are innervated by the dynamic<unk> s, receive Ia sensory innervation. Furthermore, the firing of dynamic<unk> s removes the slack in dynamic nuclear bags, bringing Ia fibers closer to the firing threshold. Dynamic<unk> s alter muscle spindle sensitivity and increases its discharge in response to velocity, the rate of change, of muscle length rather than simply the magnitude as it is with static<unk> s. Therefore, this type of<unk>  can be used for activities requiring quick changes in muscle length to adjust such as balancing on a rail. Effects of nuclear chain fibers The effect of nuclear chain fibers on primary endings is to drive the discharge up to a frequency of around 60 Hz in a linear fashion, above which the discharge can become irregular. The activities of bag2 fibers show an initial sharp peak in discharge, which gets less as the receptor adapts. Bag2 fibers also reduce the dynamic sensitivity of the Ia afferent and sometimes also reduce the length sensitivity. Activation of bag1 fibers has the effect of increasing both the length sensitivity and the dynamic sensitivity of the primary ending. It is believed that the secondary sensory endings serve to measure length and muscle contractions of nuclear chain fibers at the pole via the static γ-motoneurons both excite the ending and increase its length sensitivity. Bag1 and bag2 fibers receive very little innervation from secondary endings, and activation of these fibers has a minimal effect on the discharge of the secondary ending. Characteristics Static<unk> s Dynamic<unk> s Innervates static nuclear bag fibers (bag2 fibers) and nuclear chain fibers dynamic nuclear bag fibers (bag1 fibers) Responds to changes in magnitude of muscle length changes in velocity of muscle length Aids in sensitivity of stretch reflex muscle spindle sensitivity Mainly used for static sustained muscle movement quick movements requiring rapid change
Gamma motor neuron
Upper motor neuron
Muscle spindle
Nociceptor
Cerebrospinal fibers
Postganglionic nerve fibers
Efferent nerve fiber
Sensory nerve