Chapter 10 Why are there Three Types of Intrafusal Muscle Fibers?

Abstract

Publisher Summary This chapter describes a working scheme for the use of the three intrafusal fiber types. The bag 1 fibers provide control of the stretch sensitivity of primary afferents. Rapid changes in dynamic fusimotor output would be pointless because of the relative slowness of response of the bag 1 fibers and their inability to effectively oppose unloading. The bias levels of primary and secondary afferents could be set by bag 2 activity, but the slowness of bag 2 fiber contraction means that quick changes could not be made in this way. Gamma axons supplying bag 2 fibers alone can be expected, therefore, like dynamic axons, to fire tonically rather than phasically. Finally, the γ motoneurons that would be expected to show rapid modulation during movements should be those jointly supplying bag and chain fibers. The proposed function for the dynamic system is already well supported by spindle recordings in natural movements, but the crucial test for a dual function within the static system is to look for two distinct patterns of natural activity among γ s axons. The prediction is that one group, directed mainly to bag fibers, will fire tonically while the other, directed to chain or to bag 2 and chain fibers, will be modulated roughly in parallel with the active muscle contractions.