Up to this point, we have described only how muscle spindles behave when there are no changes in γ motor neuron activity. The efferent innervation of muscle spindles is extremely important, however, because it determines the sensitivity of muscle spindles to stretch. For example, in
Fig. 9.4A, the activity of a muscle spindle afferent fiber is shown during
CHAPTER 9 Organization of Motor Function •Fig. 9.3 Responses of a Primary Ending (Group Ia) and a Secondary Ending (Group II) to Changes in Muscle Length. Notethedifferenceindynamicandstaticresponsivenessoftheseendings.Thewaveformsatthetoprepresentthechangesinmusclelength.ThemiddleandbottomrowsshowthedischargesofagroupIafiberandagroupIIfiber,respectively,duringthevariouschangesinmusclelength.
•Fig. 9.4 The Activity of γ Motor Neurons Can Counteract the Effects of Unloading on the Discharge of a Muscle Spindle Afferent Fiber. A, Theactivityofamusclespindleafferentfiberduringsteadystretch.B, Stimulationofα motorneuronat0mseccausescontractionoftheextrafusalfibers,whichleadstomuscleshorteningandincreasedmuscletensionbutunloadingofthetensionacrossthemusclespindle,whichinturninducestheafferentfibertostopfiring.Uponrelaxation,themusclereturnstoitsoriginallength,andtensionisrestoredontheintrafusalfibers,causingthereturnofactivityinthegroupIaafferentfiber.C, Coactivationofα andγ motorneuronscausesshorteningofbothextrafusalandintrafusalfibers.Thusthereisnounloadingofthespindle,andtheafferentfibermaintainsitsspontaneousactivity.(RedrawnfromKufflerSW,NichollsJG.From Neuron to Brain. Sunderland,MA:Sinauer;1976.) a steady stretch. If only the extrafusal muscle fibers were may stop discharging and become insensitive to further to contract (this can be done experimentally by selective decreases in muscle length. However, the unloading of the stimulation of α motor neurons; see
Fig. 9.4B ), the muscle spindle can be prevented if α and γ motor neurons are spindle would be unloaded by the resultant shortening of stimulated simultaneously. Such combined stimulation the muscle. If this happens, the muscle spindle afferent fiber causes the intrafusal muscle fibers of the spindle to shorten along with the extrafusal muscle fibers, maintaining the baseline tension on the equatorial portion of the intrafusal fibers (see
Fig. 9.4C •Fig. 9.5 Effects of Static and Dynamic γMotor Neurons on the Responses of a Primary Ending to Muscle Stretch. A, Thetimecourseofthestretch.B, ThedischargeofgroupIafibersintheabsenceofγmotorneuronactivity.C, Stimulationofastaticγmotoraxon.D, Stimulationofadynamicγmotoraxon.(RedrawnfromCroweA,MatthewsPBC.J Physiol. 1964;174:109.)