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The behavior of muscle spindles in the absence of changes in γ motor neuron activity has been described so far. However, the efferent innervation of muscle spindles is crucial as it determines their sensitivity to stretch. Fig. 9.4A shows the activity of a muscle spindle afferent fiber during steady stretch, while Fig. 9.3 illustrates the responses of a primary ending (Group Ia) and a secondary ending (Group II) to changes in muscle length, highlighting the differences in their dynamic and static responsiveness.

Furthermore, Fig. 9.4 demonstrates how the activity of γ motor neurons can counteract the effects of unloading on the discharge of a muscle spindle afferent fiber. Part A shows the activity during steady stretch, while part B illustrates the consequences of stimulating α motor neurons, resulting in muscle shortening and increased tension but unloading of the tension across the muscle spindle. On relaxation, the muscle returns to its original length, restoring tension on the intrafusal fibers and causing the return of activity in the group Ia afferent fiber.

However, if α and γ motor neurons are coactivated, as depicted in part C of Fig. 9.4, both the extrafusal and intrafusal fibers shorten, preventing unloading of the spindle and allowing the afferent fiber to maintain its spontaneous activity.

Finally, Fig. 9.5 depicts the effects of static and dynamic γ motor neurons on the responses of a primary ending to muscle stretch. The time course of the stretch is shown in part A, while part B demonstrates the discharge of group Ia fibers in the absence of γ motor neuron activity. Parts C and D represent the stimulation of static and dynamic γ motor axons, respectively.

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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.)