CHAPTER 9 Organization of Motor Function flexion movement results from relaxation of antagonist •Fig. 9.17 A, Experimentalarrangementforrecordingfromacorticospinalneuronwhileamonkeyperformstrainedwristmovements.Astimulationelectrodeisusedtoelicitantidromicspikesthatareusedtoidentifythemotorcortexneuronspecificallyasapyramidaltractneuron.Stimuliarenotappliedwhilethemonkeyisperformingmovements.B, Thepyramidaltractneuron(PTN)dischargesbeforetheonsetofmovementorEMGactivitywhenflexorsneedtogenerateforce(noloadandflexorloadconditions).Moreover,thefiringrateiscorrelatedwiththelevelofflexorforcethatisneeded.Intheextensorloadcondition,flexorsdonotneedtocontracttogeneratemovement,andthusthereisnoactivityinthisPTN.Thetoptraceshowswristmovement,whichisessentiallyidenticalforallthreeexperimentalconditions.Thusthiscell’sactivityinvolvesencodingforcemagnitudeanddirection,butnotdisplacement.(Figure basedonworkofEvartsandcolleagues.) movement, and the activity is correlated with movement parameters, such as displacement, force, and target location, just as primary motor cortex activity can be, which is consistent with the distributed network view of the cortical motor areas. There do, however, appear to be some real differences between the areas as well, although these differences may be more quantitative than qualitative. For example, the percentage of cells in the premotor and supplementary motor areas that show activity related to earlier motor planning stages is higher than that of such cells in the primary motor cortex. In addition, the premotor and supplementary motor areas can be distinguished from each other by the apparently greater involvement of the premotor area in movements made to external cues (such as in the task shown in
Fig.
9.18 ) and the greater involvement of the supplementary motor area in movements made in response to internal cues (i.e., self-initiated). Research has also revealed that each of these areas is functionally heterogeneous and can therefore be further subdivided; however, such details are beyond the scope of this discussion.
Motor Control by the Cerebellum
Overview of the Role of the Cerebellum in Motor Control