Flexion movement in motor function is a result of the relaxation of the antagonist. In an experimental arrangement for recording from a corticospinal neuron, a stimulation electrode is used to elicit antidromic spikes. These spikes help identify the motor cortex neuron specifically as a pyramidal tract neuron. The firing rate of the pyramidal tract neuron is correlated with the level of flexor force needed for movement. When there is no load or flexor load conditions, the pyramidal tract neuron discharges before the onset of movement or EMG activity. In the extensor load condition, there is no activity in this pyramidal tract neuron as flexors do not need to contract for movement generation. Wrist movement is essentially identical in all three experimental conditions, indicating that this cell's activity encodes force magnitude and direction but not displacement. The primary motor cortex activity can also be correlated with movement parameters such as displacement, force, and target location, supporting the distributed network view of cortical motor areas. However, there are some real differences between the areas, with the premotor and supplementary motor areas showing a higher percentage of cells related to earlier motor planning stages compared to the primary motor cortex. The premotor area is more involved in movements made to external cues, while the supplementary motor area is more involved in movements made in response to internal cues. Research has revealed that each of these areas is functionally heterogeneous and can be further subdivided. The cerebellum plays a role in motor control and its specific functions will be discussed in detail.
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