Motor cortexSlide 2Cortical areas involved in motor controlSlide 4Slide 5Stimulation of motor cortex can cause muscle activityEvidence for motor cortical mapSlide 8Slide 9Slide 10Slide 11Slide 12Motor neuron spiking: coding force or position? Experiment: fix position of movement (wrist rotation), change force applied to the rodSlide 14Slide 15Slide 16Motor cortex•Organization of motor cortex•Motor cortical map•Effect of cortical motor neuron activation on muscle contraction•Population codingCortical areas involved in motor controlCortical areas involved in motor controlPrimary motor cortex (M1) - initiation and execution of movementPremotor and Supplementary motor cortex - initiation of complex movement, planning the movementActivity detected in the motor area (by fMRI)Flexing the finger -- M1 onlyWriting a letter with finger (complex sequence of movement) – M1, premotor and supplementary cortexThink about writing with the finger - premotor and supplementary cortex, not M1., (Association cortex)Basic movement, posturePlanning, initiation of voluntary movementReflex (involuntary movement) Sensory-motor integration, motor learning(premotor and supplementary motorareas)Connections between different motor areas(integrating all sensory informatioCortico-spinal tractStimulation of motor cortex can cause muscle activityEMG (electromyogram) – recording of muscle contraction activity using extracellular or surface electrode.Evidence for motor cortical mapBrain stimulation–Intracortical stimulation–TMS (transcranial magnetic stimulation)Jacksonian march (propagation of seizure activity) - progressive activation of motor cortexFunctional Brain Imaging (detection of the active brain areas)1. Positron Emission Tomography (PET) -- Detection of activity-related glucose or O2 use by radiation due to positron emission from radioactive non-metabolizable glucose (16O, 18F labeled) or radioactive O2 (16O) 2. Functional Magnetic Resonance Imaging (fMRI)-- Magnetic resonance resonance of the ratio of oxygenated-nonoxygenated hemoglobin as an indication of increase flow of oxygenated blood flow to the active brain regions.TMSIntracortical stimulationSomatotopic map in primary motor cortexDistorted map: disproportionally large representation of parts requiring greater precisionSomatotopic maps also exist in premotor cortex & supplementary motor cortex. Stimulation induces complex movements involving multiple joints and even bilateral movementDivergence and convergence of cortical control of muscles:-- The same muscle is controlled by several cortical sites-- One corticospinal axon control many muscles (combinatorial control)Experiment:• Microelectrode stimulation over a grid area of motor cortex• Recording from a shoulder muscle (deltoid) and a wrist muscle (ECR) Finding:• Same muscle can be activated from multiple stimulation sites• Overlap between shoulder and wrist muscle representationsImplication:• Such overlap may allow coordination of multiple muscles for motor tasksEffectiveness of cortical stimulation at different sitesUse-dependent plasticity of the motor map(a) Deprivation causes reduction of representationHuman hand injuryRat whisker denervationUse-dependent plasticity of the motor map(b) Practice causes expansion of representation-- Finger opposition training – touching thumb with finger in a particular sequence. Following 3 weeks of training, fMRI showed larger cortical area activated by performing the trained sequence.-- fMRI studies showed larger cortical representation of left figures for string player who has an earlier inception of practice, although string players in general have higher representation than non-string players (controls) in the same orchestra.Edward V. Evarts (NIH) developed technique to record from motor cortical neuron from awake monkey performing motor tasks Information coding by motor cortical neuronsIn primary motor cortex, neuron fires before movementFour types of neurons:1. Dynamic neuron – code the rate of force2. Static neuron – code steady level of force3. Mixed neuron – code both rate and level of force4. Directional neuron – code for direction of movementMotor neuron spiking: coding force or position?Experiment: fix position of movement (wrist rotation), change force applied to the rod Conclusion: Firing of motor cortical neurons codes the force generated by the muscle. This particular neuron recorded activates flexor muscleWristExtensor load“Spike triggered average” demonstrate that a single spike from a single motor neuron can exert significant effect on muscle activityResponse correlated with each spikePopulation coding of movement direction-Direction of the movement coded by a population of neurons, rather than a single neuronGeorgopoulos et al., 1982Experimental setupDirection tuning of individual neuronMotor cortical neurons signal both force and direction!Actual direction of movement can be predicted by the vector sum of multiple neurons:- Each vector represents one neuron- Vector direction: preferred direction of the neuron- Vector length: firing rate of that neuron during the trialPopulation coding of movement