Motor cortex • Organization of motor cortex • Motor cortical map • Effect of cortical motor neuron activation on muscle contraction • Population coding
Cortical areas involved in motor control (Association cortex) Primary motor cortex (M1) - initiation and execution of movement Premotor and Supplementary motor cortex - initiation of complex movement, planning the movement Activity detected in the motor area (by fMRI) Flexing the finger -- M1 only Writing a letter with finger (complex sequence of movement) – M1, premotor and supplementary cortex Think about writing with the finger - premotor and supplementary cortex, not M1.,
Planning, initiation of voluntary movement Sensory-motor integration, motor learning Basic movement, posture Reflex (involuntary movement)
Connections between different motor areas (integrating all sensory informatio (premotor and supplementary motorareas) Cortico- spinal tract
Stimulation of motor cortex can cause muscle activity EMG (electromyogram) – recording of muscle contraction activity using extracellular or surface electrode.
Evidence for motor cortical map Intracortical stimulation • Brain stimulation • Intracortical stimulation • TMS (transcranial magnetic stimulation) • Jacksonian march (propagation of seizure activity) • - progressive activation of motor cortex • Functional Brain Imaging (detection of the active brain areas) • 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. TMS
Somatotopic map in primary motor cortex Distorted map: disproportionally large representation of parts requiring greater precision Somatotopic maps also exist in premotor cortex & supplementary motor cortex. Stimulation induces complex movements involving multiple joints and even bilateral movement
Divergence and convergence of cortical control of muscles: -- The same muscle is controlled by several cortical sites -- One corticospinal axon control many muscles (combinatorial control) Effectiveness of cortical stimulation at different sites • 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 representations • Implication: • Such overlap may allow coordination of multiple muscles for motor tasks
Use-dependent plasticity of the motor map (a) Deprivation causes reduction of representation Human hand injury Rat whisker denervation
Use-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.
Information coding by motor cortical neurons • In primary motor cortex, neuron fires before movement • Four types of neurons: • Dynamic neuron – code the rate of force • Static neuron – code steady level of force • Mixed neuron – code both rate and level of force • Directional neuron – code for direction of movement Edward V. Evarts (NIH) developed technique to record from motor cortical neuron from awake monkey performing motor tasks
Motor neuron spiking: coding force or position?Experiment: fix position of movement (wrist rotation), change force applied to the rod Wrist Extensor load Conclusion: Firing of motor cortical neurons codes the force generated by the muscle. This particular neuron recorded activates flexor muscle
“Spike triggered average” demonstrate that a single spike from a single motor neuron can exert significant effect on muscle activity Response correlated with each spike
Population coding of movement direction -Direction of the movement coded by a population of neurons, rather than a single neuron Experimental setup Georgopoulos et al., 1982
Population coding of movement 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 trial Direction tuning of individual neuron Motor cortical neurons signal both force and direction!