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

Cortical areas involved in motor control

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.,

(Association cortex)

Basic movement, posture

Planning, initiation of voluntary movement

Reflex (involuntary movement)

Sensory-motor integration,

motor learning

(premotor and supplementary motorareas)

Connections between different motor areas

(integrating all sensory informatio

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

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)

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.

TMS

Intracortical stimulation

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)

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 tasks

Effectiveness of cortical stimulation at different sites

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.

Edward V. Evarts (NIH) developed technique to record from motor cortical neuron from awake monkey performing motor tasks

Information coding by motor cortical neurons

In primary motor cortex, neuron fires before movement

Four types of neurons:

1. Dynamic neuron – code the rate of force

2. Static neuron – code steady level of force

3. Mixed neuron – code both rate and level of force

4. Directional neuron – code for direction of movement

Motor 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 muscle

WristExtensor load

“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

Georgopoulos et al., 1982

Experimental setup

Direction tuning of individual neuron

Motor 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 trial

Population coding of movement direction