Download 04/12 PPT

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)
(Association cortex)
- 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)
Corticospinal 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
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 nonmetabolizable glucose (16O, 18F labeled) or radioactive
O2 (16O)
2. Functional Magnetic Resonance Imaging (fMRI)
-- Magnetic resonance resonance of the ratio of oxygenatednonoxygenated 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 nonstring players (controls) in the same orchestra.
Information coding by motor cortical neurons
In primary motor cortex, neuron fires before movement
Edward V. Evarts
(NIH) developed
technique to record
from motor cortical
neuron from awake
monkey performing
motor tasks
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
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!