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Voluntary Movement
From Ch. 38
“Principles of Neural Science”, 4th Ed.
Kandel et al
Voluntary movement
• Voluntary movements are organized in cortex
• Sensory feed back
– Visual information
– Proprioceptive information
– Sounds and somatosensory information
• Goal of movement
– Vary in response to the same stimulus depending on behavioral task (precision
vs. power grip)
• Improves with learning/ experience
• Can be generated in response to external stimuli or internally
Cortical organization
• Hierarchical organization of motor control and task features
– Populations of neurons encode motor parameters e.g. force, direction, spatial
patterns
– The summed activity in a population determines kinematic details of movement
– Voluntary movement is highly adaptable
•
Novel behavior requires processing in several motor and parietal areas as it is continuously monitored
for errors and then modified
– Primary motor cortex
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Fires shortly before and during movement
Fires only with certain tasks and patterns of muscle activation
– Premotor areas encode global features of movement
•
Set-related neurons
– Sensorimotor transformations (external environment integrated into motor programs)
– Delayed response
Motor cortex
• Primary motor cortex
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Activated directly by peripheral stimulation
Executes movements
Adapt movements to new conditions
• Premotor areas (Motor planning)
– Dorsal premotor area (dPMA)
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Selection of action; Sensorimotor transformations; Externally triggered
movements; external cues that do not contain spatial information
– Ventral premotor area (vPMA)
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Conforming the hand to shape of objects; Mirror neurons; Selection of
action; Sensorimotor transformations; Externally triggered movements
– Supplementary motor area (SMA)
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Preparation of motor sequence from memory (internally not in response
to external information)
– Pre-supplementary motor area (pre-SMA)
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Motor sequence learning
– Cingulate motor area (CMA)
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Dorsal and ventral portions of caudal and roastral CMA (along the
cingulate sulcus)
Functions: to be determined
Somatotopical organization
Sequence in human and monkey M1
similar
Face and finger representations are
much bigger than others
Greater motor control required for
face and fingers
Motor cortex stimulation
Historical perspective
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1870 Discovery of electrical excitability of cortex in the dog;
first brain maps (Fritsh and Hitzig)
1875 First motor map of the primate brain (Ferrier)
1926 Recording of extracellular spike activity of a nerve fiber (Adrian)
1937 First experimentally derived human motor map (Penfield and
Boldrey)
1957 Microelectrode recordings to map primary somatosensory area
(Mountcastle et al.)
1958 First recordings from neurons in awake monkeys (Jasper)
1967 Intracortical microstimulation for mapping of cortical motor output
(Asanuma)
1985 TMS is used to activate motor cortex noninvasively (Barker et al.)
Transcranial stimulation
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TES – transcranial electrical stimulation (Merton and Morton 1980)
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TMS – transcranial magnetic stimulation (Barker 1985)
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Discharge of large capacitive currents (5-10kA, 2-300us) through a coil producing high magnetic field (1-2T).
Stimulus site depends on coil design, coil orientation and stimulus intensity
Non-invasive techniques to study
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High voltage (1-2kV), short duration pulses (10-50us), low resistance electrodes.
Direct stimulation occurs at the anode
Current passes through skin and scalp (resistance) before reaching cortex.
Structure-function relationship (e.g. rTMS virtual lesion)
Map brain motor output (typically averaged EMG as output =MEP)
Measure conduction velocity
TMS has advantages over TES
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No discomfort (no current passes through skin and high current densities can be avoided)
No attenuation of field when passing through tissue
No skin preparation (conduction gel)
Transcranial magnetic
stimulation
Principles of TMS
Coil design
Motor cortex stimulation
Latency
difference
• Movements can be evoked by
direct stimulation of motor cortex
• Activates corticospinal fibers
– Direct from motor cortex to spinal
motor neurons or interneurons
• Evokes a short latency EMG
response in contralateral muscles
• Latency depends on corticospinal
distance impulses have to travel
Cortex-muscle connections
Shoulder muscle
Wrist muscle
Maps can be generated by intracortical microstimulation
Sites controlling individual muscles are distributed over a wide area of motor
cortex
Muscle representations overlap in cortex
Stimulation of single sites activates several muscles (diverging innervation)
Many motor cortical neurons contribute to multijointed movements
Cortical projections
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Premotor cortex and primary motor cortex has reciprocal connections
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Parietal projections to premotor areas (sensorimotor transformations)
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Prefrontal projections to some premotor areas (cognitive-affective control and learning)
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Premotor areas and primary motor areas have direct projections to spinal motor neurons
Other projections
• Inputs from cerebellum
– Do not project directly to spinal cord
• Inputs from basal ganglia
– Do not project directly to spinal cord
• Cortico-striatal pathways
– Motor loops
– Motor cortex => striatum => globus pallidus
=> Thalamus => motor cortex
Motor cortex plasticity
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The functional organization of M1 changes after transection of facial nerve
Practiced movements
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M1 representation becomes more dense with practice
PET data
Pyramidal tract
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Bilateral sectioning of the pyramidal tract
removes the ability of fine movements
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Successive cortical stimuli result in progressively
larger EPSP in spinal motor neurons
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Make it possible to make individual movement of
digits and isolated movements of proximal joints
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Direct corticospinal control is necessary for fine control of
digits
Ia spinal circuits
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Type Ia sensory fibers are primary afferent fibers
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Spinal Ia neurons are inhibitory interneurons
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Agonist muscle: generates specific movement
Antagonist muscle: acts opposite the specific movement
Proprioceptor
Component of the muscle spindle
Conveys information about the velocity of stretch and change in
muscle length
Can respond directly to changes in somatosensory input
Cortical centers do not need to respond to minor changes
Sends inhibitory signals to antagonist motor neurons when
muscle spindles in the agonist muscle are activated
Spinal Ia neurons also inhibits spinal reflexes
Spinal circuits are used as components of complex
behaviors
Direction of movement
Activity in individual neurons in M1 is related to
muscle force and not joint displacement
Flexor muscle: decreases joint angle
Extensor muscle: increases joint angle
Increased
activity
with load
Wrist displacement
constant but load is
different
Postspike facilitation
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Cortical motor neuron
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EPSP have fixed latency
One EPSP increases the probability of spinal
motor neuron firing. It does not fully depolarize
the motor neuron
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The EMG is the sum of spike trains of a
population of motor units within a muscle
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The EMG is an indicator of firing of
spinal motor neuron
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Spike-triggered averaging
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Averaging the EMG profile over thousands of
discharges from a single cortical neuron
Cancels out random noise
Peak in EMG profile at 6ms latency = postspike
facilitation
Indicator of connectivity between cortical neuron
and the motor neuron
M1 and force
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Two types of cortical motor neurons
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Isometric wrist torques: torque
level is reached and held
Phasic-tonic: initial dynamic burst
Tonic: tonic high level
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Linear relationship between M1 firing
rate and force generation
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In both types of neurons activity
increases with torque
Direction of movement
Single neuron response to 8 directions
Population vector
Many neurons with different
preferred direction
Major response: 90- 225 deg
Predicted from vector
Actual movement
Direction of movement is encoded by a population of neurons
Motor cortical neurons are broadly tuned to directions but have a preferred
direction
Direction of movement
M1 encoding of force required to maintain a direction
Single
8 directions of
movement
Arm movements without and with external loads
(a) Unloaded: preferred direction to the upper left
(b) Loaded: opposite, preferred direction to the lower right
A cells firing rate increases if a load opposes movement
in preferred direction and decreases if load pulls in
preferred direction
Activity of a single motor
neuron
Length of vector =
discharge magnitude
Activity depends on motor task
Precision grip: same activity whether force is light or heavy
Power grip: No activity, but EMG activity the same
Complexity of movement
Internal and external information
Influence of visual cue and prior training in motor cortex
16 trials
Before
movement
After
movement
Task: press 3 buttons in a sequence either guided by (a) light or (b) previously
learned
Motor preparation
Laterality specific response
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Dorsal premotor area is active during
preparation
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Fires according to different delay times
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Fires during the whole period of anticipation
Visuomotor transformations
• Separate but parallel fronto-parietal projections
Ventral premotor cortex
Specific hand tasks activate vPMC
Mirror neurons
Ventral premotor area
• Observed movement
• Observed human movement
• Self-performed movement
Summary
• Hierarchical organization of motor control and task
features
–
Populations of neurons encode motor parameters e.g. force, direction, spatial patterns
–
The summed activity in a population determines kinematic details of movement
–
Voluntary movement is highly adaptable
•
–
Primary motor cortex
•
•
–
Novel behavior requires processing in several motor and parietal areas as it is continuously monitored for errors and
then modified
Fires shortly before and during movement
Fires only with certain tasks and patterns of muscle activation
Premotor areas encode global features of movement
• Set-related neurons
–
–
Sensorimotor transformations (external environment integrated into motor programs)
Delayed response