Transcript Slide 1

Voluntary Movement
From Ch. 38
“Principles of Neural Science”, 4th Ed.
Kandel et al
May 13, 2009
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
May 13, 2009
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
• 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 (different aspects of motor planning)
– Dorsal premotor area (dPMA)
•
Selection of action; Sensorimotor transformations; Externally triggered movements; external cues that
do not contain spatial information
– Ventral premotor area (vPMA)
•
Conforming the hand to shape of objects; Mirror neurons; Selection of action; Sensorimotor
transformations; Externally triggered movements
– Supplementary motor area (SMA)
•
Preparation of motor sequence from memory (internally not in response to external information)
– Pre-supplementary motor area (pre-SMA)
•
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
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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):
– 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.
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TMS – transcranial magnetic stimulation (Barker 1985)
– 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
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Non-invasive techniques to study
– Structure-function relationship (e.g. rTMS virtual lesion)
– Map brain motor output (typically averaged EMG as output =MEP)
– Measure conduction velocity
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TMS has advantages over TES
– 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
May 13, 2009
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
May 13, 2009
Motor cortex
• Primary motor cortex
–
–
–
Activated directly by peripheral stimulation
Executes movements
Adapt movements to new conditions
• Premotor areas (different aspects of motor planning)
– Dorsal premotor area (dPMA)
•
Selection of action; Sensorimotor transformations; Externally triggered movements; external cues that
do not contain spatial information
– Ventral premotor area (vPMA)
•
Conforming the hand to shape of objects; Mirror neurons; Selection of action; Sensorimotor
transformations; Externally triggered movements
– Supplementary motor area (SMA)
•
Preparation of motor sequence from memory (internally not in response to external information)
– Pre-supplementary motor area (pre-SMA)
•
Motor sequence learning
– Cingulate motor area (CMA)
•
•
Dorsal and ventral portions of caudal and roastral CMA (along the cingulate sulcus)
Functions: to be determined
Cortical projections
•
•
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Premotor cortex and primary motor cortex has reciprocal connections
Parietal projections to premotor areas (sensorimotor transformations)
Prefrontal projections to some premotor areas (cognitive-affective control and
learning)
Premotor areas and primary motor areas have direct projections to spinal motor
neurons
May 13, 2009
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
May 13, 2009
Motor cortex plasticity
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The functional organization of M1 changes after transection of facial nerve
May 13, 2009
Practiced movements
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M1 representation becomes more dense with practice
PET data
May 13, 2009
Pyramidal tract
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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
Bilateral sectioning of the pyramidal tract
removes the ability if fine movements
Ia spinal circuits
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Spinal Ia neurons are inhibitory interneurons
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May 13, 2009
Can respond directly to changes in somatosensory input
Cortical centers do not need to respond to minor changes
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The Ia inhibitory neurons in the spinal cord sends
inhibitory signals to antagonist motor neurons when
muscle spindles in the agonist muscle are activated
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Ia neurons also inhibits spinal reflexes
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Spinal circuits are used as components of complex
behaviors
Direction of movement
Increased
activity
with load
Wrist displacement
constant but load is
different
•
May 13, 2009
Activity in individual neurons in M1 is related to muscle force
and not direction
Postspike facilitation
• Spike-triggered averaging
May 13, 2009
M1 and force
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Linear relationship between M1 firing
rate and force generation
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Two types of motor cortical neurons
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May 13, 2009
Phasic-tonic: initial dynamic burst
Tonic: tonic high level
Direction of movement
Population vector
Single neuron
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
May 13, 2009
Direction of movement
M1 encoding of force required to maintain a direction
Single
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
May 13, 2009
Activity depends on motor task
Precision grip: same activity whether force is light or heavy
Power grip: No activity, but EMG activity the same
May 13, 2009
Complexity of movement
May 13, 2009
Internal and external information
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Influence on visual cue and prior training in motor cortex
May 13, 2009
Motor preparation
May 13, 2009
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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
May 13, 2009
Ventral premotor cortex
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Specific hand tasks activate vPMC
May 13, 2009
Mirror neurons
Ventral premotor area
• Precision grip
• Observed movement
• Observed human
movement
• Self-performed movement
May 13, 2009
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
• Novel behavior requires processing in several motor and parietal areas as it is
continuously monitored for errors and then modified
– Primary motor cortex
• 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