Voluntary Movement

1
Voluntary Movement

• From Ch. 38
• Principles of Neural Science, 4th Ed.
• Kandel et al

2
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

3
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

4
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

5
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
6
Motor cortex stimulation

• Historical perspective
• 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.)

7
Transcranial stimulation

• 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.
• 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
• 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
• 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)

8
Transcranial magnetic stimulation
Principles of TMS
Coil design
9
Motor cortex stimulation

• 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

10
Cortex-muscle connections
Wrist muscle
Shoulder 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
11
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

12
Cortical projections

• 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

13
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 gt striatum gt globus pallidus gt
  Thalamus gt motor cortex

14
Motor cortex plasticity

• The functional organization of M1 changes after
  transection of facial nerve

15
Practiced movements

• M1 representation becomes more dense with
  practice

PET data
16
Pyramidal tract

• Successive cortical stimuli result in
  progressively larger EPSP in spinal motor neurons
• Make it possible to make individual movement of
  digits and isolated movements of proximal joints
• Direct corticospinal control is necessary for
  fine control of digits
• Bilateral sectioning of the pyramidal tract
  removes the ability if fine movements

17
Ia spinal circuits

• Spinal Ia neurons are inhibitory interneurons
• Can respond directly to changes in somatosensory
  input
• Cortical centers do not need to respond to minor
  changes
• The Ia inhibitory neurons in the spinal cord
  sends inhibitory signals to antagonist motor
  neurons when muscle spindles in the agonist
  muscle are activated
• Ia neurons also inhibits spinal reflexes
• Spinal circuits are used as components of complex
  behaviors

18
Direction of movement

• Activity in individual neurons in M1 is related
  to muscle force and not direction

19
Postspike facilitation

• Spike-triggered averaging

20
M1 and force

• Linear relationship between M1 firing rate and
  force generation
• Two types of motor cortical neurons
• Phasic-tonic initial dynamic burst
• Tonic tonic high level

21
Direction of movement
Population vector
Single neuron
Direction of movement is encoded by a population
of neurons Motor cortical neurons are broadly
tuned to directions but have a preferred direction
22
Direction of movement
M1 encoding of force required to maintain a
direction
Single

• Arm movements without and with external loads
• Unloaded preferred direction to the upper left
• 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

23
Activity depends on motor task
Precision grip same activity whether force is
light or heavy Power grip No activity, but EMG
activity the same
24
Complexity of movement
25
Internal and external information

• Influence on visual cue and prior training in
  motor cortex

26
Motor preparation

• Dorsal premotor area is active during preparation
• Fires according to different delay times
• Fires during the whole period of anticipation

27
Visuomotor transformations

• Separate but parallel fronto-parietal projections

28
Ventral premotor cortex

• Specific hand tasks activate vPMC

29
Mirror neurons

• Precision grip
• Observed movement
• Observed human movement
• Self-performed movement

30
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