The primary motor cortex (M1)

Question 1

historical view of the primary motor cortex

Answer 1

Edouard Hitzig

• specific areas of the brain stimulated caused involuntary twitches in dogs

David Ferrier

• specific regions of monkey cortex where a lesion caused inability to move body part

Wilder Penfield

- mapped M1 with a small electrical current across the cortex and observed how the person would respond

• more neurons dedicated to other body parts like hands vs legs

Question 2

topography and projections to spinal cord

Answer 2

• 50-60% of the projections to the spinal cord are from M1
• primary motor cortex (M1)
• primary somatosensory cortex (S1)
• posterior parietal cortex (PPC)
• supplementary motor area (SMA)
• pre-motor area (PM)

Question 3

lesions impair independent digit

Answer 3

• precision grip impaired whereas powergrip remains
• persons with stroke often have difficulty performing activities of daily living such as buttoning a shirt

Question 4

historical view of organization

Answer 4

1. precentral gyrus (M1) contains topographic map on the body
2. each point in the map specifies tension in a single muscle
3. cortical motor areas are organized hierarchically (promotor, primary motor, spinal cord
4. organization is invariant (environment can’t modify how M1 is organized)

Question 5

rethinking classic view with evidence

Answer 5

1. motor map of a control rat vs a kindled rat is different suggesting that the cortex of people with epilepsy could be different than normal
2. dysmelia, incomplete development of upperlimbs, smaller amount of neurons dedicated to the arms and neurons are organized differently
3. phantom limb pain, with a missing limb, reorganization causes activation of neurons of missing limbs
4. no discrete representations of body parts, for monkeys and humans (pinky, thumb) more discrete representations in S1 than M1

Question 6

is the orientation of the face relative to the hand correct?

Answer 6

the orientation of the head needs to be inverted 180 degrees

the area that is lost is encroached by the areas that are near, neurons that senses the lost limb is now then responsible for sensing the low face

Question 7

pathway to an action

Answer 7

• movement in a particular direction is determined by a population of neurons (neuron fires more when the monkey points towards the left side)
• vector lengths equals level of activity for a particular movement direction

Question 8

rethinking the classic view: movement specificity

Answer 8

• specific neuron is active for a precision grip, but not when power gripping
• specific neurons perform specific actions