chapter 14: brain control of movement

what is the overall information loop

basal ganglia/cerebellum → motor cortex/brainstem centers → local circuit neurons (spinal cord) → lower motor neurons → skeletal muscle

where is the motor cortex located

part of the frontal lobe

what 3 cortexes make up the motor cortex

1. primary motor cortex (precentral gyrus)

2. premotor cortex (PMA)

3. supplementary motor cortex (SMA)

what is in layer 5 of the primate MC

betz cells

pyramidal tract

corticospinal and corticoulbar tract

main descending motor cortex pathway

1. corticospinal tract: from motor cortex through the internal capsule, midbrain, and medulla to the spinal cord as the corticospinal tract

2. corticoulbar tract: motor cortex → cranial nerves (muscles of face, head, neck)

function of the motor cortex (Penfield experiements)

- brief electrical stimulation causes muscle twitches

- somatotopic organization similar to sensory cortex

functions of the motor cortex (long stimuation)

- longer stimulation in mobkeis caused complex movements directed to a particular end posture (goal-orientated)

-

how do M1 neurons encode movement directions

each M1 neuron has a direction preference and activity drops symmetrically from that preferred direction

how do m1 cells use population coding to accurately encode movement direction

cells that have different preferred directions fire based on where the final direction is meant to go

→ think adding vectors, if you want 20º, you go a little more to the side than up and make hypotenuse

motor cortex plasticity

your cortex can adapt to changes based on what you learn

ex. mastering a sport, losing a limb

volitional control of motor neurons

you can control the activity of your motor cortex through classical conditioning

premotor cortex

plans actions (increased activity when you get the "go" cue to do something, decreased triggering when you actually move)

PMA mirror neurons

respond when doing an action and when watching the same action

*motor specific (looks at the movement, not the task)

basal ganglia structures

caudate, putamen, globus pallidus (GPe and GPi)

striatum = caudate + putamen

how does the motor cortex and prefrontal cortex project onto the basal ganglia

caudal putamen → primary motor cortex?

caudal caudate → PMA

rostral putamen → dorsal lateral prefrontal cortex (DL-PFC)

rotral caudate → OMPFC

basal ganglia loop

cerebral cortex → striatum → pallidum → ventral thalamus/substantia nigra → cerebral cortex

what do thalamocortical-BG loops do

selects appropriate actions to the direct pathway and innappropriate actions to indirect pathway (suppressing)

how do thalamocortical-BG loops work

prefrontal cortex + motor cortex + sensory cortex give all possible actions to the basal ganglia

2. all possible actions go through to the ventralateral thalamus and only the selected action goes back up the the motor cortex

→ actions that aren't selected are inhibited by GPi

3. selected action is signaled down to the spinal cord

what is the BG direct pathway

1. possible actions are sent down from cerebral cortex to striatum

2. striatal neurons express D1 (excitatory) input from substantia nigra → more movement

3. striatum disinhibits VL thalamus by inhibiting GPi

what is the BG indirect pathway

1. striatal neurons express D2 (inhibitory)

2. striatum disinhibits GPi by inhibiting GPe

3. GPi inhibits VL thalamus

parkinson's disease

no dopamine input → difficulty initiating/transitioning among movements, resting tremor, muscle rigidity LESS MOVEMENT

- no initiation of direct or indirect pathway

parkinson's disease treatment

deep brai stimulation (DMS)

→ stimulating sites in STN or GPi

huntington's disease

neurodegenerative disease with coordinated purposeless movements; cognitive and emotional disturbances

→ loss of neurons in striatum → less able to inhibit GPi, more inhibition in VL thalamus

→ stronger indirect pathway → inability to suppress unwanted movements

cerebellum functions

1. adjusts for differences between an intended movement and an actual movement

2. timing movements

3. motor learning

cerebellum structures

vermis, cerebellar hemisphere, deep cerebellar nucleus, cerebellum peduncles

what are the three layers of the cerebellum

(outer) molecular layer, purkinje layer, granular layer (inner)

cerebellum circuits

purkinje cells integrate inputs from cortex and PNS, and outputs back to cortex through thalamus

efferent copy theory

purkinje cells integrate input from cortex and PNS and output back to cortex

- cerebellum adjusts for differences between an intended movement and an actual movement

how does efference copy work

1. motor system sends an efference copy (forward model/sensory prediction) of the motor command to the cerebellum

2. this prediction is compared to sensory info coming from the spine

3. if there is a sensory discrepancy, there is a motor adjustment to the motor command

ataxia

unsteady, wide based gate

dysarthia

splurred speech

dysmetria

uncoordinated target reaches, over or undershooting

dysdiadochokinesia

uncoordinated rapid alternating movement