Brain control of Movement - Motor control 2 Flashcards
Where does directional tuning occur?
Primary motor cortex (area 4)
What can be said about the neurons in M1 and the direction of movement they control?
Each neuron has a preferred direction but the responses of all neurons are combined to produce a population vector
What kind of loop mechanisms control movement?
Both feedback and feedforward mechanisms control movement
What are examples of feedback mechanisms in movement?
Change in body position initiates rapid compensatory feedback messages from brainstem vestibular nuclei to spinal cord motor neurons to correct postural instability
In addition, before movement begins brainstem reticular formation nuclei (controlled by the cortex) initiate feedforward anticipatory adjustements to stabilise posture
In damage to descending motor pathways, what does the injury site determine?
The symptoms
In terms of movement, what does cortical damage cause?
Immediate flaccidity of contralateral muscles:
initial hypotonia due to “spinal shock” where spinal circuits are deprived of cortical input
days later spinal motor reflexes re-emerge in a consistent pattern as spared connections strengthen and new connections sprout
what does actual change in body position initiate?
initiates rapid compensatory feedback messages from brainstem vestibular nuclei to spinal cord motor neurones to correct postural instability
what do brainstem reticular formation nuclei do before movements are carried out?
initiate feedforward anticipatory adjustments to stabilize posture in preparation
what is evidence for anticipatory maintenance of body posture?
Hear sound - pull on handle - contracts biceps. BUT - gastrocnemius muscle in leg contracts first to ensure stability
how can a Cat strike with a front paw and not fall over?
due to anticipatory feedforward adjustments
- Contralateral front & ipsilateral back legs contract, shift weight to diagonal distribution
where does major subcortical input to area 6 come from?
ventral lateral nucleus in dorsal thalamus (VLo)
where does Input to VLo?
Input to VLo comes from basal ganglia
what are basal ganglia?
Basal ganglia are targets of frontal, prefrontal and parietal cortex
describe the course for basal ganglia motor loop?
Cortex 🡪 thalamus & basal ganglia🡪
SMA (cortex area 6)
what does basal ganglia motor loop initiate?
willed movements
what are the major components of the basal ganglia?
Corpus striatum (“striped body”)
what does the Corpus striatum (“striped body”) include?
includes two principal nuclei the caudate and the putamen
what are the caudate and the putamen of corpus striatum known for?
input zone of the basal ganglia
where does corpus striatum receive input from?
corticostriatal pathway - multiple parallel pathways with different functions
what is the appearance of neurones in putamen and caudate?
Medium spiny neurones
what do Medium spiny neurones in putamen and caudate receive?
receive excitatory (glutamatergic) cortical inputs on dendrites
describe neurones & circuits of basal ganglia?
Large dendritic trees and integrate massive somatosensory, premotor and motor cortical inputs
Each cortical axon contacts 1000s of spiny neurones, so integrating influence of 1000s of cortical cells
describe the axons of the neurons of basal ganglia?
Axons are inhibitory (GABAergic). Project to globus pallidus and substantia nigra pars reticulata
where do axons of the basal ganglia fire to?
Project to globus pallidus and substantia nigra pars reticulata
when does putamen fire?
before limb/trunk movements
when does caudate fire?
before eye movements
what do both putamen and caudate do?
predict movement
what is the motor loop?
Cortex → Basal ganglia → Cortex
how would you describe the type of pathway between cortex to putamen?
excitatory pathway
how would you describe the type of pathway between putamen to globus pallidus?
inhibitory pathway
describe the type of pathway between Globus pallidus to VLo neurones?
inhibitory pathway
describe the type of pathway between VLo back to SMA?
excitatory pathway
what is the functional consequence of cortical activation of putamen?
excitation
what do globus pallidus neurones do at rest?
At rest globus pallidus neurones are spontaneously active and inhibit VLo
describe cortical excitation?
Excites putamen, which
Inhibits (inhibitory) globus pallidus, which therefore
Reduces inhibition of VLo cells, so
Activity in VLo boosts SMA activity
Positive feedback loop for focussing or funnelling activation of widespread cortical areas back onto cortical SMA
what does ‘Gating’ by basal ganglia depend on?
disinhibitory arrangement
describe the direct pathway through basal ganglia?
positive feedback loop, a “GO” signal to the SMA in cortex
Enhances initiation of movements by the SMA
Globus pallidus neurones are spontaneously active at rest, so tonically inhibit (restrain) VL thalamus
input from the cortex releases this inhibition
describe the indirect pathway through basal ganglia?
Striatum inhibits GPe (globus pallidus, external) which then inhibits both GPi (GP, internal) & STN (subthalamic nuclei)
Cortex excites STN; this excites Gpi; which inhibits thalamus
action of direct pathway?
Direct pathway - selects specific motor actions
action indirect pathway?
Indirect pathway - suppresses other/inappropriate action
what are some disorders of the basal ganglia?
Parkinson’s disease
who does parkinsons disease affect?
affects 1% over 60 years
what is hypokinesia?
slowness, difficult to make voluntary movements, increased muscle tone (rigidity), tremors of hand and jaw.
what is the cause of parkinsons disease?
degeneration of neurones in substantia nigra (SN), so loss of their dopaminergic (excitatory) inputs to the striatum
what is the function of dopamine?
(normally) enhances cortical inputs through the “direct” pathway and suppress inputs through “indirect” pathway
what results from depletion of dopamine?
(Parkinson’s) closes down focussed motor activities that funnel through thalamus to SMA
what is huntingtons disease?
Hyperkinesia with dementia and personality disorders
is huntingtons disease hereditary?
rare (5-10/100,000) progressive and fatal
what is a characteristic symptoms of huntingtons disease?
Characteristic chorea – spontaneous, uncontrolled, rapid flicks and major movements with no purpose
what is the cause of huntingtons disease?
Cause - profound loss of caudate, putamen and globus pallidus
i.e. loss of ongoing (tonic) inhibition by basal ganglia
how much of brain volume does the cerebellum take up?
only 10% of brain volume, but
- 50% of total CNS neurones
what is the involvement of cerebellum in movement?
‘Commanding’ a contraction is not enough - skilled movements need detailed sequence of timed contractions of varied intensity and duration
what do lesions in cerebellum result in?
uncoordinated, inaccurate movements = ataxia:
- e.g. fail to touch nose with eyes shut (similar to alcohol which depresses cerebellar circuits)
what are M1 Layer 5, areas 4 & 6, somatosensory cortex all are part of?
HUGE cortico-ponto-cerebellar projection (connects cortex, pontine nuclei & cerebellum) - 20 mill axons (20x more than CST)
what does cerebellum instruct?
cerebellum instructs direction, timing and force
describe the motor loop through lateral cerebellum?
what are Brain-machine interfaces (BMI)?
BMI - allow patients to voluntarily control prosthetic limbs and walk again following spinal cord injuries
Sensory properties are being built into prosthetic limbs, so patients can get sensory feedback when their prosthetic limb makes contact with objects or people.
what are brain-brain interfaces (BBI)?
BBI - use brain signals from one animal to teach another animal a task it has never seen.
