Motor control - 3 Flashcards
Describe directional tuning in primary motor cortex
Each neuron has preferred direction but responses of all neurons combine to produce popular vector - commands to perform precise movements are encoded in integrated activity of neurons
What does an actual change in body position cause?
Initiates rapid compensatory feedback massages from brainstem vestibular nuclei to spinal motor neurons to correct postural instability
What is the feedforward mechanism which control movement?
Before movement begins, the brainstem reticular formation nuclei initiates feedforward anticipatory adjustments to stabilise posture
Where does the loop of information cycle?
Cortex to thalamus and basal ganglia to supplementary motor cortex
Where does major subcortical input to area 6 come from?
Ventral lateral nucleus in dorsal thalamus (VLo)
Where does input from VLo come from?
Basal ganglia
Where are basal ganglia targets of?
Frontal, prefrontal and parietal cortex
What are the major components of basal ganglia?
Corpus striatum - includes 2 principal nuclei
Caudate and Putamen
What are the caudate and putamen?
Are input zone of the basal ganglia
Where does corpus striatum receive inputs from?
All over cortex
Corticostriatal pathway - multiple parallel pathways with different functions
What do medium spiny neurons in putamen and caudate receive?
Excitatory cortical inputs on dendrites
What do large dendrite trees integrate?
Massive somatosensory, premotor and motor cortical inputs
Describe the axons of the basal ganglia
Are inhibitory
Contacts 1000s of spiny neurons so integrating influence of 1000 cortical cells
Axons project to globus pallidus and SN pars reticulata
When do putamen and caudate fire?
Putamen - before limb/trunk movements
Caudate - before eye movements
Both predict movements
What type of pathway if from cortex to putamen?
Excitatory
What type of pathway is between putamen to globus pallidus?
Inhibitory
What type of pathway is between globus pallidus to VLo neurons?
Inhibitory
What type of pathway is between VLo and back to SMA?
Excitatory
What is the functional consequence of cortical activation of putamen?
Excitation
At rest, what are the globus pallidus neurons?
Spontaneously active and inhibit VLo
Explain cortical excitation
Excites putamen which inhibits globus pallidus - reduces inhibition of VLo cells - activity in VLo boosts SMA activity
What does gating by basal ganglia depend on?
Dis-inhibitory arrangement
What are the upper motor neurons in SMA like at rest?
Not excited as little cortical input
What are the upper motor neurons in SMA like when excited?
Lots of cortical input into globus pallidus which is inhibited so this dis-inhibits the thalamus
Excites upper motor neurons in SMA
What is the function of the direct pathway in cortical input flow?
Selects specific motor actions
What is the function of the indirect pathway in cortical input flow?
Suppresses other/ inappropriate actions
Describe the direct pathway of cortical input flow
Positive feedback loop
Enhances initiation of movements by SMA
Input from cortex releases inhibition on VLo
Describe indirect pathways of cortical input flow
Antagonises the direct route
Cortex excites subthalamic nuclei which excites GP internal which inhibits thalamus
Describe Parkinson’s disease
Hypokinesia - slowness, difficult to make voluntary movements, increased muscle tone (rigidity) and tremors in hand or jaw
What is the cause of Parkinson’s disease?
Degeneration of neurons in substantia nigra so loss of their dopaminergic excitatory inputs to striatum
What is the function of dopamine?
Enhances cortical inputs through the direct pathway ad suppress inputs through indirect pathway
What does depletion of dopamine cause?
Parkinson’s - closes down focussed motor activities that funnel through thalamus to SMA
Describe Huntington’s disease
Hyperkinesia with dementia and personality disorders
Is hereditary, progressive and rare
Chorea - spontaneous uncontrolled rapid flicks and major movements of no purpose
What is the cause of Huntington’s disease?
Profound loss of caudate, putamen and globus pallidus
So loss of ongoing inhibition by basal ganglia
How many CNS neurons does the cerebellum have?
50%
What is ataxia?
Uncoordinated inaccurate movements
Can be due to lesions to cerebellum
What structures are a part of cortico-ponto-cerebellar projection?
M1 layer 5, areas 4 and 6 and somatosensory cortex
20 mill axons
What connects the cerebellum to cortex?
Ventrolateral thalamus
What is the function of the cerebellum?
Instructs direction, timing and force
Describe the motor loop through lateral cerebellum
Motor loop of voluntary movement through basal ganglia and VLo undergoes ongoing refinement
Via involvement of feedback loop through pons, cerebellum and thalamus then back to cortex
Describe Brain machine interfaces
BMI - allow patients to voluntary control prosthetic limbs and walk again after spinal cord injuries
Sensory properties in prosthetics
Describe brain brain interfaces
BBI - brain signals from one animal to teach another animal a task it has never seen
