Motor Control 2 Flashcards
Overall movement direction is encoded by?
The integrated activity of all the neurones
What type of mechanisms control movement?
Feedback and feedforward
What role does feedback play in the control of movement?
A change in body position initiates rapid compensatory feedback messages from brainstem vestibular nuclei to spinal cord motor neurones to correct postural instability
What role does feedforward play in the control of movement?
Before movements begin, brainstem reticular formation nuclei (controlled by the cortex) initiate feedforward anticipatory adjustments to stabilize posture
What determines the symptoms of an UMN lesion?
Injury site
What does cortical damage cause?
Immediate flaccidity of contralateral muscles.
What is normal Babinksi sign?
Plantar flexion
What Babinskin sign would be seen in babies and those with cortical damage?
Plantar extension
What are the signs of UMN syndrome?
- Flaccidity
- Hypotonia
- Babinksi sign
- Spasticity
- Loss of fine finger movements
What spasticity can be seen in the UMN syndrome?
- Increased muscle tone
- Hyperactive stretch reflex
- Clonus oscillatory contract/relax muscles in response to stretch- due to removal of cortical suppressive influences
What do basal ganglia motor loops do?
Selects an initiates willed movements
Where does major subcorctical input to area 6 come from?
Ventral lateral nucleus in dorsal thalamus
What is input to area 6 known as?
VLo and arises from basal ganglia
What is the basal ganglia targets of?
Frontal, prefrontal and parietal cortex
What is the loop of information cycles?
Cortex through thalamus and basal ganglia back to SMA in cortex
What are the major components of the basal ganglia?
- Corpus striatum (striped body) - includes two principal nuclei the caudate and the putamen
- They are the input zone of the basal ganglia
Where does the corpus striatum receive input from?
All over the cortex
What are the corticostriatal pathways?
Multiple parallel pathways with different functions
What fires before limb/trunk movements?
Putamen
What fires before eye movements?
Caudate
What in the putamen and caudate receive excitatory (glutamatergic) cortical inputs on dendrites?
Medium spiny neurones
What do the medium spiny neurones have?
- They have large dendritic trees and integrate somatosensory, premotor and motor cortical inputs
- Each cortical axon contacts 1000s of spiny neurones so integrating the influence of 1000s of cortical cells
- Their axons are inhibitory (GABAergic) and project to globus pallidus and to substantia nigra pars reticulata
Motor Loop: Cortex to putamen
Excitatory
Motor Loop: putamen to globus pallidus
Inhibitory
Motor Loop: globus pallidus to VLo neurones
Inhibitory
Motor Loop: VLo to SMA
Excitatory
What is the functional consequence of cortical activation of the putamen?
Excitation
What are the globus pallidus neurones at rest?
At rest globus pallidus neurones are spontaneously active and inhibit VL
Why does cortical activation of the putamen boost cortical excitation?
- Cortical excitation excites the putamen
- This inhibits the inhibitory Globus pallidus which therefore
- Releases cells in VLo from inhibition so
- Activity in VLo boosts SMA activity
- Acts as a positive feedback loop focussing or funnelling activation of widespread cortical areas onto cortical SMA
When may the go signal for voluntary movement occur?
May occur when the SMA is boosted beyond a threshold level by activity coming through the basal ganglia funnel
What type of arrangement does the gating operation of the basal ganglia depend on?
A chain of neurones arranged in a disinhibitory circuit
How does cortical input flow through the basal ganglia?
By direct and indirect loops
How does cortical input flow by direct loop through the basal ganglia?
-Direct pathway acts as a positive feedback loop, a “GO” signal to the SMA in cortex
-It enhances the initiation of movements by the SMA
-Globus pallidus neurones are spontaneously active at rest so they tonically inhibit (restrain) VL thalamus
-Input from cortex releases this inhibition
(Direct pathway slects specific motor actions)
How does cortical input flow by indirect loop through the basal ganglia?
-Indirect pathway – antagonizes the direct route
-Striatum inhibits GPe (globus pallidus external) which then inhibits both GPi (GPinternal) and STN (subthalamic nuclei)
-Cortex excites STN; this excites Gpi; which inhibits thalamus
(Indirect pathway suppress competing/inappropriate action)
Give examples of basal ganglia disorders.
- Parkinson’s
- Huntington’s chorea
Who does Parkinson’s affect?
1% of the over 60s
What is Parkinson’s caused by?
Caused by degeneration of neurones in substrantia nigra (SN) and their dopaminergic (excitatory) inputs to the striatum
How can dopamine enhance cortical inputs?
Through the direct pathway
How can dopamine suppress cortical inputs?
Through the indirect pathway
What does the depletion of dopamine do?
The depletion of dopamine closes down activation of the focussed motor activities that funnel through thalamus to SMA
What is Parkinson’s characterised by?
Hypokinesia
What is hypokinesia?
- Slowness
- Difficult to make voluntary movements
- Increased muscle tone (rigidity)
- Tremors of hand and jaw
What is Huntington’s characterised by?
Characteristic chorea
Who does Huntington’s affect?
It is hereditary, rare 5-10/100,000, progressive and fatal
What does Huntington’s involve?
Involves hyperkinesia with dementia and personality disorders
What is Huntington’s caused by?
Caused by profound loss of caudate, putamen and globus pallidus so loss of the ongoing inhibitory effects of the basal ganglia
What is characteristic chorea?
Spontaneous uncontrolled rapid flicks and major movements with no purpose
How much of the brain volume is accounted for by cerebellum?
10% of brain volume but 50% of total CNS neurones
What do lesions to the cerebellum produce?
Uncoordinated inaccurate movements , ataxia, fail to touch nose with eyes shut (similar to alcohol which depresses cerebellar circuits)
What are part of the HUGE cortico-ponto cerebellar projection?
Layer 5, areas 4 & 6, somatosensory cortex
What does the cortico-ponto-cerebellar projection do?
Connects cortex, pontine nuclei and cerebellum - 20 mill axons, 20x more that CST
How is the cerebellum connected to the cortex?
Via the ventrolateral thalamus
What does the cerebellum instruct n terms of movement?
Direction, timing and force
Describe the motor loop through the lateral cerebellum.
Motor loop for voluntary movement through basal ganglia and VLo undergoes ongoing refinement via involvement of feedback loop through pons, cerebellum, thalamus and back to cortex
