Motor Control II Flashcards
Does each neurone have a preffered direction?
Each neurone has a preferred direction but the responses of all neurons are combined to produce a population vector so commands to perform precise movements are encoded in the integrated activity of large populations of neurones in M1
What does a change in body position lead to?
A change in body position initiates rapid compensatory feedback messages from brainstem vestibular nuclei to spinal cord motor neurones to correct postural instability
What orrucs before a movement in anticipation?
In addition before movements begin, brainstem reticular formation nuclei (controlled by the cortex) initiate feedforward anticipatory adjustments to stabilize posture
Damage to descending motor pathways causes upper motor neuron syndrome which causes cortical damage causes immediate flaccidity of contralateral muscles
initial hypotonia : “spinal shock” - spinal circuits are deprived of cortical input
but what happens a few days later?
days later spinal motor reflexes re-emerge in a consistent pattern – as spared connections strengthen and new connections sprout
WHat happens to tendon reflexes in upper motor neuron syndrome?
Spasticity - increased muscle tone, hyperactive stretch reflex, clonus oscillatory contract/relax muscles in response to stretch - due to removal of cortical suppressive (inhibitory) influences
What is the loop of information from the cortex to the SMA?
Major subcortical input to area 6 comes from ventral lateral nucleus in dorsal thalamus (Vlo)
Input to VLo comes from basal ganglia
Basal ganglia are targets of frontal, prefrontal and parietal cortex
So a loop of information cycles - from the cortex through thalamus and basal ganglia and back to the SMA in cortex (area 6)
One major component of the basal ganglia is the corpus striatum (striped body), what are the 2 principal nuclei that make it up?
caudate
putamen
What are the input zones of the basal ganglia?
caudate
putamen
That caudate and putamen are _____ zones for _____
That caudate and putamen are modulation zones for movement
WHat are the neurones found in the putamen and caudate?
medium spiny neurones
What do the medium spicy neurons in the caudate and putamen do?
receive excitatory (glutamatergic) cortical inputs on dendrites
have large dendritic trees and integrate massive somatosensory, premotor and motor cortical inputs
their axons are inhibitory (GABAergic) and project to globus pallidus and to substantia nigra pars reticulata
the putamen fires before limb/trunk movements
the caudate fires before eye movements
both are predictive of movements
Describe the motor loop: Cortex - Basal ganglia - Cortex
Cortex to putamen – is an excitatory pathway
Putamen to Globus pallidus – is inhibitory
Globus pallidus to VLo neurones – is inhibitory
VLo back to SMA – is excitatory
(vlo = ventral laternal nucleus)
What are the globus pallidus neurones doing at rest?
At rest globus pallidus neurones are spontaneously active and inhibit VLo
How do cortical excitation lead to the VLo not being inhibited by the globus pallidus?
- excites putamen which
- inhibits the inhibitory Globus pallidus which therefore
- releases cells in VLo from inhibition so
- activity in VLo boosts SMA activity
This acts as a positive feedback loop focussing or funnelling activation of widespread cortical areas back onto cortical SMA
The gating operation of the basal ganglia depends on what type of arrangement?
disinhibitory arrangement
This shows the disinhibitory circuit in the motor loop
A is at rest – there is little cortical input so upper motor neurones in SMA (D) are NOT excited
A is excited – there is a lot of cortical input to B (the globus pallidus) which is inhibited
C (the thalamus) therefore is disinhibited and so excites D – the upper motor neurones in SMA
What part of the motor loop at rest is causing inhabition of the VLo and therefore the SMA?
globus pallidus
What inhibits the globus pallidus?
cortical excitation to the putamen which then inhibits the globus pallidus
What happens after the globus pallidus is inhibited?
allows the VLo to be excited to then pass signals and exctie the SMA
Cortical input flows through the basal ganglia through a direct pathway but what is the other pathway?
An indirect pathway
What is the direct pathway of cortical flow 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
And inputs from the cortex releases this inhibition
What is the indirect pathway of cortical flow through the basal ganglia?
Indirect pathway – antagonizes the direct route
Striatum inhibits GPe (should be reed arrow in diagram)(globus pallidus external) which then inhibits both GPi (GP internal) and STN (subthalamic nuclei)
Cortex excites STN; this excites Gpi; which inhibits thalamus
What is the function of the indirect pathway?
helps to prevent unwanted muscle contractions from competing with voluntary movements. It operates in conjunction with the direct pathway
What is the function of the direct and indirect pathways?
- Direct pathway selects specific motor actions
- Indirect pathway suppresses competing/inappropriate action
name a basal ganglia disorder?
parkinsons
Parkinsons causes hypokinesia, what is it?
slowness, difficult to make voluntary movements, increased muscle tone (rigidity), tremors of hand and jaw
What is the cause of hypokinesia in parkinsons?
caused by degeneration of neurones in substrantia nigra (SN) and their dopaminergic (excitatory) inputs to the striatum
dopamine can enhance cortical inputs through the “direct” pathway and suppress inputs through “indirect” pathway (there to supress non-essential functions)
the depletion of dopamine closes down activation of the focussed motor activities that funnel through thalamus to SMA
NHS definition of parkinsons
Parkinson’s disease is caused by a loss of nerve cells in the part of the brain called the substantia nigra.
Nerve cells in this part of the brain are responsible for producing a chemical called dopamine.
Dopamine acts as a messenger between the parts of the brain and nervous system that help control and co-ordinate body movements.
If these nerve cells die or become damaged, the amount of dopamine in the brain is reduced.
This means the part of the brain controlling movement cannot work as well as normal, causing movements to become slow and abnormal.
The loss of nerve cells is a slow process. The symptoms of Parkinson’s disease usually only start to develop when around 80% of the nerve cells in the substantia nigra have been lost.
What is the nae of a second basal ganglia disorder?
Huntingtons disease
What are the symptoms of huntingtons disease?
involves hyperkinesia with dementia and personality disorders
Is huntingtons disease hereditary?
yes
In huntingtons you get characteristic chorea, what is it?
spontaneous, uncontrolled, rapid flicks and major movements with no purpose
What causes characteristic chorea in huntingtons disease?
caused by profound loss of caudate, putamen and globus pallidus, so loss of the ongoing inhibitory effects of the basal ganglia
What is the definition of huntingtons disease?
Huntington’s disease is a condition that stops parts of the brain working properly over time. It’s passed on (inherited) from a person’s parents.
a progressive brain disorder that causes uncontrolled movements, emotional problems, and loss of thinking ability (cognition). Adult-onset Huntington disease, the most common form of this disorder, usually appears in a person’s thirties or forties
Does the cerebellum have a lot of neurons?
Cerebellum 10% of brain volume but 50% of total CNS neurones
What is the cerebellum involved in?
movement
WHat do lesions on the cerebellum cause?
uncoordinated inaccurate movements
ataxia
fail to touch nose with eyes shut
similar to alcohol which depresses cerebellar circuits
Wht is the cerebellum in connection with
connects cortex, pontine nuclei and cerebellum
How does the cerebellum relay information back to the cortex?
cerebellum back to cortex via ventrolateral thalamus
What does the cerebellum instruct?
instructs direction, timing and force
What makes up motor learning in the cerebellum?
based on predictions
calculations
experience (too fast for feedback control directly) – compares what is intended with what happened and compensates
This photo is showing both the pathways together
