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 is the Babinski sign?
Stimulation of lateral plantar aspect of foot leads to extension of the big toe, there may also be fanning of the other toes
What is supposed to occur with stimulation of the lateral plantar aspect of the foot in adults?
Planter flexion
When does planter extension occur on stimulation to the lateral planter aspect of the foot?
In baby or with cortical damage
What are examples of things that damage to descending motor pathways causes?
Flaccidity of contralateral muscles
Babinski sign (tendon reflexes impacted)
Loss of fine finger movement
Spasticity (due to removal of cortical suppressive influences)
What does the basal ganglia motor loop do?
Selects and initiates willed movements
What does Vlo stand for?
Ventral lateral nucleus in dorsal thalamus
Where does major subcortical input to area 6 comes from?
Ventral lateral nucleus in dorsal thalamus
Where does input to Vlo come from?
Basal ganglia
Where does input to the basal ganglia come from?
Frontal, prefrontal and parietal cortex
Explain the process of the basal ganglia motor loop?
Cortex > thalamus and basal ganglia > back to the SMA in cortex (area 6)
What are some major components of the basal ganglia?
Corpus striatum (stripped body)
This includes two principal nuclei, the caudate and the putamen
What are the input zones for the basal ganglia, and where do they recieve input from?
Caudate and putamen
From all over the cortex
What kind of inputs does the medium spiny neurons of the putamen and caudate receive?
Excitatory (glutamatergic) cortical inputs on dendrites
they have large dendritic trees and integrate massive somatosensory, premotor and motor cortical inputs
Are the axons of the putamen and caudate excitatory or inhibitory?
Inhibitory (GABAergic)
Where does the putamen and caudate project to?
Globus pallidus and to substantia nigra pars reticulata
What does the putamen fire before?
Limb/trunk movements
What does the caudate fire before?
Eye movements
The motor loop is cortex > basal ganglia > cortex, which of these pathways are excitatory and which are inhibitory?
Cortex to putamen - excitatory
Putamen to globus pallidus - inhibitory
Globus pallidus to VLo neurons - inhibitory
VLo back to SMA - excitatory
this means functional consequence of cortical activation of putamen is excitation
Explain how cortical activation of putamen boosts cortical excitation?
At rest, globus pallidus neurons are spontaneously active and inhibit VLo
So cortical excitation:
1) Excites putamen which
2) Inhibits the inhibitory globus pallidus which
3) Releases cells in VLo from inhibition so
4) Activity in VLo boosts SMA activity
At rest, what state are neurons in the globus pallidus?
Spontaneously active and inhibit VLo
What kind of loop does the motor loop of the basal ganglia act as?
Positive feedback loop funnelling activation of widespread cortical areas back onto cortical SMA
When does the “go” signal for voluntary movement occur?
When the SMA is boosted beyong threshold level by activity coming through the basal ganglia funnel
What kinds of loops do cortical inputs flow as through basal ganglia (2 different kinds)?
Direct pathway
Indirect pathway
What is the function of direct pathway through basal ganglia?
Acts as positive feedback loop causes a “go” signal to the SMA in cortex
Enhances the initiation of movements by the SMA
What is the function of the indirect pathway through the basal ganglia?
Antagonises the direct root
Inhibits the thalamus, suppreses competing/inappropriate action
What is the process of the indirect pathway through the basal ganglia?
1) Striatum inhibits GPe (globus pallidus external)
2) This inhibits both GPi (GP internal) and STN (subthalamic nuclei)
3) Cortex excites STN, this excites GPi which inhibits thalamus
What are examples of basal ganglia disorders?
Parkinson disease (PD)
Huntington disease
What does GPe stand for?
Globus pallidus external
What does GPi stand for?
Globus pallidus internal
What does STN stand for?
Subthalamic nuclei
What causes parkinson disease?
Degeneration of neurons in substantia nigra (SN) and their dopaminergic (excitatory) inputs to the striatum
Depletion of dopamine closes down the activation of the focussed motor activites that funnel through thalamus to SMA
What does SN stand for?
Substantia nigra
Are dopaminergic inputs excitatory or inhibitory?
Excitatory
Are GABAergenic inputs excitatory or inhibitory?
Inhibitory
How does dopamine impact the direct and indirect pathways through the basal ganglia?
Enhances cortical inputs through direct pathway
Suppresses inputes through the indirect pathway
What is a major symptom of PD?
Hypokinesia
What does hypokinesia describe?
Slowness
Difficult to make voluntary movements
Increased muscle tone (rigidity)
Tremors of hand and jaw
What is Huntington’s disease?
Hyperkinesia with dementia and personality disorders
Progressive and fatal
Hereditary
What is the prevalence of Huntington’s disease?
5-10/100,000
What is Huntington’s disease caused by?
Profound loss of caudate, putamen and globus pallidus
So loss of ongoing inhibitory effects of basal ganglia
Huntington’s disease shows characteristic chorea, what does this describe?
Spontaneous, uncontrolled, rapid flicks and major movements with no purpose
What percent of brain volume is the cerebellum?
10%
What percentage of CNS neurons are in the cerebellum?
50%
What do lesions to the cerebellum cause?
Uncoordinated inaccurate movements:
ataxia
fail to touch nose with eyes shut
similar to alcohol which represses cerebellar circuits
What is ataxia?
A term for a group of disorders that affect co-ordination, balance and speech
What connects the cortex, pontine nuclei and cerebellum?
Cortico-ponto-cerebellar projection
What parts of the cortex are part of the cortico-ponto-cerebellar projection?
Layer 5
Areas 4 and 6
Somatosensory cortex
What does information travel from the cerebellum back to the cortex via?
Ventrolateral thalamus
What information does the cerebellum give the cortex?
Instructs direction, timing and force required to coordinate movement
What does the motor loop for voluntary movement through basal ganglia and VLo undergo ongoing refinement from?
Involvement of feedback loop through pons, cerebellum, thalamus and back to cortex
