How many neurons does the cerebellum have?
- about as many as the rest of the brain due to lots of tiny inhibitory neurons
- optimisation structure - optimisation of movement
What are the basal ganglia?
- a collection of nuclei mainly in the forebrain but also in the brain stem and near the thalamus
- way of regulating cortical activity including movement
- subcortical grey matter
Do the cerebellum and basal ganglia directly influence the motor neurons?
- no
- their influence is on the motor cortex (basal ganglia) or cortex and brain stem (cerebellum)
What are the functions of the basal ganglia in regards to movement?
- allow the selection of complex patterns of voluntary movements (multiple muscles in a particular sequence; selects sub-routines and organises them together to make an appropriate movement)
- evaluate the success of actions in achieving the goals of those actions (e.g. becoming better at scales on the piano)
- initiating movements (damaged in parkinson’s disease, they know what they want to do but they can’t get the movement going)
What is a ganglia?
- collection of neurons in the periphery
What is a nucleus?
- a collection of neurons in the CNS
What is a ganglia in the CNS?
an interconnected lot of nuclei
What are the nuclei that make up the basal ganglia?
- caudate - C shaped tail that follow the lateral ventricles
- putamen - same type of tissue structure as caudate - both are input structures to the basal ganglia, outer segment of the globus pallidus/putamen triad , in the telencephalon
- globus pallidus, external and internal segments
- subthalamic nuclei
- substantia nigra pars compacta, substantia nigra pars reticulata (same sort of tissue as globus pallidus) (in the brainstem) (dopamine secreting cells); black in colour due to melanin
What is the anatomical organisation of the inputs to the basal ganglia?
- caudate and putamen are the input regions of the basal ganglia, which in terms of movement, get their input from the motor cortex regions, also get input from the substantia nigra (dopaminergic cells/projections come up from the brain stem)
- the basal ganglia receive inputs from most of the cortex except the primary auditory cortex and the primary visual cortex
What is the functional organisation of intrinsic circuitry and outputs of basal ganglia?
- intrinsic connections from caudate/putamen to globus pallidus
- some of those go to the thalamic nuclei and back to the globus pallidus
- output is through the thalamus and back to the cortex
- like a sort of loop - tripartite loop (cortex, basal ganglia, thalamus, cortex)
- millions of fibres doing this loop
How do we get disinhibition in the direct pathway through the basal ganglia?
- substantia nigra pars compacta and cerebral cortex both excite the caudate/putamen transiently
- caudate/putamen transiently inhibits the globus pallidus, internal segment; the globus pallidus is now v. quiet
- the globus pallidus normally tonically inhibits VA/VL complex of thalamus - disinhibited
- thalamic activity increases because released from inhibition and therefore excites that frontal cortex transiently
- this pathway represents the movements you have selected
How do we get disinhibition in the indirect pathway in the basal ganglia?
- input from cerebral cortex to caudate/putamen
- makes them better at inhibiting globus pallidus external segment
- normally inhibits the subthalamic nucleus
- released - excites the globus pallidus Internal -> therefore good at inhibiting the thalamus again
- indirect pathways suppress aspects of movement - likely suppressing similar movements to the one we are trying to complete to make it more accurate
What is the centre-surround functional organisation of the direct and indirect pathways?
- direct pathway is selected out some intended motor pathways
- indirect pathway is inhibiting the similar movements to sharpen the movement
What is the action of dopamine in the basal ganglia pathways?
- modulator of excitatory neurons
- all the neurons in the direct pathway get excited by dopamine (D1 receptor)
- people with parkinson’s can’t produce dopamine therefore
- indirect pathway neurons have D2 receptors which means they get inhibited by dopamine
- dopamine makes movement happen
- dopamine spreads its influence over huge areas
- almost like hormones instead of neurotransmitters
If you cut through the brainstem of someone with parkinson’s disease, what would you see?
- a lack of the black crescents normally seen in the brainstem - the substantia nigra - due to lack of ability to produce dopamine
- black would be due to melanin, a chemical in the pathway
- not clinically noticeable until late in life
- cause largely unknown
- associated with a tremor
How does huntington’s disease affect the brain?
- loss of caudate/putamen
- specific genetic disease
- sudden spontaneous unplanned movements
- ongoing suppression becomes less effective
What are the functions of the cerebellum?
- coordinating the timing and sequence of muscle actions and movements
- the maintenance of muscle tone
- motor learning
- planning sequences of muscle activation for complex movements
- significant role in motor control
- optimiser of timing
- part of getting better at movements
- stringing movements together, getting better at sequencing
What is the overall organisation and subdivisions of the cerebellum?
- cortex on the outside
- tightly folded
- lots of white matter underneath
- subcortical grey matter
- folded up sheets - shapes that look like teeth
- i.e. deep cerebellar nuclear
What is the functional organisation of the inputs to the cerebellum?
- inputs come from cortex (presumably planned movements) and sensory info from spinal cord (what did happen)
- lots of muscle proprioceptors (direct and indirect)
- vestibular
- how can we make them match up
How do we test for cerebellar dysfunction?
- e.g. tracing a square
- lose the precision of the movement
What are the major outputs of the cerebellum?
- output goes to deep nuclei and back to the cerebral cortex (via thalamus) or brainstem nuclei
- descending motor pathways of the brainstem
- can adjust assets of the motor plan going down the spinal cord to try and correct errors
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