basal ganglia and cerebellum

Question 1

Basal ganglia and cerebellum

Answer 1

Control of movement - but no direct descending projections to spinal cord
- Modify motor performance by feedback to motor cortex
- Receive “copy” of motor command going to motor neurons
- Cortex gives rise to motor output thru corticospinal and corticobulbar fibres
- Processed before returning to cortical origin
- Components of neuronal circuits - activity constantly modified

Question 2

Basal ganglia

Answer 2

Main structures:
Striatum or caudate/putamen (C/P)
- Cortex projects to C/P via glutamate
Globus pallidus
- External division: GPe
- Internal division: GPi
- Output of basal ganglia is GPi
- Inhibitory (GABA) to thalamus (VL and VA, mainly VA)
- Thalamus back to cortex is excitatory (glutamate)
Substantia Nigra (SN)
Subthalamic nucleus (ST)

Question 3

Pathways between C/P and GPi

Answer 3

Direct pathway: On top, single inhibitory neuron
- Stimulates movement
Indirect: on bottom, 2 inhibitory neurons, impinging on excitatory neuron in subthalamic nucleus
- Reduces movement

• Act upon output cell in GPi
  
  • Output cell (inhibits thalamus) is under 2 opposing influences
    If activity of output cell is decreased:
    (by decreasing drive from ST)
    (or incr drive from direct pathway)
• Thalamus disinhibited
• Incr activity to cortex (more movement)

Dopaminergic projection from SN:
- Stimulates direct pathway
- Inhibits indirect pathway
- Loss of dopamine: Parkinson’s
- Excessive movements in Huntington’s (degen of indirect pathway, damage to ST)

Question 4

Cerebellum

Answer 4

Regulates motor ouput of cortex via projection from dentate nucleus -> thalamus -> back to cerebral cortex -> corticospinal tract
- Copy of info reached cerebellum thru corticopontine fibres, pontine nuclei, middle cerebellar peduncle
- Movement detected by proprioceptors -> spinocerebellar tract -> inferior cerebellar peduncle -> cerebellum
- Compares command to performance (feedback)
- Result leaves via dentate nucleus and superior cerebellar peduncle
- Destination: thalamus, mainly VL, then back to cerebral cortex
- Minor projection to red nucleus (does not influence cerebral cortex)
- Mods motor function by minor projection to spinal cord and reticular formation