Basal Ganglia, Cerebellum & Diseases of Motor System

Question 1

Describe the basal ganglia

Answer 1

no direct input/projections with SC

Input via cerebral cortex
Outputs via thalamic nuclei

encodes start/stop/direction/amplitude/expression of movement

Question 2

Describe the 4 subcortical nuclei of the basal ganglia

Answer 2

Striatum: caudate nucleus/putamen/nucleus accumbens (main input for BG)

Globes pallidus: internal (output nuclei sends GABA to thalamus) and external segment

Substantia nigra: pars reticulata (inhibitory output) and pars compacta (dopaminergic cells to striatum)

Subthalamic nucleus: BG connectivity, pacemaker defines sync/timing of BG outputs

BG: large grey matter masses control thalamus activity

Question 3

Describe basal ganglia circuitry

Answer 3

complex wiring ensures well integrated input from different brain regions, ensures output is regulated

thalamus activity managed via balance of excitatory/inhibitory (tonic) pathways

direct pathway facilitates movement
indirect pathway inhibits movement

Question 4

Describe the direct pathway

Answer 4

releases thalamus from tonic inhibition - voluntary movement
(inhibition of inhibitory GPi neurons)

+ve feedback loop

striatal neurons (D1 dopamine) receive input from dopaminergic neurons from SubNigra enhancing excitation

Question 5

Describe the indirect pathway

Answer 5

increases tonic inhibition of thalamus - suppress movement
(inhibition of inhibitory GPe neurons)

-ve feedback loop

striatal neurons (D2 dopamine) input from SubNigra counters excitatory input, weaker pathway

Question 6

What are the basal ganglia loops?

Answer 6

Motor: skeletomotor/oculomotor loops
Cognitive (prefrontal): higher cortical functions
Limbic: emotion

Question 7

Describe the cerebellum

Answer 7

posterior fossa (10% of brain, 75% of brain neurons); evaluates difference between intended movement/actual motor response subconsciously

maintains balance/coordinates voluntary movements/motor learning/cognition

Question 8

What are some cerebllar damage effects?

Answer 8

Reduced muscle tone
Impaired balance/motor learning
Disrupted spatial accuracy
Disrupted temporal coordination
Impaired cognition

Question 9

Describe the cerebellum anatomy

Answer 9

2 symmetrical cerebellar hemispheres with parallel convolutions (folia) each with 3 lobes separated by fissures

thin outer grey matter, internal white matter

3 grey matter masses (dentate/interposed/fastigal nuclei)

Question 10

Describe the cerebellar peduncles

Answer 10

Superior: somatosensory info to ant. lobe, afferent visual/auditory fibres to post. lobe, efferent from dentate to cerebral motor cortex

Middle: afferent carry voluntary movement info from motor cortex pons to post. lobe
no efferent

Inferior: afferent carry sensory info from vestibular nuclei in brainstem/muscle proprioreceptors via medulla to flocculonodular lobe
efferent carry info back and from vermis to inferior olivary nucleus

Question 11

Describe the 3 functional regions of the cerebellum

Answer 11

Flocculondular lobe: vestibulocerebellum afferent input to lateral vestibular nuclei (balance/eye movement control)

Vermis (mid regions - spine cerebellum): vermis receives sensory input to fastigal nucleus (gaze/posture), mid region receives somatosensory input from limbs to interposed nucleus (limb coordination)

Cerebrocerebellum: input from cerebral cortex via dentate nucleus to PF, M, PM cortices (mental performance of complex motor actions/distal limb coordination)

Question 12

Describe the microscopic layers of the cerebellum

Answer 12

Molecular layer: granule axons form parallel fibres, purkinje cells orient dendritic fields perpendicular to parallel fibres

Purkinje layer: receive input from climbing/parallel fibres, send GABA to deep nuclei (dentate) - inhibitory output

Granule layer: mossy fibres (main input) terminate here, contact granule cells/golgi neurons regulate cerebellar glomeruli (200 granule cells for each mossy fibres)

Question 13

Describe 2 input sources for the cerebellum

Answer 13

Mossy fibres (SC/brainstem nuclei): excitatory connections to granule cell dendrites - parallel fibres - purkinje cell action potential

Climbing fibres (inferior olivary nucleus): around purkinje neurons; depolarises them/complex spike

Question 14

Explain long-term depression

Answer 14

Climbing fibres reduce parallel fibre input to purkinje creating depression in synapses between them via calcium influx

Question 15

Explain myasthenia gravis

Answer 15

signal transmission failure at neuromuscular junction
autoimmune cause, congenital cause, inherited
reduced ACh receptors/muscle activation

abnormal muscle fatigue, remission, relapse

Question 16

Explain Guillain-Barré Syndrome

Answer 16

autoimmune disorder with rapid onset of muscle weakness where immune system damages peripheral nervous system (myelin sheaths of lower motor neuron axons)

treated with plasmapheresis/iv administration of immunoglobulins reducing antibodies effecr

Question 17

Describe features of lower/upper moptom neuron disease

Answer 17

Lower: muscle weakness/atrophy, decreased muscle tone/tendon reflexes, muscle twitch/fasciculations

Upper: muscle weakness (little atrophy/paralysis), increased tendon reflexes/muscle tone
Babinski reflex

Question 18

Which tests distinguish neurogenic/myopathic diseases?

Answer 18

Clinic observations: distribution of muscle weakness, muscle pain/twitch

Blood/urine for soluble enzyme (myoglobin)
Electromyography
Nerve conduction studies
Muscle biopsy
DNA studies

Question 19

What are some symptoms of cerebellar disorders?

Answer 19

Cerebellar hypotonia
Dysmetria
Ataxia
Dysdiadochokinesia
Scanning speech
Nystagmus
Intention tremor

Question 20

Describe Parkinson’s

Answer 20

hypokinetic disorder (dementia)

resting tremor, akinesia, stiffness, slow movement (bradykinesia)
Unstable posture/walking/balance

substantia nigra death:
less dopaminergic input, under active direct/overactive indirect pathway
increased GPi output suppressed thalamus/movement (treated with L-dopa)

Question 21

Explain ballistic (hyperkinetic basal ganglia disorder)

Answer 21

damage to STN reduces GPi excitation/thalamus inhibition
excessive motor cortex activation
Dyskinesia (ballistic)
treat with DA receptor antagonists

Question 22

Explain Huntington’s disease

Answer 22

inherited autosomal dominant disorder
striatal death (mainly indirect pathway(

Reduced GPe inhibition
Increased STN inhibition
GPi under activated
Thalamus under inhibited

Motor cortex over activated
Dyskinesia (chorea)
Treat with DA receptor antagonists