what is the general function of the cerebellum
motor control and coordination
label this image of the cerebellum
describe the functions of the 3 main parts of the cerebellum
1- VERMIS: coordinates trunk and leg movements
2- LATERAL HEMISPHERES: quick and finely coordinated movements, esp of arms, of ipsilateral side
3- FLOCCULONODULAR LOBE: maintains balance, and coordinates eye, head and neck movements
a patient with a cerebellum lesion presents with right sided limb ataxia. where is the lesion likely located?
right lateral hemisphere
a patient with a cerebellum lesion presents with truncal ataxia and abnormal gait. where is the lesion likely located?
which brain lobes is the cerebellum in close contact with, and how is it separated from these?
- parietal and occipital lobes
- separated by tentorium cerebelli
via which structures does the cerebellum communicate with the cortex and brainstem?
Communicates with cortex via thalamus.
Communicates with brainstem via cerebellar peduncles (CPs):
- superior CPs: cerebellum to midbrain
- middle CPs: cerebellum to pons
- inferior CPs: cerebellum to medulla
why can cerebellar pathology sometimes cause hydrocephalus
- Cerebellum is separated from pons by 4th ventricle.
- So extension of midline cerebellar lesion can cause compression of 4th ventricle and thus hydrocephalus.
name possible causes of cerebellar pathology
- Posterior fossa tumour
- multiple Sclerosis
- Inherited (e.g. Friedreich's ataxia)
- Epilepsy medication (carbamzepine, phenytoin toxicity)
describe the symptoms associated with cerebellar pathology
- Dysdiadochokinesis (inability to perform rapid alternating movements)
- Ataxia: gait (reeling and broad-based), truncal (can't sit or stand unsupported) and limb (ipsilateral lack of voluntary movement coordination)
- Intention tremor
- Slurred speech
label the basal ganglia
what are the basal ganglia
Group of subcortical nuclei that are part of the motor system.
Communicate with cortex and cerebellum to aid in the initiation of movement.
Include: caudate nucleus, putamen, globus pallidus (interna and externa), subthalamic nucleus and substantia nigra (pars reticularis and pars compacta)
describe the composition of the substantia nigra
Made up of:
- pars reticulatis ventrally
- pars compacta dorsally - pigmented dopaminergic neurones
In the normal brain, what is the effect of the 2 pathways from the putamen on the cortex? How does the activity of the substantia nigra pars compacta change this?
DIRECT PATHWAY (excitatory): putamen releases GABA which inhibits GPi and SNpr... prevents the inhibitory effects of GPi and SNpr on the thalamus (via GABA)... allows thalamus to activate cortex via glutamate release
INDIRECT PATHWAY (inhibitory): putamen releases GABA which inhibits GPe... prevents inhibitory effect of GPe on STN (via GABA)... allows STN to activate GPi and SNr via glutamate release... so GPi and SNr are able to inhibit thalamus via GABA release... thalamus unable to activate cortex
Dopamine released by the SNpc acts on putamen to enhance cortex activation:
- stimulates direct pathway (more movement) - D1 Rs
- inhibits indirect pathway (allows movement) - D2 Rs
what is the pathophysiology of Parkinson's disease?
Degeneration of dopaminergic neurones in substantia nigra... loss of putamen-thalamus-cortex indirect pathway inhibition and direct pathway enhancement... decreased stimulation of motor cortex by thalamus... defects in mov. initiation and fine-tuning.
Name the 4 cardinal symptoms of Parkinson's disease. What other symptoms might a P present with?
1. RESTING TREMOR - "pill-rolling"
2. HYPERTONIA - rigidity, e.g. flexed posture when walking
3. BRADYKINESIA - slow mov., inc. festinating gait
4. POSTURAL INSTABILITY
Ps can also present with hypophonia, reduced facial expression, micrographia, sleep disturbance, depression and dementia.
describe the most common drug treatment for Parkinson's
LEVODOPA: dopamine precursor able to cross BBB (converted to dopamine by dopa decarboxylase within remaining nigrostriatal neurones).
Must be administed with CARBIDOPA (cannot cross BBB) to prevent metabolism of levodopa by peripheral dopa decarboxylase.
Explain the pathophysiology of Huntington's disease.
i. CAG trinucleotide repeat expansion (36+) in HTT gene on chromo. 4...
ii. Huntingtin protein aggregates in putamen and caudate nucleus (striatum) causing neuronal cell death (excitotoxicity?)...
iii. Loss of movement inhibition via indirect putamen-thalamus-cortex pathway, and cognitive decline.
what is hemiballismus and how might a P present?
Subthalamic nucleus damage (e.g. stroke, non-ketotic hyperglycaemia, etc.)... closs of mov. inhibition via indirect putamen-thalamus-cortex pathway... contralateral unilateral movement abnormalities.
Symptoms: chorea and incoordination.