Cerebellum and basal ganglia
Cerebellum and basal ganglia: cerebellar signs, Parkinson disease, chorea and tremor
The cerebellum makes movement smooth and accurate on the same side; the basal ganglia select wanted movements, with dopamine tipping the balance. Covers Purkinje circuitry, cerebellar signs, direct and indirect pathways, Parkinson drugs, Huntington, hemiballismus and tremor types.
The core idea
Neither the cerebellum nor the basal ganglia start movement or connect directly to lower motor neurones. Both loop back to the motor cortex through the thalamus. The cerebellum compares intended with actual movement and corrects errors; the basal ganglia release wanted movement programmes and suppress unwanted ones.
Cerebellum: functional divisions
| Division | Parts | Connections | Function | Lesion |
|---|---|---|---|---|
| Vestibulocerebellum | Flocculonodular lobe | Vestibular nuclei | Balance, eye movements | Truncal ataxia, nystagmus |
| Spinocerebellum | Vermis and paravermis | Spinal cord (proprioception) | Posture, tone, gait | Gait ataxia; alcohol damages the anterior vermis |
| Cerebrocerebellum | Lateral hemispheres | Cortex via pons; output via dentate | Planning and timing of skilled limb movement | Limb ataxia, intention tremor, dysdiadochokinesia |
Cerebellar circuitry
- Three layers: molecular (outer), Purkinje cell layer, granular (inner). Five neurone types: Purkinje, granule, basket, stellate and Golgi cells. Granule cells are the only excitatory neurones (glutamate); the rest are inhibitory.
- The only output of the cerebellar cortex is the Purkinje cell, which is inhibitory (GABA) onto the deep nuclei (dentate, emboliform, globose, fastigial) and vestibular nuclei. Purkinje cells are very vulnerable to hypoxia, alcohol and heat.
- Inputs: mossy fibres (pons, spinal cord, vestibular nuclei) excite granule cells, whose parallel fibres excite many Purkinje cells; climbing fibres come only from the inferior olive, one per Purkinje cell, signalling errors for motor learning.
- Peduncles: superior carries most output (dentate to contralateral red nucleus and ventrolateral thalamus, then motor cortex); middle carries pontine input; inferior carries spinal (dorsal spinocerebellar) and vestibular input.
Why cerebellar signs are on the same side
Cerebellar output crosses to the opposite motor cortex, and the corticospinal tract crosses back. A right cerebellar lesion causes right-sided incoordination, unlike cortical and basal ganglia lesions, which act on the opposite side.
Cerebellar signs
- Dysdiadochokinesia (clumsy rapid alternating movements)
- Ataxia (wide-based gait, falling towards the lesion)
- Nystagmus (fast phase towards the side of the lesion)
- Intention tremor and past-pointing (dysmetria)
- Slurred, scanning speech
- Hypotonia with pendular reflexes: a tapped knee swings back and forth several times because hypotonic muscles fail to damp it. A patient with nystagmus and past-pointing is likely to have a pendular knee jerk.
Distinguishing ataxias
| Type | Romberg | Clues |
|---|---|---|
| Cerebellar | Usually negative (unsteady even with eyes open) | Nystagmus, intention tremor, dysarthria |
| Sensory | Positive (worse with eyes closed) | Lost vibration and position: B12 deficiency (macrocytic anaemia with ataxia, subacute combined degeneration), tabes dorsalis, neuropathy |
| Vestibular | Falls towards the lesion | Vertigo, nausea, vomiting, tinnitus, nystagmus with normal limb coordination (labyrinthitis, Ménière disease) |
Multiple sclerosis commonly produces cerebellar signs (scanning speech, intention tremor, nystagmus) with spastic legs and urinary symptoms; heat worsens symptoms. Pilocytic astrocytoma and medulloblastoma are childhood posterior fossa tumours.
Basal ganglia: parts and pathways
Caudate and putamen (the striatum, main input), globus pallidus external (GPe) and internal (GPi, main output), subthalamic nucleus, substantia nigra (pars compacta makes dopamine; pars reticulata is an output).
- Direct pathway: striatum inhibits GPi, releasing the thalamus: more movement. Dopamine excites it via D1.
- Indirect pathway: striatum inhibits GPe, freeing the subthalamic nucleus to excite GPi, which inhibits the thalamus: less movement. Dopamine inhibits it via D2.
- Net effect of nigral dopamine: facilitates movement. Striatal cholinergic interneurones oppose dopamine, so loss of dopamine leaves a relative ACh excess; anticholinergics (trihexyphenidyl) therefore help tremor.
Disorders
| Disorder | Lesion | Result |
|---|---|---|
| Parkinson disease | Loss of dopaminergic neurones in the substantia nigra pars compacta; pale nigra with Lewy bodies (eosinophilic inclusions of alpha-synuclein) | Bradykinesia, lead-pipe or cogwheel rigidity, resting pill-rolling tremor (4–6 Hz), stooped posture, shuffling gait, mask face, micrographia |
| Dementia with Lewy bodies | Cortical Lewy bodies | Dementia, visual hallucinations, parkinsonism |
| Huntington disease | Loss of GABAergic neurones in the caudate (and putamen); CAG repeat, autosomal dominant (50% of children), anticipation | Chorea from loss of indirect-pathway inhibition, dementia, psychiatric change |
| Sydenham chorea | Rheumatic fever (antibody cross-reaction with basal ganglia) | Chorea in a child with fever and nodules; check ASO titre |
| Hemiballismus | Contralateral subthalamic nucleus (lacunar stroke) | Violent flinging of one arm and leg |
| Wilson disease | Copper in the lenticular nucleus | Tremor, dystonia, Kayser–Fleischer rings, liver disease |
| Drug-induced parkinsonism | D2 blockade (haloperidol, metoclopramide) | Symmetrical; DaT SPECT normal because nigrostriatal terminals are intact; resolves slowly after stopping |
Hypertensive haemorrhage most often affects the putamen (lenticulostriate arteries).
Treating Parkinson disease
- Levodopa crosses the blood–brain barrier on the large neutral amino acid transporter; carbidopa (peripheral decarboxylase inhibitor) stops conversion outside the brain, reducing nausea and allowing a lower dose. Protein meals (meat, daal, eggs) supply amino acids that compete for gut and brain transport, so take it 30–60 minutes before meals. Side effects: nausea, postural hypotension, dyskinesias, hallucinations, dark urine.
- Dopamine agonists: pramipexole, ropinirole, bromocriptine (D2 agonist, also used for hyperprolactinaemia), and transdermal rotigotine, which maintains treatment when the patient is nil by mouth (for example ileus after laparotomy).
- MAO-B inhibitors (selegiline, rasagiline) and COMT inhibitors (entacapone) slow dopamine breakdown.
- Antiemetics: avoid metoclopramide and prochlorperazine; domperidone (does not cross the blood–brain barrier) or ondansetron are safer.
Tremors compared
| Tremor | When | Seen in |
|---|---|---|
| Resting | At rest, eases with movement | Parkinson disease |
| Intention | Worse approaching a target | Cerebellar disease, MS |
| Postural (essential) | Holding a posture, family history, improves with alcohol, no rigidity | Essential tremor (first-line propranolol); thyrotoxicosis, anxiety, beta-agonists |
Remember Cerebellum: same side, smooth and accurate. Basal ganglia: opposite side, dopamine says go (D1) and lifts the brake (D2).
Exam trap Purkinje cells are the only output of the cerebellar cortex and are inhibitory; granule cells are the only excitatory cortical neurones.
Exam trap Hemiballismus is the subthalamic nucleus; Huntington the caudate; Parkinson the substantia nigra pars compacta.
How FCPS asks about this
- Nystagmus and past-pointing: cerebellar lesion; expect pendular reflexes and ipsilateral signs.
- Lesion in resting tremor with mask face and shuffling gait: substantia nigra pars compacta; Lewy bodies contain alpha-synuclein.
- Why anticholinergics help Parkinson tremor: relative ACh excess in the striatum.
- Levodopa failing with protein meals: dietary amino acids compete for transport.
- Parkinson patient nil by mouth: transdermal rotigotine; antiemetic domperidone.
- Parkinsonism on haloperidol with normal DaT scan: drug-induced; nigrostriatal terminals intact.
- Climbing fibres: from the inferior olive, for motor learning.
Test yourself on this topic
5 of the 27 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
At post-mortem, a man with Parkinson disease has a pale substantia nigra with round eosinophilic inclusions in its neurones. What do these inclusions mainly contain?
Answer: α-Synuclein
Lewy bodies are round, eosinophilic cytoplasmic inclusions made mainly of aggregated α-synuclein with ubiquitin. They are the hallmark of Parkinson disease in pigmented brainstem neurones, and of dementia with Lewy bodies when widespread in the cortex. Loss of dopaminergic neurones in the pars compacta makes the nigra pale. Tau forms neurofibrillary tangles in Alzheimer disease and progressive supranuclear palsy.
Lewy bodies
- Made of α-synuclein and ubiquitin
- In pigmented nigral neurones
- Pale nigra from dopamine neurone loss
- Cortical spread: dementia with Lewy bodies
Remember Lewy body means α-synuclein.
Exam trap Multiple system atrophy also involves α-synuclein, but in glial rather than neuronal inclusions.
Why the other options are wrong
- Tau protein Tau forms tangles in Alzheimer disease and PSP.
- β-Amyloid β-Amyloid forms plaques in Alzheimer disease.
- Huntingtin Huntingtin aggregates in Huntington disease.
- Prion protein Prion protein is the agent of CJD.
A 38-year-old man has early Huntington disease with restless, jerky movements of his face and hands. The first striatal neurones to die are those projecting to the external globus pallidus. Which of the following explains why this causes excess movement?
Answer: Loss of the indirect pathway disinhibits the thalamus
Indirect-pathway striatal neurones (GABA and enkephalin) normally inhibit the external globus pallidus, which allows the subthalamic nucleus to drive the internal pallidus and brake the thalamus. When these neurones die early in Huntington disease, the external pallidus becomes overactive and suppresses the subthalamic nucleus. Output from the internal pallidus falls, the thalamus is released, and unwanted movements (chorea) appear. Later loss of direct-pathway neurones contributes to rigidity and slowness.
Chorea in Huntington disease
- Early loss: indirect-pathway neurones
- GPe overactive, STN suppressed
- GPi output falls: thalamus released
- Result: chorea
Remember Lose the brake (indirect pathway), gain movement.
Exam trap Juvenile Huntington disease often presents with rigidity rather than chorea, because both pathways are lost early.
Why the other options are wrong
- Loss of the direct pathway excites the thalamus Loss of the direct pathway reduces movement.
- Dopamine neurones in the nigra degenerate Nigral dopamine loss causes Parkinson disease.
- The subthalamic nucleus becomes overactive The subthalamic nucleus becomes underactive, as in hemiballismus.
- Cerebellar output to the thalamus increases The cerebellum is not the primary site in Huntington disease.
A 6-year-old has a midline posterior fossa tumour. He walks with a broad-based, unsteady gait and has nystagmus, but limb coordination is fairly normal. Which part of the cerebellum is mainly affected?
Answer: Flocculonodular lobe and vermis
The flocculonodular lobe (vestibulocerebellum) and vermis control balance, posture and eye movements. Midline lesions such as medulloblastoma in children cause truncal ataxia, a wide-based gait and nystagmus, with relatively spared limbs. Hemisphere lesions cause ipsilateral limb ataxia, intention tremor and dysdiadochokinesia.
Cerebellar lesions
- Midline: truncal ataxia, nystagmus
- Flocculonodular: vestibulocerebellum
- Hemisphere: ipsilateral limb ataxia
- Child midline tumour: medulloblastoma
Remember Middle shakes the trunk; sides shake the limbs.
Exam trap Cerebellar signs are on the same side as the lesion.
Why the other options are wrong
- Lateral cerebellar hemisphere Ipsilateral limb ataxia and intention tremor.
- Dentate nucleus Limb coordination output.
- Middle cerebellar peduncle Limb ataxia.
- Cerebellar tonsil only Herniation, not this picture.
A 66-year-old man with Parkinson disease notices that his levodopa–carbidopa works poorly when he takes it with a lunch of meat, daal and eggs, but well when he takes it 45 minutes before meals. What is the reason?
Answer: Dietary amino acids compete for its transport
Levodopa is a large neutral amino acid. It is absorbed from the small intestine and crosses the blood–brain barrier on the same L-type amino acid transporter used by dietary amino acids such as leucine and phenylalanine. A high-protein meal floods these carriers, so less levodopa is absorbed and less reaches the brain, and the dose seems to fail. Taking doses 30–60 minutes before meals, or moving most protein to the evening meal, improves the response.
Levodopa and protein
- Levodopa is a large neutral amino acid
- Shares the L-type amino acid transporter
- Protein competes in gut and at the BBB
- Take it before meals
Remember A protein-rich meal can block levodopa at the door.
Exam trap Delayed gastric emptying in Parkinson disease also makes levodopa absorption erratic.
Why the other options are wrong
- Protein speeds peripheral decarboxylation Carbidopa already blocks peripheral decarboxylation.
- Protein binds carbidopa in the gut Protein does not bind carbidopa.
- Food makes the stomach too acidic Acidity is not the problem, though meals also slow gastric emptying.
- Protein increases dopamine breakdown by MAO-B Dietary protein does not induce MAO-B.
A man with a right cerebellar hemisphere tumour reaches for a cup. Which signs are expected, and on which side?
Answer: Right intention tremor, ataxia
The cerebellar hemispheres coordinate ipsilateral limb movements (the pathways cross twice—at the superior cerebellar peduncle and in the corticospinal tract), so a hemisphere lesion causes ipsilateral signs: intention tremor, dysmetria (past-pointing on finger–nose test), dysdiadochokinesia, rebound, hypotonia, pendular reflexes, and nystagmus towards the side of the lesion. Midline (vermis) lesions cause truncal ataxia and a broad-based gait (e.g. alcohol). Remember DANISH: dysdiadochokinesia, ataxia, nystagmus, intention tremor, slurred (scanning) speech, hypotonia. A resting tremor suggests Parkinson disease (basal ganglia).
Cerebellar signs
- Hemisphere lesion → ipsilateral limb signs
- Intention tremor, past-pointing, dysdiadochokinesia
- Vermis → truncal ataxia, broad gait
- DANISH
- Resting tremor = basal ganglia
Remember Cerebellum: same side.
Why the other options are wrong
- Left intention tremor Cerebellar signs are ipsilateral.
- Right resting tremor Basal ganglia (Parkinson).
- Right hemiplegia, upgoing toe Corticospinal lesion.
- Left-sided sensory loss Not cerebellar.
Where this sits in the CPSP syllabus
- Paper 1: Nervous system, Cerebellum and basal ganglia function
- Surgery & Allied, Paper 2: Nervous system, Cerebellum and basal ganglia function
- Medicine & Allied, Paper 2: Nervous system, Cerebellum and basal ganglia function
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