Basal Ganglia & Cerebellum

The basal ganglia and cerebellum are the two great subcortical motor regulators: neither initiates movement, yet both sculpt it. The basal ganglia gate and select movements through a dopamine-tuned cortico-striato-pallido-thalamo-cortical loop, while the cerebellum corrects movements in real time and stores motor learning. This is a high-yield physiology block because Parkinson disease, Huntington disease, and cerebellar ataxia syndromes follow directly from the circuit logic — once you understand the wiring, the clinical pictures fall out automatically.

Basal ganglia — components & classification

The "basal ganglia" is a functional, not strictly anatomical, family of nuclei buried in the cerebral white matter and midbrain.

Structure	Subdivisions	Functional role
Striatum (neostriatum)	Caudate nucleus + Putamen	Main input station; receives cortex + thalamus
Globus pallidus	External (GPe) + Internal (GPi)	GPi = major output; GPe = relay in indirect path
Subthalamic nucleus (STN)	—	Excitatory drive within indirect pathway
Substantia nigra	Pars compacta (SNc) + Pars reticulata (SNr)	SNc = dopamine source; SNr = output (with GPi)

Useful groupings:

• Corpus striatum = caudate + putamen + globus pallidus.
• Lentiform (lenticular) nucleus = putamen + globus pallidus (lens-shaped).
• Neostriatum / striatum = caudate + putamen (the input layer).
• Output nuclei = GPi + SNr (functionally the same, split by the internal capsule). Both are tonically active, inhibitory (GABAergic), and tonically clamp the thalamus.

High-yield: The basal ganglia output (GPi/SNr) is GABAergic and tonically inhibitory to the ventral lateral/ventral anterior (VL/VA) thalamus. Movement requires this brake to be released. "Movement = disinhibition of the thalamus."

The two pathways — circuit physiology

All cortical input enters at the striatum (glutamatergic, excitatory). From the striatum, two opposing projections emerge, distinguished by dopamine receptor type and net effect on the thalamus.

Direct pathway — facilitates movement ("Go")

Cortex →(+)→ Striatum (D1) →(−)→ GPi/SNr →(−)→ Thalamus (VL/VA) →(+)→ Cortex (motor)

The striatum inhibits the GPi. Since GPi normally inhibits the thalamus, inhibiting the GPi disinhibits the thalamus → thalamus excites motor cortex → movement is facilitated. Two negatives make a positive.

• Uses D1 receptors (Gs-coupled, excitatory, stimulated by dopamine).
• Co-expresses substance P and dynorphin.

Indirect pathway — suppresses movement ("Stop")

Cortex →(+)→ Striatum (D2) →(−)→ GPe →(−)→ STN →(+)→ GPi/SNr →(−)→ Thalamus

The striatum inhibits GPe; GPe normally inhibits the STN, so inhibiting GPe releases the STN; the disinhibited STN strongly excites GPi; an over-driven GPi clamps the thalamus harder → movement is suppressed.

• Uses D2 receptors (Gi-coupled, inhibitory, inhibited by dopamine).
• Co-expresses enkephalin.

High-yield: D1 = Direct = Drive (excitatory, "Go"). D2 = inDirect ("Stop"). Dopamine from SNc excites the direct path (D1) and inhibits the indirect path (D2) — both actions converge to promote movement.

Dopaminergic nigrostriatal modulation

The substantia nigra pars compacta (SNc) sends dopaminergic fibres (the nigrostriatal tract) to the striatum. Net effect of dopamine = pro-kinetic, because it simultaneously turns up the "Go" path and turns down the "Stop" path. Loss of this dopamine (as in Parkinson disease) tips the balance toward the indirect/"Stop" pathway → hypokinesia.

Mnemonic for transmitters: "Glutamate Goes in (cortex → striatum and STN → GPi are excitatory); GABA is the Guard at every other relay." The only excitatory intrinsic projection is the STN → GPi glutamatergic link.

Functional loops of the basal ganglia

The basal ganglia run several parallel, segregated loops, all sharing the same architecture but different cortical territories:

1. Motor loop (putamen-based) — scaling and selection of movement; lesion → movement disorders.
2. Oculomotor loop (caudate-based) — saccadic eye movements.
3. Prefrontal / associative loop — executive function, working memory.
4. Limbic loop (ventral striatum / nucleus accumbens) — motivation, reward, addiction.

High-yield: Functions of basal ganglia = planning and scaling/amplitude of movement, suppression of unwanted movement, procedural (habit) learning, and posture. They do not initiate voluntary movement (the cortex does) and they have no direct connection to the spinal cord — output is funnelled through the thalamus back to cortex.

Cerebellum — structure & functional divisions

The cerebellum sits dorsal to the pons/medulla and contains more than half of all CNS neurons. It has three layers in its cortex and a deep set of nuclei that form its sole output.

Cerebellar cortex layers (superficial → deep)

Layer	Key cells	Note
Molecular	Stellate, basket cells; parallel fibres	Outermost
Purkinje	Purkinje cells	Single layer; sole OUTPUT of cerebellar cortex
Granular	Granule cells (most numerous neuron in CNS), Golgi cells	Innermost

High-yield: Purkinje cells are the only output of the cerebellar cortex and they are inhibitory (GABAergic). They project to the deep cerebellar nuclei (chiefly the dentate nucleus), which provide the actual excitatory output of the cerebellum to the thalamus/red nucleus.

Inputs — the two afferent systems

• Climbing fibres — from the inferior olivary nucleus; each makes a powerful one-to-one synapse on a Purkinje cell → produces a complex spike. Central to error signalling and motor learning (long-term depression, LTD).
• Mossy fibres — from everywhere else (pontine nuclei, spinal cord, vestibular nuclei) → granule cells → parallel fibres → Purkinje cells → simple spikes.

Functional (and anatomical) divisions

Division	Anatomical name	Connections	Function	Lesion
Vestibulocerebellum	Flocculonodular lobe	Vestibular nuclei	Balance, equilibrium, eye movements	Truncal ataxia, nystagmus
Spinocerebellum	Vermis + paravermis	Spinal cord	Posture, muscle tone, gait	Truncal/gait ataxia, hypotonia
Cerebrocerebellum (pontocerebellum)	Lateral hemispheres	Cortex via pons → dentate	Planning, timing, coordination of skilled limb movement	Limb ataxia, intention tremor, dysdiadochokinesia

High-yield: Vermis lesion → truncal ataxia (patient cannot sit/stand steadily, e.g. medulloblastoma in children, alcoholic anterior vermis degeneration → gait ataxia). Lateral hemisphere lesion → ipsilateral limb (appendicular) ataxia + intention tremor.

Laterality — a favourite MCQ point

• Cerebellum acts IPSILATERALLY. Cerebellar signs appear on the same side as the lesion. (Double-crossing of the cortico-ponto-cerebellar and dentato-thalamo-cortical pathways.)
• Basal ganglia / cortex act CONTRALATERALLY for motor effects.

High-yield: A right cerebellar hemisphere lesion → right-sided limb ataxia, and the patient falls toward the side of the lesion. Past-pointing is also toward the lesion side.

Clinical features — putting the circuits to work

Hypokinetic vs hyperkinetic disorders

Basal ganglia disease splits cleanly into "too little movement" (hypokinetic, e.g. Parkinson) and "too much movement" (hyperkinetic, e.g. Huntington, hemiballismus, chorea).

Feature	Parkinson disease	Huntington / hemiballismus
Category	Hypokinetic	Hyperkinetic
Core lesion	SNc dopamine ↓	Striatum loss (HD) / STN (hemiballismus)
Pathway balance	Indirect path overactive	Indirect path underactive
Movement	Bradykinesia, rigidity, resting tremor	Chorea, ballism, dance-like
Tone	Increased (cogwheel/lead-pipe)	Decreased / variable

Parkinson disease — pathophysiology in detail

Degeneration of dopaminergic neurons in the SNc → loss of nigrostriatal dopamine → the direct (D1, "Go") pathway is underdriven and the indirect (D2, "Stop") pathway is disinhibited. Net result: the GPi/SNr becomes overactive, the thalamus is excessively clamped, and the motor cortex is under-excited → hypokinesia.

Flow: SNc dopamine ↓ → direct path ↓ + indirect path ↑ → GPi overactivity → thalamic inhibition ↑ → reduced cortical drive → bradykinesia/rigidity.

Cardinal features (TRAP):

• Tremor — resting tremor, "pill-rolling", 3–5 Hz, decreases on action.
• Rigidity — cogwheel (lead-pipe + tremor).
• Akinesia/bradykinesia — slowness, masked facies, micrographia.
• Postural instability — late, stooped posture, festinant gait.

Pathology: loss of pigmented neurons in SNc; Lewy bodies (α-synuclein, ubiquitin inclusions).

High-yield: In Parkinson, drug of choice is levodopa + carbidopa (carbidopa = peripheral DOPA-decarboxylase inhibitor, ↑ central dopamine, ↓ peripheral side effects). Surgical/refractory target for deep brain stimulation = STN (or GPi).

Huntington disease & hemiballismus

• Huntington disease (HD): autosomal dominant CAG trinucleotide repeat expansion in the huntingtin gene (chromosome 4), shows anticipation. Early loss of GABAergic/enkephalin striatal neurons of the indirect pathway → indirect "Stop" path fails → chorea. Caudate atrophy → "box-car" / boxing-glove-shaped lateral ventricles on imaging. Features: chorea, dementia, behavioural change.
• Hemiballismus: lesion (classically infarct) of the contralateral subthalamic nucleus (STN) → loss of STN drive to GPi → GPi underactive → thalamus disinhibited → violent, large-amplitude flinging movements of one half of the body.

High-yield: Subthalamic nucleus lesion → contralateral hemiballismus. This is the single most repeated basal-ganglia localisation MCQ.

Cerebellar signs

A useful umbrella mnemonic — DANISH:

• Dysdiadochokinesia (impaired rapid alternating movements)
• Ataxia (gait/truncal/limb)
• Nystagmus
• Intention tremor
• Scanning/staccato dysarthria (slurred, sing-song speech)
• Hypotonia / Heel-shin incoordination

Other classic cerebellar signs: dysmetria (past-pointing, finger-nose test), rebound phenomenon (Holmes), pendular knee jerk, and a wide-based, drunken (ataxic) gait that does not worsen with eyes closed (distinguishing it from sensory ataxia, where Romberg is positive).

Tremor distinctions — the most-tested table

Feature	Resting tremor	Intention (action) tremor
Lesion site	Basal ganglia (Parkinson, SNc)	Cerebellum (dentate/superior peduncle)
When present	At rest, abolished by movement	Appears/worsens on target-directed movement
Frequency	3–5 Hz, "pill-rolling"	Worsens near target (terminal)
With sleep	Disappears	—
Associated	Rigidity, bradykinesia	Dysmetria, dysdiadochokinesia, ataxia

High-yield: Resting tremor → Parkinson (basal ganglia). Intention tremor → cerebellum. Essential tremor is an action/postural tremor, often familial (AD), improves with alcohol and propranolol, and worsens with posture-holding — distinct from both.

Diagnosis & investigation of choice

• Parkinson disease: primarily a clinical diagnosis (bradykinesia + ≥1 of rigidity/resting tremor, asymmetric onset, levodopa-responsive). Supportive imaging = DaTscan (¹²³I-FP-CIT SPECT) showing reduced striatal dopamine transporter uptake; distinguishes degenerative parkinsonism from essential tremor/drug-induced. MRI mainly excludes mimics.
• Huntington disease: genetic testing for CAG repeat expansion (≥36 repeats; ≥40 fully penetrant) is confirmatory; MRI shows caudate atrophy with box-car ventricles.
• Wilson disease (a key young-patient differential, copper deposition affecting lentiform nucleus/putamen): serum ceruloplasmin ↓, 24-h urinary copper ↑, Kayser–Fleischer rings (slit-lamp), MRI "face of the giant panda" sign in midbrain.
• Cerebellar lesions: MRI brain is the investigation of choice for structural lesions (tumour, infarct, MS plaque, atrophy).

Management / drug of choice (quick map)

• Parkinson disease: Levodopa–carbidopa (most effective). Adjuncts: dopamine agonists (pramipexole, ropinirole — preferred in younger patients), MAO-B inhibitors (selegiline, rasagiline), COMT inhibitors (entacapone), amantadine (for dyskinesia), anticholinergics (for tremor-predominant). DBS of STN for advanced disease.
• Huntington chorea: tetrabenazine / deutetrabenazine (VMAT-2 inhibitors) for chorea.
• Hemiballismus: dopamine blockers (haloperidol) / tetrabenazine.
• Essential tremor: propranolol (first line) or primidone.
• Wilson disease: copper chelation — D-penicillamine / trientine, zinc.

Complications & associations

• Parkinson: levodopa-induced dyskinesias and "on–off" / wearing-off phenomena; postural hypotension; dementia (Parkinson disease dementia, Lewy body disease); depression; aspiration from dysphagia.
• Huntington: progressive dementia, psychiatric disease (high suicide risk), aspiration; uniformly fatal over 15–20 years.
• Cerebellar disease: falls, aspiration, and posterior-fossa mass effect → tonsillar herniation (acute hydrocephalus from 4th-ventricle compression — a neurosurgical emergency in cerebellar haemorrhage/medulloblastoma).

Key differentials

• Resting vs intention vs essential tremor (table above) — the highest-yield differential.
• Cerebellar (sensory-independent) ataxia vs sensory ataxia: Romberg negative in cerebellar, positive in sensory (dorsal column/tabes); cerebellar ataxia has nystagmus and dysarthria, sensory ataxia has loss of proprioception/vibration and a stamping gait.
• Parkinson disease vs Parkinson-plus / drug-induced parkinsonism: symmetry, poor levodopa response, early falls (PSP), autonomic failure (MSA), and metoclopramide/antipsychotic exposure favour secondary causes.
• Chorea differentials: Sydenham chorea (post-streptococcal, rheumatic fever), Wilson disease, SLE, drug-induced.

Recently asked / exam angle

• STN lesion → contralateral hemiballismus (repeated almost every cycle).
• Sole output of cerebellar cortex = Purkinje cell, and it is inhibitory (GABA).
• Cerebellum produces ipsilateral signs; patient falls toward the side of the lesion.
• Direct pathway uses D1 (excitatory), indirect uses D2 (inhibitory); dopamine net effect is pro-kinetic.
• Basal ganglia output (GPi/SNr) is tonically inhibitory to thalamus → movement = disinhibition.
• Matching transmitter to pathway: substance P/dynorphin = direct; enkephalin = indirect.
• Resting tremor = basal ganglia; intention tremor = cerebellum (1-line MCQ).
• Climbing fibres from inferior olive; mossy fibres from elsewhere — afferent source identification.
• Vermis lesion → truncal ataxia; lateral hemisphere → limb ataxia localisation.
• Box-car ventricles + CAG repeats = Huntington.
• DaTscan / DBS target = STN in Parkinson management questions.

Rapid revision

1. Striatum = caudate + putamen = input; GPi + SNr = output (GABAergic, tonically inhibitory to thalamus).
2. Direct path (D1, +): Cortex → Striatum ⊣ GPi ⊣ Thalamus → Cortex → facilitates movement (two negatives).
3. Indirect path (D2, −): Striatum ⊣ GPe ⊣ STN → GPi ⊣ Thalamus → suppresses movement.
4. Dopamine (SNc) excites D1, inhibits D2 → net pro-kinetic; its loss → Parkinson hypokinesia.
5. Only intrinsic excitatory intra-BG link = STN → GPi (glutamate).
6. STN lesion → contralateral hemiballismus.
7. Parkinson cardinal signs = TRAP; pathology = SNc loss + Lewy bodies (α-synuclein); DOC = levodopa–carbidopa; DBS target = STN.
8. Huntington = AD, CAG repeat, chromosome 4, indirect-path neuron loss, box-car ventricles, chorea + dementia; chorea Rx = tetrabenazine.
9. Purkinje cell = sole cerebellar cortical output, inhibitory; deep nucleus (dentate) gives excitatory output.
10. Cerebellum = ipsilateral signs, falls toward lesion; vermis → truncal, hemisphere → limb ataxia; flocculonodular → balance/nystagmus.
11. Cerebellar signs = DANISH; Romberg negative (vs positive in sensory ataxia).
12. Resting tremor = basal ganglia/Parkinson; intention tremor = cerebellum; essential tremor = action, propranolol-responsive, alcohol-relieved.