In one breath
The basal ganglia are a ring of deep grey masses — caudate nucleus, putamen, globus pallidus, plus the subthalamic nucleus and substantia nigra — that take their input almost entirely from the cerebral cortex and send it back to the cortex through the thalamus. That circle is the motor loop: cortex → striatum → globus pallidus internus → thalamus → cortex. Inside the loop a direct pathway releases the thalamus from inhibition and lets movement through, while an indirect pathway does the opposite and holds movement back; dopamine from the substantia nigra tilts the balance towards the direct one. So these nuclei plan and program movement, set muscle tone and posture, and when they fail the result is either too little movement or too much.
Builds on: Movement and organization of the motor system · Rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts · Thalamus: nuclei, functions and thalamic syndrome · Leads to: Parkinson’s and Huntington’s disease · Cerebellar functions, lesions and tests
Parts of the basal ganglia
| Group | What it contains |
|---|---|
| The three main masses | Caudate nucleus, putamen, globus pallidus |
| Also counted in | Subthalamic nucleus, substantia nigra |
| Striatum (neostriatum) | Caudate nucleus + putamen |
| Lenticular nucleus | Putamen + globus pallidus |
Two of them are split in two, and the split matters for every circuit on this page:
- Globus pallidus → an external part (globus pallidus externus, GPe) and an internal part (globus pallidus internus, GPi).
- Substantia nigra → pars compacta (the dopamine cells) and pars reticulata.
Striatum or lenticular nucleus?
Both contain the putamen, which is why they are mixed up. Striatum = caudate + putamen (a functional unit, where nearly all input arrives). Lenticular nucleus = putamen + globus pallidus (an anatomical lump on a coronal section). Learn the putamen as the member of both.
Inputs
Unlike the cerebellum, the basal ganglia get no direct sensory input from the periphery, the spinal cord or the brainstem sensory nuclei. Everything reaches them second-hand, and mostly from the cortex.
- The cerebral cortex is the main source.
- Smaller inputs arrive from the thalamus, the dorsal raphe nucleus and the pedunculopontine region of the brainstem.
- Almost all of it enters through the striatum.
The four afferent projections:
| Projection | From | To |
|---|---|---|
| Corticostriate | All areas of cortex | Striatum |
| Thalamostriate | Centromedian nucleus | Striatum |
| Raphestriate | Dorsal raphe nucleus | Striatum |
| Pedunculostriate | Pedunculopontine nucleus | Basal ganglia |
Within the corticostriate projection the two halves of the striatum are not fed alike: the putamen is supplied mainly by the sensorimotor cortex, and the caudate nucleus by the rest of the cortex.
Outputs
The internal segment of the globus pallidus is the principal output nucleus.
- Its fibres go to the ventral lateral, ventral anterior and centromedian nuclei of the thalamus, and from there on to the prefrontal and premotor cortices — which is where the extrapyramidal pathways largely begin (Rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts).
- Polarity of the last two steps: GPi → thalamus is inhibitory; thalamus → cortex is excitatory. Remember this pair and the direct and indirect pathways both fall out of it.
- The pars reticulata of the substantia nigra also projects to the thalamus, so it is a second, smaller exit.
- Other targets: pedunculopontine nucleus, habenula and superior colliculus.
The motor loop
Put input and output end to end and the circuit closes on itself:
cortex → striatum → globus pallidus internus → thalamus → cortex
That is the motor loop, and it is the single most quotable line in this chapter.
The book's figure labels the thalamic target VPL
The text of your chapter names the ventral lateral (VL), ventral anterior (VA) and centromedian (CM) nuclei, while the caption of the output figure reads VPL. Go with the text: Worth knowing, though not in your pages: VPL is the somatosensory relay for the spinothalamic and dorsal-column pathways (Thalamus: nuclei, functions and thalamic syndrome); the motor relay to the cortex is VL and VA.
Connections inside the basal ganglia
| Projection | From → To | Transmitter / effect | Failure causes |
|---|---|---|---|
| Nigrostriatal | SN pars compacta → striatum | Dopaminergic | Parkinsonism |
| Striatonigral | Striatum → substantia nigra | GABA, inhibitory | Huntington’s disease |
| Subthalamic → GPi | Subthalamic nucleus → GPi | Excitatory | Ballism (when the nucleus is lost) |
Those two dopamine-and-GABA arrows running in opposite directions between the striatum and the substantia nigra, and which disease follows the loss of each, are exam currency. See Parkinson’s and Huntington’s disease.
The two pathways through the basal ganglia
The inputs, the internal connections and the outputs together build two routes from striatum to thalamus, and they pull in opposite directions.
Direct pathway
Cortex → striatum → GPi → thalamus → motor cortex.
Step by step, with the sign of each synapse:
| Step | Effect | Transmitter |
|---|---|---|
| Cortex → striatum | Excitatory | Glutamate |
| Striatum → GPi | Inhibitory | GABA |
| GPi → thalamus | Inhibitory | GABA |
| Thalamus → cortex | Excitatory | Glutamate |
Two inhibitory synapses in series make an excitation. So stimulating the striatum ends up exciting the thalamus, by disinhibition.
How it behaves during a movement:
- Striatal neurons sit almost silent at rest, with very little background firing.
- The cortex and thalamus fire them up as the movement starts.
- The active striatum switches off the GPi.
- The GPi was itself holding the thalamus down, so removing it frees the thalamic neurons.
- The freed thalamus drives the motor cortex through the thalamocortical projection, and the movement is permitted.
The direct pathway is the one turned on during movement. Read it as the accelerator.
Indirect pathway
Cortex → striatum → GPe → subthalamic nucleus → GPi → thalamus.
The extra loop through GPe and the subthalamic nucleus inserts one more inhibitory synapse, and one inversion changes everything:
| Step | Effect |
|---|---|
| Striatum → GPe | Inhibitory |
| GPe → subthalamic nucleus | Inhibitory |
| Subthalamic nucleus → GPi | Excitatory |
| GPi → thalamus | Inhibitory |
- The striatum inhibits GPe; GPe stops inhibiting the subthalamic nucleus, so the subthalamic nucleus becomes more active.
- The subthalamic nucleus drives the GPi.
- An active GPi clamps down harder on the thalamus, so the thalamocortical projection is suppressed.
- Net result: the striatum’s output through the indirect pathway is inhibitory. Read it as the brake.
Count the inhibitory synapses
Never memorise “direct excites, indirect inhibits” alone — the examiner asks why. Between striatum and thalamus the direct route has two inhibitory synapses (striatum→GPi, GPi→thalamus), so the thalamus is released. The indirect route has three, with an excitatory subthalamic step in the middle, so the thalamus is held down. The odd or even count of inversions is the whole trick.
Nigrostriatal modulation
The two pathways are normally in balance. Tip the activity of either one and the motor output of the basal ganglia becomes lopsided — which is why basal ganglia disease shows both hypokinetic and hyperkinetic features at the same time, in the same patient.
What sets the balance is the dopaminergic nigrostriatal projection from pars compacta to the striatum:
- on the direct pathway its influence is excitatory;
- on the indirect pathway it is inhibitory;
- it also modulates the corticostriate input itself.
So dopamine pushes the accelerator and eases the brake at once — both effects favour movement. That is why losing it is so disabling.
Alongside dopamine sits acetylcholine:
- The striatal neurons are cholinergic.
- It is the ratio of acetylcholine to dopamine in the striatum that keeps those neurons working properly.
- Disturb the ratio and motor activity becomes abnormal — the reason anticholinergic drugs help in Parkinsonism even though the missing transmitter is dopamine (Parkinson’s and Huntington’s disease).
Two zones of the striatum
The striatum is not uniform. It is split into striosomes and matrix, and they belong to two different worlds:
| Zone | Input from | Concerned with |
|---|---|---|
| Striosomes | Limbic system | Limbic functions |
| Matrix | Motor cortex | Motor functions |
Functions of the basal ganglia
- Planning and programming of movement. Basal ganglia neurons start firing before the movement itself begins, which is the evidence that they prepare it rather than execute it.
- Control of posture. The basal ganglia–thalamus–cortex projection reaches the brainstem and from there shapes the descending pathways that hold posture. Disease here produces gross postural abnormality.
- Damping of stretch reflexes. Stimulating the caudate nucleus suppresses the stretch reflex, partly through the thalamocortical feedback loop onto the inhibitory areas of motor cortex, and partly by driving the inhibitory reticular formation (Stretch reflex).
- Subconscious gross movement is regulated by the neostriatum.
- Cognition. The caudate nucleus, through its links with the frontal neocortex, contributes to cognitive function; a caudate lesion leaves a deficit in tasks that depend on what has been learnt.
- Speech. A lesion of the head of the left caudate nucleus gives dysarthritic aphasia — words cannot be articulated properly.
- Muscle tone for skilled movement is supplied by the globus pallidus.
- Coordination of the impulses needed for skilled movement is the job of the substantia nigra.
- Associated movements — the ones that accompany a voluntary act automatically, such as arm swing in walking — are controlled here.
- Species difference in lesions. Destroying the basal ganglia in animals causes little motor deficit; in humans, basal ganglia disease is severely disabling.
Applied: why the deficit is "release", not weakness
The basal ganglia never reach the spinal motor neuron themselves — their only way out is back through the thalamus to the cortex. So their diseases produce no paralysis and no loss of reflexes. What appears instead is abnormal tone, abnormal posture and involuntary movement: the cortex is being driven wrongly rather than disconnected. Compare an upper motor neuron lesion, where power itself is lost.
Draw it: the motor loop with the direct and indirect pathways
One box-and-arrow diagram earns most of the marks in this chapter. Draw it the same way every time.
- Top of the page: a wide box labelled CORTEX (add “PMC, SMA, motor cortex”).
- Middle row, left: STRIATUM (write “caudate + putamen” inside it).
- Middle row, right: GPe above, subthalamic nucleus below it, and GPi to the right of both.
- Bottom: THALAMUS (label “VL, VA, CM”).
- Left of the striatum: substantia nigra, drawn as two halves — pars compacta and pars reticulata.
- Now the arrows, and mark every one with + or −:
- cortex → striatum: + (glutamate);
- direct: striatum → GPi −, GPi → thalamus −;
- indirect: striatum → GPe −, GPe → subthalamic nucleus −, subthalamic nucleus → GPi +, then the same GPi → thalamus −;
- thalamus → cortex: +, closing the loop;
- pars compacta → striatum: label it dopamine, and write beside it ”+ on direct, − on indirect”;
- striatum → substantia nigra: label it GABA.
- Write the two conclusions under the diagram: direct pathway = thalamus released = movement allowed; indirect pathway = thalamus suppressed = movement held back.
- Keep the same drawing for Parkinson’s and Huntington’s disease and just cross out the failing arrow: the dopamine arrow in Parkinson’s disease, the striatal GABA output in Huntington’s, the subthalamic nucleus in ballism.
Exam-answer skeleton: "Describe the parts, connections and functions of the basal ganglia" (long essay)
- Name the parts: the three main masses, plus subthalamic nucleus and substantia nigra; define striatum and lenticular nucleus; the GPe/GPi and pars compacta/pars reticulata splits.
- Inputs: no direct sensory input, unlike the cerebellum; cortex is the chief source and the striatum the gateway; list the four afferent projections.
- Outputs: GPi as the principal output nucleus → VL, VA and CM thalamus → prefrontal and premotor cortex; GPi → thalamus inhibitory, thalamus → cortex excitatory; pars reticulata as second exit; other targets.
- State the motor loop in one line: cortex → striatum → GPi → thalamus → cortex.
- Internal connections: nigrostriatal (dopamine), striatonigral (GABA), subthalamic → GPi (excitatory), naming the disease attached to each.
- Direct and indirect pathways, with the sign of every synapse, and the disinhibition argument; then the dopaminergic modulation (+ direct, − indirect) and the acetylcholine : dopamine ratio. Draw the diagram here.
- Striosomes (limbic) and matrix (motor).
- Functions: planning and programming, posture, stretch-reflex damping, subconscious gross movement, cognition and speech, muscle tone, skilled-movement coordination, associated movements.
- Close with dysfunction in two lines: hypokinetic Parkinson’s disease from loss of the dopamine arrow, hyperkinetic chorea and ballism from loss of the striatal and subthalamic ones (Parkinson’s and Huntington’s disease).
Asked in exams
- Basal ganglia: parts, connections, functions: Jul/Aug 2014, 10 marks · Oct/Nov 2018, 10 marks · Jan/Feb 2022, 2 marks · May 2022, MCQ · Jan 2023, MCQ · Aug 2024, MCQ · Oct 2024, MCQ