The Nervous System · Part Two · How It Senses and Moves
Lesson 37 / 61
The Basal Ganglia: Nuclei, Pathways, and the Threshold for Movement
Five nuclei, three routes, and the standing brake that decides whether you move.
For the Unified Model of Tone, the basal ganglia are the body's motor gate. They are five subcortical nuclei whose output neurons fire continuously and hold the thalamus under inhibition, so a movement begins when that brake lifts and not when a command arrives. Three routes work the brake: a direct one that lifts it, an indirect one that raises it, and a hyperdirect one that arrives before either. The model reads most movement disorders as that threshold sitting too high or too low.
Output nuclei
Internal globus pallidus and substantia nigra pars reticulata
Striatal receptors
D1 on the direct route, D2 on the indirect
Output bottleneck
About 900 to 1, neostriatum to internal pallidum, one rat hemisphere
Dopamine source
Pars compacta, reaching the striatum by the nigrostriatal pathway
Basal ganglia
A group of nuclei lying deep to the cerebral hemispheres. Five nuclei make up the group: the caudate, the putamen, the globus pallidus in its external and internal divisions, the subthalamic nucleus, and the substantia nigra in its pars compacta and pars reticulata. Caudate and putamen together with the pallidum form the corpus striatum, the putamen and pallidum together forming the lenticular nucleus. The dorsal striatum serves motor and executive work. The ventral striatum, built from the nucleus accumbens and olfactory tubercle, serves reward and aversion.
The gate and tone
The threshold for release is not fixed. Dopamine shifts it over minutes, cortical drive shifts it in milliseconds, and afferent traffic from joints and skin shifts it whenever the body moves. All three act on the same few thousand output neurons, which is why influences running on such different clocks are read as one setting.
01The nuclei and their counts
The basal ganglia are five nuclei whose whole influence leaves through a few thousand output cells
The basal ganglia are a cluster of subcortical nuclei lying deep to the cerebral hemispheres, built for motor control and drawn into reward and cognition by the same wiring. Five nuclei make up the group, and every command the group issues leaves through a few thousand cells. They are wired intricately onto one another, some promoting movement and others antagonizing it, and the naming matters clinically because signs map onto specific nodes.
Unbiased stereology in six young adult rats gives the cluster its proportions. Per hemisphere the neostriatum held 2.79 million neurons, the external globus pallidus 46,000 and the subthalamic nucleus 13,600. The substantia nigra held 26,300 neurons in the pars reticulata and 7,200 in the pars compacta. The entopeduncular nucleus, which is the rat's internal pallidum, held 3,200 Oorschot 1996. Cavalieri and optical disector methods produced those counts, so they do not depend on cell size or section thickness.
The output bottleneck decides how selection has to work
Those counts set the compression close to nine hundred to one on one side of the rat brain Oorschot 1996. Cortical intention arrives wide and leaves narrow. A channel that thin cannot describe a movement in any detail. It can permit or refuse, which is why this circuit selects by veto and never by instruction, and why one small nucleus can cancel a whole field of competing programs at once.
The chemistry that biases the whole cluster comes from a small population. Stereology in seven human control brains counted about 550,000 pigmented neurons in the substantia nigra, against 260,000 non-pigmented neurons in the same structure Pakkenberg 1991.
The split between dorsal and ventral striatum predicts which signs travel together. When the dorsal division fails, movement slows, posture stiffens and automatic gestures drop out. When the ventral division fails, drive falls away while strength is intact, and the picture is apathy rather than paralysis. Both divisions receive input from the cortex, the limbic system and the deep gain-setting nuclei of the brainstem, including the ventral tegmental area. The architecture that times a reaching arm also weighs whether the reach is worth making.
02Findings
What the research shows
03Gating by disinhibition
The basal ganglia release movement by lifting a brake that is already on
The internal globus pallidus and the substantia nigra pars reticulata fire tonically onto the ventral anterior and ventrolateral nuclei of the thalamus, so resting motion sits under a standing brake Chevalier 1990. Every movement starts with a withdrawal of that inhibition. The striatum quiets those output neurons, the thalamus is freed, and the cortex receives its permission to act. Chevalier and Deniau went further and proposed that basal ganglia output works as a movement template, specifying which motor elements are engaged in directing movement through space.
Silencing one output nucleus shows what its firing was holding back. Muscimol placed in the substantia nigra pars reticulata of the monkey produced irrepressible saccades toward the opposite visual field Hikosaka 1985. Nothing was added to the eye muscles or to the colliculus, and the movement came out anyway because the standing inhibition was gone. Saccades to remembered targets were more vulnerable than saccades to a visible one, so internally generated movement depends most on the brake. The authors extended the same logic to skeletal movement. The saccade generator itself is described in saccades and pursuit.
Single reticulata neurons split into an inhibited group and an excited group
Driving each striatal route selectively with optogenetics in mice, while recording single units in the pars reticulata, produced both excitations and inhibitions from each route Freeze 2013. Movement initiation correlated with the inhibited subpopulation of reticulata neurons, and motor suppression with the excited one. The authors read that as support for an inhibitory gate opened by the direct pathway and closed by the indirect. The gate appears in single cells only as two opposed groups, and neither group on its own is the gate.
Because the default is suppression, the question at every instant is which single program earns release while its rivals stay clamped. When the tonic brake fails to lift, posture freezes into rigidity and steps shorten. When it lifts indiscriminately, unwanted movements leak out and the body performs gestures the will never ordered.
04The two striatal routes
Two striatal populations, opposite chemistry, opposite effect on the brake
Dopamine biases two striatal populations against each other. The direct route runs from striatal GABAergic neurons onto the internal globus pallidus and lifts the tonic brake on the thalamus, which is the disinhibition that permits a chosen movement. The indirect route projects instead onto the external globus pallidus and engages the subthalamic nucleus, raising inhibitory output and suppressing competing programs Albin 1989.
The two routes are separate populations with separate chemistry. Striatonigral neurons carry D1 receptors along with substance P and dynorphin, and striatopallidal neurons carry D2 receptors along with enkephalin Gerfen 1990. Destroying the dopamine supply in the rat with 6-hydroxydopamine moved those markers in opposite directions in the two populations, raising D2 receptor and enkephalin messenger RNA while lowering D1 receptor and substance P. Each shift reversed with the matching receptor agonist. One transmitter reaches both routes and pushes them apart.
Dopamine arrives from the substantia nigra pars compacta, whose neurons project to the striatum as the nigrostriatal pathway. Where that transmitter is made, how far it spreads, and which subtype reads it are set out in dopamine and the monoamines.
Driving one route alone produces the syndrome
Bilateral optogenetic excitation of indirect-pathway striatal projection neurons in the mouse elicited a parkinsonian state, with increased freezing, bradykinesia and fewer locomotor initiations Kravitz 2010. Driving the direct pathway did the reverse, reducing freezing and raising locomotion, and in a mouse model of Parkinson's disease direct-pathway activation rescued the deficits in freezing, bradykinesia and locomotor initiation completely. Neither muscle nor cortex was touched in either case.
The Unified Model of Tone reads that balance as where the motor threshold sits. Dopamine raises the weight of the direct route and lowers the weight of the indirect one. Losing it moves both at once, which is why the transmitter loss produces a body that is slow rather than one that is weak. The standing balance across these nuclei decides whether a movement is released, and it decides it before a single muscle is recruited.
Both routes rise together at the moment of action
Recording both striatal populations with a genetically encoded calcium indicator in freely moving mice showed them increasing activity together instead of in opposition. Concurrent activation in one hemisphere preceded contraversive movements and predicted which specific movement would occur within 500 ms Cui 2013. Neither population fell silent while the other rose. A route that only releases and a route that only restrains would not behave that way, so the pair arms together and the balance between them settles which of the competing programs survives.
05The hyperdirect route
A third route reaches the output nuclei before the striatum has answered
Cortical fibers reach the subthalamic nucleus without touching the striatum. From there the nucleus drives the internal globus pallidus and the substantia nigra pars reticulata to a broad inhibition that lands ahead of any striatal signal Nambu 2002. Its conduction time is shorter than anything routed through the striatum. Where the direct pathway opens one gate, the hyperdirect and indirect pathways shut the surrounding ones.
Nambu's sequence runs in three signals. The hyperdirect pathway inhibits the selected program and its competitors together. The direct pathway then disinhibits the selected program alone. The indirect pathway follows with extensive inhibition that terminates it. The authors named the arrangement a dynamic center-surround model, which is surround inhibition applied to motion: an early broad veto arriving ahead of the slower striatal loop.
Silencing the subthalamic nucleus abolished the cortical excitation of pallidal cells
In awake monkeys, blocking subthalamic activity with muscimol abolished both the early and the late excitation that cortical stimulation evoked in pallidal neurons Nambu 2000. Early excitation appeared in subthalamic neurons before it appeared in pallidal ones, which fixes the order of the relay. Injecting the NMDA antagonist CPP into the subthalamic nucleus attenuated both the early and the late excitation while the non-NMDA antagonist NBQX did not, so cortico-subthalamic transmission is carried mainly by NMDA receptors.
The reticulata also reaches beyond the thalamus. It projects to the superior colliculus, the pedunculopontine nucleus and the medullary reticular formation, threading eye movement, gait and brainstem tone into the same gating economy. Its GABAergic neurons hold those targets under restraint exactly as they hold the thalamus. Motor disinhibition is carried by the direct pathway, while surround and competing motor inhibition are carried by the indirect and hyperdirect pathways. Reading which route is failing separates a body that cannot start from one that cannot stop.
06Joint traffic at the gate
Traffic from joints and skin reaches the basal ganglia gate and moves where the threshold sits
Single-cell recording in behaving primates found short-latency responses to passive joint rotation, and a somatotopic organization of movement-related neurons in the external pallidum, the internal pallidum and the subthalamic nucleus DeLong 1984. The proximal limb was better represented than the distal one. In the same recordings, bradykinesia tracked the loss of pallidal output coding movement amplitude. Cortex is not the only thing that addresses this gate, because mechanoreceptive and proprioceptive traffic arrives at the same cells.
Here the Unified Model of Tone takes that recording one step past what the paper claims. If pallidal and subthalamic cells answer passive joint rotation at short latency, then the limbs already hold a vote on where the release threshold sits, and the vote is cast segment by segment. Chiropractic care works those joint afferents on purpose, at the segments whose report is distorted, which is why a practice organized around the nervous system attends to the spine at all. What follows at the gate depends on the state that gate is already holding, so the same contact leaves one threshold where it was and moves another.
Input entering at the spinal cord changed the firing state of the gate
Epidural electrical stimulation of the dorsal columns restored locomotion in acutely dopamine-depleted mice and in chronically 6-hydroxydopamine-lesioned rats Fuentes 2009. Recovery ran alongside the disruption of aberrant low-frequency synchronous corticostriatal oscillations, which let activity patterns resembling the normal pre-locomotion state re-emerge.
The same input has been tried in people. A review that screened 433 records found 25 unique studies with 103 participants in total Streumer 2023. Gait improved in almost all Parkinson's patients who also had pain, most often low back pain, and the improvement held regardless of stimulation parameters or electrode location. Patients without pain have not been studied in a double-blind design, so that group carries no controlled result yet. The split is what the model expects. One input met two different states, and where nociceptive traffic was already loading the circuit, altering that traffic moved the gate.
The claim is written in units this circuit already reports. Local field potentials recorded from deep-brain electrodes inside the human subthalamic nucleus carry a pathological rhythm in the 8 to 35 Hz band. In nine patients recorded on and off levodopa, the fall in that peak tracked improvement in akinesia and rigidity at r = 0.835 across 17 sides, with no correlation to tremor Kühn 2006. Afferent traffic addresses the same threshold that rhythm reports. Added traffic moves the peak downward in a patient who cannot start and upward in one whose movements escape unbidden. The direction belongs to the state the input meets, and not to the input.
07Hypokinetic and hyperkinetic
Movement disorders divide by which route fails, and the dopamine balance predicts which appears
Basal ganglia disorders divide into hypokinetic and hyperkinetic forms, and Albin named the population behind each. His original proposal put the hyperkinetic disorders down to selective impairment of striatal neurons projecting to the lateral globus pallidus. The hypokinetic disorders came from changes in striatal projection neurons producing a net increase in basal ganglia output Albin 1989. Those two populations are differentially regulated by striatal afferents, so what arrives at the striatum sets the balance that decides which picture appears.
In Parkinson's disease the net effect of dopamine loss is increased inhibitory output from the internal globus pallidus and the substantia nigra pars reticulata, and thus decreased activity in thalamocortical neurons, expressed clinically as bradykinesia. Lesioning the subthalamic nucleus in MPTP-treated monkeys reduced akinesia, rigidity and tremor together in the contralateral limbs Bergman 1990. Removing the excess output removed the signs.
Symptoms begin after two thirds of the lateral ventral tier is gone
The same stereological count, made in seven patients and seven controls, found pigmented nigral neurons reduced by 66 percent in Parkinson's disease while non-pigmented neurons fell only 24 percent Pakkenberg 1991. One population is targeted and the other is largely left. Counted at the onset of symptoms, cell loss reaches 68 percent in the lateral ventral tier and 48 percent across the caudal nigra as a whole. Pigmented nigral neurons fall 45 percent in the first decade of disease Fearnley 1991. Normal aging strips pigmented nigral neurons at 4.7 percent per decade and spares the lateral ventral tier most, the exact reverse of the disease pattern, so ordinary attrition is not what drives it.
The terminal field goes before the cell bodies do. Striatal dopamine is depleted on the order of 70 to 80 percent by the time motor signs appear Bernheimer 1973. Dopaminergic markers in the dorsal putamen are virtually gone by four years after diagnosis, while surviving melanized nigral neurons are down only 30 to 60 percent across those same early years Kordower 2013. Cells lose the dopamine phenotype before they die, so the gate fails at the terminals before the cell count says it should. The presymptomatic phase runs about five years, which means the gate stiffens silently long before movement changes.
Tremor is generated outside this cluster
One parkinsonian sign keeps its own company. Tremor did not track the subthalamic rhythm that akinesia and rigidity followed, and the oscillation behind it is generated in a different loop. Cerebellar outflow to the thalamus and cortex helps generate thalamocortical oscillations through T-type calcium channel activation. In two mouse models of essential tremor, five T-type antagonists at non-sedating doses cut harmaline tremor by at least 50 percent, with maximal suppression between 53 and 81 percent. The same compounds cut tremor in GABA-A alpha1-null mice by at least 70 percent Handforth 2010. No single channel subtype owns the rhythm, since Cav3.1-null mice and heterozygotes trembled as much as wild-type animals and two of the antagonists still worked in Cav3.1-null mice. A coupled olivocerebellar system is oscillating, and the cerebellum holds the other half of that loop.
The opposite imbalance frees movement that should stay clamped. Analysis of 17 Huntington's disease specimens found the enkephalin-containing neurons projecting to the external pallidum much more affected in early and middle stages. The substance P-containing neurons projecting to the internal segment were relatively spared Reiner 1988. All targets were depleted only at advanced stages. The restraining route degenerates first while the releasing route stays intact, which is the imbalance that produces chorea. Dystonia and related hyperkinetic syndromes arise from the same failure of restraint, with unwanted programs escaping the gate.
Parkinson's disease starts later in women and presents more often with tremor
Risk of Parkinson's disease is twice as high in men as in women, while women carry higher mortality and faster progression. Motor symptoms, nonmotor symptoms and treatment response all differ by sex Cerri 2019. In 253 untreated patients, onset came 2.1 years later in women, at 53.4 years against 51.3, and tremor was the presenting sign in 67 percent of women against 48 percent of men Haaxma 2007. Women carried 16 percent higher striatal dopamine transporter binding on SPECT at onset and throughout, while UPDRS-III scores at onset and the rate of deterioration were equal. Women begin with more of the circuit intact.
Read as one threshold instead of a list of diseases, these syndromes line up on a single axis. A threshold that tracks the demand in front of it is what health looks like in motor terms, because movement can be released on time and withheld on time. In Parkinson's disease 66 percent of the pigmented nigral population is gone and the brake has grown too heavy to lift, which reaches the clinic as bradykinesia, rigidity and freezing. In Huntington's disease the enkephalin route is lost first and the brake has grown too light to hold, which reaches the clinic as chorea. Between them sit the cases where the reserve differs and the wiring does not, such as the 16 percent higher striatal transporter binding women carry at onset. The clinical question is never whether the gate works. It is where the gate is sitting and what is still able to move it.
The basal ganglia never tell a muscle what to do. They decide, instant by instant, what the rest of the brain is allowed to finish.
08Tone
How this system expresses tone
The basal ganglia express tone through what they forbid. Read this circuit at its output cells, where a standing inhibition is held, lifted for one program at a time, and re-set by dopamine over minutes and by joint traffic within a single step.
Constraint
Everything this cluster decides leaves through about 3,200 internal pallidal cells per hemisphere in the rat, so it can permit or refuse and never specify how a movement is made.
Set point
The gate defends a range, not a value. Pigmented nigral neurons fall 4.7 percent per decade in normal aging and movement holds, because the threshold re-sets as the cells go.
Input quality
Pallidal and subthalamic neurons in behaving primates answer passive joint rotation at short latency, and dorsal column stimulation improved gait across 25 human studies of Parkinson's disease.
Each of the others leaves a specific mark on this circuit. Gain: dopamine scales how strongly striatal neurons answer the same cortical drive, and losing it drives the D1 and D2 populations in opposite directions at once. Prediction: both striatal routes rise together up to 500 ms before a movement, so the gate has committed before the limb has moved. Oscillation: subthalamic activity locks into an 8 to 35 Hz rhythm whose suppression tracks rigidity at r = 0.835. Time course: the hyperdirect veto arrives in milliseconds while the nigral loss beneath a first symptom takes about five years. Load: the output neurons hold their inhibition by firing continuously, and a brake that never rests is paid for in energy. Coupling: one population of reticulata neurons restrains the thalamus, the superior colliculus, the pedunculopontine nucleus and the medullary reticular formation, so gaze and gait and posture shift as a set.
09Across the library
How this page relates to the rest of the library
Where dopamine is made, how far it spreads from release sites that form no synapse, and which receptor subtype reads it at each address in the striatum.
The forward models the cortex runs, and how a movement is predicted and corrected before and after this gate is asked to release it.
The other motor loop, its three cortical layers and four deep nuclei, and the olivocerebellar circuit where tremor rhythms are generated.
The saccade system held under the reticulata's GABAergic brake, and the collicular motor map that fires the moment that brake lifts.
The cortex that supplies the intention this gate rules on, and what behavior looks like when its own control processes fail.
That page owns the silent decade before the first symptom and the reserve it spends, staged as an illness through the gut, the sense of smell and dream sleep. This lesson owns the counts, the regional selectivity of the loss, and the gate those counts move.
10Frequently asked
Questions about this topic
What do the basal ganglia actually do?
The basal ganglia decide which movement is permitted. Their output cells never stop firing, and that steady inhibition holds the thalamus down until the striatum quiets them and the movement is released. The channel they speak through is narrow. About 2.79 million neostriatal neurons in one rat hemisphere leave through roughly 3,200 internal pallidal cells, a compression near nine hundred to one. A channel that thin can permit or refuse and nothing finer, so selection happens by veto, one program released while its rivals stay clamped.
What is the difference between the direct and indirect pathways?
The direct pathway runs from striatal GABAergic neurons onto the internal globus pallidus and removes the tonic brake on the thalamus, releasing the chosen movement. The indirect pathway projects onto the external globus pallidus and engages the subthalamic nucleus, which raises inhibitory output and suppresses competing programs. The populations differ chemically. Striatonigral neurons carry D1 receptors with substance P and dynorphin, and striatopallidal neurons carry D2 receptors with enkephalin. Dopamine reaches both and pushes them apart. Albin and colleagues described the two routes in 1989.
What is the hyperdirect pathway?
The hyperdirect pathway carries powerful excitation from motor cortical areas to the subthalamic nucleus and on to the pallidum, bypassing the striatum with a shorter conduction time. It produces a fast, broad inhibition that reaches the output nuclei before the striatal loop can answer. Blocking subthalamic activity with muscimol in awake monkeys abolished the early and late excitation that cortical stimulation evoked in pallidal neurons. Transmission from cortex to subthalamic nucleus runs mainly through NMDA receptors. Nambu and colleagues named the arrangement a dynamic center-surround model.
Why does losing dopamine make movement slow?
Dopamine sets the balance between the two striatal routes. When it is lost the indirect pathway dominates, inhibitory output from the internal globus pallidus and the pars reticulata rises, and thalamocortical activity falls. The clinical result is bradykinesia, a body that is slow rather than weak. Driving indirect-pathway neurons alone in mice reproduces the picture, with increased freezing and fewer locomotor initiations, and driving the direct pathway alone reverses it. Lesioning the subthalamic nucleus in MPTP-treated monkeys reduced akinesia, rigidity and tremor together in the contralateral limbs.
How much of the substantia nigra is lost before Parkinson's symptoms appear?
At the onset of symptoms about 68 percent of the lateral ventral tier of the substantia nigra is gone, and 48 percent of the caudal nigra as a whole. Loss concentrates in that tier rather than spreading evenly across the region. Pigmented nigral neurons fall about 45 percent in the first decade of disease. The terminals run ahead of the cell bodies, with striatal dopamine depleted roughly 70 to 80 percent by the time motor signs begin, so the gate fails before the cell count says it should.
Why does Huntington's disease cause too much movement?
Huntington's disease attacks the restraining route first. In 17 human specimens, enkephalin-containing neurons projecting to the external globus pallidus were much more affected in early and middle stages than substance P-containing neurons projecting to the internal segment. All targets were depleted only at advanced stages. The indirect pathway therefore fails while the releasing direct pathway stays intact. Unwanted programs escape a gate that can no longer clamp them, which is the imbalance expressed as chorea. Dystonia and related hyperkinetic syndromes come from the same failure of restraint.
Do the basal ganglia listen to the limbs, or only to the cortex?
The basal ganglia answer the limbs directly. Single-cell recording in behaving primates found short-latency responses to passive joint rotation. The same work mapped movement-related neurons somatotopically in the external pallidum, internal pallidum and subthalamic nucleus, with the proximal limb better represented than the distal. Epidural stimulation of the dorsal columns restored locomotion in dopamine-depleted mice and 6-hydroxydopamine rats, and disrupted the aberrant low-frequency corticostriatal synchrony alongside it. In people, gait improved in almost all Parkinson's patients who also had pain, across 25 studies and 103 participants.
Do the basal ganglia only control movement?
Movement is the best-mapped of the basal ganglia's jobs, not the only one. The ventral division takes its dopamine from the ventral tegmental area instead of the pars compacta. It gates reward and aversion the way the dorsal division gates motion, by releasing one option and clamping the rest. That shared architecture is why apathy and slowness turn up in the same illnesses, and why a drug that shifts striatal dopamine changes what a person will start as well as what they can start.
What does the Unified Model of Tone say the basal ganglia are measuring?
The model reads this cluster as a threshold device. Tone is the integrated organization of the body's interacting state, the chord and not the notes, and this circuit meets that state at the point where movement is released or withheld. Where the threshold sits is one reading of it, taken from the output neurons' firing and the 8 to 35 Hz subthalamic rhythm. A threshold that tracks the demand in front of it releases movement on time. Held too high it gives bradykinesia and freezing, held too low chorea and dystonia.
11The sources
References
Sources: primary literature, linked inline.