Basal ganglia anatomy .

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Basal Ganglia Anatomy

The basal ganglia are a collection of gray matter nuclei located deep within the white matter of the cerebral hemispheres. They do not project directly to the periphery, but participate in complex networks that modulate the descending motor systems, as well as cognitive and affective functions.

1. Components and Terminology

The main components are:

Grouping	Structures
Striatum (neostriatum)	Caudate nucleus + Putamen
Lenticular (lentiform) nucleus	Putamen + Globus pallidus
Pallidum	Globus pallidus - external segment (GPe) + internal segment (GPi)
Associated nuclei	Subthalamic nucleus (STN), Substantia nigra (SNc + SNr)
Ventral striatum	Nucleus accumbens + ventral putamen/caudate

The striatum receives virtually all inputs to the basal ganglia. Outputs leave via the GPi and substantia nigra pars reticulata (SNr).

Neuroanatomy through Clinical Cases, 3rd Ed., p. 768
Costanzo Physiology 7th Ed., p. 3467

2. Three-Dimensional Spatial Relationships

Basal ganglia spatial relationships - lateral and anterolateral views showing caudate, putamen, globus pallidus, thalamus, nucleus accumbens, and amygdala

Fig. 16.1 - Spatial relationships of the basal ganglia, thalamus, and amygdala. (A) Lateral view; (B) Anterolateral view. From Neuroanatomy through Clinical Cases, 3rd Ed.

Caudate Nucleus

A C-shaped structure that runs along the lateral wall of the lateral ventricle
Divided into head, body, and tail (no sharp boundaries between them)
The head bulges into the lateral wall of the anterior horn of the lateral ventricle
The tail curves inferiorly and anteriorly through the temporal lobe, ending just anterior to the amygdala
Connected to the putamen by cellular bridges that cross the internal capsule - these give rise to the "striped" appearance and the name "striatum"

Putamen

The largest and most lateral nucleus of the basal ganglia
Anteriorly and ventrally it fuses with the head of the caudate - this region is the ventral striatum
The nucleus accumbens forms the bulk of the ventral striatum and is important in limbic circuitry

Globus Pallidus

Lies just medial to the putamen
Named "pale globe" due to the many myelinated axons traversing it
Divided into external segment (GPe) and internal segment (GPi) by the internal medullary lamina
GPi is structurally and functionally homologous with the SNr

Lenticular (Lentiform) Nucleus

The putamen + globus pallidus together form a lens- or lentil-shaped structure
Lateral to the internal capsule in all planes of section

Internal Capsule Relationships

Fig. 16.3 - Basal ganglia and thalamus in relation to the internal capsule. From Neuroanatomy through Clinical Cases, 3rd Ed.

Moving from lateral to medial (on axial section):

Insula → extreme capsule → claustrum → external capsule → Putamen → external medullary lamina → GPe → internal medullary lamina → GPi → internal capsule

Key landmarks:

Anterior limb of the internal capsule: between the head of caudate (medially) and the lentiform nucleus (laterally)
Posterior limb of the internal capsule: between the thalamus (medially) and the lentiform nucleus (laterally)
The caudate and thalamus are always medial to the internal capsule; the lentiform nucleus is always lateral

Subthalamic Nucleus (STN)

A small lens-shaped structure at the border between the cerebrum and brainstem
Located ventral to the thalamus, dorsal to the substantia nigra

Substantia Nigra

Extends the length of the midbrain, ventral to the red nucleus and dorsal to the cerebral peduncles
Contains about 500,000 dopaminergic neurons normally
Divided into pars compacta (SNc) - dopamine-producing - and pars reticulata (SNr) - output nucleus
A melanin-containing (pigmented) nucleus; the dark coloration is from neuromelanin

3. Afferent (Input) Connections

All inputs arrive via the striatum. Sources include:

Nearly all areas of the cerebral cortex (topographically organized)
Intralaminar nuclei of the thalamus
Substantia nigra pars compacta (SNc) - dopaminergic input
Locus coeruleus (noradrenergic) and raphe nuclei (serotonergic)
There is also a direct cortical projection to the STN (hyperdirect pathway)

4. Efferent (Output) Connections

The GPi and SNr are the output nuclei:

GPi → ventral anterior (VA) and ventrolateral (VL) thalamus → motor and premotor cortex (including SMA)
GPi → centromedian thalamic nucleus
SNr → mediodorsal thalamus and superior colliculus (eye movements)
Outputs are also sent to the pedunculopontine nucleus (PPN) in the brainstem

Schematic of basal ganglia afferent, intrinsic, and efferent connections including cortex, striatum, GPe, GPi, STN, SNc, SNr, and thalamus

Fig. 96.1 - Afferent (blue), intrinsic (dashed red), and efferent (green) connections of the basal ganglia. C,P = caudate/putamen; GPe = external globus pallidus; GPi = internal globus pallidus; STN = subthalamic nucleus; SNc = substantia nigra pars compacta; SNr = pars reticulata; VA/VL = thalamic nuclei; SC = superior colliculus; CM = centromedian nucleus. From Bradley and Daroff's Neurology in Clinical Practice.

5. Direct and Indirect Pathways

These two opposing pathways are fundamental to basal ganglia function and are carefully balanced:

Direct Pathway (net excitatory effect on motor cortex)

Cortex → Striatum (D1 receptors) → GPi/SNr → Thalamus → Motor Cortex

Striatum sends inhibitory (GABA) projections to GPi/SNr
This reduces tonic inhibition of the thalamus, thereby disinhibiting it
Result: increased thalamic output → increased cortical excitation → facilitation of movement

Indirect Pathway (net inhibitory effect on motor cortex)

Cortex → Striatum (D2 receptors) → GPe → STN → GPi/SNr → Thalamus → Motor Cortex

Striatum inhibits GPe (GABA)
GPe then releases its inhibitory tone on STN
STN increases excitatory (glutamate) drive to GPi/SNr
GPi/SNr increases inhibition of thalamus
Result: reduced thalamic output → suppression of movement

Dopamine's Role (nigrostriatal projection)

D1 receptors (direct pathway): dopamine is excitatory - facilitates movement
D2 receptors (indirect pathway): dopamine is inhibitory - reduces indirect pathway activity, further facilitating movement
Net effect: dopamine from SNc promotes movement through both pathways

6. Parallel Functional Circuits

There are several parallel cortico-basal ganglia-thalamo-cortical loops:

Circuit	Input cortex	Output target	Function
Motor	Sensorimotor cortex	SMA / premotor cortex	General movement
Oculomotor	Frontal eye fields	Superior colliculus / frontal eye fields	Saccadic eye movements
Prefrontal	Prefrontal cortex	Prefrontal cortex	Cognitive/executive functions
Limbic	Anterior cingulate, orbitofrontal	Limbic areas	Emotion, motivation

7. Clinical Relevance

Disorder	Pathology	Mechanism	Features
Parkinson disease	Degeneration of SNc dopaminergic neurons	Decreased D1 activation (less facilitation) + decreased D2 inhibition (more indirect pathway activity) → net increased GPi inhibition of thalamus	Resting tremor, rigidity, bradykinesia, shuffling gait
Huntington disease	Destruction of striatal (GABAergic/cholinergic) neurons	Loss of indirect pathway striatal neurons → decreased GPe inhibition → decreased STN drive → less GPi inhibition → excessive movement	Chorea (involuntary writhing movements), dementia
Hemiballismus	STN lesion (usually infarct)	Loss of STN excitatory drive to GPi → reduced thalamic inhibition → involuntary movement	Wild flinging movements of contralateral limbs

Sources:

Neuroanatomy through Clinical Cases, 3rd Ed. (Blumenfeld), pp. 767-775
Bradley and Daroff's Neurology in Clinical Practice, Ch. 96
Costanzo Physiology, 7th Ed., pp. 3467-3482
Neuroscience: Exploring the Brain, 5th Ed. (Bear, Connors, Paradiso), Ch. 14

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