Lecture #31: Introduction to Supraspinal Motor Systems Part II: Basal Ganglia

What are the basal ganglia?

The basal ganglia are a collection of interconnected gray matter nuclei that communicate with the cerebral cortex via the thalamus and are essential for selecting, initiating, and suppressing actions and behaviors.

What is the primary function of the basal ganglia?

The basal ganglia regulate action selection by promoting desired movements or behaviors while suppressing competing or unwanted actions.

How do the basal ganglia communicate with the cerebral cortex?

The basal ganglia form closed-loop circuits with the ipsilateral cerebral cortex through relay nuclei in the thalamus.

What are the four functional categories of basal ganglia nuclei?

Input nuclei, intrinsic nuclei, modulatory nuclei, and output nuclei.

Which nuclei make up the striatum?

The striatum consists of the caudate nucleus, putamen, and nucleus accumbens.

What is the role of the striatum in basal ganglia circuitry?

The striatum serves as the main input nucleus of the basal ganglia and integrates excitatory cortical signals.

What neurotransmitter is released by striatal neurons?

Striatal neurons primarily release GABA, providing inhibitory output to downstream nuclei.

What structures form the pallidum?

The pallidum consists of the globus pallidus external segment (GPe), globus pallidus internal segment (GPi), and ventral pallidum.

What is the role of the globus pallidus internal segment (GPi)?

The GPi is a major output nucleus of the basal ganglia that inhibits thalamic targets.

What neurotransmitter do pallidal neurons release?

Pallidal neurons release GABA.

What is the subthalamic nucleus (STN)?

The STN is a diencephalic nucleus that provides excitatory input to basal ganglia output nuclei.

What neurotransmitter does the STN release?

The subthalamic nucleus releases glutamate.

What is the role of the substantia nigra pars compacta (SNpc)?

The SNpc provides dopaminergic modulation to the striatum.

What is the role of the substantia nigra pars reticulata (SNr)?

The SNr is an output nucleus of the basal ganglia that inhibits thalamic and brainstem targets.

What is the ventral tegmental area (VTA)?

The VTA is a dopaminergic modulatory nucleus involved primarily in limbic and reward-related basal ganglia circuits.

What is the direct pathway of the basal ganglia?

The direct pathway promotes selected actions by reducing inhibitory output from the basal ganglia to the thalamus.

How does the direct pathway facilitate movement?

Striatal inhibition of GPi and SNr reduces thalamic inhibition, allowing cortical excitation to initiate movement.

What is the indirect pathway of the basal ganglia?

The indirect pathway suppresses competing actions by increasing inhibitory output to the thalamus.

How does the indirect pathway inhibit movement?

Striatal inhibition of GPe disinhibits the STN, which excites GPi and increases thalamic inhibition.

What is the hyperdirect pathway?

The hyperdirect pathway rapidly suppresses ongoing or planned actions through direct cortical excitation of the STN.

Why is the hyperdirect pathway important?

It allows rapid global inhibition of actions, enabling sudden stopping of behavior.

What are medium spiny neurons (MSNs)?

Medium spiny neurons are GABAergic projection neurons in the striatum that form the core of basal ganglia processing.

What distinguishes D1 and D2 MSNs?

D1 MSNs facilitate the direct pathway, while D2 MSNs facilitate the indirect pathway.

How does dopamine affect the direct pathway?

Dopamine increases excitability of D1 neurons, enhancing action promotion.

How does dopamine affect the indirect pathway?

Dopamine decreases excitability of D2 neurons, reducing action suppression.

What is the overall effect of dopamine on movement?

Dopamine biases basal ganglia output toward movement initiation.

How does acetylcholine affect basal ganglia pathways?

Acetylcholine has effects opposite to dopamine, decreasing direct pathway activity and increasing indirect pathway activity.

What are the major basal ganglia functional channels?

Motor, oculomotor, prefrontal, and limbic channels.

Which dopaminergic system modulates motor basal ganglia circuits?

The nigrostriatal pathway from the substantia nigra pars compacta.

Which dopaminergic system modulates limbic basal ganglia circuits?

The mesolimbic pathway from the ventral tegmental area.

What happens when the direct pathway is underactive?

Hypokinetic movement disorders with reduced voluntary movement occur.

What happens when the indirect pathway is underactive?

Hyperkinetic movement disorders with excessive involuntary movements occur.

What is Parkinson’s disease?

A neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra pars compacta.

What are the hallmark motor features of Parkinson’s disease?

Resting tremor, rigidity, bradykinesia or akinesia, and postural instability.

How does dopamine loss affect basal ganglia circuitry in Parkinson’s disease?

Reduced direct pathway activity and excessive indirect pathway activity suppress thalamic output.

What are Lewy bodies?

Eosinophilic intracellular inclusions containing alpha-synuclein found in Parkinson’s disease.

What are common non-motor symptoms of Parkinson’s disease?

Anosmia, sleep disturbances, autonomic dysfunction, mood disorders, and cognitive impairment.

What is hemiballismus?

A hyperkinetic movement disorder characterized by violent flinging movements of one side of the body.

Which nucleus is damaged in hemiballismus?

The subthalamic nucleus.

Why does STN damage cause hyperkinetic movement?

Loss of STN excitation reduces GPi inhibition of the thalamus, leading to excessive cortical activation.

What is Huntington’s disease?

An inherited neurodegenerative disorder characterized by progressive chorea and cognitive decline.

What genetic mutation causes Huntington’s disease?

Expansion of CAG repeats in the huntingtin gene.

What basal ganglia structure degenerates in Huntington’s disease?

The striatum, particularly the caudate nucleus.

How does Huntington’s disease affect basal ganglia pathways?

Preferential loss of indirect pathway neurons leads to excessive movement.

What is tardive dyskinesia?

A delayed-onset movement disorder characterized by repetitive involuntary movements, often caused by chronic dopamine receptor blockade.

Why can antipsychotic drugs cause Parkinsonian symptoms?

D2 receptor antagonism mimics dopamine depletion in the basal ganglia.

What is the overarching role of the basal ganglia?

The basal ganglia coordinate action selection by balancing promotion and suppression of behaviors through inhibitory control.