Basal Ganglia

Major components involved in motor control: - Motor cortex - Basal ganglia - Cerebellum - Thalamus - Pyramidal tracts - Brainstem - Spinal cord - Extrapyramidal tracts - Muscles of the body - Muscles of the head and neck

The basal ganglia modulate the activity of upper motor neurons, without direct connections to lower motor neurons.
Functional mechanisms: - Reciprocal regulatory loops: Involve feedback and feedforward mechanisms that regulate motor functions.

Striatum: Comprised of: - Caudate nucleus - Putamen - Ventral striatum (nucleus accumbens)
Globus Pallidus (GP): Includes: - External segment - Internal segment
Subthalamic nucleus
Substantia nigra (SN): - Pars reticulata (related to Globus pallidus) - Pars compacta (dopaminergic neurons)
Connections to: - Motor cortices - Thalamic nuclei (VA, VL)

Structure: - Cortex provides modulatory inputs to the caudate and putamen. - Responsible for short-term motor memory or echo.

Execution of Sequential Movements: - Involved in the selection and initiation of intended movements. - Firing patterns change with the anticipation, execution, and cessation of movements.
Learning and Reinforcement: - The striatum signals aspects of learning and reinforcement related to motor actions. - Tonic firing of striatal neurons in response to conditioned sensory stimuli linked to rewards (functions as an “error signal”).
Procedural Learning and Habit Formation: - Neurons exhibit changes in firing patterns as tasks are learned or when novel stimuli are introduced.

Beyond motor functions, the basal ganglia also play roles in: - Emotion regulation - Memory

Corticostriatal pathway: Projections from all association cortices: - Caudate nucleus: Inputs from multimodal frontal and motor cortices (controls eye movement). - Putamen: Inputs from higher-order sensory, premotor, and motor cortices. - Parallel cortico-basal ganglia projections: Segregation maintained in basal ganglia outputs.

Projection neurons: Medium spiny neurons - Constitute 80% to 95% of all neurons in the striatum. - Utilize GABA (inhibitory) as their transmitter, exhibiting low levels of basal activity.
Local circuit neurons: - Medium aspiny cells: Use GABA. - Giant aspiny cells: Use acetylcholine.

MSNs integrate inputs from various cortical and subcortical structures.
Inputs from: - Local circuitry - Thalamic neurons - Dopaminergic neurons - Various neuromodulators.
Outputs are directed as follows: - Putamen → Globus Pallidus - Caudate → Substantia Nigra

Excitatory inputs (Glutamate): From frontal (motor) cortices including: - Primary motor cortex - Premotor cortex - Additional regions: prefrontal, parietal, temporal, and cingulate cortices.
Intrinsic inhibitory projections (GABA): BG functions primarily through an inhibitory network.
Modulatory inputs (Dopamine): Primarily from the substantia nigra, pars compacta.

Neurons in the substantia nigra and globus pallidus tonically inhibit: - Cells in the superior colliculus - VA/VL thalamus.
This inhibition prevents unwanted movement.
Disinhibition mechanism: - GABAergic feedback from medium spiny neurons in the striatum inhibits this tonic inhibition, allowing for the initiation of movement.

Transient inhibition by MSNs in the striatum leads to:
Inhibition of tonically active inhibitory neurons in the internal segment of GP.
Resulting in disinhibition of excitatory projections from the thalamus to the cortex.

Transient inhibition from MSNs in the striatum leads to:
Inhibition of tonically active inhibitory neurons in the external segment of GP.
Resulting in disinhibition of excitatory projections from the subthalamic nucleus to the internal segment of GP.
This pathway has the opposite effect of the direct pathway, acting to inhibit movement.
Dopaminergic modulation from SN impacts both pathways, leading to a net effect of increased movement.

Major output nuclei include: - Internal segment of Globus Pallidus (GPi) - Substantia Nigra Pars Reticulata (SNr)
Both nuclei tonically inhibit target nuclei in the thalamus and brainstem.
Inhibitory and excitatory balance between direct and indirect pathways influences movement initiation.
The involvement of dopamine receptors (D1 for direct pathway and D2 for indirect pathway) modulates the overall effect on motor control.

Movement disorders correlate with dysfunction in the basal ganglia. Key diseases include: 1. Parkinson's Disease 2. Huntington's Disease 3. Hemiballismus
Characteristic motor disturbances observed in these conditions include: 1. Tremors and involuntary movements 2. Postural and muscle tone alterations 3. Poverty of movement (hypokinesia) without paralysis (as seen in Parkinson's).
Conversely, disorders like Huntington's may lead to excessive movements (hyperkinesia).

Characterized by the loss of dopaminergic neurons in the substantia nigra.
Age-related decline in the number of dopaminergic neurons leading to reduced control over thalamic and cortical regions that initiate movement.

The basal ganglia play a complex role in modulating motor activity, with implications for emotional and cognitive processes as well. Future discussions will delve deeper into these aspects.