Basal Ganglia Lecture Notes

Basal Ganglia Overview

  • Definition: The basal ganglia refers to a collection of nuclei in the brain that play a crucial role in the regulation of motor control and various cognitive processes.

Collection of Nuclei

  • The primary components of the basal ganglia include:

    1. Striatum: Comprised of the caudate nucleus and the putamen.

    2. Globus Pallidus: Divided into the internal and external segments.

    3. Substantia Nigra: Contains dopaminergic neurons, crucial for movement regulation.

    4. Subthalamic Nucleus: Plays a role in controlling motor functions.

Anatomical Structure

Sagittal View of Basal Ganglia

  • Key components visible in a sagittal view include:

    • Striatum (including caudate and putamen)

    • Globus pallidus (both lateral and medial parts)

    • Thalamus

    • Subthalamic nucleus

    • Substantia nigra

    • Amygdala

Input and Output Pathways

Inputs to the Basal Ganglia

  • The main input to the striatum comes from various sources:

    • From the cortex and brainstem.

    • Caudate Nucleus: Receives input from:

      • Multimodal association cortices.

      • Motor areas of the frontal lobe controlling eye movements.

    • Putamen: Receives input from:

      • Primary and secondary somatosensory cortices.

      • Secondary visual cortices.

      • Premotor and motor cortices.

      • Auditory association areas.

Outputs from the Basal Ganglia

  • The principal output nuclei of the basal ganglia are:

    • Globus Pallidus (GP)

    • Substantia Nigra (SN)

  • Output is primarily inhibitory, employing GABAergic signaling.

    • SN includes two regions:

      • Pars Reticulata.

      • Pars Compacta (contains dopaminergic cells).

Electrophysiology of Striatal Neurons

Striatum's Neuron Composition

  • A significant majority (approximately 95%) of neurons in the striatum are GABAergic, medium spiny neurons (MSNs).

  • Function of MSNs:

    • Receive inputs via their dendrites from various cortical regions.

    • Project inhibitory signals to both internal and external segments of the GP.

Synaptic Connections

  • Dendritic Spines:

    • Medium spiny neurons receive excitatory inputs from:

      • Corticocortical pathways.

      • Corticothalamic pathways.

      • Corticospinal pathways, which form excitatory glutamatergic synapses on dendritic spines.

  • Input Structure of MSNs:

    • Low number of contacts from a single axon leads to divergence; many axons can also contact a single MSN, allowing for integration from thousands of cortical cells.

Basal Ganglia Pathways

Direct and Indirect Pathways

  • Direct Pathway:

    • Project from striatum to internal globus pallidus (GPi) and then to thalamus.

  • Indirect Pathway:

    • Project from striatum to external GP and then indirectly to GPi, influencing thalamic output.

  • Both pathways mediate outputs which modulate motor control.

BP Circuitry Dynamics

  • At rest:

    • GP is tonically active, inhibiting motor cortex via thalamic inhibition.

  • With excitation of the striatum, a cascade of disinhibition ensues:

    • GP is transiently inhibited, allowing for enhanced excitatory output from VA/VL thalamus to the motor cortex.

Disinhibition Mechanism

  • Disinhibition of VA/VL Thalamus:

    • Requires excitation of striatum, which inhibits GP outputs that usually inhibit thalamic areas, thus facilitating movement.

  • Saccadic Eye Movements:

    • Involves activation of the caudate, leading to inhibition of SNpr, which relieves tonic inhibition of the superior colliculus.

Dopamine Influence

Role of Dopamine in Pathways

  • Dopaminergic Inputs:

    • Dopamine modulates both pathways via D1 and D2 receptors.

    • D1 receptors facilitate movement by enhancing striatal inhibition of GP.

    • D2 receptors inhibit striatal neurons involved in the indirect pathway, thus reducing overall basal ganglia output.

Disorders Related to Basal Ganglia

Hyperkinesia and Hypokinesia

  • Two main groups of basal ganglia disorders:

    1. Hyperkinesia:

      • Involves uncontrolled movements (dyskinesia). Example: Huntington's Disease.

    2. Hypokinesia:

      • Difficulty in initiating movement (akinesis). Example: Parkinson's Disease.

Parkinson's Disease

  • Pathophysiology:

    • Degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc).

    • Results in an overall loss of dopaminergic signaling, leading to increased motor inhibition.

  • Symptoms:

    • Muscle rigidity, bradykinesia, and tremor.

  • Treatment:

    • L-Dopa: Widely used but has limitations due to decreasing efficacy over time as dopaminergic neurons degenerate.

    • Deep Brain Stimulation (DBS): Involves implanting electrodes to stimulate basal ganglia outputs.

Huntington