The Refinement of Movement

Basal Ganglia and Cerebellum

• Basal Ganglia

  • Key roles:

    • Movement ON/OFF control

    • Movement specificity

    • Learning of movements

• Cerebellum

  • Functions include:

    • Movement error detection

    • Movement error correction

    • Learning from movement errors

Circuitry and Inputs

Basal Ganglia and Cerebellar Circuits

• Receive input from the cortex

• Provide output to upper motor neurons in cortex and brainstem

• Predominantly inhibitory circuits that require disinhibition to promote movement

• Integration of multiple areas:

  • Cerebral Cortex

  • Basal Ganglia

  • Thalamus

  • Cerebellum

  • Spinal Cord

  • Brain Stem

Basal Ganglia Functions

Voluntary Movement Control

• Essential for normal voluntary movement

• Mediates smooth transitions between commands for movement initiation and termination

• Disfunctions note:

  • Parkinson’s Disease (PD): Affects movement initiation

  • Huntington’s Disease (HD): Causes unwanted movements

Input Structures

• Corpus Striatum

  • Receives cortical inputs consisting of caudate and putamen

    • Caudate: Involved in eye, head, and body movements; processes sensory input

    • Putamen: Involved in body movements; processes motor inputs

Globus Pallidus Role

• Acts as a critical point for movement inhibition

• Receives input from the corpus striatum

• Must be "turned off" to allow thalamic excitation and thereby movement

Thalamus and Motor Cortex Interaction

Inhibition and Relay

• Thalamus receives inhibitory signals from the internal globus pallidus (GPi)

• Responsible for providing excitatory drive to the motor cortex;

• Disinhibition is necessary for movement

Basic Loop: Direct Pathway Through Basal Ganglia

Sequence of Actions

1. Cortical input generates a motor plan.

2. Basal ganglia modulate inhibition - "Go" / "Stop" commands.

3. Thalamic integration relays the motor commands.

4. Motor cortex executes the resultant action.

Medium Spiny Neurons (MSNs)

• Critical for decision making regarding movement

• Prior to movement, MSNs fire action potentials to signal movement intent

• Inhibit globus pallidus neurons to facilitate movement

Pathway Projections

Direct, Indirect, and Hyper-Direct Pathways

• Direct Pathway: Excitation via caudate/putamen to GPi, leading to thalamic disinhibition.

• Indirect Pathway: Increases inhibitory output to suppress unwanted movements.

• Hyper-Direct Pathway: Inhibitory pathway through the subthalamic nucleus, counteracting movement commands.

Neurotransmitters

Dopamine in the Basal Ganglia

• Substantia Nigra: Provides dopaminergic input to caudate/putamen.

  • Enhances activity of the direct pathway and inhibits the indirect pathway.

• Loss of dopaminergic neurons leads to movement difficulties (e.g., in Parkinson's Disease).

Clinical Applications

Parkinson's and Huntington's Disease

• Parkinson’s Disease: Loss of dopamine neurons results in impaired initiation and control of movements.

• Huntington’s Disease: Degeneration of caudate-to-GP connections increases unwanted movements and ballistic actions.

Deep Brain Stimulation (DBS)

• Initiated to alleviate motor symptoms in diseases like Parkinson’s.

• Effective at normalizing abnormal activity in the basal ganglia circuit.

• Target Areas: Internal Globus Pallidus or Subthalamic Nucleus.