Motor Systems and Basal Ganglia: Structure, Function, and Disorders

Skilled Performance and Motor Learning

Introduction to Motor Systems

The motor system is responsible for the planning, initiation, and execution of voluntary movements. It involves multiple brain regions, including the cerebral cortex and subcortical structures such as the basal ganglia and cerebellum.

Cerebral Cortex – Motor Areas

Primary Motor Cortex (M1)

• Location: Precentral gyrus of the frontal lobe.

• Function: Initiates voluntary movements by sending signals to lower motor neurons.

• Somatotopic Organization: Different body parts are represented in specific cortical areas (motor homunculus).

Supplementary Motor Area (SMA)

• Function: Involved in planning and coordinating complex, self-initiated movements.

• Role: Especially important for movements that are internally generated rather than triggered by external stimuli.

Premotor Cortex (PMC)

• Function: Involved in the planning of movements, especially those guided by sensory information.

• Role: Integrates sensory cues to guide movement selection and preparation.

Differences in Motor Areas

• Different types of movements (e.g., simple, sequential, or imagined) activate distinct regions within the motor cortex.

• Functional neuroimaging studies (e.g., Roland, 1980) show that planning, execution, and decision-making involve overlapping but distinct cortical areas.

Descending Motor Systems

Lateral (Dorsal) Motor Systems

• Lateral Corticospinal Tract: Main pathway for voluntary control of distal limb muscles.

• Rubrospinal Tract: Assists in movement of the limbs.

• Function:

  • Goal-directed limb movements

  • Control of distal muscles (extremities)

  • Contralateral projections (crosses to opposite side of body)

Medial (Ventral) Motor Systems

• Anterior (Ventral) Corticospinal Tract

• Vestibulospinal Tract

• Reticulospinal Tract

• Tectospinal Tract

• Function:

  • Postural tone

  • Control of proximal and axial muscles

  • Ipsilateral and bilateral projections

Basal Ganglia: Structure and Function

Overview

The basal ganglia are a group of subcortical nuclei at the base of the cerebral cortex, crucial for the control of movement, especially the initiation and regulation of voluntary motor activity.

Main Functions

• Preparation and execution of movement

• Planning and control of complex movement sequences

• Programming self-initiated (internally generated) movements

• Control of habitual, skill-based behaviors

• Selective activation of some movements while suppressing others (inhibition of unwanted movements)

Anatomical Components

• Striatum: Caudate nucleus and putamen (main input nuclei)

• Lentiform (Lenticular) Nucleus: Putamen and globus pallidus

• Globus Pallidus: Internal (GPi) and external (GPe) segments

• Subthalamic Nucleus: Located inferior to the thalamus, communicates with globus pallidus

• Substantia Nigra: Pars compacta (dopaminergic neurons) and pars reticulata (output, inhibits thalamus)

Inputs to the Basal Ganglia

• Input nuclei: Striatum (putamen and caudate)

• Sources:

  • Widespread areas of the cerebral cortex (sensory, motor, association areas)

  • Thalamus

Outputs from the Basal Ganglia

• Output nuclei (inhibitory, GABAergic):

  • Globus pallidus internal segment (GPi)

  • Substantia nigra pars reticulata (SNpr)

• Targets:

  • Thalamus

  • Superior colliculus

Basal Ganglia Pathways

The basal ganglia modulate movement through two main pathways, both of which influence the thalamus and, consequently, motor cortex activity.

• Direct Pathway: Facilitates movement by increasing thalamic output to the cortex.

• Indirect Pathway: Inhibits movement by decreasing thalamic output to the cortex.

Direct Pathway (Facilitates Movement)

• Cortex excites striatum

• Striatum inhibits GPi/SNpr

• GPi/SNpr inhibition of thalamus is reduced (disinhibition)

• Thalamus excites motor cortex, promoting movement

Indirect Pathway (Inhibits Movement)

• Cortex excites striatum

• Striatum inhibits GPe

• GPe inhibition of subthalamic nucleus is reduced

• Subthalamic nucleus excites GPi/SNpr

• GPi/SNpr increases inhibition of thalamus

• Thalamic excitation of motor cortex is reduced, suppressing movement

Effect of Dopamine

• Dopamine from substantia nigra pars compacta has different effects on the two pathways:

• Direct pathway: Dopamine excites striatal neurons (via D1 receptors), facilitating movement

• Indirect pathway: Dopamine inhibits striatal neurons (via D2 receptors), reducing inhibition of movement

• Loss of dopamine (as in Parkinson's disease) disrupts this balance, leading to motor deficits

Basal Ganglia Disorders

Parkinson's Disease

• Cause: Degeneration of dopaminergic neurons in the substantia nigra pars compacta (unknown etiology)

• Pathophysiology: Loss of dopamine disrupts normal modulation of basal ganglia output, leading to excessive inhibition of the thalamus and reduced movement

• Classic Symptoms:

  • Akinesia and Bradykinesia: Difficulty initiating movement; slow, small movements

  • Resting Tremor: Rhythmic oscillation in distal limbs (usually hands or upper extremities)

  • Rigidity: Increased muscle tone and stiffness

  • Postural Instability: Unsteady gait, increased risk of falls

• Other Symptoms:

  • Stooped posture

  • Shuffling gait with freezing and difficulty turning

  • Diminished arm swing while walking

  • Micrographia (small handwriting) and hypophonia (soft speech)

  • Diminished facial expressions (masked facies)

Huntington's Disease

• Cause: Degeneration of striatal neurons in the caudate and putamen, especially those of the indirect pathway

• Pathophysiology: Reduced inhibition of unwanted movements due to loss of indirect pathway neurons

• Symptoms:

  • Hyperkinetic movements, including chorea (involuntary, dance-like movements) and dystonia (sustained muscle contractions)

Summary Table: Basal Ganglia Pathways and Disorders

Pathway	Effect on Movement	Neurotransmitter	Disorder (Example)
Direct	Facilitates movement	Dopamine (D1 receptors, excitatory)	Impaired in Parkinson's (reduced activity)
Indirect	Inhibits movement	Dopamine (D2 receptors, inhibitory)	Impaired in Huntington's (reduced inhibition)

Conclusion

The basal ganglia are essential for the control and regulation of voluntary movement. Disorders such as Parkinson's and Huntington's disease highlight the importance of these structures in both the initiation and suppression of movement. Although much is known about their anatomy and function, the basal ganglia's full role in motor control remains an active area of research.

Additional info: The notes above expand on the original slides by providing definitions, context, and examples for key terms and concepts, as well as a summary table for clarity.