PSYC2101 week 12 motor system

Foundations of Neuroscience - Lecture Overview

• Instructor: Dr. Dorita Chang

• Course: PSYC2101

• Week: Apr 8

Week 12 Roadmap

• Topics Covered:

  • Systems Neuroscience III

  • The Central Motor System

  • Major Components:

  • Primary and Secondary Motor Cortex

  • Spinal Tracts

  • Integration of Concepts:

  • Back to the Motor Hierarchy

  • Movement Planning and Selection

  • Coordination and Monitoring

  • Properties of the Cortex

Motor Systems Overview

• Function: Generate coordinated movements resulting in observable behaviors (e.g., walking, running, speaking)

• Control Sources:

  • Spinal Cord:

  • Local coordination and control of muscle action: reflexes

  • Brain:

  • Management of complex, planned, and learned skills; utilizes lower-level motor programs for muscle selection

The Central Motor System

• Hierarchy Overview:

  • Planning/Strategy: (e.g., grasping a cup)→ prefrontal cortex, posterior parietal cortex, basal ganglia

  • Tactics: plan the action, select the right muscles (e.g., coordinated arm and finger movements). → motor and premotor cortex, cerebellum, basal ganglia

  • Execution: muscle movements; controls position and force→ brain stem, spinal cord

Major Components of the Motor Cortex

• Area 4 (Primary Motor Cortex - M1):

  • Located on the precentral gyrus, rostral to the central sulcus.

  • Somatotopic Organization: Controls contralateral side through a motor homunculus, similar to the somatosensory cortex.

  • Laminar Structure: 6-layer organization, particularly notable in layer V, which contains Betz cells (large pyramidal neurons)

  • Function: from motor cortex to spinal cord through corticospinal tract

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Area 6 - Secondary Motor Cortex

• Divisions:

  • Premotor Area (PMA): preparation for movement.

  • Supplementary Motor Area (SMA): Medial part focusing on complex movement sequences and coordination.

• Function: Preparatory role

Descending Motor Pathways in the Spinal Cord

1. Lateral Pathways

• Function: Facilitate voluntary limb movements.

• Components:

  • Corticospinal Tract (Pyramidal Tract):

  • Originates from areas 4 & 6, crosses at medulla-spinal cord junction.

  • Left motor cortex controls the right body side and vice versa.

  • Rubrospinal Tract:

  • Originates from red nucleus in midbrain, an indirect pathway from motor cortex

2. Ventromedial Pathways

• Function: Maintain balance, posture and some reflexes.

• Components:

  • Vestibulospinal tract: Maintains head balance.

  • Tectospinal tract: from tectum (superior colliculus) Mediates responses to visual stimuli.

  • Pontine and Medullary Reticulospinal Tracts: Manage antigravity reflexes and muscle tone adjustments.

Neuronal Characteristics in Motor Control

• Somatic/ Lower Motor Neurons: Located in the ventral horn of spinal cord, exits through ventral root ; control skeletal muscles.

  • Alpha Motor Neurons: Innervate extrafusal muscle fibers—mainly responsible for generating force in contractions and stretching

  • Gamma Motor Neurons: Innervate intrafusal fibers of muscle spindles—modulating spindle sensitivity (muscle tone)

Motor hierarchy

Planning of movements also involve other brain parts

• posterior parietal: spatial and 3D mental representation (eg. imaging a dance movement)

• prefrontal cortex: anticipate results of actions

Monkey “press button” experiment: anticipation of movement

After knowing which button he should press but not yet time to press the button, his premotor area fires

Firing stop quickly after the movement starts

Conclusion: premotor area is for planning

Mirror neurons in premotor area, fires when see other people performing similar actions as they do

Basal ganglia: selection and initiation of willed movements

Motor Planning and Basal Ganglia

• Function: Initiation and selection of movements.

• Components:

  • Striatum: Caudate + Putamen

  • Globus Pallidus & Substantia Nigra: Key roles in movement regulation and dopamine transmission.

  • Connections with Thalamus (VLo): Information relays to motor cortex to influence movement execution.

Clinical Considerations

• Parkinson's Disease:

  • Symptoms include difficulty initiating movements (akinesia), slowness (bradykinesia), and reduced movement (hypokinesia).

  • Caused by degeneration of dopaminergic neurons in substantia nigra affecting striatal function.

• Huntington's Disease:

  • Causes hyperkinesia, chorea, dementia due to loss of basal ganglia neurons.

Cerebellum

• Role: Adjusts timing and sequencing of muscle contractions for voluntary movements.

• Cerebellar Lesions: Can lead to ataxia, which is characterized by uncoordinated movements.

Cortical Properties

• Direction Tuning: M1 neurons peak firing based on movement direction—reflecting a population coding method where numerous neuron activations represent collective movement intentions.

• Illustration of Population Vector Coding: Each neuron contributes to the overall movement direction through individual “votes.”

Summary Notes

• The interplay of various cortical areas influences movement, with a critical focus on inhibition and disinhibition via the globus pallidus.

• Understanding of these neural circuits is essential for recognizing motor control and planning mechanisms.

Next Week Topics

• Introduction to Plasticity I: Genesis, Pruning

• Exploration of mirror neurons and the implications on social cognition and understanding actions in others.