Motor Cortex Lecture Notes

Overview of Motor Cortex

• The motor cortex consists of two primary areas: Area 4 (Primary Motor Cortex) and Area 6.
• Primary Motor Cortex (M1) is located in the precentral gyrus in front of the central sulcus.
• Area 6 is anterior to Area 4 and is divided into:
  • Supplementary Motor Area (SMA)
  • Premotor Area (PMA)

Structure and Function of Motor Cortex

Primary Motor Cortex (Area 4)

• Location: Precentral gyrus, right in front of the central sulcus.
• Function: Contains neuron cell bodies that form descending spinal tracts, arranged in a somatotopic map.
• Neurons controlling adjacent body parts are located next to each other.
  • Example: Neurons for the shoulder, elbow, wrist, and hand are sequentially arranged.
• Controls the opposite side of the body (left motor cortex controls right side of body).

Somatosensory Cortex (Postcentral Gyrus)

• Located just behind the primary motor cortex.
• Controls sensory input and has a similar somatotopic arrangement.
• The primary somatosensory cortex receives touch input from regions corresponding to the motor cortex.

Representation of Body Parts

• Overrepresentation: Hand, fingers, lips, jaw, and tongue have a higher degree of representation in the motor cortex due to the need for fine control.
• Example of fine control: The hand requires more precise muscle control than larger body parts like the arms.

Areas 5 & 7: Posterior Parietal Cortex

• Integrates sensory input and spatial awareness, crucial for coordinating movement.
• Damage can lead to hemispatial neglect affecting perception on the opposite side of the body.

Area 6: Supplementary Motor Area (SMA) and Premotor Area (PMA)

• SMA: Involved in planning movements, particularly for distal motor units (arms and legs).
• PMA: More associated with proximal muscles (shoulder, hips).
• Neurons in areas 4 and 6 control spinal cord motor neurons for muscle movements.

Planning and Initiation of Movement

• Experiment on monkey: Neurons in PMA become active during a waiting period before the monkey triggers movement. This indicates planning activity.
• Instruction stimulus prepares the motor cortex for action before the actual movement occurs.

Inputs to Motor Cortex

1. Posterior Parietal Areas: Integrate sensory information to inform motor actions.
2. Prefrontal Cortex: Involved in higher-level planning and decision-making regarding movements. Can inhibit non-goal-oriented movements.

Encoding Movements in Motor Cortex

• Neurons in the primary motor cortex do not send commands directly to individual muscles.
• Instead, they have direction vectors indicating preferred movement directions.
• Population coding: Movement direction is determined by the combined activity of neuron populations with different direction preferences.

Plasticity in Motor Cortex

• The motor cortex exhibits plasticity, allowing it to adapt based on experience or training.
• Examples include:
  • Rat experiment: Cutting nerve connections to vibrissae (whiskers) leads to reorganization in the motor cortex.
  • Braille training in humans: Increased activity in the motor cortex associated with the trained hand after extensive practice.

Conclusion

• The motor cortex is complex and adaptable, influencing both basic motor functions and higher cognitive planning.
• Future lectures will explore non-cortical parts of the brain involved in movement.