Cortical Motor Areas

PSL300 Overview

• Course: PSL300 Physiology at University of Toronto

Cortical Motor Areas

• Key Areas Involved:

  • Red Nucleus

  • Synergy

  • Motor Cortex

  • Motor Field

  • Premotor Area

  • SMA (Supplementary Motor Area)

  • CMA (Cingulate Motor Area)

Hierarchical Organization of Motor Control

• Simple Reflexes:

  • Involve sensory neurons synapsing with spinal motoneurons.

  • Basic form of motor control.

• Central Pattern Generators (CPGs):

  • Networks of interneurons in spinal cord/brainstem for coordinated motor functions (e.g., locomotion, respiration).

• Complex/Volitional Movement:

  • Involves planning and refining motor output via motor cortex, basal ganglia, and cerebellum.

Red Nucleus

• Rubrospinal Cells:

  • Activate localized synergies, particularly in distal limbs and face (e.g., gripping, twisting).

• Contrasts with Other Pathways:

  • Reticulospinal and vestibulospinal tracts organize broader postural and locomotory synergies.

• Rubrospinal Tract:

  • Crosses midline, terminates in intermediate zones of spinal cord.

Synergy

• Definition:

  • Group of muscles contracting together for a specific task.

• Reticulospinal vs Rubrospinal Synergies:

  • Reticulospinal: widespread, covering half the body for support postures.

  • Rubrospinal: highly localized.

Motor Cortex

• Location:

  • Precentral gyrus.

• Somatotopic Organization:

  • Arrangement does not correlate with body part size.

• Corticospinal Pathways:

  • Major axon pathways descend from motor cortex to spinal cord, mainly synapsing with interneurons.

Multiple Representation of Motor Control

• Motor Nuclei Representation:

  • Muscles represented in columns at various sites in the motor cortex.

• Neighbored Muscle Columns:

  • Muscle columns are adjacent and represent different synergies.

• Experience-Based Representation:

  • Muscles involved in numerous synergies occupy the largest cortical area.

  • E.g., unique synergies illustrate complex movements like a thumbs up.

Motor Field

• Definition:

  • Connectivity of one corticospinal axon with multiple motor nuclei across spinal segments.

• Corticospinal Synaptic Contacts:

  • Comprise motor fields, with many synapses remaining silent, allowing for plasticity.

Somatosensory Inputs

• Input Types:

  • Proprioceptors offer direct sensory access to motor cortex.

  • Cutaneous inputs arise from somatosensory association areas for posture and motion information.

• Proprioceptive Input:

  • Direct influence from thalamus and somatic association cortex.

Premotor Areas

• Function:

  • Project to motor cortex, sequencing motor cortical synergies for movements.

• Pattern Generation Role:

  • Critical for learned/complex motor tasks.

Premotor Cortex

• Sensory Inputs Processing:

  • Important for movement cues (e.g., visual and auditory cues).

• Broca's Area:

  • Related to sequencing language elements (speech/writing), with input from Wernicke's area.

Sensorimotor Cues

• Environmental Cues:

  • Trigger motor reactions; recognized by sensory association areas and relayed to frontal lobe.

• Motor Cortex Response Selection:

  • Premotor cortex chooses responses based on input cues.

Visuomotor Response

• Coordinated Activities:

  • Initiate movement synergies through cue recognition (e.g., hand positioning).

Preparatory Activity in Motor Control

• Role of Premotor Neurons:

  • Activate prior to movement execution, facilitating appropriate motor synergies.

• Cueing Mechanism:

  • Involves a ‘warning’ cue followed by a ‘go’ cue, with prolonged delays activating premotor neurons.

Supplementary Motor Area (SMA)

• Location:

  • Medial wall of the hemisphere; less detailed somatotopic representation relative to the motor cortex.

• Function:

  • Controls bilateral limb coordination during different actions.

• Internal Signals Processing:

  • Engages volitional signals conducive to movement execution.

Cingulate Motor Area (CMA)

• Location:

  • Within the cingulate sulcus.

• Function:

  • Involved in processing emotional and motivational drivers for movements.