Brain Control of Movement: Motor Systems and Skilled Performance

Skilled Performance and Motor Learning

Introduction to Motor Systems

The control of skilled movement is a central topic in biological psychology, focusing on how the brain plans, initiates, and executes voluntary actions. This section explores the major cortical regions and descending pathways involved in motor control.

Brain Control of Movement

Motor Planning and Execution

• Motor plan: An abstract representation of an intended movement, involving the specification of a goal, creation of a plan, initiation and execution, and online comparison of actual movement with the plan.

• Key steps:

  1. Specify a goal for the action

  2. Create the plan (primarily SMA and PMA)

  3. Initiate and execute the movement (M1)

  4. Compare executed movement with the plan (feedback mechanisms)

• Example: Reaching for a cup involves planning the trajectory, executing the reach, and adjusting based on sensory feedback.

Cerebral Cortex – Motor Areas

Overview of Motor Cortical Regions

• Primary Motor Cortex (M1): Located in the precentral gyrus; responsible for initiation and execution of movement.

• Supplementary Motor Area (SMA): Involved in planning internally generated movements and bimanual coordination.

• Premotor Area (PMA): Involved in planning movements guided by external sensory cues, especially visual signals.

Primary Motor Cortex (M1)

Location and Function

• Location: Precentral gyrus of the frontal lobe.

• Function:

  • Initiation and execution of voluntary movement

  • Specifies how actions are to be carried out

  • Direct and indirect excitation of motor neurons in the spinal cord

  • Neurons fire immediately before and during movement

  • Encodes direction and force of movement

• Example: Electrical stimulation of M1 elicits contralateral muscle contraction.

Somatotopic Organization of M1

Motor Homunculus

• Somatotopic mapping: The motor cortex is organized such that different regions correspond to control of specific body parts.

• Homunculus: Areas requiring fine motor control (e.g., hands, face) are disproportionately represented.

• Example: The hand and face occupy large regions of M1 due to their complex movements.

Major Inputs and Outputs for M1

Connectivity of Primary Motor Cortex

• Inputs:

  • Premotor and supplementary motor areas

  • Primary somatosensory cortex

  • Parietal cortex (area 5)

  • Basal ganglia and cerebellum (via thalamus)

• Outputs:

  • Motor nuclei in brainstem and spinal cord

  • Basal ganglia

  • Cerebellum

Additional (Association) Motor Areas

Premotor and Supplementary Motor Areas

• Premotor Area (PMA): Lateral surface, projects to M1 and spinal cord, receives multisensory inputs.

• Supplementary Motor Area (SMA): Superior and medial surface, involved in internally generated movement sequences.

• Both receive inputs from thalamus and other cortical areas.

Function of Motor Association Areas

Movement Planning and Selection

• Involved in planning movements before execution.

• Neurons fire as soon as a goal is set and continue during movement.

• Encode desired actions; M1 specifies execution details.

• Select appropriate movements or sequences based on context.

Premotor Area (PMA)

Role in Sensory-Guided Movements

• Receives multisensory inputs, especially visual signals.

• Sequences motor synergies using sensory cues.

• Prepares for voluntary movements; stimulation causes muscle contractions at multiple joints.

• Affects contralateral limbs.

• Influenced by cerebellum via thalamus.

• Example: Catching a ball by visually tracking its trajectory and planning the grasp.

Supplementary Motor Area (SMA)

Role in Internally Initiated Movements

• Used in self-motivated, internally initiated movements.

• Sequences movements based on internal cues (e.g., learned sequences).

• Associated with bimanual control; interhemispheric connections between SMAs.

• Influenced by basal ganglia via thalamus.

• Example: Playing a piano piece from memory.

Differences in Motor Areas

Functional Specialization

• Different motor areas are activated for different types of movements (e.g., simple vs. complex sequences).

• Some PMA neurons are mirror neurons: they fire both when performing an action and when observing the same action performed by others.

• Example: Observing someone grasp an object activates similar neural circuits as performing the grasp oneself.

Descending Spinal Tracts

Pathways from Brain to Spinal Cord

• Axons descend from the brain to the spinal cord via two major groups: lateral and medial pathways.

• Some tracts originate in the cerebral cortex, others in the brainstem.

Lower vs Upper Motor Neurons

Classification and Function

• Lower Motor Neurons: Final common pathway; alpha motor neurons originate in spinal cord and brainstem, innervate skeletal muscle.

• Upper Motor Neurons: Originate in cortex and brainstem, terminate in spinal cord and brainstem.

• Affected by descending motor tracts, sensory neurons, and interneurons.

Descending Motor Systems

Lateral vs. Medial Pathways

System	Main Tracts	Function	Muscle Groups
Lateral (dorsal)	Lateral Corticospinal, Rubrospinal	Goal-directed limb movements, distal muscles, contralateral projections	Distal (limbs)
Medial (ventral)	Anterior (Ventral) Corticospinal, Vestibulospinal, Reticulospinal, Tectospinal	Postural tone, proximal and axial muscles, ipsilateral and bilateral projections	Axial (trunk, proximal)

Lateral Corticospinal Tract

Structure and Function

• Largest descending tract (~750,000 fibers per hemisphere).

• Originates from primary motor, premotor, and somatosensory cortex.

• Contacts interneurons and motor neurons of distal muscles in lateral ventral horn.

• Crosses at pyramidal decussation (cervicomedullary junction).

• Controls distal muscles on contralateral side.

Anterior (Ventral) Corticospinal Tract

Structure and Function

• ~250,000 fibers per hemisphere.

• Originates from primary motor and premotor areas.

• Remains uncrossed until spinal cord.

• Bilaterally activates interneurons and motor neurons of proximal and axial muscles.

Brainstem (Indirect) Pathways

Overview

• Brainstem nuclei project to spinal motor neurons.

• Medial motor systems: Vestibulospinal (medial/lateral), Reticulospinal, Tectospinal.

• Lateral motor system: Rubrospinal.

Vestibulospinal Tracts

Lateral and Medial Vestibulospinal Tracts

• Lateral Vestibulospinal Tract: Originates from lateral vestibular nuclei, travels ipsilaterally to entire cord, helps extensor muscle tone, involved in balance.

• Medial Vestibulospinal Tract: Originates from medial vestibular nuclei, travels bilaterally to cervical and thoracic levels, positions head and neck.

Reticulospinal Tract

Function

• Originates from reticular formation.

• Travels in anterior column to entire cord (ipsilateral).

• Involved in stabilizing posture.

Tectospinal Tract

Function

• Originates from superior colliculus.

• Connects with motor neurons in cervical spinal cord (contralateral).

• Coordinates head and eye movement.

Rubrospinal Tract

Function

• Originates from red nucleus.

• Crosses midline in brainstem (midbrain).

• Travels in lateral column (contralateral).

• Goal-directed limb movements (e.g., reaching, manipulation).

• Influences distal muscles.

Summary of Descending Motor Systems

Lateral (dorsal) Motor Systems	Medial (ventral) Motor Systems
Lateral Corticospinal tract Rubrospinal tract Mainly DISTAL muscles (limbs)	Anterior (Ventral) Corticospinal tract Vestibulospinal tract Reticulospinal tract Tectospinal tract Mainly AXIAL muscles

Summary: Brain Control of Movement

• M1: Executes movement; receives convergent inputs from multiple cortical regions (S1, PPC, SMA, PMA) and indirectly from cerebellum and basal ganglia via thalamus.

• Premotor cortex: Responsible for movement planning and sequences; sensory-guided.

• SMA: Internally-guided and bimanual coordination.

• Descending tracts: Contact interneurons and motor neurons; function and pathway depend on medial/lateral, direct/indirect routes.

Additional info: This material is highly relevant to Biological Psychology (Ch. 3), especially the neural basis of movement and motor control. Concepts such as motor planning, cortical organization, and descending pathways are foundational for understanding voluntary movement and skilled performance in humans.