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Chapter 9: Organization of the Motor System

Chapter 9: Organization of the Motor System

Nervous System as Information Processor

  • Input: External world

  • Output: Effector organs

    • Motor behaviors

    • Skeletal muscles

  • Integration and Processing:

    • Internal body milieu

    • Cardiac muscles for homeostatic adjustments

    • Smooth muscles

    • Exocrine glands

    • Endocrine glands

The Motor System

  • Initiating and Producing Movement:

    • Interaction of information from the senses and the entire brain.

    • Specific brain regions involved:

    • Motor cortex: Plans and initiates movement.

    • Basal ganglia: Coordinates movement.

    • Cerebellum: Coordinates movement.

    • Spinal cord: Conducts information to the muscles.

Sequential Production of Movement

  1. Visual Information: Required to locate target.

  2. Frontal-Lobe Motor Areas: Plan the reach and command the movement.

  3. Spinal Cord: Carries information to the hand.

  4. Motor Neurons: Carry message to muscles of the hand and forearm.

  5. Sensory Receptors: On the fingers send message to sensory cortex saying that the cup has been grasped.

  6. Brainstem and Spinal Cord: Carries sensory information to the brain.

  7. Basal Ganglia: Judge accuracy of grasp forces.

  8. Cerebellum: Corrects movement errors.

Illustrative Sequence

  • Motor Nerve and Sensory Nerve involved in communication between the brain and hand.

Initiating a Motor Sequence

  1. Parietal Cortex: Receives and integrates sensory information, initiates movement by sending goals.

  2. Prefrontal Cortex: Plans movements.

  3. Premotor Cortex: Organizes movement sequences.

  4. Motor Cortex: Produces elementary movements.

Diversity of Motor Control Areas

  • Motor Planning Areas: Integrate and refine basic movements such as walking or climbing, involving the neocortex.

Motor Plans

  • Execution: Movement of muscles is planned and executed through a network of brain areas:

    • Supplementary Motor Area: Involved in planning.

    • Premotor Cortex: Coordinates sequences.

    • Prefrontal Cortex: Responsible for planning.

    • Primary Motor Cortex: Executes movements.

  • Related Areas:

    • Parietal Lobe: Handles perception of space and location of limbs, auditory and visual perceptions.

    • Temporal Lobe: Involved in memory processes.

    • Occipital Lobe: Processes visual information.

Subcortical Motor Control

  • The Basal Ganglia, Cerebellum, and Brainstem play crucial roles in the control of movement.

  • Basal Ganglia:

    • Receives input from the cortex and the substantia nigra in the midbrain.

    • Sends projections back to these areas supporting modulation of motor activity.

Basal Ganglia Functionality and Disorders

  • Movement Disorders:

    • Hyperkinetic Symptoms:

    • Result from damage to the caudate and putamen, leading to dyskinesias (unwanted writhing and twitching movements).

    • Examples include: Huntington's disease and Tourette syndrome.

    • Hypokinetic Symptoms:

    • Result from loss of dopamine input from the substantia nigra, associated with difficulties in making voluntary movements.

    • Example includes Parkinson's disease.

    • Basal Ganglia Functionality: Involved in modulating force, suggesting both too much and too little force lead to coordination issues.

Regulating Movement Force

  • Direct Pathway:

    • Excitatory connections onto thalamic neurons which, in turn, excite neurons in motor and cognitive cortices.

  • Indirect Pathway:

    • Inhibits thalamic neurons, preventing them from exciting cortical neurons, influencing movement regulation.

The Cerebellum

  • Anatomy: Composed of two hemispheres; lateral parts manage body appending movement, while the floccular lobe manages eye movements and balance.

  • Cerebellum Role in Motor Learning:

    • Important for acquiring and maintaining motor skills.

  • Movement Timing and Accuracy:

    • Adjusts movements based on sensory input and desired output by comparing intention (motor commands) with actual results.

Cerebellum and Motor Feedback

  • Adjustments are made when movements do not reach intended targets.

  • Communication pathway:

    • Cortex sends motor commands to spinal cord and a copy to cerebellum.

    • Sensory feedback from muscles and visual input inform the cerebellum for corrections.

  • Cognitive Processes: Similar error correction strategies are used in other cognitive functions, such as language processing.

Intention, Action, Feedback Cycle

  • Pathway Components:

    • Motor instructions reach the spinal cord via corticospinal tracts, while feedback from actual movements is integrated for error correction through inputs to the cerebellum.

Brainstem and Movement Control

  • The brainstem features multiple pathways that refine movements specified by the cortex, especially in coarse movements of the body such as balance and posture.

Communicating with the Spinal Cord

  • The spinal cord is the terminal point for converting motor plans into movements.

  • Pathways from Brain to Spinal Cord:

    • Tracts project from the cortex to the spinal cord, facilitating movement execution.

Corticospinal-Tract Pathway

  • Left Hemisphere Connections:

    • Involves the left-hemisphere motor cortex, which influences skilled limb and digit movements through its lateral corticospinal tract.

    • Anterior corticospinal tract affects muscles in the body's midline.

Interneurons, Motor Neurons, and Muscles

  • Includes interactions between interneurons, efferent neurons, and effector muscles in executing movements at the neuromuscular junction, highlighting the complexity of motor signaling.