NSB: Motor System

Muscle Structure and Function

  • Muscles

    • What is the fundamental molecular mechanisms that converts neural signals into physical force?

    • Motor Neurons

    • How does a motor neuron rapidly and synchronously trigger muscle contraction?

    • Movement Autopilot

    • How do our central nervous system circuits create the coordinated, rhythmic patterns needed for automatic actions like breathing?

Hierarchical Organization in Motor Control

  • Motor Control

    • Orchestrated via a hierarchical organization

    • Cortex Functionality

    • How does the cortex encode voluntary action?

      • Planning complex movements without accidental early movement.

Muscle Fiber Composition

  • Muscle Fiber

    • Defined as a single elongated cell specialized for contraction.

    • Myofibril

    • A cylindrical protein structure made of repeating units called sarcomeres.

Sarcomere Structure and Function

  • Sarcomere

    • The smallest functional unit of muscle contraction.

    • Contains overlapping thick (myosin) and thin (F-actin) filaments arranged in an orderly fashion.

    • Molecular Basis of Contraction

    • Explains actin and myosin dynamics based on the Slide Filament Model.

      • Myosin and F-actin filaments slide relative to each other.

      • The power stroke of myosin provides a mechanical force.

Myosin Characteristics

  • Myosin

    • An F-actin binding motor protein with a long tail and globular head forming the cross bridge between thick and thin filaments.

    • Power Stroke

    • The myosin head pivots, forcefully pulling the actin filament towards the center of the sarcomere.

  • Myosin as an ATPase

    • Contains an ATPase domain in its head that hydrolyzes ATP into ADP.

    • Steps in ATP action on myosin include:

    1. ATP binding to myosin triggers dissociation of myosin and actin.

    2. ATP hydrolysis causes a conformational change in the myosin head.

    3. ADP-myosin binds actin again.

    4. Release of ADP and inorganic phosphate ($Pi$) induces a power stroke.

Excitation-Contraction Coupling (ECC)

  • ECC

    • Links motor neuron activity to muscle contraction via Ca2+ regulation.

    • Involves synaptic transmission between the axon terminal of a motor neuron and a muscle fiber.

    • Action potential triggers Ca2+ release:

    • Muscle depolarization propagates into the cell interior via T tubules, a network of tunnels.

    • Depolarization triggers Ca2+ release from the sarcoplasmic reticulum (SR).

    • Results in rapid and synchronous contraction of sarcomeres.

    • T tubule and SR Interaction:

    • Calcium ions ($Ca^{2+}$) interact with F-actin filaments during contraction.

Sequence of Events in Muscle Contraction

  • Order of Events for muscle contraction:

    1. An action potential in the motor neuron.

    2. Release of acetylcholine.

    3. Flow of K+ and Na+ ions through their receptors.

    4. Activation of the nicotinic acetylcholine receptor.

    5. Release of Ca2+ from the sarcoplasmic reticulum.

    6. ATP hydrolysis.

    7. ADP-myosin binds actin.

    8. Movement of troponin/tropomyosin.

    9. Power stroke.

Coordination of Muscle Activation

  • Muscle Coordination

    • Activation of different muscles coordinated to achieve desired movement.

    • Antagonistic Pairs:

    • Alternate contraction of extensors and flexors around joints.

    • Rhythmic Movements:

    • Require coordinated and rhythmic contraction patterns.

Central Pattern Generators (CPGs)

  • CPGs

    • Defined as central nervous system circuits capable of producing rhythmic outputs for coordinated muscle contractions without sensory feedback.

    • Pacemaker Cells:

    • Neurons that generate rhythmic output even in the absence of sensory input.

    • Function in Respiration:

    • Diaphragm contraction and relaxation, the primary muscle for respiration.

    • Rhythm Generators:

    • Set the beat (inspiration-expiration timing) via neurons with intrinsic bursting or pacemaker properties.

    • Pattern Generators:

    • Shape the output pattern to motor neurons regulating how muscles contract.

    • Organize the activation of different muscle groups.

Experimental Evidence for CPGs

  • Identification of the Inspiratory CPG:

    • The pre-Bötzinger complex (pre-BotC) identified as a central rhythm generator.

    • Following specific sectioning, rhythmic output in animals can dramatically decrease.

    • Studies have shown the necessity of specific progenitor-derived excitatory neurons for rhythm generation.

Molecular Basis of Rhythm Generation

  • Molecular/Developmental Aspects of CPGs:

    • Tidal volume of airflow measured in genetically modified mice expressing channelrhodopsin in specific neurons.

    • Photostimulation at certain phases of breathing influences rhythmic activity, confirming the necessity of specific neurons in rhythm generation.

    • Breathing Center:

    • Functions through three interacting oscillator circuits located in the brainstem (medulla).

    • Core rhythm generation is largely network-driven, indicating the complexity of CPG networks.