3.7b Muscle Physiology of Skeletal Muscle

Objectives

• Describe a motor unit.
• Define the principles of muscle mechanics.
• Define the term; muscle twitch.
• Explain how tension develops in a muscle twitch.

The Motor Unit

• Definition of Motor Unit: A motor unit consists of a motor neuron and all muscle fibers it supplies.
    - The number of muscle fibers per motor unit influences the capability for fine control: a smaller number allows for finer control.
• Distribution of Muscle Fibers: Muscle fibers belonging to a motor unit are distributed throughout the muscle, which means that when a single motor unit is activated, it results in a weak contraction of the entire muscle.
• Asynchronous Contraction of Motor Units: In general, motor units within a muscle contract asynchronously, which is a mechanism that helps to prevent muscle fatigue by allowing different motor units to take turns contracting.

Structure of Motor Units

• Each muscle is served by at least one motor nerve, comprising various components such as:
    - Spinal Cord: Origin of motor neurons.
    - Motor Neuron: Cell body and axon responsible for transmitting signals to muscle fibers.
      - Axon terminals form neuromuscular junctions that connect with muscle fibers to communicate and stimulate contraction.

Principles of Muscle Mechanics

• The principles of contraction apply both to individual muscle fibers and to whole muscles.
• Muscle Contraction Basics:
    - Contractions produce muscle tension, which is the force exerted upon a load or object intended to be moved.
    - Types of Muscle Contraction:
      - Isometric Contraction: There is no shortening of the muscle, but muscle tension increases without exceeding the load.
      - Isotonic Contraction: The muscle shortens when muscle tension exceeds the load.
• Response to Stimuli: The force and duration of muscle contractions vary according to the frequency and intensity of the stimuli received.

Muscle Twitch

• Definition of Muscle Twitch: A muscle twitch is a motor unit's response to a single action potential from its motor neuron.
    - Latent Period: This initial phase occurs during excitation-contraction coupling and is characterized by no measurable muscle tension.
    - Period of Contraction: Characterized by cross-bridge formation and an increase in tension within the muscle.
    - Period of Relaxation: Occurs when calcium ions return to the sarcoplasmic reticulum, leading to a decrease in muscle tension down to zero.
    - It is noted that muscle contraction occurs more rapidly than relaxation.

Myogram of an Isometric Twitch

• A myogram capturing an isometric twitch will show the following phases:
    - Latent Period: No tension; transition of internal events.
    - Period of Contraction: Represents increasing tension.
    - Period of Relaxation: Shows the decline of tension as the muscle relaxes, returning to the baseline.

Development of Tension in a Twitch

• Different skeletal muscles exhibit variations in tension over time during a twitch.
    - Historical myographic data representing the tension variations, particularly in different muscle types (for example, eye muscle, gastrocnemius, and soleus), highlights the latent phase linked to action potentials and calcium release.

Muscle Metabolism: Energy for Contraction

• ATP as Energy Source: ATP is the only source used directly for muscle contraction functions including cross-bridge movement and calcium pump activity within the sarcoplasmic reticulum.
    - Available ATP stores are depleted within 4–6 seconds of initiation of maximal exercise.
• ATP Regeneration Methods:
    1. Direct Phosphorylation: Involves the regeneration of ATP via creatine phosphate (CP).
    2. Anaerobic Pathway: This includes glycolysis, which results in lactic acid production.
    3. Aerobic Respiration: Generates ATP in the presence of oxygen.

ATP Regeneration Pathways

• Two major types of pathways for regenerating ATP during muscle activity:
    1. Direct Phosphorylation: Coupled reaction of creatine phosphate and ADP utilizing creatine kinase to yield ATP.
    2. Anaerobic Pathway:
       - Glycolysis results in the formation of lactic acid when ATP is generated without oxygen.
       - Minimum output yields 2 ATP per glucose.
    3. Aerobic Pathway:
        - This pathway utilizes fatty acids, glucose, and amino acids to generate approximately 32 ATP per glucose, requiring oxygen.

Energy Demand in Varying Activities

• Resting Muscle Fiber: During low demand for ATP, sufficient oxygen is available and creates energy reserves in the form of glycogen and creatine phosphate (CP).
• Moderate Activity: ATP demand increases but sufficient oxygen allows for aerobic ATP generation primarily from glycogen.
• Peak Activity: ATP needs exceed oxygen diffusion rates, leading to high lactic acid buildup due to reliance on anaerobic glycolysis and resulting in increased acidity and muscle fatigue.

Influence of Load on Muscle Contraction

• Muscles will contract fastest when there is no load.
• Increasing load leads to several physiological changes:
    - Increased latent period.
    - A slower contraction rate.
    - Decreased duration of contraction.
• Heavy loads hinder muscle shortening and decrease contraction duration.

Factors Increasing Force of Contraction

• Multiple factors influence the strength of muscle contraction including:
    - A larger number of muscle fibers,
    - Increased muscle size,
    - High frequency of stimulation (such as wave summation).
• When muscle sarcomeres are slightly stretched over resting length, contractile force is enhanced due to improved cross-bridge attachment.

Graded Muscle Responses

• Graded responses refer to the muscle's ability to alter contraction strength to match different demands:
    - Achieved by changing both frequency and strength of stimulation.
• Single Stimulus: Results in individual contractile responses, termed muscle twitches.

Changes in Stimulus Frequency

• The muscle can achieve greater force by increasing the firing rate of motor neurons through wave summation:
    - Increased frequency results in a situation where the muscle does not completely relax between stimuli, leading to enhanced force of contraction.
• Effects of Constant Stimuli: If stimuli are applied at an accelerating rate without sufficient time for relaxation, this leads to unfused (incomplete) tetanus.
• In cases of rapid stimuli, the muscle reaches maximum tension, leading to fused (complete) tetanus characterized by a smooth, sustained contraction that risks entering muscle fatigue.

Response to Changes in Stimulus Strength

• Changes in stimulus strength contribute to muscle contraction quality and force:
    - Wave summation and recruitment (multiple motor unit summation) are key processes controlling contraction force.
• Recruitment Order:
    - Smaller muscle fibers are recruited first, followed by larger fibers, proportional to the intensity of the stimulus. Larger units are activated primarily for powerful contractions.

Muscle Fiber Types and Properties

• Muscle fibers can be categorized based on their contraction speed and metabolic characteristics:
    - Slow Oxidative Fibers: Designated for endurance, utilizing aerobic pathways, fatigue-resistant.
    - Fast Oxidative Fibers: Somewhat fatigue-resistant and can use both aerobic and anaerobic pathways.
    - Fast Glycolytic Fibers: Fatiguable, primarily utilizing anaerobic respiration.

• Comparative Characteristics of Fiber Types:
    | Property | Slow Fibers | Fast Fibers |
    |-----------------------------------|---------------------------|------------------------------|
    | Cross-sectional diameter | Small | Large |
    | Contraction speed | Slow | Fast |
    | Fatigue resistance | High | Low |
    | Myoglobin content | High | Low |
    | Capillary supply | Dense | Scarce |
    | Mitochondria | Many | Few |
    | Glycolytic enzyme concentration in | Low | High |

• Muscle fiber distinctions contribute significantly to the functional capacity of skeletal muscles, impacting endurance, strength, and overall performance during various activities.