CH 8

EXSC 4400 - Exercise Physiology: Chapter 8 - Skeletal Muscle: Structure & Function

Overview

  • Instructor: Emily Post, PhD, CSCS

  • Key Term: ATP (Adenosine Triphosphate)

Lecture Outline

  • Structure of Skeletal Muscle

  • Neuromuscular Junction

  • Muscular Contraction

  • Exercise and Muscle Fatigue

  • Exercise-Associated Muscle Cramps

  • Fiber Types

  • Muscle Actions

  • Speed of Muscle Action and Relaxation

  • Force Regulation in Muscle

  • Force-Velocity/Force-Power Relationships

Structure of Skeletal Muscle

  • General Information:

    • The human body contains over 600 skeletal muscles.

    • Skeletal muscle constitutes 40-50% of total body weight.

  • Functions of Skeletal Muscle:

    • Force production for locomotion and breathing.

    • Force production for postural support.

    • Heat production during cold stress.

  • Muscle Actions:

    • Flexors: Decrease joint angle.

    • Extensors: Increase joint angles.

Connective Tissue Covering Skeletal Muscle

  • Epimysium: Surrounds the entire muscle.

  • Perimysium: Surrounds bundles of muscle fibers known as fascicles.

  • Endomysium: Surrounds individual muscle fibers.

  • Basement membrane: Located just below the endomysium.

  • Sarcolemma: Muscle cell membrane.

Satellite Cells

  • Function: Precursor cells to skeletal muscle cells, crucial for muscle growth and repair.

  • Role in Muscle Growth: Satellite cells can increase the number of nuclei in mature muscle fibers during growth:

    • Myonuclear domain: Defined as the volume of cytoplasm surrounding each nucleus, influencing muscle fiber's ability to synthesize proteins.

    • More nuclei enhance protein synthesis, critical for growth from strength training.

Myofibrils

  • Function: Contain contractile proteins, specifically:

    • Actin: Thin filament.

    • Myosin: Thick filament.

  • Sarcomere: Contractile unit of muscle.

    • Components include:

    • Z line: Defines the end limits of the sarcomere.

    • M line: Middle of the sarcomere.

    • H zone: Area of thick filaments within the A band that does not overlap with thin filaments.

    • A band: Region containing both actin and myosin.

    • I band: Region containing thin filaments only.

  • Sarcoplasmic Reticulum: Storage site for calcium ions vital for muscle contractions.

  • Terminal Cisternae: Enlarged areas of the sarcoplasmic reticulum.

  • Transverse Tubules (T-tubules): Extensions of the sarcolemma that extend into the muscle fiber, facilitating the conduction of action potentials.

Check In Questions

  • What are the primary functions of skeletal muscle?

  • What role do satellite cells play?

  • Identify the two main contractile proteins.

  • Where is calcium stored in muscle cells? SR

Neuromuscular Junction (NMJ)

  • Definition: The junction between a motor neuron and a muscle fiber.

  • Components:

    • Motor Unit: Comprising of a single motor neuron and all the muscle fibers it innervates.

    • Motor End Plate: Specialized structure that surrounds the end of a motor neuron.

    • Neuromuscular Cleft: The small gap between the neuron and muscle fiber.

  • Mechanism:

    • Acetylcholine (Ach) is released from the motor neuron, resulting in an end-plate potential (EPP).

NMJ Process

  1. Motor neuron action potential arrives.

  2. Calcium ions (Ca2+Ca^{2+} ) enter through voltage-gated channels.

  3. Release of acetylcholine.

  4. Binding of acetylcholine opens ion channels, allowing sodium ions (Na+Na^+) to enter.

  5. Propagation of action potential in muscle plasma membrane.

  6. Muscle fiber action potential initiation.

  7. Muscle fiber repolarization following cessation of acetylcholine release.

  8. Calcium ions pumped back into the sarcoplasmic reticulum (SR).

Sliding Filament Model of Muscle Contraction

  • Also referred to as the swinging lever-arm model.

  • Muscle shortening results from:

    • Movement of the actin filament over the myosin filament.

    • Formation of cross-bridges between actin and myosin filaments, decreasing the distance between Z lines of the sarcomere.

  • Key Points:

    • Actin and myosin do not change size; they slide past each other.

    • A band remains constant, I band decreases, and H zone decreases during contraction.

Relationship Among Troponin, Tropomyosin, Myosin, & Calcium

  • Troponin Complex: Composed of three proteins; binds to actin, tropomyosin, and calcium.

  • Calcium Binding: When Ca2+Ca^{2+} binds to troponin, it triggers a conformational change that moves tropomyosin, exposing myosin binding sites on actin.

Energy for Muscle Contraction

  • ATP: Critical for muscle contraction; energy produced via ATP hydrolysis, facilitating the power stroke of myosin.

  • ATP Hydrolysis Reaction: ATP → ADP + Pi

  • Sources of ATP:

    1. Phosphocreatine (PCr)

    2. Glycolysis

    3. Oxidative phosphorylation

  • Muscle metabolism includes various substrates such as glycogen, glucose, fatty acids, and amino acids used for ATP generation.

Check In Questions

  • Outline the steps involved in excitation at the NMJ.

  • Explain the changes in A band, I band, and H zone during muscle contraction.

  • Identify sources of ATP for muscle contraction.

Excitation-Contraction Coupling

  • Describes how excitation (depolarization of the motor end plate) is linked to muscular contraction.

  • Process:

    • Action potential travels down T-tubules, causing Ca2+Ca^{2+} release from the SR.

    • Calcium binds to troponin, resulting in a shift of tropomyosin, exposing myosin binding sites.

    • Cross-bridge formation occurs, leading to contraction (power stroke).

Step-by-Step Summary: Excitation-Contraction Coupling

Excitation

  • Nerve signal reaches synaptic knob.

  • Acetylcholine is released into the synaptic cleft, binds to receptors, opening sodium channels.

  • Sodium influx leads to depolarization across the muscle fiber, conducted via T-tubules.

Contraction

  • Depolarization triggers calcium release from the SR.

  • Calcium binds to troponin, uncovering myosin binding sites on actin.

  • Cross-bridge forms, followed by power stroke.

  • ATP binds to myosin, breaking the cross-bridge.

Relaxation

  • Stimulation of motor neuron ceases, reducing acetylcholine release.

  • Muscle fiber repolarizes; calcium is pumped back into SR, tropomyosin re-covers binding sites, leading to muscle relaxation.

Check In Questions

  • Recite steps in excitation-contraction coupling (excitation, contraction, relaxation).

Cross-Bridge Cycling

  • Mechanism:

    • Resting state: no force is generated.

    • ATP hydrolysis causes the cross-bridge to cock and energize.

    • Cross-bridge binds to actin; ADP and Pi are released causing a power stroke, sliding filaments past one another.

    • A new ATP must attach to myosin for it to release from actin, resetting the cycle.

Muscle Fatigue

  • Definition: Decline in muscle power output, with decreases in force generation and shortening velocity.

  • Factors involved:

    • High-intensity exercise (around 60 seconds) leads to lactate, H+, ADP, Pi, and free radical accumulation, diminishing cross-bridge binding.

    • Long-duration exercise (2-4 hours) also leads to muscle fatigue due to factors like free radicals, electrolyte imbalance, and glycogen depletion.

Exercise-Associated Muscle Cramps

  • Definition: Spasmodic, involuntary muscle contractions often precipitated by prolonged, high-intensity exercise.

  • Typical causes are not related to electrolyte balance but involve excessive motor neuron firing, affecting muscle spindle and Golgi tendon organ functions.

  • Relief: Often achieved through passive stretching.

Help for People with Muscle Cramps?

  • Treatment strategies may include sending strong inhibitory stimuli to the spinal cord to prevent motor neuron firing.

  • Recent studies indicate activation of mouth/throat ion channels (Transient receptor potentials) can send inhibitory signals to counteract cramping.

    • Examples: Garlic, wasabi, pepper, cinnamon, mustard oil may inhibit over-activity in motor neurons.

Characteristics of Muscle Fiber Types

  • Biochemical Properties:

    • Oxidative capacity, number of capillaries, mitochondria, amounts of myoglobin.

    • Type of myosin ATPase isoform influences speed of ATP degradation and contractile protein abundance.

  • Contractile Properties:

    • Force production, muscle fiber types (I, IIa, IIb), and fatigue resistance contribute to overall contractile performance.

Skeletal Muscle Fiber Types

  • muscle biopsy: may not be representative of entire body

  • immunohistochemical staining → staining for type of myosin ATPase isoform

    • fiber types differienctiates by color

  • gel electrophoresis: a technique used to separate proteins based on their size and charge, allowing for the analysis of specific muscle fiber types and their characteristics.

Muscle Fiber Types Defined

  • Type I Fibers:

    • Slow-twitch, oxidative, high fatigue resistance.

  • Type IIa Fibers:

    • Fast-oxidative glycolytic fibers, intermediate characteristics.

  • Type IIx Fibers:

    • Fast-twitch, glycolytic, lower fatigue resistance.

Fiber Types and Performance

  • Nonathletes typically have about 50% slow (Type I) and 50% fast fibers (Types IIa and IIx).

  • Power athletes possess a higher proportion of fast fibers.

  • Endurance athletes show greater amounts of slow fibers.

  • Important Note: Fiber type is only one factor influencing athletic performance.

Check In Questions

  • Describe characteristics of each muscle fiber type.

Types of Muscle Action

  • Isometric: Muscle exerts force without changing length.

  • Dynamic (Isotonic):

    • Concentric: Muscle shortens while producing force.

    • Eccentric: Muscle produces force while lengthening, often associated with muscle injury and soreness.

Speed of Muscle Action and Relaxation

  • Muscle Twitch: Reaction from a single stimulus consisting of:

    • Latent period: ~5 ms

    • Contraction phase: ~40 ms

    • Relaxation phase: ~50 ms

  • Contracting muscle fibers speed up release of calcium and have higher ATPase activity in faster fibers.

Force Regulation in Muscle

  • Influencing factors:

    • Number and types of motor units recruited; increased number/fast motor units generate greater force.

    • Initial muscle length affects optimal force generation and cross-bridge formation.

    • Frequency of neural stimulation affects contraction strength.

    • Warm-up exercises facilitate "postactivation potentiation."

Length-Tension Relationships in Skeletal Muscle

  • Key Point: Relative tension produced depends on muscle length at the time of contraction.

    • Normal: Optimal cross-bridge interactions occur at 2.25 µm for maximal tension development.

    • Shorter/Longer lengths: Deviations lead to reduced tension due to fewer cross-bridge interactions or limits on interaction.

Aging and Disease Affecting Muscle Function

  • Sarcopenia:

    • Muscle mass loss approximately 10% between ages 25-50, and an additional 40% lost from 50-80 years, marked by loss of fast fibers.

    • Resistance training can mitigate age-related muscle loss.

  • Diabetes: Associated with muscle mass decline and sarcopenia.

  • Cancer Cachexia: Causes approximately 50% of cancer patients to experience muscle wasting, significantly impacting health and mortality rates.

Force-Velocity and Force-Power Relationships

  • Force-Velocity Relationship: Higher percentages of fast-twitch fibers correlate with speed in muscle movement at any absolute force exerted.

  • Force-Power Relationship: Fast-twitch muscles exhibit greater peak power output at higher velocities (up to 200-300 degrees/sec). Beyond these velocities, power output falls as force generation decreases.

Lecture Summary

  • Structure of Skeletal Muscle

  • Neuromuscular Junction

  • Muscular Contraction

  • Exercise and Muscle Fatigue

  • Exercise-Associated Muscle Cramps

  • Fiber Types

  • Muscle Actions

  • Speed of Muscle Action and Relaxation

  • Force Regulation in Muscle

  • Force-Velocity/Force-Power Relationships