Muscle physiology handouts

Muscle Physiology Overview

• Overview of muscle physiology based on content from Sherwood Chapter 8.

Lecture Objectives

• Types of Muscles and Functions:

  • Different types of muscles based on location, structure, and control.

• Skeletal Muscle Structure and Function:

  • Structure of skeletal muscles and mechanisms of contraction and relaxation.

• Muscle Collaboration Mechanics:

  • How skeletal muscles work together mechanically.

• Metabolism of Skeletal Muscles:

  • Types categorized by metabolism.

• Smooth Muscle Structure and Function:

  • Structure and contraction mechanisms of smooth muscles.

• Cardiac Muscle Comparison:

  • Comparison of cardiac muscle with skeletal and smooth muscles.

Introduction to Muscles

• Muscles account for approximately 50% of human body weight.

• Functions of Muscle Contraction:

  • Enables purposeful movement.

  • Manages manipulation of external objects.

  • Assists in propulsion of contents through hollow internal organs (like digestion and circulation).

  • Facilitates emptying of internal contents (urination, childbirth).

Types of Muscles

• Based on Location:

  • Skeletal Muscle: 32-40% of total body muscle.

  • Cardiac Muscle.

  • Smooth Muscle.

• Based on Striations:

  • Striated Muscle: Has dark and light bands.

    • Includes skeletal and cardiac muscle.

  • Unstriated Muscle: No banding, refers to smooth muscle.

• Based on Control:

  • Voluntary: Skeletal muscles under somatic innervation.

  • Involuntary: Cardiac and smooth muscle regulated by autonomic nervous system.

Skeletal Muscle Structure

• Composition and Arrangement:

  • Skeletal muscles consist of numerous muscle fibers arranged parallel, bundled by connective tissue.

• Muscle Fiber Characteristics:

  • Large, elongated, cylindrical cells (10-100 μm diameter, up to 750,000 μm length).

  • Multi-nucleated due to fusion during development.

  • High quantity of mitochondria for energy production.

Muscle Cell/Fiber Structure

• Contractile Elements:

  • Myofibrils make up 80% of muscle fiber volume, regular arrangement of thick (myosin) and thin (actin) filaments.

• Sarcomere:

  • The functional unit of myofibrils, containing actin and myosin, around 2.5 μm long.

• Regions of Sarcomeres:

  • A Band: Containing myosin with some overlapping actin.

  • H Zone: Myosin only, central region of A band devoid of actin.

  • I Band: Contains actin only, no myosin overlap.

• Key Features of Muscle Fiber:

  • Myosin filaments are composed of several hundred myosin molecules, forming cross bridges.

  • Actin filaments comprise spherical actin joined into twisted strands, responsible for binding with myosin.

Skeletal Muscle Contraction Mechanism

• Sliding Filament Mechanism:

  • Contraction occurs via cycles of cross-bridge binding and bending, resulting in thin filaments pulling inward towards the center of the sarcomere.

  • Consequences include shortening of Z-lines and reduction of H-zone and I-band.

• Steps of Contraction:

  • Binding: Myosin cross-bridge binds to actin.

  • Power Stroke: Myosin head bends, pulling actin.

  • Detachment: Cross-bridge released, returning to original conformation until it binds again.

• Excitation-Contraction Coupling:

  • Action potential leads to muscle contraction through Ca2+ release and subsequent binding with troponin, facilitating tropomyosin displacement and exposing actin binding sites.

Role of ATP in Muscle Contraction

• ATP Dynamics:

  • ATP hydrolysis necessary for myosin binding and power stroke execution.

  • Rigor mortis occurs due to failure of ATP production after death, locking actin and myosin together.

Muscle Relaxation Mechanism

• Ca2+ Reuptake:

  • Calcium uptake by sarcoplasmic reticulum post-contraction; results in decreased cross-bridge cycling and muscle relaxation.

• Contraction Duration:

  • Electrical activity is short-lived compared to muscle contraction duration.

Mechanics of Skeletal Muscle

• Twitch Overview:

  • Single action potentials generate brief muscular contractions (twitch).

  • Multiple fibers contracting simultaneously produces stronger contractions.

• Motor Units Concept:

  • Comprised of a single motor neuron and the muscle fibers it innervates, influencing overall muscle strength and recruitment patterns.

Muscle Physiology Overview

Overview

This outline offers a comprehensive understanding of muscle physiology as detailed in Sherwood Chapter 8.

Lecture Objectives

• Types of Muscles and Functions:

  • Analyze the different types of muscles classified by their location in the body, structure, and the mechanisms by which they are controlled.

• Skeletal Muscle Structure and Function:

  • Explore the structural characteristics of skeletal muscles, examining how they contract and relax, as well as the role of motor units in muscle function.

• Muscle Collaboration Mechanics:

  • Investigate how different skeletal muscles work together to create movement, including the concepts of synergists and antagonists in muscle action.

• Metabolism of Skeletal Muscles:

  • Classify skeletal muscles based on their metabolic pathways and energy sources, differentiating between slow-twitch and fast-twitch fibers.

• Smooth Muscle Structure and Function:

  • Detail the unique structure and contraction mechanisms seen in smooth muscles, focusing on their role within various organ systems.

• Cardiac Muscle Comparison:

  • Compare and contrast cardiac muscle with skeletal and smooth muscles, highlighting the unique characteristics that support its function in the heart.

Introduction to Muscles

• Muscles constitute approximately 50% of the human body weight and play a crucial role in facilitating movement and bodily functions.

Functions of Muscle Contraction:

• Purposeful Movement: Enables voluntary movements such as walking and grasping.

• Manipulation of External Objects: Controls actions such as holding, lifting, or moving objects in the environment.

• Assists in Propulsion: Aids in moving contents through hollow organs, crucial in processes like digestion and circulation.

• Facilitates Emptying: Involves actions such as urination and childbirth, demonstrating the essential functions of muscular contractions in physiological processes.

Types of Muscles

1. Based on Location:

   • Skeletal Muscle: Comprises 32-40% of total body muscle, attached to bones, allowing for voluntary movement.

   • Cardiac Muscle: Found only in the heart, responsible for pumping blood.

   • Smooth Muscle: Located in walls of hollow organs, such as the intestines and blood vessels, aiding involuntary movement.

2. Based on Striations:

   • Striated Muscle: Exhibits dark and light bands (striations); includes both skeletal and cardiac muscle types.

   • Unstriated Muscle: Lacks striations; typically refers to smooth muscle tissue.

3. Based on Control:

   • Voluntary Muscle: Skeletal muscles, which are controlled consciously under somatic innervation.

   • Involuntary Muscle: Cardiac and smooth muscles regulated by the autonomic nervous system, functioning without conscious control.

Skeletal Muscle Structure

• Composition and Arrangement:

  • Skeletal muscles are composed of numerous parallel-arranged muscle fibers, each bundled together by connective tissue layers known as epimysium, perimysium, and endomysium.

• Muscle Fiber Characteristics:

  • Comprise large, elongated, and cylindrical cells, with diameters ranging from 10-100 μm and lengths extending up to 750,000 μm.

  • Muscle fibers are multi-nucleated due to the fusion of myoblasts during development, allowing for synchronized contraction.

  • Contains a high density of mitochondria, critical for ATP production, reflecting the energy demands during contraction.

Muscle Cell/Fiber Structure

• Contractile Elements:

  • Myofibrils account for approximately 80% of muscle fiber volume, organized into a regular pattern of thick (myosin) and thin (actin) filaments.

• Sarcomere:

  • The functional unit of myofibrils, sarcomeres are roughly 2.5 μm in length and consist of overlapping actin and myosin filaments, essential for contraction.

• Regions of Sarcomeres:

  • A Band: Region where myosin and actin overlap, contributing to muscle contraction.

  • H Zone: Central part of the A band, consisting solely of myosin when muscle is relaxed.

  • I Band: Area of actin only, which shortens during contraction as filaments slide past each other.

• Key Features of Muscle Fiber:

  • Myosin filaments are composed of numerous myosin molecules, forming cross-bridges with actin during contraction.

  • Actin filaments are formed from globular actin subunits which polymerize into twisting strands, crucial for the binding process with myosin heads during contraction.

Skeletal Muscle Contraction Mechanism

• Sliding Filament Mechanism:

  • Contraction results from cycles of cross-bridge formation and bending, causing thin filaments to move toward the sarcomere center.

  • The contraction leads to the shortening of Z-lines, resulting in diminished H-zone and I-band lengths.

• Steps of Contraction:

  1. Binding: Myosin cross-bridge binds to actin filaments.

  2. Power Stroke: The myosin head bends, pulling the actin filaments toward the center.

  3. Detachment: The cross-bridge releases, allowing myosin to return to its resting state to bind again.

• Excitation-Contraction Coupling:

  • A muscle action potential triggers the release of Ca2+ ion from the sarcoplasmic reticulum, which binds to troponin. This causes a conformational change that displaces tropomyosin and exposes the binding sites on actin filaments, leading to cross-bridge cycling.

• Role of ATP in Muscle Contraction:

  • The hydrolysis of ATP is necessary for both the binding of myosin to actin and the execution of the power stroke, providing the energy required for contraction.

  • Rigor mortis occurs post-mortem due to the cessation of ATP production, locking the actin and myosin in a contracted state until decomposition.

Muscle Relaxation Mechanism

• Ca2+ Reuptake:

  • After contraction, calcium is actively transported back into the sarcoplasmic reticulum, causing a decline in myosin-actin interactions and allowing muscle relaxation to occur.

• Contraction Duration:

  • The duration of muscle contractions often exceeds the brief electrical impulses that stimulate them, allowing sustained muscle activity for various functional needs.

Mechanics of Skeletal Muscle

• Twitch Overview:

  • A single action potential can evoke a brief and isolated muscle contraction known as a twitch, leading to varying levels of muscle tension.

• Motor Units Concept:

  • Comprised of a single motor neuron and all muscle fibers it innervates, motor units are fundamental in determining the strength of muscle contractions and the pattern of recruitment during muscle tasks.

1. What are the different types of muscles classified by their location in the body, structure, and the mechanisms by which they are controlled?

   • Muscles can be classified based on location (skeletal, cardiac, smooth), structure (striated vs. unstriated), and control methods (voluntary vs. involuntary).

2. How do skeletal muscles contract and relax, and what is the role of motor units in muscle function?

   • Skeletal muscles contract through a mechanism called the sliding filament model, where myosin and actin filaments slide past each other. Motor units, which consist of a motor neuron and the muscle fibers it innervates, determine the strength and coordination of muscle contractions.

3. How do different skeletal muscles work together to create movement, and what concepts of muscle action apply?

   • Skeletal muscles work together through the concepts of synergists (muscles that assist in movement) and antagonists (muscles that oppose movement). This collaboration is essential for coordinated movements.

4. What metabolic pathways and energy sources are involved in classifying skeletal muscles, specifically in differentiating between slow-twitch and fast-twitch fibers?

   • Skeletal muscles are classified based on their metabolic pathways as slow-twitch (type I fibers, aerobic metabolism, endurance) and fast-twitch (type II fibers, anaerobic metabolism, rapid force production).

5. What is the unique structure and contraction mechanism seen in smooth muscles, and how do they function within various organ systems?

   • Smooth muscle is non-striated and operates involuntarily. Its contraction mechanism involves the interaction between actin and myosin filaments, regulated by calcium levels, playing a crucial role in organ systems like the digestive tract and blood vessels.

6. How does cardiac muscle compare to skeletal and smooth muscles in terms of structure and function?

   • Cardiac muscle is striated like skeletal muscle but involuntary like smooth muscle, characterized by intercalated discs that facilitate synchronized contraction, essential for effective heart function.