Skeletal Muscle Structure and Contraction: Anatomy & Physiology Study Notes

MUSCLE

Overview of Muscle Types

Muscle tissue is essential for movement, posture, and various physiological functions. There are three main types of muscle in the human body: skeletal muscle, cardiac muscle, and smooth muscle. Each type has distinct anatomical and functional characteristics.

• Skeletal Muscle: Voluntary, striated muscle responsible for body movement.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs.

Skeletal Muscle Anatomy

Organization of Skeletal Muscle

Skeletal muscle is organized in a hierarchical structure, from the whole muscle down to the molecular components that enable contraction.

• Muscle Tissue: Composed of bundles called muscle fascicles.

• Muscle Fascicles: Contain individual muscle fibers (cells).

• Muscle Fibers: Multinucleated cells containing specialized organelles for contraction.

• Key Components:

  • Sarcolemma: The plasma membrane of a muscle fiber.

  • T-tubules: Invaginations of the sarcolemma that help transmit action potentials.

  • Sarcoplasm: Cytoplasm of muscle cells, containing organelles and myofibrils.

  • Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores calcium ions.

  • Myofibrils: Cylindrical structures composed of repeating units called sarcomeres.

  • Mitochondria: Provide ATP for muscle contraction.

Table: Skeletal Muscle Organization

Level	Component	Function
Whole Muscle	Skeletal Muscle	Produces movement
Bundle	Muscle Fascicle	Group of muscle fibers
Cell	Muscle Fiber	Contractile cell
Organelle	Myofibril	Contains contractile proteins
Molecular	Myofilaments (Actin, Myosin, etc.)	Enable contraction

Myofibrils and Sarcomeres

Myofibrils are long, cylindrical organelles within muscle fibers, composed of repeating units called sarcomeres. The sarcomere is the fundamental contractile unit of skeletal muscle.

• Sarcomere: Defined as the region between two Z-lines.

• M-line: Center of the sarcomere, composed of myomesin.

• Myofibrils: Contain multiple, repeating sarcomeres.

• Contraction: Sarcomeres shorten during muscle contraction, producing movement.

Myofilaments: Thin and Thick Filaments

Myofibrils are composed of two main types of myofilaments: thin filaments and thick filaments. These filaments are responsible for the contractile properties of muscle.

• Thin Filaments: Primarily composed of actin and associated regulatory proteins (troponin and tropomyosin).

• Thick Filaments: Composed mainly of myosin molecules, which have heads and tails that interact with actin during contraction.

Table: Myofilament Components

Filament Type	Main Proteins	Function
Thin Filament	Actin, Troponin, Tropomyosin	Provides binding sites for myosin; regulates contraction
Thick Filament	Myosin	Motor protein; interacts with actin to produce contraction

Muscle Contraction

Sliding Filament Theory

The sliding filament theory explains how muscles contract at the molecular level. During contraction, thin filaments slide past thick filaments, shortening the sarcomere and generating force.

• Thin filaments (actin) slide toward the center of the sarcomere.

• Thick filaments (myosin) remain stationary.

• H zone and I band decrease in width; A band remains unchanged.

• Z-lines move closer together.

Molecular Basis of Contraction

Muscle contraction is driven by the interaction between actin and myosin, regulated by calcium ions and ATP.

1. Calcium Release: Action potential triggers release of Ca2+ from the sarcoplasmic reticulum.

2. Troponin Activation: Ca2+ binds to troponin, causing a conformational change.

3. Tropomyosin Shift: Tropomyosin moves away from actin's myosin-binding sites.

4. Cross-Bridge Formation: Myosin heads bind to exposed sites on actin.

5. Power Stroke: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere.

6. ATP Binding: ATP binds to myosin, causing it to release actin and reset for another cycle.

Key Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

This reaction provides the energy required for the myosin head to perform the power stroke.

Role of Calcium and ATP

• Calcium (Ca2+): Essential for exposing myosin-binding sites on actin.

• ATP: Required for both the power stroke and the release of myosin from actin.

Application: Rigor Mortis

After death, ATP production ceases, preventing myosin heads from detaching from actin. This results in muscle stiffening known as rigor mortis.

Key Concepts Summary

• Muscle fibers are filled with myofibrils, which contain thick (myosin) and thin (actin) filaments.

• The sliding filament theory describes how myofilaments interact to produce contraction of the sarcomere and, consequently, the muscle.

• Contraction requires both calcium ions and ATP.

Additional info: Cardiac and smooth muscle have unique structural and functional properties compared to skeletal muscle, including differences in control (involuntary vs. voluntary), appearance (striated vs. non-striated), and location in the body.