Muscle Tissue and Skeletal Muscle Anatomy

Chapters 9 and 10: Muscle Tissue

  • Muscle Tissue Overview

    • Makes up approximately 1/2 the body’s mass.

    • Most distinguishing characteristic is the ability to transform ATP into directed mechanical energy.

Types of Muscle Tissue

  • There are 3 types of muscle tissue:

    1. Skeletal Muscle

    • Characteristics:

      • Voluntary

      • Striated

      • Multinucleate

    1. Smooth Muscle

    • Characteristics:

      • Involuntary

      • Smooth (non-striated)

      • Uninucleate

    1. Cardiac Muscle

    • Characteristics:

      • Involuntary

      • Striated

      • Uninucleate

      • Intercalated discs

Muscular System Overview

  • Focus primarily on skeletal muscle, with cardiac muscle discussed later.

  • Functions:

    • Required for body movement.

    • Along with the skeleton, acts as a system of levers and pulleys to move body parts either with great force or very precisely.

Major Functions of Skeletal Muscle

  • The primary functions of skeletal muscle include:

    1. Movement

    2. Posture and body position, stabilizing joints

    3. Respiration

    4. Heat production to maintain temperature

Properties of Skeletal Muscle

  • Essential properties to know include:

    1. Excitability (Irritability):

    • Ability to respond to a stimulus.

    1. Contractility:

    • Capability to shorten in length.

    1. Extensibility:

    • Ability to stretch.

    1. Elasticity:

    • Capacity to return to the original shape after stretching.

    1. Conductivity:

    • Transmission of electrical impulses along the muscle cells.

Gross Anatomy of Skeletal Muscle

  • Skeletal muscle is a discrete organ, composed of hundreds of thousands of muscle fibers.

  • Comprehensive structure includes:

    • Mostly muscle fibers

    • Connective tissue

    • Blood vessels

    • Nerve fibers

Structure of a Skeletal Muscle

  • Components include:

    1. Muscular Fascia:

    • Connective tissue that encloses muscles.

    1. Tendon:

    • Connective tissue that attaches muscle to bone.

    1. Epimysium:

    • Connective tissue that wraps around the entire muscle.

    1. Perimysium:

    • Connective tissue that encloses a fascicle (bundle of fibers).

    1. Endomysium:

    • Surrounds each muscle fiber (or cell).

Purpose of Connective Tissue in Skeletal Muscle

  • Functions include:

    • Nerves:

      • Each muscle fiber has a neuromuscular junction, necessary for initiating muscle contraction.

    • Blood Supply:

      • Supplies glucose, O2, and ATP, essential for muscle contraction.

    • Muscle Attachments:

      • Connective tissues enable attachments via tendons and aponeuroses to muscle insertion and muscle origin.

Interaction with Other Systems

  • Skeletal muscle contraction involves:

    • Receiving signals from the central nervous system, which leads to muscle contractions.

    • Blood provides the necessary nutrients and energy for contractions.

    • The arrangements of muscle attachments determine which part of the body or skeleton moves during contraction.

Motor Units

  • The motor unit is defined as the interaction between a motor neuron and muscle cell.

  • Components include:

    • Motor Neuron:

      • The nerve cell that sends signals to muscle fibers.

    • Muscle Cells:

      • All muscle cells connected to a single motor neuron.

    • Characteristics of motor units:

      • Some have only a few muscle cells, while others can have many.

      • Fewer muscle cells in a motor unit results in more precise movements.

      • More muscle cells allow for larger possible force during contractions.

Skeletal Muscle on a Cellular Level

  • Questions to consider:

    1. How does skeletal muscle appear at the cellular level?

    2. What molecules are crucial for muscle contraction and their arrangements?

    3. What signals govern contraction, and how does this system operate?

    4. How does the sarcomere shorten during contraction?

    5. How is skeletal muscle specialized for varying types of movement?

Anatomy of a Skeletal Muscle Fiber

  • Components include:

    1. Single muscle cell:

    • Also referred to as a muscle fiber.

    1. Plasma Membrane (Sarcolemma):

    • Surrounds the muscle fiber and is involved in the conduction of impulses.

    1. Cytoplasm (Sarcoplasm):

    • Contains the organelles and proteins necessary for muscle contraction.

    1. Sarcoplasmic Reticulum:

    • A specialized type of smooth endoplasmic reticulum involved in calcium ion storage and release.

    1. Myofibrils:

    • Composed of myofilaments (actin and myosin) and regulatory proteins (troponin and tropomyosin).

Diagrammatic Representation of Muscle Fiber Components

  • The detailed structure of a muscle fiber includes:

    • T-tubules:

      • Extensions of the sarcolemma that penetrate into the muscle cell.

    • Mitochondria:

      • Provide ATP for energy required for contraction.

    • Triad:

      • The arrangement of one T-tubule and two terminal cisternae of the sarcoplasmic reticulum.

Filament Structure

  • Within myofibrils, the arrangement of filaments is critical:

    1. Thick Filament (Myosin):

    • Consists of myosin heads and tails with functional characteristics.

    1. Thin Filament (Actin):

    • Composed of actin subunits with specific active sites for binding to myosin.

    1. Elastic Filament (Titin):

    • Helps in maintaining structure and elasticity of the muscle.

  • While characteristics such as the diameter of the thick and thin filaments are described, the typical dimensions include:

    • Thick filament diameter: 15 nm

    • Thin filament diameter: 8 nm

    • Actin active site size: 1 nm

  • This comprehensive overview provides critical insights into the structure, organization, and function of skeletal muscle tissue, essential for understanding human anatomy and physiology related to the musculoskeletal system.