10.1 & 10.2

The Different Types of Muscle

  • Primary Types of Muscle:

    • Skeletal Muscle: Visible under the skin, particularly in limbs; moves the skeleton.

    • Cardiac Muscle: Found in the heart; responsible for pumping blood through the circulatory system.

    • Smooth Muscle: Involved in involuntary movements (e.g., goosebumps, digestion).

10.1 Overview of Muscle Tissues

  • Learning Objectives:

    • Describe the different types of muscle.

    • Explain contractibility and extensibility.

  • Muscle Tissue Properties:

    • Muscle is one of the four primary tissue types in the body.

    • Contains three types of muscle tissue: skeletal, cardiac, and smooth.

  • Common Properties:

    • Excitability: Plasma membranes can change electrical states and send action potentials along their length.

    • Skeletal muscle relies on nervous system signaling for contraction.

    • Cardiac and smooth muscle respond to hormones and local stimuli as well.

  • Muscle Contraction Mechanism:

    • Contraction occurs when actin is pulled by myosin, activated by calcium ions (Ca²⁺) accessing binding sites on actin.

    • All muscles require ATP for contraction.

    • Relaxation occurs as Ca²⁺ is removed and actin-binding sites are re-shielded.

  • Additional Muscle Properties:

    • Elasticity: Ability to return to original length after stretching.

    • Extensibility: Ability to stretch or extend.

    • Contractility: Power to pull on attachment points and generate force.

Differences Among Muscle Types

  • Skeletal and cardiac muscles show a striated appearance due to regularly arranged actin and myosin.

    • Skeletal Muscle Fibers:

    • Multinucleated.

    • Voluntary control.

    • Cardiac Muscle Fibers:

    • 1-2 nuclei; connected for synchronization.

    • Involuntary control.

    • Smooth Muscle:

    • Non-striated appearance; single nucleus; less organized.

    • Involuntary and essential for organ function.

10.2 Skeletal Muscle

  • Learning Objectives:

    • Describe layers of connective tissue in skeletal muscle.

    • Explain function with tendons.

    • Identify areas of skeletal muscle fibers.

    • Describe excitation-contraction coupling.

  • Key Features of Skeletal Muscle:

    • Controls voluntary movement and posture; prevents excessive movement and maintains stability.

    • Protects internal organs.

    • Generates heat during contraction (homeostasis).

  • Structure:

    • Composed of skeletal muscle fibers, blood vessels, nerve fibers, and connective tissues.

    • Three Connective Tissue Layers:

    • Epimysium: Dense connective tissue wrapping the entire muscle.

    • Perimysium: Encloses bundles of muscle fibers (fascicles).

    • Endomysium: Surrounds each muscle fiber, containing nutrients and extracellular fluid.

  • Muscle Fiber Characteristics:

    • Long and cylindrical, called muscle fibers, can be large (up to 100 μm in diameter, 30 cm in length).

    • Formed by the fusion of myoblasts during development.

    • Specialized terms: Sarcolemma (plasma membrane), sarcoplasm (cytoplasm), and sarcoplasmic reticulum (stores calcium).

  • Functional Unit - Sarcomere:

    • Sarcomeres are the contractile units made of actin (thin filament) and myosin (thick filament).

    • Striated appearance from organized arrangement of myofilaments.

    • Z-disc: Borders of the sarcomere, anchoring actin.

The Neuromuscular Junction (NMJ)

  • Specialized site where motor neurons meet muscle fibers.

  • Every skeletal muscle fiber is innervated by a motor neuron at NMJ.

  • Excitation-Contraction Coupling:

    • Cells have membrane potentials allowing electrical signaling.

    • Action Potential: Electrical wave traveling along the membrane, triggering muscle contraction.

    • NMJ: Motor neuron’s axon releases acetylcholine (ACh), causing muscle fiber depolarization.

  • Depolarization Process:

    • ACh binds to receptors, triggering voltage-gated sodium channels to open and allow Na⁺ entry.

    • Results in an action potential that travels along the muscle fiber's sarcolemma.

  • Calcium Release:

    • An action potential stimulates calcium release from the SR. Ca²⁺ initiates contraction by exposing actin-binding sites for myosin.

  • T-Tubules and Triad:

    • T-tubules allow action potentials to reach the SR from the sarcolemma.

    • A triad consists of two terminal cisternae of SR flanking one T-tubule, enhancing the calcium release for muscle fiber contraction.