Overview of Muscle Tissues

Muscular System Overview

Introduction to Muscles

Muscles play critical roles in body movements, joint stabilization, and thermoregulation.

How Muscles Work

  • Muscles generate force for movement through contraction.

  • Muscle contraction involves the overlapping of proteins within muscle fibers, enhancing their interaction.

Why Muscles are Important

  • Muscles facilitate not only body movements but also the transport of substances within the body, such as:

    • Air (in and out of lungs)

    • Food (through the digestive tract)

    • Blood (through the heart and vessels)

Etymology

  • The term muscle originates from the Latin word "mus," meaning "little mouse," due to the appearance of contracting muscles resembling mice scurrying under skin.

Characteristics of Muscle Tissue

  • Muscle tissue is the dominant tissue in various organs such as the heart and hollow organs, comprising nearly half of the body’s mass.

  • The unique characteristic of muscle tissue is its ability to contract.

6.1 Overview of Muscle Tissues

Learning Objectives

  • Compare and contrast structure and function of three types of muscle tissue.

  • Define muscular system and explain the roles of endomysium, perimysium, epimysium, tendon, and aponeurosis.

6.1a Muscle Types

  1. Skeletal Muscle

    • Body Location: Attached mainly to bones; some attach to skin (facial muscles).

    • Cell Shape: Long, cylindrical, multinucleate cells with striations.

    • Regulation: Voluntary, controlled by the nervous system.

    • Contractile Speed: Ranges from slow to fast.

    • Unique Features: Striated and major component of muscular system.

  2. Cardiac Muscle

    • Body Location: Walls of the heart.

    • Cell Shape: Branching chains of cells; uninucleate with striations and intercalated discs.

    • Regulation: Involuntary, with internal pacemaker and influenced by the nervous system and hormones.

    • Contractile Speed: Generally slow, but rhythmic.

  3. Smooth Muscle

    • Body Location: Walls of hollow organs (e.g., intestines, blood vessels).

    • Cell Shape: Fusiform, uninucleate; no striations.

    • Regulation: Involuntary, influenced by nervous system, hormones, chemicals, and stretch.

    • Contractile Speed: Very slow, with some rhythmic contractions.

Similarities Among Muscle Types

  • All muscle cells (skeletal and smooth) are elongated and referred to as muscle fibers.

  • Muscle cell contraction depends on myofilaments similar to microfilaments.

  • Terms: "myo-" or "mys-" and "sarco-" indicate association with muscle tissue.

Definitions of Key Terms

  • Endomysium: Connective tissue surrounding each muscle fiber.

  • Perimysium: Connective tissue that wraps around groups of muscle fibers (fascicles).

  • Epimysium: Connective tissue that covers the entire muscle.

  • Tendon: Connective tissue that attaches muscle to bone.

  • Aponeurosis: Sheet-like connective tissue that attaches muscle to another muscle or to bone.

6.1b Muscle Functions

Categories of Muscle Functions

  1. Produce Movement

    • Enables locomotion and manipulation of environment.

    • Important for reflexes and emotional expressions.

  2. Maintain Posture and Body Position

    • Functions continuously to adjust posture, allowing for maintenance of erect or seated positions.

  3. Stabilize Joints

    • Muscles support joints, crucial in areas with poorly articulating surfaces (e.g., shoulders, knees).

  4. Generate Heat

    • Muscle contractions create heat, crucial for maintaining normal body temperature. Skeletal muscle accounts for at least 40% of body mass, making it a significant source of heat.

Additional Functions

  • Smooth muscles: help regulate openings, dilate/constrict pupils, raise hair through arrector pili.

  • Skeletal muscles: control voluntary functions and protect internal organs.

6.2 Microscopic Anatomy of Skeletal Muscle

Learning Objective

  • Describe microscopic structure of skeletal muscle and the roles of actin and myosin.

Structure of Skeletal Muscle Fibers

  • Skeletal muscle fibers are multinucleated with the nucleus beneath the plasma membrane (sarcoplasmic).

  • Elongated organelles known as myofibrils fill the muscle fiber and are responsible for contractions.

  • Striated Appearance: Caused by light (I) and dark (A) bands formed by thin (actin) and thick (myosin) filaments.

Banding Pattern Elements

  • I Band: Contains thin filaments; Z-disc marks the midline interruption of the light band.

  • A Band: Contains thick filaments; H zone is the lighter central area within.

  • M Line: Holds adjacent thick filaments together in the center of the H zone.

Sarcomeres

  • Definition: Contractile units in myofibrils; arranged end-to-end.

  • Myofilaments within sarcomeres lead to visible striations in skeletal muscles.

Myofilament Types

  1. Thick Filaments: Made mostly of myosin; important for cross-bridge formation.

  2. Thin Filaments: Composed of actin, plus regulatory proteins that permit bonding with myosin heads.

Sarcoplasmic Reticulum (SR)

  • Specialized smooth endoplasmic reticulum responsible for calcium storage and release during muscle contraction.

6.3 Skeletal Muscle Activity

Learning Objectives

  • Describe action potential initiation in muscle cells.

Electrical Properties of Muscle Fibers

  • Irritability: Ability to respond to stimulus.

  • Contractility: Ability to shorten forcefully when stimulated.

  • Extensibility: Ability to stretch.

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

Neuromuscular Junction and Action Potential

  1. Nerve Impulse Reaches Axon Terminal: Calcium channels open and calcium ions enter.

  2. Acetylcholine (ACh) Release: Synaptic vesicles fuse with the membrane, releasing ACh into the synaptic cleft.

  3. ACh Binding: It binds to receptors on the sarcolemma, leading to increased permeability to sodium ions (Na+).

  4. Depolarization: This ion movement generates action potential, causing contraction.

  5. Relaxation: Breakdown of ACh by acetylcholinesterase (AChE) ceases stimulation and allows relaxation.