Chapter 9.1-9.3 Study Notes

Chapter 9: Muscular System Histology and Physiology

9.1 Learning Outcomes

  • Learning Outcomes

    1. Summarize the functions of the different muscle tissues in the body.

    2. List and explain the 4 functional properties of all muscle tissue.

    3. Detail the organization of skeletal muscle from skeletal muscle fiber to whole muscle including associated connective tissues.

    4. Describe the function and label the components within a skeletal muscle fiber:

    • Sarcolemma

    • Transverse (T) tubules

    • Sarcoplasmic reticulum

    • Myofibrils

    1. Describe the function and label the sarcomere, including:

    • Thin filaments (actin)

    • Thick filaments (myosin)

    • Elastic filaments (titin)

    • Troponin

    • Tropomyosin

    • Z disc

    • M line

    • I band

    • A band

    1. Describe the function and label the neuromuscular junction (NMJ).

9.1 Functions of the Muscular System

  • The body has 3 types of muscle tissue:

    1. Skeletal Muscle: Most abundant.

    2. Cardiac Muscle: Found only in the heart.

    3. Smooth Muscle: Found throughout various organs.

Skeletal Muscle

  • Characteristics:

    • Can be voluntary or involuntary (reflexes).

    • Nervous system controls voluntary skeletal muscle.

    • Striated appearance (stripes or lines perpendicular to the length of the cells).

    • Composed of multinucleated fibers; nuclei are scattered along the length of cells.

Cardiac Muscle

  • Characteristics:

    • Involuntary; not under conscious control.

    • Exclusively found in the heart and responsible for pumping blood.

    • Features faint striations similar to skeletal muscle.

    • Intercalated discs separate individual cells.

    • Not multinucleated and exhibits branching connections between cells.

Smooth Muscle

  • Characteristics:

    • Involuntary; not consciously controlled.

    • Most widely distributed muscle type; found in walls of hollow organs (e.g., intestines, uterus, blood vessels).

    • Responsible for movements such as food transit through the digestive tract.

    • Composed of spindle-shaped, tapered cells.

Functions of Muscle Tissue

  • All 3 muscle types share 7 main functions:

    1. Movement of the body.

    2. Maintenance of posture.

    3. Respiration through contraction of skeletal muscles of the thorax and diaphragm.

    4. Production of body heat through skeletal muscle contraction.

    5. Communication (facial expressions, speech).

    6. Constriction of organs and vessels (smooth muscle contractions).

    7. Contraction of the heart.

9.2 General Properties of Muscle Tissue

  • Muscle tissue has 4 functional properties:

    1. Contractility: Ability to shorten forcefully, causing movement of attached structures (like bones) or increasing pressure within vessels or organs.

    2. Excitability: Capacity to respond to electrical stimuli from nerves; usually under conscious control.

    • Example: Deciding to pick up an object involves coordinating voluntary skeletal muscle movements.

    1. Extensibility: Muscles can be stretched beyond their normal resting length but still contract (e.g., bending over to touch toes stretches hamstrings).

    2. Elasticity: Ability to return to original resting length after being stretched.

9.3 Whole Skeletal Muscle Anatomy

  • Organization from muscle fiber to muscle:

    • A muscle cell is referred to as a muscle fiber.

    • Bundles of fibers form tissues; these tissues combine to create organs (e.g., biceps muscle, composed of muscle, nerve, connective, and adipose tissues).

    • An organ consists of two or more tissues working together for a common function.

Muscle Diameter and Composition

  • Large muscles have many large-diameter fibers while smaller muscles possess smaller-diameter fibers.

  • Most muscles contain a mixture of both.

  • The number of skeletal muscle fibers remains stable after birth; however, the size of muscle fibers increases, resulting in larger muscles.

Connective Tissue Layers Surrounding Skeletal Muscles

  • Skeletal muscles are surrounded by 3 connective tissue layers:

    1. Epimysium: Surrounds the entire muscle.

    2. Perimysium: Envelopes the fascicles (bundles of fibers).

    3. Endomysium: Surrounds each individual muscle fiber.

  • Whole muscles consist of fascicles; fascicles comprise individual muscle fibers.

  • Muscle fibers are composed of myofibrils, which in turn are made up of myofilaments.

Muscle Fiber Composition

  • Specialized nerve cells responsible for skeletal muscle contraction are termed motor neurons.

  • Motor neurons originate in the brain and spinal cord, extending to skeletal muscle fibers via nerves.

  • Each motor neuron typically controls several muscle fibers.

  • An artery and 1 or 2 veins accompany a nerve through muscle connective tissue layers.

Skeletal Muscle Fiber Anatomy

  • Skeletal muscle contraction involves:

    1. An electrical event.

    2. A mechanical event.

Electrical Component Structures in Muscle Fibers

  • The three components of muscle fibers that respond to and transmit electrical signals include:

    1. Sarcolemma: The plasma membrane of muscle fibers.

    2. Transverse tubules (T tubules): Inward folds of the sarcolemma that project into the muscle fiber, conducting electrical impulses.

    3. Sarcoplasmic reticulum: The smooth endoplasmic reticulum of muscle fibers, storing calcium ions.

  • Sarcoplasmic Reticulum Function: Calcium release from this structure acts as a "switch" to initiate muscle contraction.

  • Triad Structure: Comprised of two terminal cisternae associated with T tubules together.

Sarcoplasm

  • Sarcoplasm: The cytoplasm of muscle fibers, containing organelles such as mitochondria and glycogen.

Mechanical Component Structures in Muscle Fibers

  • Muscle contractility is attributed to:

    • Myofibrils: Bundles of protein filaments extending the full length of muscle fibers, facilitating interactions that shorten fibers during contraction.

    • Myofilaments: Composed of two types:

    1. Actin: Thin filaments.

    2. Myosin: Thick filaments, arranged in sarcomeres.

Structure of Sarcomeres

  • Sarcomeres form the segments of myofibrils and join end to end.

  • Each sarcomere is defined by two main regions:

    1. I Bands: Lighter staining regions (only contain actin).

    2. A Band: Central darker-staining region (contains both actin and myosin).

  • The H Zone is a smaller central band in A bands containing only myosin.

  • Z Disks: Serve as anchors for actin myofilaments; span the length between Z disks defines a sarcomere.

  • The M Line: Central dark line within the H Zone, assisting in anchoring myosin filaments.

Proteins of Myofilaments

  • Actin Myofilaments Composition:

    • Composed of three proteins:

    1. Globular (G) Actin: Forms actin filaments.

    2. Tropomyosin: Binds along actin, blocking active sites.

    3. Troponin: Binds to tropomyosin and calcium, regulating contraction.

  • Myosin Myofilaments Composition:

    • Each myosin molecule consists of:

    • Two heavy chains intertwined to form a rod-like structure.

    • Two myosin heads that extend from the rod.

Neuromuscular Junction Structure

  • The neuromuscular junction consists of:

    1. Axon terminals: The endings of motor neuron axons.

    2. Muscle Fiber Area: The muscle fiber area that the axon innervates.

Sliding Filament Model

  • Sliding Filament Model: Describes the interaction of myofilaments during muscle contraction.

    • In a relaxed sarcomere:

    1. Actin and myosin myofilaments overlap slightly.

    2. The length of the sarcomere is at the normal resting length.

    • Upon contraction, the actin slides past myosin, pulling Z disks closer together, shortening the sarcomere.

  • In a fully contracted sarcomere:

    1. A bands do not narrow; neither myosin nor actin filament lengths change.

    2. Actin moves past myosin, increasing overlap in the center of the sarcomere.