Functions of the Muscular System

FUNCTIONS OF THE MUSCULAR SYSTEM

Learning Outcome

  • After reading this section, you should be able to:

    • A. List the functions of the muscular system.

Overview of Muscle Activity

  • A vivid example demonstrating muscle activity includes a runner sprinting towards the finish line:

    • Arms and legs pumping to achieve maximum speed.

    • Rapid heart rate alongside deep and regular breathing.

    • Blood being redirected from digestive organs to skeletal muscles to enhance oxygen supply.

  • Muscle tissue is the most abundant and adaptable tissue in the body.

  • Key activities executed by the muscular system include:

    • Postural maintenance, even without conscious movement.

    • Respiratory functions ensuring continuous breathing.

    • Heart's continuous pumping action.

    • Regulation of blood flow through vessel contraction and relaxation.

  • Body movement originates from:

    • Cilia or flagella on cells, gravity, and muscle contraction.

Major Functions of the Muscular System

  1. Movement of the Body: Skeletal muscles contract to facilitate overall movements like walking and running.

  2. Maintenance of Posture: Skeletal muscles maintain tone necessary for standing and sitting erect.

  3. Respiration: Contraction of thoracic cage and diaphragm muscles aids in breathing.

  4. Production of Body Heat: Heat is released as a by-product of skeletal muscle contraction, vital for body temperature maintenance.

  5. Communication: Essential for speaking, writing, gesturing, and facial expressions.

  6. Constriction of Organs and Vessels: Smooth muscles regulate food propulsion and blood flow.

  7. Contraction of the Heart: Cardiac muscle contractions propel blood throughout the body.

OVERVIEW OF THE MUSCULAR SYSTEM

Types of Muscle Tissue

Characteristic

Skeletal Muscle

Cardiac Muscle

Smooth Muscle

Location

Attached to bones

Heart

Walls of hollow organs

Appearance

Striated

Striated

Non-striated

Cell Shape

Long, cylindrical

Branched

Spindle-shaped

Nucleus

Multiple, peripheral

Single, central

Single, central

Control

Voluntary

Involuntary

Involuntary

Special Features

No intercalated disks

Intercalated disks

Gap junctions

  • Functional Characteristics:

    1. Contractility: Ability of muscle to shorten forcefully.

    2. Excitability: Capacity to respond to stimuli.

    3. Extensibility: Ability to stretch beyond normal resting lengths and still contract.

    4. Elasticity: Ability of muscle to return to original resting length post-stretching.

SKELETAL MUSCLE ANATOMY

Connective Tissue Coverings

  1. Epimysium: Connective tissue sheath encasing each skeletal muscle.

  2. Perimysium: Subdivides muscles into visible bundles called fascicles; acts as passageways for vessels and nerves.

  3. Endomysium: Delicate layer separating individual muscle fibers within fascicles.

Histology and Electrical Components

  • Sarcolemma: The cell membrane of a muscle fiber.

  • Sarcoplasm: Muscle cell cytoplasm containing mitochondria and glycogen.

  • Transverse Tubules (T Tubules): Tubelike inward folds of the sarcolemma that carry electrical signals into the interior.

  • Sarcoplasmic Reticulum: Specialized smooth ER that stores high levels of Ca^{2+}.

Mechanical Component Structures (The Sarcomere)

  • Myofibrils: Bundles of protein filaments (actin and myosin).

  • Sarcomere Components:

    • Z Disks: Protein network forming the boundaries of a sarcomere; anchors actin.

    • I Band: Lighter region containing only actin myofilaments.

    • A Band: Darker region in the center; contains overlapping actin and myosin.

    • H Zone: Center of the A band where only myosin exists.

    • M Line: Dark line in the middle of the H zone that anchors myosin.

  • Detailed Myofilament Structure:

    • Actin (Thin Filament): Composed of G-actin molecules with active sites, tropomyosin (covers sites), and troponin (binds Ca^{2+}).

    • Myosin (Thick Filament): Shaped like golf clubs with a myosin rod (tail) and myosin heads. Heads contain ATPase to break down ATP for energy.

Neuromuscular Junction (NMJ)

  • Presynaptic Terminal: End of the nerve cell containing synaptic vesicles filled with Acetylcholine (ACh).

  • Synaptic Cleft: The gap between the terminal and the muscle fiber.

  • Motor End-Plate: The specialized area of the sarcolemma containing receptors for ACh.

Sliding Filament Model: Visual Changes During Contraction

  • When muscles contract, the actin and myosin filaments slide past each other:

    1. Z disks move closer together.

    2. Sarcomere shortens.

    3. I bands narrow/shorten.

    4. H zone narrows or disappears entirely.

    5. A band remains the same length (since myosin length does not change).