The Muscular System: Structure, Function, and Physiology

The Muscular System

Introduction to Mobility

Mobility refers to purposeful physical movement, encompassing both simple and complex actions as well as coordination. It is essential for daily activities and overall health. Mobility depends on the integrated function of bones, joints, articular cartilage, tendons, and skeletal muscle, as well as the mechanics of muscle contraction.

• Clinical Relevance: Nurses play a key role in promoting optimal mobility, preventing immobility (e.g., fall prevention), and minimizing complications such as pressure ulcers when immobility occurs.

Muscle Tissue Types

Overview of Muscle Tissue

There are three main types of muscle tissue in the human body, each with distinct structure and function.

• Skeletal Muscle: Attached to bones and skin; long, striated, multinucleated fibers; voluntary control.

• Smooth Muscle: Found in walls of hollow organs; spindle-shaped, non-striated, uninucleated cells; involuntary control.

• Cardiac Muscle: Found in the heart; branching, striated, mainly uninucleated cells; involuntary control; highest density of mitochondria.

Comparison Table: Skeletal, Cardiac, and Smooth Muscle

Characteristic	Skeletal	Cardiac	Smooth
Location	Attached to bones or skin	Walls of the heart	Walls of hollow organs
Cell Shape & Appearance	Long, cylindrical, multinucleated, striated	Branching chains, uni- or binucleated, striated	Spindle-shaped, uninucleated, non-striated
Control	Voluntary	Involuntary	Involuntary

General Characteristics of Muscle

Key Properties

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length after being stretched.

Muscle Functions

Primary Roles of Muscle Tissue

• Movement: Produces body movements.

• Maintaining Posture: Stabilizes body position.

• Stabilizing Joints: Reinforces and supports joints.

• Heat Generation: Especially by skeletal muscle during contraction.

• Other Functions: Moves substances through organs, forms valves, controls pupil size, and causes "goosebumps."

Skeletal Muscle Structure

Components of Skeletal Muscle

• Each muscle is an organ composed of:

  • One artery, one nerve, and one or more veins

  • Muscle fibers (cells)

  • Connective tissue (covering, protecting, reinforcing)

Connective Tissue Coverings

• Epimysium: Surrounds the whole muscle; dense irregular connective tissue.

• Perimysium: Surrounds fascicles (groups of muscle fibers); fibrous connective tissue.

• Endomysium: Surrounds each muscle fiber; fine areolar connective tissue.

Microscopic Anatomy of a Skeletal Muscle Fiber

• Long, cylindrical cell (up to 30 cm); multiple peripheral nuclei.

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm containing glycosomes (glycogen storage) and myoglobin (oxygen storage).

• Myofibrils: Rod-like elements filling most of the cell volume; contain contractile units called sarcomeres.

• Sarcoplasmic Reticulum (SR): Specialized smooth endoplasmic reticulum; regulates intracellular Ca2+.

• T Tubules: Invaginations of the sarcolemma; conduct impulses to every sarcomere.

Sarcomere Structure

• Sarcomere: The functional contractile unit of muscle; composed of thick (myosin) and thin (actin) myofilaments.

• Myofilaments are organized in a repeating pattern, creating striations.

Thick and Thin Filaments

• Thick Filaments: Composed of myosin molecules with heads that bind to actin and ATP.

• Thin Filaments: Composed of actin, tropomyosin, and troponin; provide binding sites for myosin heads.

Muscle Contraction: The Sliding Filament Model

Overview of Contraction

Muscle contraction occurs when myosin heads bind to actin, forming cross-bridges and pulling thin filaments toward the center of the sarcomere. This process shortens the muscle fiber.

• Requires both Ca2+ and ATP.

• Four major phases:

  1. Activation at the neuromuscular junction (NMJ)

  2. Muscle fiber excitation

  3. Excitation-contraction coupling

  4. Cross-bridge cycling

Neuromuscular Junction (NMJ)

• Site where a motor neuron communicates with a muscle fiber.

• Separated by a synaptic cleft filled with gel-like extracellular substance.

• Axon terminals contain vesicles of acetylcholine (ACh), which is released into the cleft upon nerve impulse arrival.

• ACh binds to receptors on the sarcolemma, opening ion channels and generating an action potential.

• ACh is quickly broken down by acetylcholinesterase to terminate the signal.

Role of Calcium (Ca2+) in Contraction

• At rest, tropomyosin blocks myosin-binding sites on actin.

• When Ca2+ is released from the SR, it binds to troponin, causing a conformational change that moves tropomyosin and exposes binding sites.

• Myosin heads bind to actin, initiating contraction.

• When stimulation ceases, Ca2+ is pumped back into the SR, ending contraction.

ATP and Muscle Contraction

• ATP is required for cross-bridge detachment and for pumping Ca2+ back into the SR.

• After death, lack of ATP leads to rigor mortis (muscle stiffness).

Motor Units and Muscle Control

Motor Unit Definition

• A motor unit consists of one motor neuron and all the muscle fibers it innervates.

• Finer control is achieved by increasing the number of motor units activated.

Muscle Mechanics

Lever Systems

• Muscles act on bones as levers to produce movement.

• Components: rigid bar (bone), fulcrum (joint), effort (muscle contraction), and load (resistance).

• Levers can be adapted for power or speed depending on the arrangement of these components.

Arrangement of Fascicles

• Fascicle arrangement affects muscle's range of motion and power.

• Common patterns: circular, convergent, parallel, fusiform, pennate.

Muscle Naming and Major Muscles

Criteria for Naming Muscles

• Shape (e.g., deltoid)

• Size (e.g., maximus, minimus, longus)

• Direction of fibers (e.g., rectus, transversus, oblique)

• Number of origins (e.g., biceps, triceps)

• Location of attachments (origin and insertion)

• Action (e.g., flexor, extensor)

Muscles are grouped by function and location. For clinical practice, it is important to know the location and function of common muscles such as the deltoid, vastus lateralis, and gluteal muscles.

Muscle Tone and Twitch

Muscle Tone

• Constant, slightly contracted state of all muscles; keeps muscles firm and ready to respond.

Muscle Twitch

• Response of a muscle to a single action potential.

• Phases: latent period (excitation-contraction coupling), contraction (cross-bridge formation), relaxation (Ca2+ re-entry into SR).

Muscle Metabolism and Energy Sources

ATP Regeneration

• ATP is the only energy source for contractile activity; depleted in 4-6 seconds.

• Regenerated by:

  • Direct phosphorylation (creatine phosphate)

  • Anaerobic pathway (glycolysis → lactic acid)

  • Aerobic respiration (mitochondria; uses oxygen, glucose, fatty acids)

Muscle Fiber Types

• Slow oxidative: Endurance, fatigue-resistant (dark meat)

• Fast oxidative: Intermediate resistance (walking)

• Fast glycolytic: Short, powerful movements (light meat)

Smooth Muscle

Structure and Function

• Found in walls of hollow organs (except heart); usually arranged in two layers (longitudinal and circular).

• Cells are spindle-shaped, uninucleated, and non-striated.

• Contraction is involuntary and can be triggered by neurotransmitters, hormones, or stretch.

• Can contract in a coordinated (single-unit) or independent (multi-unit) manner.

Special Features of Smooth Muscle

• Can contract when stretched and adapt to new lengths (stress-relaxation response).

• Capable of hyperplasia (increase in cell number).

• Uses less ATP and contracts more slowly than skeletal muscle.

Summary Table: Muscle Types and Key Features

Feature	Skeletal	Cardiac	Smooth
Control	Voluntary	Involuntary	Involuntary
Striations	Yes	Yes	No
Cell Shape	Long, cylindrical	Branching	Spindle-shaped
Number of Nuclei	Multinucleated	Uni- or binucleated	Uninucleated
Location	Bones, skin	Heart	Hollow organs