The Muscular System

Introduction to Muscle Tissue

The muscular system is responsible for all types of body movement and is composed of three basic muscle types: skeletal, cardiac, and smooth muscle. Each type has unique structural and functional characteristics that enable the body to perform a wide range of activities.

• Skeletal muscle: Attached to bones, responsible for voluntary movements.

• Cardiac muscle: Found only in the heart, responsible for pumping blood.

• Smooth muscle: Located in walls of hollow organs, responsible for involuntary movements.

Muscle Types: Structure and Function

Skeletal Muscle

Skeletal muscle fibers are large, multinucleate, and have a striated appearance due to the arrangement of myofibrils. They are under voluntary control and are responsible for body movement, posture, joint stabilization, and heat generation.

• Striated and voluntary

• Attached to bones by tendons (cordlike) or aponeuroses (sheetlike)

• Surrounded by connective tissue layers:

  • Endomysium: Encloses a single muscle fiber

  • Perimysium: Wraps around a fascicle (bundle) of fibers

  • Epimysium: Covers the entire muscle

  • Fascia: On the outside of the epimysium

Smooth Muscle

Smooth muscle is found mainly in the walls of hollow visceral organs such as the stomach, urinary bladder, and respiratory passages. It is involuntary, non-striated, and composed of spindle-shaped, uninucleate fibers. Contractions are slow and sustained.

• No striations

• Involuntary control

• Spindle-shaped, single nucleus

Cardiac Muscle

Cardiac muscle is found only in the heart. It is striated, involuntary, and composed of branching cells joined by gap junctions called intercalated discs. Cardiac muscle contracts at a steady rate set by the heart's pacemaker.

• Striated

• Involuntary control

• Uninucleate, branching cells

• Intercalated discs for cell-to-cell communication

Comparison of Muscle Types

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

Skeletal muscle fibers contain specialized structures that enable contraction:

• Sarcolemma: Specialized plasma membrane

• Myofibrils: Long organelles inside muscle cell, responsible for striations

• Light (I) bands and dark (A) bands: Give muscle its banded appearance

Sarcomere: The Contractile Unit

The sarcomere is the structural and functional unit of skeletal muscle. It is defined as the segment between two Z discs and is responsible for muscle contraction.

• Thick filaments: Composed of myosin, contain ATPase enzymes and myosin heads (cross bridges)

• Thin filaments: Composed of actin, anchored to the Z disc

• H zone: Lighter central area within the A band, disappears during contraction

• M line: Center of the H zone

Sarcoplasmic Reticulum (SR)

The SR is a specialized smooth endoplasmic reticulum that surrounds myofibrils and stores/releases calcium ions, which are essential for muscle contraction.

Physiology of Skeletal Muscle Contraction

Functional Properties

• Irritability (Responsiveness): Ability to receive and respond to a stimulus

• Contractility: Ability to forcibly shorten when adequately stimulated

• Extensibility: Ability to be stretched

• Elasticity: Ability to recoil and resume resting length after stretching

Nerve Stimulus and Action Potential

Skeletal muscles must be stimulated by a motor neuron to contract. The motor unit consists of one motor neuron and all the muscle fibers it stimulates.

• Neuromuscular junction: Site where the axon terminal of a motor neuron meets the sarcolemma of a muscle fiber

• Neurotransmitter: Chemical messenger (acetylcholine, ACh) released to transmit the nerve impulse

• Synaptic cleft: Gap between nerve and muscle, filled with interstitial fluid

Steps in Muscle Stimulation

1. Nerve impulse reaches axon terminal of motor neuron

2. Calcium channels open; Ca2+ enters axon terminal

3. Ca2+ entry causes synaptic vesicles to release ACh

4. ACh diffuses across synaptic cleft, binds to receptors on sarcolemma

5. Sarcolemma becomes more permeable to Na+; depolarization occurs

6. Action potential is generated and spreads along the muscle fiber

7. Acetylcholinesterase (AChE) breaks down ACh, ending contraction

Restoration of Resting State

• K+ diffuses out of the cell

• Sodium-potassium pump restores original ion concentrations

Sliding Filament Theory of Contraction

Muscle contraction occurs as myosin heads bind to actin, forming cross bridges, and pull the thin filaments toward the center of the sarcomere. This process is powered by ATP and regulated by Ca2+ ions.

• Ca2+ binds to regulatory proteins on actin, exposing binding sites

• Myosin heads attach, pivot, and pull actin filaments inward

• ATP is required for both the power stroke and detachment of myosin heads

Contraction of a Skeletal Muscle as a Whole

Graded Responses

Muscle fibers contract in an all-or-none fashion, but whole muscles can produce graded responses by varying the frequency and number of fibers stimulated.

• Muscle twitch: Single, brief contraction (not typical in normal function)

• Summing of contractions: Rapid, repeated stimulation leads to stronger contractions

• Unfused (incomplete) tetanus: Partial relaxation between contractions

• Fused (complete) tetanus: No relaxation, smooth sustained contraction

Muscle Force and Recruitment

• Force depends on the number of fibers stimulated

• Maximum contraction occurs when all motor units are active

Energy for Muscle Contraction

ATP: The Immediate Energy Source

ATP is the only direct energy source for muscle contraction, but is stored in limited amounts. Muscles regenerate ATP through three main pathways:

1. Direct phosphorylation of ADP by creatine phosphate (CP):

   • CP + ADP → Creatine + ATP

   • Provides energy for about 15 seconds

2. Aerobic respiration:

   • Occurs in mitochondria, requires oxygen

   • Glucose + O2 → CO2 + H2O + energy (about 32 ATP)

   • Slower, but efficient and used during rest or moderate activity

3. Anaerobic glycolysis and lactic acid formation:

   • Glucose → Pyruvic acid → Lactic acid + 2 ATP (when O2 is limited)

   • Fast, but less efficient and leads to muscle fatigue

Muscle Fatigue and Oxygen Deficit

Muscle fatigue occurs after prolonged, strenuous activity due to factors such as ion imbalances, oxygen deficit, lactic acid accumulation, and decreased ATP supply. After exercise, rapid, deep breathing helps repay the oxygen deficit.

Types of Muscle Contractions

• Isotonic contractions: Muscle shortens and movement occurs (e.g., lifting weights)

• Isometric contractions: Muscle tension increases, but muscle does not shorten (e.g., pushing against an immovable object)

Muscle Tone and Effects of Exercise

Muscle Tone

Muscle tone is the state of continuous partial contraction, keeping muscles firm, healthy, and ready for action.

Effects of Exercise

• Aerobic (endurance) exercise: Increases muscle strength, flexibility, and resistance to fatigue; improves metabolism, digestion, and coordination

• Resistance (isometric) exercise: Increases muscle size and strength by causing muscle fiber hypertrophy

Additional info: The notes above are based on standard college-level Anatomy & Physiology content and have been expanded for clarity and completeness.