The Muscular System: Structure, Function, and Physiology

The Muscular System

Muscle Tissue Types

Muscle tissue is one of the four primary tissue types in the human body, specialized for contraction. There are three main types of muscle tissue, each with distinct structures and functions:

• Skeletal Muscle Tissue: Moves the body by pulling on bones of the skeleton; under voluntary control.

• Cardiac Muscle Tissue: Found only in the heart; responsible for propelling blood throughout the body; involuntary control.

• Smooth Muscle Tissue: Located in the walls of hollow organs (e.g., digestive tract, blood vessels); moves fluids and solids; regulates diameter of small arteries; involuntary control.

Functions of Skeletal Muscle Tissue

Skeletal muscle tissue performs several essential functions:

• Produce Skeletal Movement: Muscle contractions pull on tendons, moving bones.

• Maintain Posture and Position: Constant muscle activity maintains body posture and position.

• Support Soft Tissues: Layers of skeletal muscle form the abdominal wall and pelvic floor, supporting visceral organs and protecting internal tissues.

• Guard Entrances and Exits: Skeletal muscles form sphincters around openings to the digestive and urinary tracts, providing voluntary control over swallowing, defecation, and urination.

• Maintain Body Temperature: Muscle contraction generates heat as a byproduct, helping to maintain body temperature.

• Provide Nutrient Reserves: Contractile proteins can be broken down into amino acids for energy when dietary intake is inadequate.

Skeletal Muscle Structure

Skeletal muscle is organized into bundles of muscle fibers, each wrapped in connective tissue layers:

• Epimysium: Dense collagenous layer surrounding the entire muscle.

• Perimysium: Fibrous layer dividing the muscle into bundles called muscle fascicles.

• Endomysium: Delicate connective tissue surrounding individual muscle fibers.

The connective tissue layers converge to form tendons (cord-like structures attaching muscle to bone) or aponeuroses (sheet-like structures attaching muscle to a broader area).

Skeletal Muscle Fiber Terminology and Structure

• Muscle Fiber: The muscle cell.

• Sarcolemma: The cell membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum for calcium storage.

Muscle fibers are multinucleated and packed with myofibrils, which contain myofilaments:

• Thin Filaments: Composed mainly of actin.

• Thick Filaments: Composed mainly of myosin.

The arrangement of myofilaments gives skeletal muscle its striated appearance.

Sarcomeres: The Functional Unit

The sarcomere is the basic contractile unit of skeletal muscle, with about 10,000 per myofibril. Key components include:

• Z lines: Boundaries between adjacent sarcomeres.

• M line: Center of the sarcomere, connecting thick filaments.

• H band: Region with only thick filaments.

• A band: Dark region containing both thick and thin filaments.

• I band: Light region with only thin filaments.

Structure of Thin and Thick Filaments

• Thin Filaments are composed of:

  • Actin: Twisted double strand with myosin binding sites.

  • Tropomyosin: Covers myosin binding sites on actin when muscle is relaxed.

  • Troponin: Binds to tropomyosin and actin; has a calcium binding site.

• Thick Filaments are composed of:

  • About 300 myosin molecules, with tails pointing toward the M line.

  • Myosin heads bind to actin during contraction, forming cross-bridges.

  • Elastic core helps maintain alignment.

Sarcolemma, T-tubules, and Sarcoplasmic Reticulum

• Membrane Potential: Uneven charge distribution across the sarcolemma.

• T-tubules (Transverse Tubules): Extensions of the sarcolemma that transmit action potentials into the muscle fiber, encircling sarcomeres and closely associated with the SR.

• Sarcoplasmic Reticulum: Stores calcium ions; releases them in response to action potentials, initiating contraction.

Skeletal Muscle Fiber Contraction: The Neuromuscular Junction

Muscle contraction is initiated by a signal from a motor neuron at the neuromuscular junction (NMJ):

1. Action potential travels down the motor neuron to the NMJ.

2. Acetylcholine (ACh) is released into the synaptic cleft.

3. ACh binds to receptors on the motor end plate, increasing membrane permeability to sodium ions.

4. Sodium influx generates an action potential in the sarcolemma.

5. Action potential travels along the sarcolemma and down T-tubules.

6. Triggers calcium release from the SR, initiating contraction.

7. Acetylcholinesterase (AChE) breaks down ACh, ending the signal.

Sliding Filament Theory

The sliding filament theory explains how muscles contract:

• Thin filaments slide past thick filaments, shortening the sarcomere.

• H bands and I bands become smaller; zones of overlap increase; Z lines move closer together.

• A bands remain constant in length.

• Muscle fiber shortens, producing movement.

The Contraction Cycle

1. Calcium ions arrive in the zone of overlap.

2. Active sites exposed: Calcium binds to troponin, shifting tropomyosin and exposing myosin binding sites on actin.

3. Cross-bridge formation: Myosin heads bind to actin.

4. Power stroke: Myosin heads pivot, pulling thin filaments toward the M line; ADP and phosphate are released.

5. Cross-bridge detachment: New ATP binds to myosin, causing it to detach from actin.

6. Myosin reactivation: ATP is hydrolyzed, re-cocking the myosin head.

The cycle repeats as long as ATP and calcium are available.

Relaxation and Return to Resting Length

• Stimulation from the motor neuron stops.

• ACh is broken down; action potentials cease.

• Calcium is pumped back into the SR.

• Troponin and tropomyosin return to their resting positions, blocking myosin binding sites.

• Muscle returns to original length by gravity, contraction of opposing muscles, and tissue elasticity.

Muscle Twitch and Tetanus

• Muscle Twitch: A single stimulus-contraction-relaxation sequence in a muscle fiber.

• Phases of a Twitch:

  • Latent Period: Time between stimulus and contraction; calcium released.

  • Contraction Phase: Tension rises to peak; cross-bridge cycling occurs.

  • Relaxation Phase: Tension falls; calcium reabsorbed; cross-bridges detach.

• Wave Summation: Additional stimuli before relaxation ends increase contraction strength.

• Incomplete Tetanus: Rapid cycles of contraction and relaxation; tension nearly peaks but some relaxation occurs.

• Complete Tetanus: No relaxation; continuous contraction due to high-frequency stimulation.

Motor Units and Muscle Tone

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Smaller motor units allow for more precise movements.

• Recruitment: Activation of additional motor units increases muscle tension.

• Muscle Tone: Resting tension in a muscle due to partial activation of motor units; maintains posture and readiness.

Types of Muscle Contraction

• Isotonic Contraction: Muscle changes length (shortens or lengthens) while tension remains constant.

• Isometric Contraction: Muscle length remains the same; tension increases but does not exceed the load.

ATP Sources in Skeletal Muscle Fibers

• At rest, ATP is produced by mitochondria using fatty acids and glucose.

• ATP is stored as creatine phosphate and glycogen.

• During moderate activity, ATP is produced aerobically from pyruvate in mitochondria.

• During peak activity, most ATP is produced anaerobically (glycolysis), resulting in lactic acid production and muscle fatigue.

Muscle Fatigue and Recovery

• Lactic Acid accumulates during anaerobic metabolism, lowering pH and inhibiting contraction.

• Cori Cycle: Lactate is transported to the liver, converted to pyruvate, and then to glucose, which is returned to muscles to replenish glycogen stores.

• Oxygen Debt: The extra oxygen required after exercise to restore normal metabolic conditions.

Muscle Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size due to repeated, exhaustive stimulation; more mitochondria, enzymes, glycogen, and myofibrils.

• Atrophy: Decrease in muscle size and strength due to lack of stimulation; can be temporary or permanent.

Rigor Mortis

• After death, ATP production ceases; calcium leaks into sarcoplasm, causing sustained contraction (rigor mortis).

• Begins 2–7 hours after death and lasts until muscle proteins break down (1–6 days).

Key Table: Comparison of Muscle Tissue Types

Muscle Type	Location	Control	Function	Appearance
Skeletal	Attached to bones	Voluntary	Moves skeleton, maintains posture	Striated, multinucleated
Cardiac	Heart	Involuntary	Pumps blood	Striated, single nucleus, intercalated discs
Smooth	Walls of hollow organs	Involuntary	Moves fluids, regulates diameter	Non-striated, single nucleus

Key Equations

• ATP Hydrolysis (for myosin head activation):

• $\mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P} + \text{energy}$

• Aerobic Glucose Metabolism:

• $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\,\mathrm{O}_2 \rightarrow 6\,\mathrm{CO}_2 + 6\,\mathrm{H}_2\mathrm{O} + 36\,\mathrm{ATP}$

• Anaerobic Glycolysis:

• $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 \rightarrow 2\,\mathrm{C}_3\mathrm{H}_6\mathrm{O}_3 + 2\,\mathrm{ATP}$

Summary Table: Muscle Contraction Types

Contraction Type	Muscle Length	Tension	Example
Isotonic	Changes	Constant after threshold	Lifting a book
Isometric	Unchanged	Increases	Holding a book steady

Additional info:

• The sarcoplasmic reticulum is functionally similar to the smooth endoplasmic reticulum in other cells, but specialized for calcium storage and release.

• Muscle fatigue is a protective mechanism to prevent damage from excessive activity.

• Muscle tone is important for maintaining posture and joint stability even at rest.