Chapter 9: Skeletal Muscle Tissue and the Muscular System – Study Guide

MUSCULAR SYSTEM

Types of Muscle Tissue

The human body contains three types of muscle tissue, each with distinct structural and functional characteristics.

• Skeletal Muscle: Voluntary, attached to bones, responsible for movement of the skeleton.

• Cardiac Muscle: Involuntary, found only in the heart, responsible for pumping blood.

• Smooth Muscle: Involuntary, located in walls of hollow organs (e.g., intestines, blood vessels).

Similarities: All muscle tissues contain muscle cells and contract to produce movement.

Differences: Skeletal muscle is voluntary; cardiac and smooth are involuntary. Cardiac is exclusive to the heart; smooth is found in hollow organs; skeletal attaches to bones.

Muscle Structure and Organization

Muscles are organized hierarchically from individual fibers to whole muscles, surrounded by connective tissue layers.

• Muscle Fiber: Single muscle cell.

• Fascicle: Bundle of muscle fibers.

• Muscle: Bundle of fascicles.

• Endomysium: Surrounds each muscle fiber.

• Perimysium: Surrounds each fascicle.

• Epimysium: Surrounds the entire muscle.

Myoblasts and Myosatellite Cells

Muscle development and repair depend on specialized cells.

• Myoblasts: Embryonic cells that fuse to form multinucleate skeletal muscle fibers during development.

• Myosatellite Cells: Myoblasts that remain unfused in adult muscle; aid in muscle repair after injury.

Tendon vs. Aponeurosis

Both structures connect muscle to other tissues, but differ in form and function.

• Tendon: Cord-like, attaches muscle to bone.

• Aponeurosis: Sheet-like, broad attachment to bones or other muscles.

Similarities: Both are made of dense irregular connective tissue and have limited blood supply.

Differences: Tendons are rope-shaped; aponeuroses are flat and sheet-like.

Muscle Fiber Components

Muscle fibers contain specialized structures for contraction.

• Sarcolemma: Plasma membrane; selective permeability; initiates muscle contraction by charge reversal.

• Sarcoplasm: Cytoplasm of muscle cell.

• Sarcoplasmic Reticulum (SR): Stores calcium ions; releases Ca2+ for contraction.

• Transverse (T) Tubules: Extensions of sarcolemma; transmit electrical signals deep into muscle fiber.

Myofibril, Myofilament, Sarcomere, Myosin, and Actin

These structures are essential for muscle contraction.

• Myofibril: Cylindrical structures within muscle fibers; responsible for striations.

• Myofilament: Protein filaments (actin = thin, myosin = thick) within myofibrils.

• Sarcomere: Functional unit; arrangement of thick and thin filaments creates striations.

• Myosin: Protein forming thick filaments.

• Actin: Protein forming thin filaments.

Sarcomere Structure:

• Z lines: Junctions between sarcomeres.

• I band: Only thin filaments.

• A band: Thick filaments.

• Zone of overlap: Both thick and thin filaments.

• M line: Center of A band.

• H band: Only thick filaments.

Sliding Filament Theory

Explains how muscles contract at the microscopic level.

• Actin (thin) filaments slide past myosin (thick) filaments, shortening the sarcomere.

• H and I bands decrease; zone of overlap increases; Z lines move closer; A band remains unchanged.

Muscle Contraction Cycle:

1. Nerve signal triggers contraction.

2. Signal travels via T-tubules.

3. SR releases Ca2+.

4. Ca2+ binds to troponin.

5. Troponin moves tropomyosin, exposing myosin binding sites on actin.

6. Myosin binds to actin.

7. Myosin head pivots, pulling Z-lines together.

Key Components: Actin, myosin, troponin, tropomyosin, ATP, calcium, SR, T-tubules, sarcolemma.

Membrane Potential and Muscle Cell Action Potential

Muscle contraction is initiated by changes in membrane potential and action potentials.

• Membrane Potential: Difference in charge across sarcolemma.

• Action Potential: Electrical signal that triggers contraction.

Neuromuscular Junction, Motor End Plate, and Motor Unit

These structures coordinate nerve signals and muscle contraction.

• Neuromuscular Junction (NMJ): Site where motor neuron communicates with muscle fiber.

• Motor End Plate: Region of muscle fiber with ACh receptors.

• Motor Unit: One motor neuron and all muscle fibers it controls.

Muscle Contraction Cycle Steps

The contraction cycle involves several steps:

1. Resting: Myosin heads are "cocked" (ATP hydrolyzed).

2. Contraction begins: Ca2+ released from SR.

3. Active sites exposed: Ca2+ binds troponin, moves tropomyosin.

4. Cross bridges form: Myosin binds actin.

5. Power stroke: Myosin pivots, pulling actin.

6. Cross bridges detach: ATP binds myosin, releases actin.

7. Myosin reactivates: ATP hydrolyzed, myosin "recocks".

8. Cycle repeats while Ca2+ and ATP are available.

9. When stimulus ends: Ca2+ pumped back to SR, active sites covered.

Optimal Resting Length

The sarcomere length that allows for maximum tension generation.

• Optimal length: Maximum cross-bridge formation.

• Normal range: 75–130% of optimal length.

Phases of a Muscle Twitch

A muscle twitch is a single contraction-relaxation event.

• Latent Period: Action potential, Ca2+ release, no tension.

• Contraction Phase: Ca2+ binds troponin, cross-bridge cycling, tension rises.

• Relaxation Phase: Ca2+ drops, cross-bridges detach, tension falls.

Fasciculation: Involuntary muscle twitch.

Myogram: Graph of muscle tension development.

Muscle Twitch, Treppe, Wave Summation, Incomplete and Complete Tetanus

Muscle responses to stimulation vary by frequency and intensity.

• Muscle Twitch: Single contraction-relaxation event.

• Treppe: Successive twitches increase tension (common in cardiac muscle).

• Wave Summation: Twitches add together if stimulus occurs before relaxation ends.

• Incomplete Tetanus: Rapid cycles, near peak tension, some relaxation.

• Complete Tetanus: No relaxation, continuous contraction, peak tension.

Motor Unit Recruitment and Asynchronous Summation

Muscle tension increases as more motor units are activated.

• Recruitment: Activation of additional motor units for increased tension.

• Asynchronous Summation: Motor units rotate activation to sustain contraction.

Muscle Tone at Rest

Muscle tone is the resting tension in skeletal muscle.

• Variable motor units active subconsciously.

• Produces low-level tension and increases resting metabolism.

Isotonic vs. Isometric Contractions

Muscle contractions are classified by changes in tension and length.

• Isotonic: Tension constant, muscle length changes.

• Concentric: Muscle shortens (e.g., lifting a weight).

• Eccentric: Muscle lengthens (e.g., lowering a weight).

• Isometric: Tension generated, muscle length unchanged (e.g., postural muscles).

Muscle Origin, Insertion, and Action

Muscle attachments and movements are defined as follows:

• Origin: Fixed attachment, usually proximal.

• Insertion: Movable attachment, usually distal.

• Action: Movement produced by muscle contraction.

Agonist, Antagonist, Synergist, and Fixator Muscles

Muscles interact to produce and control movement.

• Agonist (Prime Mover): Main muscle responsible for movement.

• Antagonist: Opposes agonist action.

• Synergist: Assists agonist, stabilizes origin.

• Fixator: Synergist that prevents movement at another joint.

Classes of Levers in the Body

Muscles and bones form lever systems to facilitate movement.

Class	Arrangement	Example
First Class	Fulcrum between force and load	Neck/head movement
Second Class	Load between force and fulcrum	Standing on tiptoe
Third Class	Force between load and fulcrum	Biceps flexing elbow

Aerobic vs. Anaerobic Metabolism

Muscle cells produce ATP via two main pathways.

• Aerobic Metabolism: Occurs in mitochondria; uses oxygen; produces up to 15 ATP per pyruvate.

• Anaerobic Metabolism: Occurs in cytosol; does not require oxygen; glycolysis yields 2 ATP per glucose.

Key Equations:

• Aerobic: $2\text{Pyruvate} + 6\text{O}_2 \rightarrow 34\text{ATP} + 6\text{CO}_2 + 6\text{H}_2\text{O}$

• Anaerobic: $1\text{Glucose} \rightarrow 2\text{Pyruvate} + 2\text{ATP}$

Metabolic Rates and Fuel Sources

Muscle energy sources vary by activity level.

Energy Source	Duration	Notes
Free ATP	~10 twitches	Immediate use
Creatine Phosphate	~15 seconds	Reforms ATP
Anaerobic Respiration	2–3 minutes	Uses glycogen
Aerobic Respiration	Long-term	Requires oxygen

Muscle Fatigue

Fatigue occurs when muscles cannot sustain required activity.

• Decreased pH impairs Ca2+/troponin binding and enzyme activity.

Muscle Recovery and Oxygen Debt

After fatigue, muscles require recovery to restore function.

• Recovery period: Time to return to pre-fatigue state.

• Oxygen debt: Extra oxygen needed to restore ATP, remove lactate.

• Lactate converted to pyruvate; pyruvate used for ATP production.

Cori Cycle

The Cori cycle shuttles lactate from muscles to liver for conversion to glucose.

1. Anaerobic respiration produces pyruvate.

2. Pyruvate converted to lactate.

3. Lactate released into blood, taken up by liver.

4. Liver converts lactate to pyruvate; 70% to glucose, 30% to ATP.

5. Glucose released to blood, reabsorbed by muscle cells.

Muscle Fiber Types

Muscle fibers differ in contraction speed, fatigue resistance, and energy use.

Type	Contraction	Energy	Appearance
Slow (Type I)	Slow, sustained, fatigue-resistant	Aerobic, high O2	Small, dark red
Intermediate (Type IIa)	Moderate speed, fatigue-resistant	Mixed	Pale, more capillaries
Fast (Type IIx)	Rapid, powerful, fatigues quickly	Anaerobic, high glycogen	Large, pale

Hypertrophy and Atrophy

Muscle size changes with use and disuse.

• Hypertrophy: Increase in muscle size due to more/wider myofibrils, myofilaments, mitochondria, glycogen, and enzyme activity.

• Atrophy: Decrease in muscle size, tone, and power due to reduced stimulation, aging, or nerve damage.

Muscle Disorders

Several disorders affect muscle function.

• Muscular Dystrophy: Inherited diseases causing progressive muscle weakness; DMD and BMD are common forms.

• Polio: Viral infection attacking CNS motor neurons, causing paralysis.

• Tetanus: Bacterial toxin suppresses inhibition of motor neurons, causing sustained contractions.

• Myasthenia Gravis: Autoimmune loss of ACh receptors at NMJ, resulting in weakness.

• Rigor Mortis: Postmortem muscle contraction due to ATP depletion and sustained Ca2+ presence.

Example: Duchenne muscular dystrophy is a childhood-onset, sex-linked disorder leading to respiratory paralysis.

Additional info: Academic context and explanations have been expanded for clarity and completeness. Tables have been recreated for comparison and classification. Equations are provided in LaTeX format as required.