Muscle Tissue and Muscle Physiology: Study Notes

Chapter 10: Muscle Tissue and Muscle Physiology

Module 10.1 Overview of Muscle Tissue

This section introduces the fundamental properties and functions of muscle tissue, highlighting the similarities and differences among the three major muscle types.

• Major Functions of Muscle Tissue:

  • Producing movement (e.g., locomotion, facial expressions)

  • Maintaining posture and body position

  • Stabilizing joints

  • Generating heat through contractions (thermogenesis)

• Structural Elements Common to All Muscle Cells:

  • Muscle cells (fibers) are elongated and contain contractile proteins (actin and myosin)

  • Excitability (ability to respond to stimuli)

  • Contractility (ability to shorten forcibly)

  • Extensibility (ability to be stretched)

  • Elasticity (ability to recoil to resting length)

• Comparison of Muscle Types:

  • Skeletal Muscle: Voluntary, striated, multinucleated, attached to bones

  • Cardiac Muscle: Involuntary, striated, single nucleus, found in heart, intercalated discs

  • Smooth Muscle: Involuntary, non-striated, single nucleus, found in walls of hollow organs

Module 10.2 Structure and Function of Skeletal Muscle Fibers

This section details the microscopic anatomy of skeletal muscle fibers and the organization of their contractile machinery.

• Structural Properties of Skeletal Muscle Fibers:

  • Long, cylindrical cells with multiple nuclei

  • Contain myofibrils, which are bundles of contractile proteins

• Organization of a Myofibril:

  • Composed of repeating units called sarcomeres (the functional unit of contraction)

  • Sarcomeres are delineated by Z-discs

• Filament Types:

  • Thick Filaments: Composed of myosin

  • Thin Filaments: Composed of actin, tropomyosin, and troponin

  • Elastic Filaments: Composed of titin, providing elasticity and stability

• Protein Components of a Sarcomere:

  • Contractile Proteins: Actin (thin) and myosin (thick)

  • Regulatory Proteins: Tropomyosin and troponin (control interaction between actin and myosin)

  • Structural Proteins: Titin, nebulin, dystrophin (maintain alignment and structure)

• Sliding-Filament Mechanism:

  • Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere

  • ATP is required for cross-bridge cycling between actin and myosin

• Example: During biceps contraction, sarcomeres shorten as actin and myosin interact.

Module 10.3 Skeletal Muscle Fibers as Electrically Excitable Cells

This section explains the electrical properties of muscle fibers and how they generate action potentials.

• Ion Concentrations:

  • High Na+ outside the cell, high K+ inside the cell

• Concentration Gradient vs. Electrical Potential:

  • Concentration Gradient: Difference in ion concentration across the membrane

  • Electrical Potential: Difference in charge across the membrane (membrane potential)

• Na+/K+ ATPase (Pump):

  • Maintains resting membrane potential by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed

  • Equation: $3\ \mathrm{Na}^+_{\text{in}} + 2\ \mathrm{K}^+_{\text{out}} + \mathrm{ATP} \rightarrow 3\ \mathrm{Na}^+_{\text{out}} + 2\ \mathrm{K}^+_{\text{in}} + \mathrm{ADP} + \mathrm{P_i}$

• Action Potential Sequence:

  • Depolarization: Na+ influx

  • Repolarization: K+ efflux

  • Restoration of resting potential by Na+/K+ pump

Module 10.4 The Process of Skeletal Muscle Contraction and Relaxation

This section describes the events from nerve signal to muscle contraction and relaxation.

• Anatomy of the Neuromuscular Junction (NMJ):

  • Synapse between a motor neuron and a muscle fiber

  • Components: axon terminal, synaptic cleft, motor end plate

• Events at the NMJ:

  1. Action potential arrives at axon terminal

  2. Acetylcholine (ACh) released into synaptic cleft

  3. ACh binds to receptors on motor end plate

  4. Na+ channels open, initiating muscle action potential

• Excitation-Contraction Coupling:

  • Link between muscle fiber excitation and contraction

  • Involves release of Ca2+ from sarcoplasmic reticulum

• Contraction Cycle:

  1. Ca2+ binds to troponin, moving tropomyosin

  2. Myosin binds to actin (cross-bridge formation)

  3. Power stroke (myosin pulls actin)

  4. ATP binds to myosin, detaching it from actin

  5. ATP hydrolysis resets myosin head

• Relaxation:

  • ACh is broken down by acetylcholinesterase

  • Ca2+ pumped back into sarcoplasmic reticulum

  • Muscle fiber returns to resting state

Module 10.5 Energy Sources for Skeletal Muscle

This section reviews how muscle fibers obtain ATP for contraction and the duration each energy source supports activity.

• Immediate Energy Sources:

  • Stored ATP (few seconds)

  • Creatine phosphate donates phosphate to ADP to form ATP

  • Equation: $\mathrm{Creatine\ phosphate} + \mathrm{ADP} \rightarrow \mathrm{Creatine} + \mathrm{ATP}$

• Glycolytic Mechanism (Anaerobic):

  • Breakdown of glucose to pyruvate, yielding ATP without oxygen

  • Supports activity for up to 1 minute

• Oxidative Mechanism (Aerobic):

  • Uses oxygen to produce ATP from glucose, fatty acids, or amino acids

  • Supports prolonged activity (minutes to hours)

Module 10.6 Muscle Tension at the Fiber Level

This section explains how muscle fibers generate tension and the factors that affect contraction strength.

• Stages of a Twitch Contraction:

  1. Latent period: time between stimulus and contraction

  2. Contraction period: tension increases

  3. Relaxation period: tension decreases

• Effect of Stimulation Frequency:

  • Increased frequency can lead to summation and tetanus (sustained contraction)

• Sarcomere Length and Tension:

  • Optimal sarcomere length produces maximal tension

  • Too short or too long reduces tension

• Muscle Fiber Types:

  • Type I (Slow-twitch): High endurance, oxidative metabolism, red color

  • Type II (Fast-twitch): Fatigue quickly, glycolytic metabolism, white color

Module 10.7 Muscle Tension at the Organ Level

This section discusses how groups of muscle fibers (motor units) work together to produce movement.

• Motor Unit:

  • A single motor neuron and all the muscle fibers it innervates

  • Small motor units: fine control (e.g., eye muscles)

  • Large motor units: gross movements (e.g., thigh muscles)

Module 10.8 Skeletal Muscle Performance

This section covers the causes of muscle fatigue and the recovery process after exertion.

• Factors Contributing to Fatigue:

  • Depletion of ATP, creatine phosphate, and glycogen

  • Accumulation of lactic acid and inorganic phosphate

  • Impaired calcium handling

• Recovery Period Events:

  • Replenishment of energy stores

  • Removal of lactic acid

  • Restoration of oxygen levels (oxygen debt repayment)

Module 10.9 Smooth and Cardiac Muscle

This section compares the structure and function of smooth and cardiac muscle, and describes their contraction mechanisms.

• Smooth Muscle:

  • Found in walls of hollow organs (e.g., intestines, blood vessels)

  • Spindle-shaped, single nucleus, non-striated

  • Involuntary control

• Cardiac Muscle:

  • Found only in the heart

  • Branched, striated, single nucleus, intercalated discs

  • Involuntary control

• Contraction Process:

  • Smooth Muscle: Uses actin and myosin, but lacks sarcomeres; contraction is slower and can be sustained longer

  • Cardiac Muscle: Similar to skeletal muscle but with unique features (e.g., pacemaker cells, gap junctions)

• Comparison Table:

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Yes	Yes	No
Control	Voluntary	Involuntary	Involuntary
Location	Attached to bones	Heart	Walls of hollow organs
Cell Shape	Long, cylindrical	Branched	Spindle-shaped
Nuclei per Cell	Multiple	Single	Single