Chapter 10: Muscle Tissue and Physiology – Structured Study Notes

Muscle Tissue and Physiology

Overview of Muscle Tissue

Muscle tissue (MT) is specialized for generating force, known as muscle tension. This force is essential for movement, posture, joint stabilization, heat production, and regulation of material flow through hollow organs. Muscle tissue consists of muscle cells (myocytes) and the surrounding extracellular matrix (endomysium), which transmits tension and holds cells together.

• Striated muscle cells: Includes skeletal and cardiac muscle.

• Smooth muscle cells: Found in smooth muscle tissue.

Three types of muscle tissue:

• Skeletal Muscle Tissue: Long, thin, striated, multinucleated fibers; voluntary control; attached to skeleton.

• Cardiac Muscle Tissue: Striated, branched, mostly single nucleus; involuntary; joined by intercalated discs (contain gap and tight junctions).

• Smooth Muscle Tissue: Long, flattened cells with pointed ends; single nucleus; involuntary; found in hollow organs, eyes, skin, and gland ducts.

Properties of Muscle Cells

• Contractility: Ability to contract and generate tension.

• Excitability: Responsive to stimuli, generating electrical changes.

• Conductivity: Electrical changes propagate along the cell membrane.

• Distensibility: Can be stretched up to three times resting length.

• Elasticity: Returns to original shape after stretching.

Muscle Cell Structure

• Sarcoplasm: Cytoplasm of muscle cell, contains myofibrils and organelles.

• Sarcolemma: Plasma membrane with specialized proteins.

• Myofibrils: Cylindrical organelles, bundles of specialized proteins, make up 50-80% of cell volume.

• Sarcoplasmic Reticulum (SR): Modified smooth ER, surrounds myofibrils, stores and releases calcium ions.

Structure and Function of Skeletal Muscle Fibers (SMF)

Skeletal muscle tissue consists of many muscle fibers surrounded by endomysium. Unique features include:

• Shape and Size: Thin cylinders, much longer than most cells.

• Striations: Striped appearance due to myofibril arrangement.

• Multinucleation: Multiple nuclei due to fusion of myoblasts.

Each fiber is surrounded by sarcolemma, contains myofibrils, and is enveloped by SR. Sarcolemma forms transverse tubules (T-tubules) that dive into the fiber, flanked by terminal cisternae of SR, forming a triad.

Myofibril Structure

• Myofilaments: Protein bundles within myofibrils.

• Contractile proteins: Produce tension (e.g., myosin, actin).

• Regulatory proteins: Control contraction (e.g., troponin, tropomyosin).

• Structural proteins: Maintain stability (e.g., dystrophin, titin).

Duchenne Muscular Dystrophy (DMD): Caused by defective dystrophin gene; leads to muscle degeneration.

Types of Myofilaments

• Thick Filaments: Composed of myosin; heads at ends, tails in middle.

• Thin Filaments: Composed of actin (with active sites), tropomyosin (covers active sites), troponin (binds calcium), and nebulin (regulates length).

• Elastic Filaments: Composed of titin; provides elasticity and structural support.

Myofilament Arrangement and Sarcomere

• Striations: Alternating light (I bands) and dark (A bands) regions.

• H zone: Lighter region in A band.

• M line: Dark line in middle of A and I bands.

• Z-disc: Zigzag line in I bands; sarcomere extends from one Z-disc to the next.

Sarcomere: Functional unit of contraction; includes full A band and half of two I bands.

Sliding Filament Mechanism

During contraction, thin filaments slide toward the M line, shortening the sarcomere. Filaments themselves do not shorten.

Skeletal Muscle Fibers as Electrically Excitable Cells

Muscle contraction is initiated by electrical changes across the sarcolemma. Electrophysiology studies these changes.

• Membrane Potential: Difference in electrical potential across the plasma membrane, measured in millivolts (mV).

• Resting Membrane Potential (RMP): Negative voltage when cell is at rest; cell is polarized.

Ion Channels and Gradients

• Leak Channels: Always open; allow passive ion movement.

• Gated Channels: Open in response to stimuli (ligand-gated or voltage-gated).

• Active Transport Pumps: Move ions against gradients using ATP (e.g., sodium-potassium pump).

Sodium-Potassium Pump: Moves 3 Na+ out and 2 K+ in, maintaining gradients.

Electrochemical Gradients

Combined effect of concentration and electrical gradients determines ion movement.

Generation of Resting Membrane Potential

• K+ diffuses out more easily than Na+ enters.

• Net loss of positive charge makes membrane potential more negative.

Action Potentials

Temporary, rapid changes in membrane potential, generated by opening/closing gated Na+ and K+ channels.

• Depolarization: Na+ channels open, Na+ enters, membrane potential becomes positive (up to +30 mV).

• Repolarization: Na+ channels close, K+ channels open, K+ leaves, membrane potential returns to negative.

Action potentials propagate rapidly along the sarcolemma and T-tubules, enabling coordinated contraction.

The Process of Skeletal Muscle Contraction and Relaxation

Contraction is initiated by stimulation from the nervous system at the neuromuscular junction (NMJ).

Neuromuscular Junction Structure

• Axon Terminal: Contains synaptic vesicles with acetylcholine (ACh).

• Synaptic Cleft: Space between neuron and muscle fiber; contains enzymes (acetylcholinesterase).

• Motor End Plate: Sarcolemma region with ACh receptors (ligand-gated channels).

Muscle Contraction Phases

1. Excitation Phase: Motor neuron releases ACh, which binds to receptors, causing local depolarization (end-plate potential).

2. Excitation-Contraction Coupling: End-plate potential triggers action potential, propagates down T-tubules, opens SR calcium channels, Ca2+ enters cytosol.

3. Contraction Phase: Ca2+ binds to troponin, tropomyosin moves, actin active sites exposed, myosin binds (crossbridge), power stroke occurs, ATP breaks crossbridge, cycle repeats.

4. Relaxation Phase: ACh degraded, sarcolemma repolarizes, Ca2+ pumped back into SR, troponin/tropomyosin block actin, muscle relaxes.

Muscle Spasm: Inability to relax, caused by dehydration, electrolyte imbalance, injury, or overload.

Myasthenia Gravis: Destruction of ACh receptors, leading to weakness and paralysis.

Energy Sources for Skeletal Muscle

Muscle fibers require rapid ATP regeneration for contraction. Three main sources:

• Immediate: Stored ATP and creatine phosphate (CP); CP donates phosphate to ADP via creatine kinase.

• Glycolytic (Anaerobic): Glycolysis in cytosol; uses glucose from blood or glycogen; does not require oxygen.

• Oxidative (Aerobic): Catabolism in mitochondria; uses glucose, fatty acids, amino acids; produces more ATP; requires oxygen (myoglobin stores O2).

Muscle Tension at the Fiber Level

A muscle twitch is the response of a fiber to a single action potential. Recorded as a myogram with three phases:

• Latent Period: Action potential spreads (1-2 ms).

• Contraction Period: Tension increases as crossbridge cycles occur.

• Relaxation Period: Tension decreases as Ca2+ concentration drops.

Refractory Period: Interval during which fiber cannot respond to another stimulus.

Tension Production and Stimulation Frequency

• Wave Summation: Increased frequency of stimulation increases tension.

• Unfused Tetanus: Partial relaxation between stimuli.

• Fused Tetanus: No relaxation; maximal tension.

Length-Tension Relationship: Optimal sarcomere length allows maximal crossbridge formation and tension.

Classes of Skeletal Muscle Fibers

• Type I (Slow-Twitch): Small/intermediate diameter, high endurance.

• Type II (Fast-Twitch): Larger diameter, rapid contraction; subtypes IIa and IIx.

Muscle Tension at the Organ Level

A motor unit consists of a motor neuron and the muscle fibers it innervates. All fibers in a motor unit contract together.

• Recruitment: Activation of more motor units increases force.

• Muscle Tone: Continuous, passive tension in resting muscle.

• Hypotonia: Abnormally low tone; muscle feels soft.

• Hypertonia: Abnormally high tone; muscle feels hard.

Types of Muscle Contractions

• Isotonic Concentric: Muscle shortens.

• Isotonic Eccentric: Muscle lengthens; greatest tension.

• Isometric: Muscle length unchanged.

Skeletal Muscle Performance

Physical training induces structural and biochemical changes in muscle fibers (myoplasticity).

• Endurance Training: High frequency, moderate force; increases mitochondria and blood supply.

• Resistance Training: Low frequency, high force; increases myofibrils and fiber diameter (hypertrophy).

• Disuse (Atrophy): Decreased diameter and myofibrils due to inactivity.

Muscular Fatigue: Inability to maintain exercise intensity; caused by metabolite depletion, oxygen shortage, chemical accumulation, or environmental factors.

Excess Postexercise Oxygen Consumption (EPOC): Recovery period; restores homeostasis (heat dissipation, ion concentration, blood pH).

Smooth and Cardiac Muscle

Both are involuntary. Smooth muscle is widely distributed, lining hollow organs, arrector pili, and iris.

• Peristalsis: Rhythmic contraction waves propel materials.

• Sphincters: Rings of smooth muscle regulate passage.

• Regulation of Flow: Smooth muscle controls diameter of passages.

Structure of Smooth Muscle Cells

Long, flattened cells with pointed ends; single, centrally located nucleus.

Key Terms and Examples

• Myocyte: Muscle cell.

• Endomysium: Connective tissue surrounding muscle cells.

• Intercalated Disc: Specialized junction in cardiac muscle.

• Creatine Phosphate: Immediate ATP source in muscle.

• Myoglobin: Oxygen-binding protein in muscle.

Important Equations

• ATP regeneration from creatine phosphate: $\text{Creatine~phosphate} + \text{ADP} \xrightarrow{\text{creatine~kinase}} \text{Creatine} + \text{ATP}$

• Sodium-potassium pump: $3\,\text{Na}^+~\text{(out)} + 2\,\text{K}^+~\text{(in)}$

• Resting membrane potential (general): $V_m = V_{\text{inside}} - V_{\text{outside}}$

Table: Comparison of Muscle Tissue Types

Type	Cell Shape	Striations	Nucleus	Control	Location
Skeletal	Long, cylindrical	Present	Multinucleated	Voluntary	Attached to skeleton
Cardiac	Short, branched	Present	Single (sometimes 2)	Involuntary	Heart
Smooth	Long, flattened	Absent	Single	Involuntary	Hollow organs, skin, eyes

Additional info: Academic context and definitions were expanded for clarity and completeness. Table entries inferred for comparison purposes.