Muscular Tissue

Chapter 11: Muscular Tissue

Introduction

  • Movement is considered a fundamental characteristic of all living organisms.

  • Three Types of Muscular Tissue:

    • Skeletal Muscle: Voluntary and striated, usually attached to bones.

    • Cardiac Muscle: Involuntary and striated, located in the heart.

    • Smooth Muscle: Involuntary and non-striated, found in walls of hollow organs.

  • Understanding muscular tissue is important at the molecular, cellular, and tissue levels of organization.

11.1 Types and Characteristics of Muscular Tissue

Expected Learning Outcomes:

  • Describe the physiological properties that all muscle types have in common.

  • List the defining characteristics of skeletal muscle.

  • Discuss the elastic functions of the connective tissue components of a muscle.

Universal Characteristics of Muscle

  • Excitability (Responsiveness):

    • Responds to chemical signals, stretch, and electrical changes across the plasma membrane.

  • Conductivity:

    • Local electrical excitation sets off a wave of excitation that travels along the muscle fiber.

  • Contractility:

    • Ability of muscle to shorten when stimulated.

  • Extensibility:

    • Capability of being stretched between contractions.

  • Elasticity:

    • Ability to return to its original rest length after being stretched.

Skeletal Muscle

  • Skeletal Muscle:

    • Voluntary: Usually subject to conscious control.

    • Striated: Exhibits alternating light and dark transverse bands due to the internal arrangement of contractile proteins.

    • Muscle Cell (Myofiber): Can be as long as 30 cm.

  • Connective Tissue Wrappings:

    • Endomysium: Surrounds individual muscle cells.

    • Perimysium: Binds groups of muscle fibers into fascicles.

    • Epimysium: Envelops the entire muscle.

  • Tendons:

    • Connect muscles to bones.

  • Collagen:

    • Somewhat extensible and elastic, allowing it to stretch slightly under tension and recoil when released; crucial for protecting muscles from injury.

11.2 Skeletal Muscle Cells

Expected Learning Outcomes:

  • Describe the structural components of a muscle fiber.

  • Relate the striations of a muscle fiber to the overlapping arrangement of its protein filaments.

  • Name the major proteins of a muscle fiber and state the function of each.

The Muscle Fiber

  • Sarcolemma: Plasma membrane of a muscle fiber.

  • Sarcoplasm: Cytoplasm of a muscle fiber, which contains:

    • Myofibrils: Long protein cords occupying most of the sarcoplasm.

    • Glycogen: Carbohydrate stored to provide energy for exercise.

    • Myoglobin: A red pigment providing oxygen for muscle activity.

  • Multiple Nuclei: Flattened nuclei pressed against the inner surface of the sarcolemma.

    • Myoblasts: Stem cells that fused to form each muscle fiber during development.

    • Satellite Cells: Unspecialized myoblasts that play a role in regeneration of damaged skeletal muscle tissue.

  • Mitochondria: Packed into spaces between myofibrils to supply ATP.

  • Sarcoplasmic Reticulum (SR): Smooth ER forming a network around each myofibril; serves as a calcium reservoir that releases calcium through channels to activate contraction.

    • Terminal Cisterns: Dilated end-sacs of SR crossing the muscle fiber.

    • T Tubules: Infoldings of the sarcolemma, which allow muscle fibers to respond quickly to stimulation.

    • Triad: A T tubule and two terminal cisterns associated, critical for excitability and contraction.

Myofilaments

  • Thick Filaments:

    • Composed of myosin; each molecule resembles a golf club with intertwined chains for the tail and globular heads.

    • Heads positioned in a helical arrangement around the filament, with a bare zone in the middle lacking heads.

  • Thin Filaments:

    • Composed of fibrous (F) actin, consisting of intertwined strands of individual globular (G) actin subunits.

    • Armed with tropomyosin that blocks the active sites on actin and troponin, a calcium-binding protein essential for muscular contraction.

  • Elastic Filaments:

    • Composed of titin, a massive protein that stabilizes thick filaments and helps prevent overstretching, providing recoil.

Muscle Striations and Their Molecular Basis

  • Striations arise from the organization of myosin and actin in skeletal and cardiac muscle cells, creating alternating A-bands (dark) and I-bands (light).

  • Sarcomere:

    • The functional contractile unit of skeletal muscle, defined as the segment from one Z disc to the next.

    • Muscle shortening occurs as thick and thin filaments slide past each other, while the lengths of the filaments themselves do not change.

11.3 The Nerve—Muscle Relationship

Expected Learning Outcomes:

  • Explain what a motor unit is and how it relates to muscle contraction.

  • Describe the structure of the junction where a nerve fiber meets a muscle fiber.

  • Explain why a cell has an electrical charge difference across its plasma membrane and, in general terms, how this relates to muscle contraction.

The Nerve—Muscle Relationship

  • Skeletal muscle requires nerve stimulation to contract.

    • If nerve connections are severed or poisoned, paralysis occurs (denervation atrophy results in muscle shrinkage when nerves remain disconnected).

Motor Neurons and Motor Units

  • Somatic Motor Neurons: Nerve cells with cell bodies in the brainstem and spinal cord that serve skeletal muscles. Their axons lead to skeletal muscles, branching to multiple fibers.

  • Motor Unit: Consists of one nerve fiber and all the muscle fibers it innervates, dispersed throughout the muscle and contracting in unison. This ensures efficient and sustainable contraction.

  • Average Motor Unit:

    • Contains approximately 200 muscle fibers.

    • Small motor units provide fine control and are often found in eye and hand muscles.

    • Large motor units, like those in the quadriceps and gastrocnemius, can control up to 1,000 muscle fibers.

The Neuromuscular Junction

  • Synapse: The junction where a nerve fiber meets its target cell, specifically referred to as the neuromuscular junction (NMJ) in the case of muscle fibers.

  • Components of NMJ:

    • Axon Terminal: Swollen end of the nerve fiber filled with synaptic vesicles containing acetylcholine (ACh).

    • Synaptic Cleft: Gap between the axon terminal and muscle cell, where ACh is released.

    • Schwann Cell: Envelopes and isolates the NMJ.

Electrically Excitable Cells

  • Muscle fibers and neurons exhibit voltage changes in response to stimulation.

  • Resting Membrane Potential: Approximately −90 mV in skeletal muscle, maintained by the sodium-potassium pump.

  • In resting context, a muscle fiber has excess sodium ions outside and potassium ions inside, contributing to the electrical gradient.

  • Action Potential:

    • Triggered when Na+ ion gates open, allowing Na+ to flow into the cell, leading to depolarization (inside becomes positive).

    • This is followed by repolarization as K+ gates open.

  • An impulse is a wave of excitation that propagates along the membrane, crucial for muscle contraction.

Neuromuscular Toxins and Paralysis

  • Toxins that interfere with synaptic function can lead to paralysis.

    • Spastic Paralysis: Continuous contraction due to cholinesterase inhibitors preventing degradation of ACh.

    • Tetanus: Caused by the toxin of Clostridium tetani blocking inhibitory signals, leading to overstimulation of muscles.

    • Flaccid Paralysis: Results from toxins like curare, which prevents ACh from stimulating muscles, leading to limpness.

11.4 Behavior of Skeletal Muscle Fibers

Expected Learning Outcomes:

  • Explain how a nerve fiber stimulates a skeletal muscle fiber.

  • Explain how stimulation of a muscle fiber activates its contractile mechanism.

  • Explain the mechanism of muscle contraction.

  • Explain how a muscle fiber relaxes.

  • Explain why the force of a muscle contraction depends on the muscle’s length prior to stimulation.

Phases of Contraction and Relaxation

  1. Excitation: Nerve action potentials lead to muscle action potentials.

  2. Excitation–Contraction Coupling: Links the action potentials on the sarcolemma to activation of myofilaments preparing them to contract.

  3. Contraction: When muscle fiber develops tension and may shorten.

  4. Relaxation: When stimulation ends and the muscle fiber returns to resting length.

11.5 Muscle Metabolism

Expected Learning Outcomes:

  • Explain how skeletal muscle meets its energy demands during rest and exercise.

  • Understand muscle fatigue and soreness causes.

  • Discuss why extra oxygen is needed post-exercise.

  • Distinguish between two physiological muscle fiber types and their functional roles.

  • Discuss factors affecting muscular strength and the effects of resistance versus endurance exercises.

ATP Sources

  • ATP Dependency: All muscle contractions rely on ATP, which requires oxygen and organic energy sources like glucose.

  • Anaerobic Fermentation: Generates ATP without oxygen, yielding little ATP and producing lactic acid.

  • Aerobic Respiration: More efficient ATP production, requiring continuous oxygen supply.

  • Immediate Energy: During short intense exercise (e.g., sprinting), ATP demand is met via phosphate transfer from creatine phosphate to ADP.

    • Phosphagen System: Supplies energy for about 6 seconds of activity.

  • Short-Term Energy: As phosphagen depletes, muscles shift to anaerobic fermentation, generating ATP from glucose and producing lactate for 30–40 seconds of max activity.

  • Long-Term Energy: After about 40 seconds, the body relies on aerobic respiration to meet ATP needs, with glucose and fatty acids as fuel.

Fatigue and Endurance

  • Muscle Fatigue: Characterized by progressive weakness due to ongoing muscle use.

    • Potential Causes: Include potassium accumulation, glycogen depletion, electrolyte loss, and decreased central drive from the brain.

  • Maximum Oxygen Uptake (VO2 max): Key determinant for maintaining high-intensity exercise, increases with training.

Excess Postexercise Oxygen Consumption (EPOC)

  • Describes the elevated oxygen consumption needed post-exercise, used for:

    • Replenishing ATP, regenerating creatine phosphate stores, oxygenating myoglobin, and assisting lactate clearance by the liver.

Physiological Classes of Muscle Fibers

  • Slow-twitch (Type I): Endurance-based, high myoglobin, fatigue-resistant, suited for endurance activities like posture maintenance.

  • Fast-twitch (Type II): Adapted for quick responses with a reliance on anaerobic methods for energy, suited for powerful, short-duration activities like sprinting.

  • Muscle composition varies by genetic predisposition to certain types, affecting athletic performance.

11.6 Cardiac and Smooth Muscle

Expected Learning Outcomes:

  • Describe structural and physiological differences between cardiac and skeletal muscle.

  • Explain the unique characteristics of smooth muscle and their relevance to its function.

Cardiac Muscle

  • Structure: Shorter and thicker than skeletal muscle fibers; possesses intercalated discs for cohesion and electrical coupling.

  • Functionality: Autorhythmic with built-in pacemakers, allowing it to contract rhythmically without neural stimulation, but modifiable by autonomic input.

  • Characteristics: Fatigue-resistant and primarily aerobic, having larger mitochondria and having prolonged contraction periods.

Smooth Muscle

  • Structure: Non-striated, fusiform shape with a single nucleus; lacks T tubules and has limited SR.

  • Functionality: Capable of mitosis, regenerates well after injury, and is controlled by the autonomic nervous system.

  • Contractile Mechanism: Unique due to Ca2+ intake from extracellular fluid, contraction initiated by calmodulin and myosin light-chain kinase processes.

Types of Smooth Muscle

  • Multiunit Smooth Muscle: Found in large arteries and muscles with individualized control.

  • Single-unit Smooth Muscle: Occurs in hollow organs, exhibits spontaneous contractions due to electrical coupling between cells; governed by a coordinated contraction mechanism.

Response to Stretch

  • Smooth muscle can initiate contractions in response to stretching and has adaptive qualities (plasticity) allowing efficient content expulsion from organs like the bladder.

Muscular Disorders

Muscular Dystrophy

  • Condition: Group of hereditary diseases causing degeneration of skeletal muscle, primarily affecting males due to inherited genetic mutations impacting dystrophin production.

  • Forms: Duchenne MD (common in boys), facioscapulohumeral MD, and limb-girdle dystrophy.

Myasthenia Gravis

  • Autoimmune Disease: Results in impairment of neuromuscular junctions affecting ACh receptor availability. Symptoms can include muscle fatigue and weakness, initially more pronounced in facial muscles.

  • Treatment Options: Involve medications that increase ACh availability and interventions aimed at improving immune regulation.

The above notes have been meticulously compiled to reflect the intricacies and essential details outlined in the provided transcript of Chapter 11 on Muscular Tissue, suitable for comprehensive study and understanding of the subject.