Muscle Tissue Study Notes

Fundamentals of Anatomy & Physiology: Chapter 10 - Muscle Tissue

Learning Outcomes

10-1

• Identify the common properties of muscle tissues and the primary functions of skeletal muscle.

10-2

• Describe the organization of muscle at the tissue level.

10-3

• Describe the characteristics of skeletal muscle fibers, and identify the components of a sarcomere.

10-4

• Identify the components of the neuromuscular junction, and summarize the events involved in the neural control of skeletal muscle contraction and relaxation.

10-5

• Describe the mechanism responsible for the different amounts of tension produced in a muscle fiber.

10-6

• Compare the different types of skeletal muscle contraction.

10-7

• Describe the mechanisms by which muscle fibers obtain the energy to power contractions.

10-8

• Relate the types of muscle fibers to muscle performance, discuss muscle hypertrophy, atrophy, and aging, and describe how physical conditioning affects muscle tissue.

10-9

• Identify the structural and functional differences between skeletal muscle fibers and cardiac muscle cells.

10-10

• Identify the structural and functional differences between skeletal muscle fibers and smooth muscle cells, and discuss the roles of smooth muscle tissue in systems throughout the body.

Introduction to Muscle Tissue

• Muscle tissue is a primary tissue that consists of three types:
  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle

10-1 Functions of Muscles

Common Properties of Muscle Tissue

• Cells are specialized for contraction.
• Skeletal muscles move the body by pulling on bones.
• Cardiac and smooth muscles control movements inside the body.
• Common properties include:
  • Excitability (responsiveness): Ability to respond to stimuli.
  • Contractility: Ability of cells to shorten and produce force.
  • Extensibility: Ability to stretch without damage.
  • Elasticity: Ability to recoil to original resting length.

Functions of Skeletal Muscle

• Producing movement: Facilitates movement of bones and joints.
• Maintaining posture and body position: Stabilizes position during various activities.
• Supporting soft tissues: Forms the abdominal wall and pelvic floor.
• Guarding body entrances and exits: Regulates passage through openings.
• Maintaining body temperature: Generates heat through muscle activity.
• Storing nutrients: Reserves amino acids that can be released into bloodstream.

10-2 Organization of Skeletal Muscle

Muscle Components

• Skeletal muscles contain:
  • Skeletal muscle tissue (primarily)
  • Connective tissues
  • Blood vessels
  • Nerves

Connective Tissue Layers

• Epimysium
  • Layer of collagen fibers that surrounds the muscle.
  • Connected to deep fascia.
  • Separates muscle from surrounding tissues.
• Perimysium
  • Surrounds muscle fiber bundles (fascicles).
  • Contains:
  • Collagen fibers
  • Elastic fibers
  • Blood vessels
  • Nerves
• Endomysium
  • Surrounds individual muscle cells (muscle fibers).
  • Contains:
  • Capillary networks
  • Myosatellite cells (stem cells) that repair damage
  • Nerve fibers

Formation of Tendons and Aponeuroses

• Collagen fibers of epimysium, perimysium, and endomysium come together at the ends of muscles to form:
  • Tendon: A bundle that attaches skeletal muscles to bones.
  • Aponeurosis: A sheet that connects muscles to bones or other muscles.

Vascular Network

• Skeletal muscles have extensive vascular networks that:
  • Deliver oxygen and nutrients.
  • Remove metabolic wastes.
• Contract only when stimulated by the central nervous system; classified as voluntary muscles.
  • Exception: Diaphragm (subconscious control).

10-3 Skeletal Muscle Fibers

Characteristics

• Skeletal muscle fibers are enormous compared to other cells.
• Contain hundreds of nuclei (multinucleate).
• Develop by fusion of embryonic cells (myoblasts).
• Known as striated muscle cells due to the presence of striations.

Components of a Muscle Fiber

• Sarcolemma: Plasma membrane of a muscle fiber, surrounding the sarcoplasm (cytoplasm).
  • A sudden change in membrane potential initiates contraction.
• Transverse tubules (T tubules): Tubes that extend from surface of muscle fiber deep into sarcoplasm, transmitting action potentials into the cell interior.
• Sarcoplasmic reticulum (SR): Tubular network surrounding each myofibril, similar to smooth endoplasmic reticulum.
  • Forms terminal cisternae that attach to T tubules (forming a triad).
  • Specialized for storage and release of calcium ions.

Myofibrils

• Lengthwise subdivisions within a muscle fiber; responsible for muscle contraction.
• Made of bundles of protein filaments (myofilaments) divided into:
  • Thin filaments: Primarily composed of actin.
  • Thick filaments: Primarily composed of myosin.

Sarcomeres

• Smallest functional units of a muscle fiber: Interactions between filaments produce contraction.
• Arrangement of filaments accounts for the striated pattern of myofibrils:
  • Dark bands (A bands): Contain thick filaments.
  • Light bands (I bands): Contain thin filaments.

A Band Structure

• M line: In center of A band; stabilizes the position of thick filaments.
• H band: On either side of M line; contains thick filaments but no thin filaments.
• Zone of overlap: Dark region where thick and thin filaments overlap.

I Band Structure

• Contains thin filaments but no thick filaments.
• Z lines: Marks boundaries between adjacent sarcomeres, bisect I bands.
• Titin: Elastic protein extending from tips of thick filaments to the Z line; keeps filaments aligned and aids in restoring sarcomere length.

Sliding-Filament Theory

• Describes contraction during which:
  1. H bands and I bands narrow.
  2. Zones of overlap widen.
  3. Z lines move closer together.
  4. Width of A band remains constant.
• Thin filaments slide toward the center of the sarcomere during contraction.

10-4 The Neuromuscular Junction

Components

• Excitable membranes: Found in skeletal muscle fibers and neurons; involved in action potentials (electrical impulses).
• Neuromuscular junction (NMJ): Synapse between a neuron and a skeletal muscle fiber where the axon terminal of the motor neuron releases acetylcholine (ACh).
  • ACh binds to and opens chemically gated Na+ channels on the muscle fiber, allowing Na+ to enter the cell, leading to depolarization of the motor end plate and action potential generation.

Excitation-Contraction Coupling

• Action potential travels down T tubules to triads.
• Ca2+ is released from terminal cisternae of the SR.
• Ca2+ binds to troponin, changing its shape and shifting the troponin-tropomyosin complex, which exposes active sites on thin filaments.
• This initiates the contraction cycle.

Contraction Cycle

1. Contraction cycle initiation.
2. Active-site exposure.
3. Cross-bridge formation (myosin binding to actin).
4. Myosin head pivoting (power stroke).
5. Cross-bridge detachment.
6. Myosin reactivation.

Muscle Tension Generation

• When muscle cells contract, they generate tension (pull).
• This tension must overcome the load (resistance) for movement to occur.
• The entire muscle shortens at the same rate due to synchronous contraction of all sarcomeres.
• Speed of shortening is dependent on the cycling rate (number of power strokes per second).

Duration of Contraction

• Depends on:
  • Duration of neural stimulus.
  • Presence of free calcium ions in cytosol.
  • Availability of ATP.

Relaxation of the Muscle

• As Ca2+ is pumped back into the SR and Ca2+ concentrations in the cytosol fall:
  1. Ca2+ detaches from troponin.
  2. Troponin returns to original position.
  3. Active sites are re-covered by tropomyosin, ceasing contraction.

Rigor Mortis

• A phenomenon occurring after death:
  • Results from ATP depletion and ion pump failure.
  • Calcium ion concentrations build up in the cytosol, causing muscles to remain contracted.

10-5 Tension Production

Mechanism of Tension Production

• The number of contracting sarcomeres in a muscle fiber is fixed; it can either produce tension or remain relaxed.
• Tension produced is influenced by:
  • Number of power strokes performed.
  • Fiber’s resting length at time of stimulation.
  • Frequency of stimulation.

Length-Tension Relationship

• Tension production relates to the length of sarcomeres.
• Maximum tension occurs with optimal thick and thin filament overlap, utilizing the maximum number of cross-bridges formed.

Frequency of Stimulation

• A single neural stimulation yields a single contraction, referred to as a twitch, lasting 7-100 msec.
• Sustained contractions require repeated stimuli.
• A myogram illustrates tension development in muscle fibers.

Phases of a Single Twitch

1. Latent period: Action potential travels across the sarcolemma, and SR releases Ca2+.
2. Contraction phase: Ca2+ binds to troponin; cross-bridges form, and tension reaches a peak.
3. Relaxation phase: Ca2+ levels decline; cross-bridges detach, causing tension to decrease.

Treppe

• Stair-step increase in tension due to repeated stimuli shortly after relaxation phase.
  • Stimulation frequency < 50/second leads to a series of increasing contractions, typical in cardiac muscle, not skeletal muscle.

Wave Summation

• Increased tension due to summation of twitches, caused by stimuli occurring before the relaxation phase ends (frequency > 50/second).

Tetanus

• Maximum tension production occurs in two forms:
  • Incomplete tetanus: Near-maximum tension due to rapid contraction-relaxation cycles.
  • Complete tetanus: Continuous contraction eliminating relaxation phases where all potential cross-bridges form.

10-6 Muscle Contractions

Tension Production Dependence

• Tension production by skeletal muscles is influenced by the number of stimulated muscle fibers.

Motor Units

• A motor unit is a motor neuron and all the muscle fibers it controls, which may range from a few to thousands.
• All fibers in a motor unit contract simultaneously.

Fasciculation

• Involuntary muscle twitches involving multiple muscle fibers but differing from a true twitch.

Recruitment

• Increase in active motor units results in a smooth and steady rise in tension, reaching maximum tension when all motor units attain complete tetanus.
• Sustained contractions produce submaximal tension with motor units resting in rotation.

Muscle Tone

• Normal resting tension and firmness in a muscle without movement; stabilizes bone/joint positions and aids balance/posture.
• Increased muscle tone leads to higher resting energy consumption.

Types of Muscle Contractions

• Classifications based on tension patterns include isotonic (muscle changes length) and isometric (muscle tension without changing length).

Isotonic Contractions

• Involves muscle length changes, resulting in motion.
  • Isotonic concentric contraction: Muscle tension exceeds load, causing shortening.
  • Isotonic eccentric contraction: Muscle tension is less than load, causing elongation.

Isometric Contractions

• Muscle develops tension but does not change length.

Load and Speed of Contraction

• They are inversely related; heavier loads take longer for movement initiation.
• Muscle tension must exceed load for shortening to occur.

Muscle Relaxation

• Elastic forces: Tendons recoil after contraction aiding the return to resting length.
• Opposing muscle contractions: Help return muscles to resting length rapidly.
• Gravity: Assists opposing muscles during relaxation.

10-7 Energy to Power Contractions

ATP Usage in Muscle Contraction

• ATP is the only direct energy source for muscle contraction; muscles require significant ATP for contraction.
• Stored ATP starts contractions, but more must be generated for prolonged activity.

ATP Generation Mechanisms

1. Direct phosphorylation of ADP by creatine phosphate.
2. Anaerobic metabolism (glycolysis).
3. Aerobic metabolism (citric acid cycle and electron transport chain).

Glycolysis

• An anaerobic process important during peak muscular activity, converting glucose from glycogen, yielding two ATP per glucose molecule.

Aerobic Metabolism

• Primary energy source during rest, breaking down fatty acids for energy.

Muscle Metabolism Details

• Skeletal muscles at rest metabolize fatty acids and store glycogen/creatine phosphate.
• Moderate activity generates ATP through aerobic glucose breakdown; peak activity converts pyruvate to lactate.

Recovery Period

• Time required after exertion for muscles to return to normal.
• Lactate removal and recycling (Cori cycle): Lactate moves to the liver, converting to pyruvate and eventually glucose, replenishing glycogen reserves.

Oxygen Debt

• Excess postexercise oxygen consumption (EPOC): Body needs more oxygen post-exertion for normalization of metabolic activities; characterized by increased breathing rate/depth.

Heat Production

• Active skeletal muscles produce significant heat, releasing up to 85% of heat necessary to maintain normal body temperature.

Hormonal Influence on Muscle Metabolism

• Various hormones enhance metabolic activity in skeletal muscles, including:
  • Growth hormone
  • Testosterone
  • Thyroid hormones
  • Epinephrine

10-8 Muscle Performance

Components of Muscle Performance

• Force: Maximum tension produced.
• Endurance: Duration an activity can be sustained.
• Both depend on muscle fiber types and physical conditioning.

Skeletal Muscle Fiber Types

• Fast fibers: Contract quickly, large diameter, high glycogen, fewer mitochondria, strong contractions but rapid fatigue.
• Slow fibers: Slow to contract and fatigue, smaller diameter, numerous mitochondria, extensive capillary supply, and high myoglobin content.
• Intermediate fibers: Mid-sized fibers, low myoglobin, fatigue slower than fast fibers.

Muscle Fiber Distribution

• White muscles: Mainly fast fibers (e.g., chicken breast).
• Red muscles: Mainly slow fibers (e.g., chicken legs).
• Most human muscles: Mixtures of fiber types (pink appearance).

Muscle Hypertrophy and Atrophy

• Hypertrophy: Muscle growth due to heavy training, increasing diameter, myofibril count, mitochondria, and glycogen reserves.
• Atrophy: Reduction in muscle size, tone, and power due to inactivity.

Aging Effects on Muscle

• Skeletal muscle fibers reduce in diameter and elasticity (fibrosis).
• Exercise tolerance and recovery ability decline with age.

Muscle Fatigue Factors

• Muscles cease to perform adequately when fatigued, correlating with:
  • Depletion of metabolic reserves.
  • Sarcolemma and SR damage.
  • pH decline affecting calcium ion binding/enzyme activities.
  • Weariness due to low blood pH and associated pain.

Physical Conditioning Effects

• Improves power and endurance:
  • Anaerobic endurance: Engages fast fibers, stimulating hypertrophy.
  • Improved by intensive workouts.
  • Aerobic endurance: Involves prolonged activities, enhancing mitochondria without stimulating hypertrophy.
  • Requires regular, low-intensity activities.

10-9 Cardiac Muscle Tissue

General Characteristics

• Cardiac muscle cells:
  • Exclusive to the heart.
  • Have excitable membranes.
  • Striated like skeletal muscle.

Structural Features

• Small size, typically branched, with single nucleus.
• Short, wide T tubules (no triads).
• SR lacks terminal cisternae; primarily reliant on aerobic metabolism.
• Contain significant myoglobin and many mitochondria.
• Connected by intercalated discs for strengthened tissue structure.

Intercalated Discs

• Specialized connections enhancing cell stability and tissue structure.
• Allow ion movement between cells for synchronized contraction.

Functional Characteristics

• Automaticity: Cardiac muscle can contract without neural stimulation, regulated by pacemaker cells.
• Nervous system can adjust contraction pacing and intensity.
• Contractions last much longer than those in skeletal muscle, with elongated refractory periods; summation and tetanic contractions are prevented.

10-10 Smooth Muscle Tissue

Structural Characteristics

• Composed of long, slender, spindle-shaped cells with a singular central nucleus.
• Lacks T tubules, myofibrils, or sarcomeres; non-striated muscle.
• Contains scattered thick filaments with numerous myosin heads.
• Thin filaments attach to dense bodies, linking adjacent cells and transmitting contractions.
• Lacks tendons or aponeuroses.

Functional Characteristics

• Differs from other muscle types in terms of:
  • Excitation-contraction coupling: Triggered by free Ca2+ in the cytoplasm, binding with calmodulin to activate myosin light chain kinase for contraction initiation.
  • Length-tension relationships: Lacks sarcomeres, so contractile tension and length are not directly related; smooth muscle exhibits plasticity, functioning effectively over extensive length ranges.
• Control of contractions:
  • Multiunit smooth muscles: Connected via motor units and influenced by various motor neurons.
  • Visceral smooth muscles: Found in sheets/layers; rhythmic activity is regulated by pacemaker cells.

Smooth Muscle Tone

• Represents the normal level of activity within smooth muscle, modifiable by neural, hormonal, or chemical factors.