Muscle Tissue Notes

Muscle Tissue

Introduction to Muscle Tissue

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• Producing movement

• Maintaining posture and body position

• Supporting soft tissues

• Guarding body entrances and exits

• Maintaining body temperature

• Storing nutrients

Organization of Skeletal Muscle

• Skeletal muscles contain:

  • Skeletal muscle tissue (primarily)

  • Connective tissues

  • Blood vessels

  • Nerves

• Skeletal muscles have three layers of connective tissue:

  • Epimysium

  • Perimysium

  • Endomysium

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

Collagen Fibers

• Collagen fibers of epimysium, perimysium, and endomysium come together:

  • At ends of muscles to form:

    • A tendon (bundle)

    • Or aponeurosis (sheet)

  • To attach skeletal muscles to bones

Skeletal Muscles

• Have extensive vascular networks that:

  • Deliver oxygen and nutrients

  • Remove metabolic wastes

• Contract only when stimulated by the central nervous system

  • Often called voluntary muscles

  • The diaphragm usually works subconsciously

Skeletal Muscle Fibers

• Are enormous compared to other cells

• Contain hundreds of nuclei (multinucleate)

• Develop by fusion of embryonic cells (myoblasts)

• Also known as striated muscle cells due to striations

Sarcolemma

• Plasma membrane of a muscle fiber

• Surrounds the sarcoplasm (cytoplasm of a muscle fiber)

• A sudden change in membrane potential initiates a contraction

Transverse Tubules (T Tubules)

• Tubes that extend from the surface of muscle fiber deep into sarcoplasm

• Transmit action potentials from sarcolemma into cell interior

  • Action potentials trigger contraction

Sarcoplasmic Reticulum (SR)

• A tubular network surrounding each myofibril

• Similar to smooth endoplasmic reticulum

• Forms chambers (terminal cisternae) that attach to T tubules

  • Two terminal cisternae plus a T tubule forms a triad

• Specialized for storage and release of calcium ions

  • Ions are actively transported from cytosol into terminal cisternae

Myofibrils

• Lengthwise subdivisions within a muscle fiber

• Responsible for muscle contraction

• Made of bundles of protein filaments (myofilaments)

• Two types of myofilaments:

  • Thin filaments

    • Composed primarily of actin

  • Thick filaments

    • Composed primarily of myosin

Sarcomeres

• Smallest functional units of a muscle fiber

• Interactions between filaments produce contraction

• Arrangement of filaments accounts for striated pattern of myofibrils

  • Dark bands (A bands)

  • Light bands (I bands)

A Band

• M line

  • In the center of the A band

  • Proteins stabilize positions of thick filaments

• H band

  • On either side of the M line

  • Has thick filaments but no thin filaments

• Zone of overlap

  • Dark region

  • Where thick and thin filaments overlap

I Band

• Contains thin filaments but no thick filaments

• Z lines

  • Bisect I bands

  • Mark boundaries between adjacent sarcomeres

• Titin

  • Elastic protein

  • Extends from tips of thick filaments to the Z line

  • Keeps filaments in proper alignment

  • Aids in restoring resting sarcomere length

Thin Filaments

• Contain F-actin, nebulin, tropomyosin, and troponin proteins

  • Filamentous actin (F-actin)

    • Twisted strand composed of two rows of globular G-actin molecules

    • Active sites on G-actin bind to myosin

  • Nebulin

    • Holds F-actin strand together

  • Tropomyosin

    • Covers active sites on G-actin

    • Prevents actin–myosin interaction

  • Troponin

    • A globular protein

    • Binds tropomyosin, G-actin, and Ca^{2+}

Thick Filaments

• Each contains about 300 myosin molecules

• Each myosin molecule consists of:

  • Tail

    • Binds to other myosin molecules

  • Head

    • Made of two globular protein subunits

    • Projects toward nearest thin filament

• Core of titin recoils after stretching

Sliding-Filament Theory

• During a contraction:

  • H bands and I bands narrow

  • Zones of overlap widen

  • Z lines move closer together

  • Width of A band remains constant

• Thus, thin filaments must slide toward the center of the sarcomere

The Neuromuscular Junction

• Excitable membranes are found in skeletal muscle fibers and neurons.

• Depolarization and repolarization events produce action potentials (electrical impulses).

• Skeletal muscle fibers contract due to stimulation by motor neurons

Neuromuscular Junction (NMJ)

• Synapse between a neuron and a skeletal muscle fiber

• Axon terminal of the motor neuron releases a neurotransmitter into the synaptic cleft

  • The neurotransmitter is acetylcholine (ACh)

• ACh binds to and opens a chemically gated Na^{+} channel on the muscle fiber

  • Na^{+} enters cell and depolarizes motor end plate

  • An action potential is generated

Excitation–Contraction Coupling

• Action potential travels down T tubules to triads

  • Ca^{2+} is released from terminal cisternae of SR

• Ca^{2+} binds to troponin and changes its shape

• Troponin–tropomyosin complex changes position

  • Exposes active sites on thin filaments

• Contraction cycle is initiated

Contraction Cycle

• Contraction cycle begins

• Active-site exposure

• Cross-bridge formation (myosin binds to actin)

• Myosin head pivoting (power stroke)

• Cross-bridge detachment

• Myosin reactivation

Generation of Muscle Tension

• When muscle cells contract, they produce tension (pull).

• To produce movement, tension must overcome the load (resistance).

• The entire muscle shortens at the same rate

  • Because all sarcomeres contract together

  • The speed of shortening depends on cycling rate (number of power strokes per second)

Duration of a Contraction

• Depends on:

  • Duration of neural stimulus

  • Presence of free calcium ions in cytosol

  • Availability of ATP

Relaxation

• As Ca^{2+} is pumped back into SR and Ca^{2+} concentration in cytosol falls

  • Ca^{2+} detaches from troponin

  • Troponin returns to original position

  • Active sites are re-covered by tropomyosin and the contraction ends

Rigor Mortis

• Fixed muscular contraction after death

• Results when:

  • ATP runs out and ion pumps cease to function

  • Calcium ions build up in cytosol

Tension Production

• The number of contracting sarcomeres in a muscle fiber is fixed

  • So, a muscle fiber is either producing tension or relaxed

• The amount of tension produced depends on the:

  • The number of power strokes performed

  • Fiber’s resting length at the time of stimulation

  • Frequency of stimulation

Length–Tension Relationship

• Tension produced by a muscle fiber relates to the length of the sarcomeres

• The amount of tension produced depends on the:

  • The number of power strokes performed by cross-bridges

  • Amount of overlap between thick and thin filaments

• Maximum tension is produced when the maximum number of cross-bridges is formed

  • Occurs when the zone of overlap is large

Frequency of Stimulation

• A single neural stimulation produces a single contraction, or twitch

  • Lasts 7–100 msec

• Sustained muscular contractions

  • Require many repeated stimuli

• A myogram is a graph showing tension development in muscle fibers

A Single Twitch Has Three Phases

• Latent period

  • Action potential moves across sarcolemma

  • SR releases Ca^{2+}

• Contraction phase

  • Calcium ions bind to troponin and cross-bridges form

  • Tension builds to a peak

• Relaxation phase

  • Ca^{2+} levels in cytosol fall

  • Cross-bridges detach and tension decreases

Treppe

• A stair-step increase in tension

• Caused by repeated stimulations immediately after relaxation phase

  • Stimulus frequency <50/second

  • Produces a series of contractions with increasing tension

• Typically seen in cardiac muscle and not skeletal muscles

Wave Summation

• Increasing tension due to summation of twitches

• Caused by repeated stimulations before the end of relaxation phase

  • Stimulus frequency >50/second

Tetanus Is Maximum Tension

• Incomplete tetanus

  • Muscle produces near-maximum tension

  • Caused by rapid cycles of contraction and relaxation

• Complete tetanus

  • Higher stimulation frequency eliminates relaxation phase

  • Muscle is in continuous contraction

  • All potential cross-bridges form

Muscle Contractions

• Tension production by skeletal muscles depends on the number of stimulated muscle fibers

Motor Unit

• A motor unit is a motor neuron and all of the muscle fibers it controls

  • May contain a few muscle fibers or thousands

  • All fibers in a motor unit contract at the same time

• Fasciculation

  • Involuntary “muscle twitch”

  • Unlike a true twitch, it involves more than one muscle fiber

Recruitment

• Increase in the number of active motor units

• Produces smooth, steady increase in tension

• Maximum tension is achieved when all motor units reach complete tetanus

  • Can be sustained for a very short time

• Sustained contractions

  • Produce less than maximum tension

  • Motor units are allowed to rest in rotation

Muscle Tone

• The normal tension and firmness of a muscle at rest

• Without causing movement, motor units actively:

  • Stabilize positions of bones and joints

  • Maintain balance and posture

• Elevated muscle tone increases resting energy consumption

Types of Muscle Contractions

• Contractions are classified based on their pattern of tension production

  • Isotonic or isometric

Isotonic Contractions

• Skeletal muscle changes length, resulting in motion

  • Isotonic concentric contraction

    • Muscle tension > load (resistance)

    • Muscle shortens

  • Isotonic eccentric contraction

    • Muscle tension < load

    • Muscle elongates

Isometric Contractions

• Skeletal muscle develops tension that never exceeds the load

• Muscle does not change length

Load and Speed of Contraction

• Are inversely related

• The heavier the load, the longer it takes for movement to begin

• Tension must exceed the load before shortening can occur

Muscle Relaxation and the Return to Resting Length

• Elastic forces

  • Tendons recoil after a contraction

  • Helps return muscle fibers to resting length

• Opposing muscle contractions

  • Opposing muscles return a muscle to resting length quickly

• Gravity

  • Assists opposing muscles

Energy to Power Contractions

• ATP (adenosine triphosphate) is the only energy source used directly for muscle contraction

  • Contracting muscles use a lot of ATP

  • Muscles store enough ATP to start a contraction

  • More ATP must be generated to sustain a contraction

ATP at Rest

• At rest, skeletal muscle fibers produce more ATP than needed

• ATP transfers energy to creatine, creating creatine phosphate (CP)

  • Used to store energy and convert ADP back to ATP

• The enzyme creatine kinase (CK)

  • Catalyzes the conversion of ADP to ATP using the energy stored in CP

• When CP is used up, other mechanisms are used to generate ATP

ATP Generation

• ATP is generated by:

  • Direct phosphorylation of ADP by creatine phosphate (CP)

  • Anaerobic metabolism (glycolysis)

  • Aerobic metabolism (citric acid cycle and electron transport chain)

Glycolysis

• Anaerobic process

• Important energy source for peak muscular activity

• Breaks down glucose from glycogen stored in skeletal muscles

• Produces two ATP per molecule of glucose

Aerobic Metabolism

• Primary energy source of resting muscles

• Breaks down fatty acids

Muscle Metabolism

• Skeletal muscles at rest metabolize fatty acids and store glycogen and CP

• During moderate activity, muscles generate ATP through aerobic breakdown of glucose, primarily

• At peak activity, pyruvate produced via glycolysis is converted to lactate

Recovery Period

• The time required after exertion for muscles to return to normal

Lactate Removal and Recycling (Cori Cycle)

• Lactate is transferred from muscles to the liver

• The liver converts lactate to pyruvate

• Most pyruvate molecules are converted to glucose

• Glucose is used to rebuild glycogen reserves in muscle cells

Oxygen Debt

• Also called excess postexercise oxygen consumption (EPOC)

• After exercise or other exertion:

  • Body needs more oxygen than usual to normalize metabolic activities

  • Breathing rate and depth are increased

Heat Production and Loss

• Active skeletal muscles produce heat

  • Release up to 85 percent of the heat needed to maintain normal body temperature

Hormones

• Several hormones increase metabolic activities in skeletal muscles:

  • Growth hormone

  • Testosterone

  • Thyroid hormones

  • Epinephrine

Muscle Performance

• Muscle performance

  • Force

    • The maximum amount of tension produced

  • Endurance

    • The amount of time an activity can be sustained

• Force and endurance depend on:

  • The types of muscle fibers

  • Physical conditioning

Three Types of Skeletal Muscle Fibers

• Fast fibers

• Slow fibers

• Intermediate fibers

Fast Fibers

• Majority of skeletal muscle fibers

• Contract very quickly

• Large diameter

• Large glycogen reserves

• Few mitochondria

• Produce strong contractions but fatigue quickly

Slow Fibers

• Slow to contract and slow to fatigue

• Small diameter

• Numerous mitochondria

• High oxygen supply from extensive capillary network

• Contain myoglobin (red pigment that binds oxygen)

Intermediate Fibers

• Are mid-sized

• Little myoglobin

• Slower to fatigue than fast fibers

Muscle Performance and the Distribution of Muscle Fibers

• White muscles

  • Mostly fast fibers

  • Pale (e.g., chicken breast)

• Red muscles

  • Mostly slow fibers

  • Dark (e.g., chicken legs)

• Most human muscles

  • Contain a mixture of fiber types and are pink

Muscle Hypertrophy

• Muscle growth from heavy training that causes increases in:

  • Diameter of muscle fibers

  • The number of myofibrils

  • The number of mitochondria

  • Glycogen reserves

Muscle Atrophy

• Reduction of muscle size, tone, and power due to lack of activity

Changes in Muscle Tissue as We Age

• Skeletal muscle fibers become smaller in diameter

• Skeletal muscles become less elastic

  • Fibrosis—Increase in fibrous connective tissue

• Tolerance for exercise decreases

• Ability to recover from muscular injuries decreases

Muscle Fatigue

• When muscles can no longer perform at a required level, they are fatigued

• Correlated with:

  • Depletion of metabolic reserves

  • Damage to sarcolemma and sarcoplasmic reticulum

  • Decline in pH, which affects calcium ion binding and alters enzyme activities

  • Weariness due to low blood pH and pain

Physical Conditioning Improves Power and Endurance

• Anaerobic endurance (e.g., 50-meter dash, weight lifting)

  • Uses fast fibers and stimulates hypertrophy

  • Improved by frequent, brief, intensive workouts

• Aerobic endurance (prolonged activities)

  • Supported by mitochondria

  • Does not stimulate muscle hypertrophy

  • Training involves sustained, low levels of activity

Effects of Training

• Improvements in aerobic endurance result from:

  • Alterations in the characteristics of muscle fibers

  • Improvements in cardiovascular performance

Cardiac Muscle Tissue

• Cardiac muscle tissue

  • Cardiac muscle cells

    • Found only in the heart

    • Have excitable membranes

    • Striated like skeletal muscle cells

Structural Characteristics of Cardiac Muscle Tissue

• Unlike skeletal muscle cells, cardiac muscle cells:

  • Are small

  • Are typically branched with a single nucleus

  • Have short, wide T tubules (no triads)

  • Have SR with no terminal cisternae

  • Are almost totally dependent on aerobic metabolism (contain lots of myoglobin, many mitochondria)

  • Contact each other via intercalated discs

Intercalated Discs

• Specialized connections

• Join sarcolemmas of adjacent cardiac muscle cells by gap junctions and desmosomes

• Functions include:

  • Stabilizing positions of adjacent cells

  • Maintaining three-dimensional structure of tissue

  • Allowing ions to move from one cell to another (so cardiac muscle cells beat in rhythm)

Functional Characteristics of Cardiac Muscle:

• Automaticity

  • Contraction without neural stimulation

  • Controlled by pacemaker cells

• The nervous system can alter pace and tension of contractions

• Contractions last 10 times longer than those in skeletal muscle, and refractory periods are longer

• Wave summation and tetanic contractions are prevented due to special properties of sarcolemma

Smooth Muscle Tissue

• Smooth muscle tissue

  • Integumentary system

    • Arrector pili muscles erect hairs

  • Cardiovascular and respiratory systems

    • Regulates blood pressure and airflow

  • Digestive and urinary systems

    • Forms sphincters

    • Moves materials along and out of the body

  • Reproductive system

    • Transports gametes and expels fetus

Structural Characteristics of Smooth Muscle

• Long, slender, spindle-shaped cells

• Single, central nucleus

• No T tubules, myofibrils, or sarcomeres (nonstriated muscle)

• Scattered thick filaments with many myosin heads

• Thin filaments attached to dense bodies

  • Dense bodies connect adjacent cells, transmitting contractions

• No tendons or aponeuroses

Functional Characteristics of Smooth Muscle Tissue

• Smooth muscle differs from other muscle tissue in:

  • Excitation–contraction coupling

  • Length–tension relationships

  • Control of contractions

  • Smooth muscle tone

Excitation–Contraction Coupling

• Free Ca^{2+} in cytoplasm triggers contraction

• Ca^{2+} binds with calmodulin

  • Activates myosin light chain kinase

  • Allows myosin heads to attach to actin

Length–Tension Relationships

• Due to the lack of sarcomeres:

  • Tension and resting length are not directly related

  • Even a stretched smooth muscle can contract

    • Plasticity—the ability to function over a wide range of lengths

Control of Contractions

• Multiunit smooth muscle cells

  • Innervated in motor units

  • Each cell may be connected to more than one motor neuron

• Visceral smooth muscle cells

  • Not connected to motor neurons

  • Arranged in sheets or layers

  • Rhythmic cycles of activity are controlled by pacesetter cells

Smooth Muscle Tone

• Normal background level of activity

• Can be decreased by neural, hormonal, or chemical factors