Chapter 9 PowerPoint

Overview of Muscle Tissue

• Nearly half of body's mass.

• Transforms chemical energy (ATP) into directed mechanical energy to exert force.

• Investigation includes types, characteristics, and functions of muscle tissue.

Muscle Tissue Terminology

• Prefixes: Myo, mys, and sarco refer to muscle.

  • Example: sarcoplasm is the muscle cell cytoplasm.

Types of Muscle Tissue

• Three types:

  • Skeletal

  • Cardiac

  • Smooth

• Skeletal and smooth muscle cells are elongated and called muscle fibers.

Skeletal Muscle

• Packaged into skeletal muscles attached to bones and skin.

• Skeletal muscle fibers are the longest with striations (stripes).

• Voluntary muscle: consciously controlled.

• Contracts rapidly, tires easily, powerful.

• Key words: skeletal, striated, and voluntary.

Cardiac Muscle

• Found only in the heart, making up bulk of heart walls.

• Striated.

• Involuntary: cannot be consciously controlled.

• Contracts at a steady rate due to heart's pacemaker; nervous system can increase the rate.

• Key words: cardiac, striated, and involuntary.

Smooth Muscle

• Found in walls of hollow organs like stomach, urinary bladder, and airways.

• Not striated.

• Involuntary: cannot be consciously controlled.

• Key words: visceral, nonstriated, and involuntary.

Characteristics of Muscle Tissue

• Excitability (responsiveness): ability to receive and respond to stimuli.

• Contractility: ability to shorten forcibly when stimulated.

• Extensibility: ability to be stretched.

• Elasticity: ability to recoil to resting length.

Muscle Functions

• Produce movement: responsible for locomotion and manipulation.

  • Example: walking, digesting, pumping blood.

• Maintain posture and body position.

• Stabilize joints.

• Generate heat as they contract.

Skeletal Muscle Anatomy

• Skeletal muscle is an organ with nerve and blood supply, connective tissue sheaths, and attachments.

Nerve and Blood Supply

• Each muscle receives a nerve, artery, and veins.

• Consciously controlled skeletal muscle has nerves supplying every fiber.

• Contracting muscle fibers need oxygen and nutrients and quick waste removal.

Connective Tissue Sheaths

• Cover each skeletal muscle and muscle fiber.

• Support cells and reinforce whole muscle.

• Sheaths (external to internal):

  • Epimysium: dense irregular connective tissue around entire muscle; blends with fascia.

  • Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers).

  • Endomysium: fine areolar connective tissue surrounding each muscle fiber.

Attachments

• Muscles span joints and attach to bones in at least two places:

  • Insertion: attachment to movable bone.

  • Origin: attachment to immovable or less movable bone.

• Attachments can be direct or indirect:

  • Direct (fleshy): epimysium fused to periosteum of bone or perichondrium of cartilage.

  • Indirect: connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis.

Muscle Fiber Microanatomy and Sliding Filament Model

• Skeletal muscle fibers are long, cylindrical cells with multiple nuclei.

• Sarcolemma: muscle fiber plasma membrane.

• Sarcoplasm: muscle fiber cytoplasm.

• Contains glycosomes for glycogen storage and myoglobin for O2 storage.

• Modified organelles: myofibrils, sarcoplasmic reticulum, T tubules.

Myofibrils

• Densely packed, rodlike elements; single muscle fiber can contain 1000s; accounts for ~80% of muscle cell volume.

• Features:

  • Striations

  • Sarcomeres

  • Myofilaments

  • Molecular composition of myofilaments

Striations

• Stripes formed from repeating series of dark and light bands.

  • A bands: dark regions

    • H zone: lighter region in middle of dark A band

    • M line: line of protein (myomesin) bisecting H zone vertically

  • I bands: lighter regions

    • Z disc (line): coin-shaped sheet of proteins on midline of light I band

Sarcomere

• Smallest contractile unit (functional unit) of muscle fiber.

• Contains A band with half of an I band at each end.

• Area between Z discs.

• Individual sarcomeres align end to end along myofibril.

Myofilaments

• Orderly arrangement of actin and myosin myofilaments within sarcomere.

• Actin myofilaments: thin filaments

  • Extend across I band and partway in A band

  • Anchored to Z discs

• Myosin myofilaments: thick filaments

  • Extend length of A band

  • Connected at M line

• Sarcomere cross section shows hexagonal arrangement of one thick filament surrounded by six thin filaments.

Molecular Composition of Myofilaments

• Thick filaments: composed of protein myosin with two heavy and four light polypeptide chains.

  • Heavy chains intertwine to form myosin tail.

  • Light chains form myosin globular head.

  • During contraction, heads link thick and thin filaments, forming cross bridges.

  • Myosins are offset, resulting in staggered array of heads.

• Thin filaments: composed of fibrous protein actin

  • Actin is polypeptide of kidney-shaped G actin (globular) subunits.

    • G actin subunits have active sites for myosin head attachment.

    • G actin subunits link to form fibrous F actin (filamentous).

    • Two F actin strands twist to form a thin filament.

  • Tropomyosin and troponin: regulatory proteins bound to actin

• Other proteins: help form the structure of the myofibril

  • Elastic filament: composed of protein titin

    • Holds thick filaments in place; helps recoil after stretch; resists excessive stretching

  • Dystrophin

    • Links thin filaments to proteins of sarcolemma

  • Nebulin, myomesin, C proteins bind filaments or sarcomeres together

    • Maintain alignment of sarcomere

Clinical - Homeostatic Imbalance 9.1

• Duchenne muscular dystrophy (DMD): most common and serious form of muscular dystrophies; muscle-destroying diseases appearing during childhood.

• Inherited as a sex-linked recessive disease, primarily in males (1 in 3600 births).

• Appears between 2 and 7 years old; clumsy, falls frequently.

• Progresses from extremities upward, affecting head, chest muscles, and cardiac muscle.

• With supportive care, people with DMD can live into their 30s and beyond.

• Caused by defective gene for dystrophin, which links thin filaments to extracellular matrix and stabilizes sarcolemma.

• Sarcolemma of DMD patients tears easily, allowing excess calcium entry, damaging contractile fibers.

• Inflammation follows; regenerative capacity is lost, leading to increased apoptosis of muscle cells and drop in muscle mass.

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic reticulum: network of smooth endoplasmic reticulum tubules surrounding each myofibril.

  • Most run longitudinally.

  • Terminal cisterns: perpendicular cross channels at the A–I band junction.

  • Functions in regulation of intracellular Ca2+ levels.

  • Stores and releases Ca2+.

• T tubules

  • Tube formed by protrusion of sarcolemma deep into cell interior.

  • Increase muscle fiber's surface area.

  • Lumen continuous with extracellular space.

  • Allow electrical nerve transmissions to reach deep into the interior of each muscle fiber.

  • Penetrate cell's interior at each A–I band junction between terminal cisterns.

  • Triad: area formed from terminal cistern of one sarcomere, T tubule, and terminal cistern of neighboring sarcomere.

Triad Relationships

• T tubule contains integral membrane proteins that protrude into intermembrane space.

  • Tubule proteins act as voltage sensors that change shape in response to electrical current.

• SR cistern membranes have integral membrane proteins that protrude into intermembrane space.

  • SR integral proteins control opening of calcium channels in SR cisterns.

• When an electrical impulse passes by, T tubule proteins change shape, causing SR proteins to change shape, releasing calcium into cytoplasm.

Sliding Filament Model of Contraction

• Contraction: the activation of cross bridges to generate force.

• Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening.

• Contraction ends when cross bridges become inactive.

• In relaxed state, thin and thick filaments overlap only slightly at ends of A band.

• Sliding filament model: during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more.

  • Neither thick nor thin filaments change length, just overlap more.

• When nervous system stimulates muscle fiber, myosin heads bind to actin, forming cross bridges, which cause sliding (contraction) process to begin.

• Cross bridge attachments form and break several times, pulling thin filaments toward center of sarcomere.

  • Causes shortening of muscle fiber

  • Z discs are pulled toward M line

  • I bands shorten

  • Z discs become closer

  • H zones disappear

  • A bands move closer to each other

Muscle Fiber Contraction

Background and Overview

• Decision to move activated by brain; signal is transmitted down spinal cord to motor neurons, which activate muscle fibers.

• Neurons and muscle cells are excitable cells capable of action potentials.

  • Excitable cells can change resting membrane potential voltages.

• AP crosses from neuron to muscle cell via neurotransmitter acetylcholine (ACh).

Ion Channels

• Play major role in changing membrane potentials.

• Two classes:

  • Chemically gated ion channels: opened by chemical messengers such as neurotransmitters

    • Example: ACh receptors on muscle cells

  • Voltage-gated ion channels: open or close in response to voltage changes in membrane potential

Anatomy of Motor Neurons and the Neuromuscular Junction

• Skeletal muscles stimulated by somatic motor neurons.

• Axons (long extensions of motor neurons) travel from CNS to skeletal muscle.

• Each axon divides into many branches as it enters muscle.

• Axon branches end on muscle fiber, forming neuromuscular junction or motor end plate.

  • Each muscle fiber has one neuromuscular junction with one motor neuron.

• Axon terminal (end of axon) and muscle fiber are separated by gel-filled space called synaptic cleft.

• Stored within axon terminals are membrane-bound synaptic vesicles.

  • Synaptic vesicles contain neurotransmitter acetylcholine (ACh).

• Infoldings of sarcolemma, called junctional folds, contain millions of ACh receptors.

• NMJ consists of axon terminals, synaptic cleft, and junctional folds.

The Big Picture

• Four steps must occur for skeletal muscle to contract:

  • Events at neuromuscular junction

  • Muscle fiber excitation

  • Excitation-contraction coupling

  • Cross bridge cycling

Events at the Neuromuscular Junction

• AP arrives at axon terminal

• Voltage-gated calcium channels open; calcium enters motor neuron

• Calcium entry causes release of ACh neurotransmitter into synaptic cleft

• ACh diffuses across to ACh receptors (Na+ chemical gates) on sarcolemma

• ACh binding to receptors opens gates, allowing Na+ to enter, resulting in end plate potential

• Acetylcholinesterase degrades ACh

Clinical - Homeostatic Imbalance 9.2

• Many toxins, drugs, and diseases interfere with events at the neuromuscular junction

  • Example: myasthenia gravis: drooping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness.

    • Involves shortage of Ach receptors because person's ACh receptors are attacked by own antibodies

    • Suggests this is an autoimmune disease

Generation of an Action Potential Across the Sarcolemma

• Resting sarcolemma is polarized; a voltage exists across membrane.

  • Inside of cell is negative compared to outside

• Action potential is caused by changes in electrical charges.

• Occurs in three steps:

  • Generation of end plate potential

  • Depolarization

  • Repolarization

End Plate Potential

• ACh released from motor neuron binds to ACh receptors on sarcolemma.

• Causes chemically gated ion channels (ligands) on sarcolemma to open.

• Na^+ diffuses into muscle fiber.

  • Some K^+ diffuses outward, but not much.

• Interior of sarcolemma becomes less negative (more positive).

• Results in local depolarization called end plate potential.

Depolarization

• Generation and propagation of an action potential (AP).

• If end plate potential causes enough change in membrane voltage to reach critical level called threshold, voltage-gated Na^+ channels in membrane will open.

• Large influx of Na^+ triggers AP that is unstoppable and will lead to muscle fiber contraction.

• AP spreads across sarcolemma from one voltage-gated Na^+ channel to next one in adjacent areas, causing that area to depolarize.

Repolarization

• Restoration of resting conditions.

• Na^+ voltage-gated channels close, and voltage-gated K^+ channels open.

• K^+ efflux out of cell rapidly brings cell back to initial resting membrane voltage.

• Refractory period: muscle fiber cannot be stimulated until repolarization is complete.

• Ionic conditions of resting state are restored by Na^+-K^+ pump.

  • Na^+ that came into cell is pumped back out, and K^+ that flowed outside is pumped back into cell

Excitation-Contraction (E-C) Coupling

• Events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction).

• AP is propagated along sarcolemma and down into T tubules, where voltage-sensitive proteins in tubules stimulate Ca^{2+} release from SR.

  • Ca^{2+} release leads to contraction

• AP is brief and ends before contraction is seen

Muscle Fiber Contraction: Cross Bridge Cycling

• At low intracellular Ca^{2+} concentration:

  • Tropomyosin blocks active sites on actin.

  • Myosin heads cannot attach to actin.

  • Muscle fiber remains relaxed.

• Voltage-sensitive proteins in T tubules change shape, causing sarcoplasmic reticulum (SR) to release Ca^{2+} to cytosol.

• At higher intracellular Ca^{2+} concentrations, Ca^{2+} binds to troponin.

• Troponin changes shape and moves tropomyosin away from myosin-binding sites.

• Myosin heads are allowed to bind to actin, forming cross bridge.

• Cycling is initiated, causing sarcomere shortening and muscle contraction.

• When nervous stimulation ceases, Ca^{2+} is pumped back into SR, and contraction ends.

• Four steps of the cross bridge cycle

  • Cross bridge formation: high-energy myosin head attaches to actin thin filament active site

  • Working (power) stroke: myosin head pivots and pulls thin filament toward M line

  • Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach

  • Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into high-energy state

    • Energy will be used for power stroke in next cross bridge cycle

Clinical - Homeostatic Imbalance 9.3

• Rigor mortis

  • 3–4 hours after death, muscles begin to stiffen

  • Peak rigidity occurs about 12 hours postmortem

  • Intracellular calcium levels increase because ATP is no longer being synthesized, so calcium cannot be pumped back into SR

    • Results in cross bridge formation

  • ATP is also needed for cross bridge detachment

    • Results in myosin head staying bound to actin, causing constant state of contraction

  • Muscles stay contracted until muscle proteins break down, causing myosin to release

Whole Muscle Contraction

• Same principles apply to contraction of both single fibers and whole muscles

• Contraction produces muscle tension: force exerted on load or object to be moved

• Contraction may/may not shorten muscle

  • Isometric contraction: no shortening; muscle tension increases but does not exceed load

  • Isotonic contraction: muscle shortens because muscle tension exceeds load

• Force and duration of contraction vary in response to stimuli of different frequencies and intensities

• Each muscle is served by at least one motor nerve

  • Motor nerve contains axons of up to hundreds of motor neurons

  • Axons branch into terminals, each of which forms NMJ with single muscle fiber

• Motor unit is the nerve-muscle functional unit

The Motor Unit

• Motor unit consists of motor neuron and all muscle fibers it supplies (four to several hundred).

  • Smaller the fiber number, the greater the fine control

• Muscle fibers from a motor unit are spread throughout the whole muscle, so stimulation of a single motor unit causes only weak contraction of entire muscle.

The Muscle Twitch

• Simplest contraction resulting from a muscle fiber's response to a single action potential from motor neuron

  • Muscle fiber contracts quickly, then relaxes

• Twitch can be observed and recorded as a myogram

  • Tracing: line recording contraction activity

• Three phases of muscle twitch:

  • Latent period: events of excitation-contraction coupling

    • No muscle tension seen

  • Period of contraction: cross bridge formation

    • Tension increases

  • Period of relaxation: Ca^{2+} reentry into SR

    • Tension declines to zero

• Muscle contracts faster than it relaxes

• Differences in strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles

  • Example: eye muscles contraction are rapid and brief, whereas larger, fleshy muscles (calf muscles) contract more slowly and hold it longer

Graded Muscle Responses

• Normal muscle contraction is relatively smooth, and strength varies with needs

  • A muscle twitch is seen only in lab setting or with neuromuscular problems, but not in normal muscle

• Graded muscle responses vary strength of contraction for different demands

  • Required for proper control of skeletal movement

• Responses are graded by:

  • Changing frequency of stimulation

  • Changing strength of stimulation

Temporal Summation

• Muscle response to changes in stimulus frequency

  • Single stimulus results in single contractile response (i.e., muscle twitch)

  • Wave (temporal) summation results if two stimuli are received by a muscle in rapid succession

    • Muscle fibers do not have time to completely relax between stimuli, so twitches increase in force with each stimulus

    • Additional Ca^{2+} stimulates more shortening

  • If stimuli frequency increases, muscle tension reaches near maximum

    • Produces smooth, continuous contractions that add up (summation)

    • Further increase in stimulus frequency causes muscle to progress to sustained, quivering contraction referred to as unfused (incomplete) tetanus

  • If stimuli frequency further increase, muscle tension reaches maximum

    • Referred to as fused (complete) tetanus because contractions “fuse” into one smooth sustained contraction plateau

    • Prolonged muscle contractions lead to muscle fatigue

Muscle Response to Changes in Stimulus Strength

• Recruitment (or multiple motor unit summation): stimulus is sent to more muscle fibers, leading to more precise control.

• Types of stimulus involved in recruitment:

  • Subthreshold stimulus: stimulus not strong enough, so no contractions seen

  • Threshold stimulus: stimulus is strong enough to cause first observable contraction

  • Maximal stimulus: strongest stimulus that increases maximum contractile force

    • All motor units have been recruited

• Recruitment works on size principle:

  • Motor units with smallest muscle fibers are recruited first

  • Motor units with larger and larger fibers are recruited as stimulus intensity increases

  • Largest motor units are activated only for most powerful contractions

  • Motor units in muscle usually contract asynchronously

    • Some fibers contract while others rest

    • Helps prevent fatigue

Muscle Tone

• Constant, slightly contracted state of all muscles

• Due to spinal reflexes

  • Groups of motor units are alternately activated in response to input from stretch receptors in muscles

• Keeps muscles firm, healthy, and ready to respond

Isotonic and Isometric Contractions

• Isotonic contractions: muscle changes in length and moves load

  • Isotonic contractions can be either concentric or eccentric:

    • Concentric contractions: muscle shortens and does work

      • Example: biceps contract to pick up a book

    • Eccentric contractions: muscle lengthens and generates force

      • Example: laying a book down causes biceps to lengthen which generating a force

• Isometric contractions

  • Load is greater than the maximum tension muscle can generate, so muscle neither shortens nor lengthens

• Electrochemical and mechanical events are same in isotonic or isometric contractions, but results are different

  • In isotonic contractions, actin filaments shorten and cause movement

  • In isometric contractions, cross bridges generate force, but actin filaments do not shorten

    • Myosin heads “spin their wheels” on same actin-binding site

Energy for Contraction and ATP

Providing Energy for Contraction

• ATP supplies the energy needed for the muscle fiber to:

  • Move and detach cross bridges

  • Pump calcium back into SR

  • Pump Na^+ out of and K^+ back into cell after excitation-contraction coupling

• Available stores of ATP depleted in 4–6 seconds

• ATP is the only source of energy for contractile activities; therefore it must be regenerated quickly

• ATP is regenerated quickly by three mechanisms:

  • Direct phosphorylation of ADP by creatine phosphate (CP)

  • Anaerobic pathway: glycolysis and lactic acid formation

  • Aerobic pathway

Direct Phosphorylation of ADP by Creatine Phosphate (CP)

• Creatine phosphate is a unique molecule located in muscle fibers that donates a phosphate to ADP to instantly form ATP

  • Creatine kinase is enzyme that carries out transfer of phosphate

  • Muscle fibers have enough ATP and CP reserves to power cell for about 15 seconds

• Creatine phosphate + ADP \rightarrow creatine + ATP

Anaerobic Pathway: Glycolysis and Lactic Acid Formation

• ATP can also be generated by breaking down and using energy stored in glucose.

  • Glycolysis: first step in glucose breakdown

    • Does not require oxygen

    • Glucose is broken into 2 pyruvic acid molecules

    • 2 ATPs are generated for each glucose broken down

  • Low oxygen levels prevent pyruvic acid from entering aerobic respiration phase.

• Normally, pyruvic acid enters mitochondria to start aerobic respiration phase; however, at high intensity activity, oxygen is not available

  • Bulging muscles compress blood vessels, impairing oxygen delivery

  • In the absence of oxygen, referred to as anaerobic glycolysis, pyruvic acid is converted to lactic acid.

• Lactic acid

  • Diffuses into bloodstream

  • Used as fuel by liver, kidneys, and heart

  • Converted back into pyruvic acid or glucose by liver

• Anaerobic respiration yields only 5% as much ATP as aerobic respiration, but produces ATP 2½ times faster

Aerobic Respiration

• Produces 95% of ATP during rest and light-to-moderate exercise

  • Slower than anaerobic pathway

• Consists of series of chemical reactions that occur in mitochondria and require oxygen

  • Breaks glucose into CO2, H2O, and large amount ATP (32 can be produced)

• Fuels used include glucose from glycogen stored in muscle fiber, then bloodborne glucose, and free fatty acids.

  • Fatty acids are main fuel after 30 minutes of exercise

• Energy systems used during sports

  • Aerobic endurance

    • Length of time muscle contracts using aerobic pathways

    • Light-to-moderate activity, can continue for hours

  • Anaerobic threshold

    • Point at which muscle metabolism converts to anaerobic pathway

Muscle Fatigue

• Fatigue is the physiological inability to contract despite continued stimulation

• Possible causes include:

  • Ionic imbalances can cause fatigue

    • Levels of K^+, Na^+, and Ca^{2+} can change disrupting membrane potential of muscle cell

  • Increased inorganic phosphate (Pi) from CP and ATP breakdown may interfere with calcium release from SR or hamper power

  • Decreased ATP and increased magnesium

    • As ATP levels drop, magnesium levels increase and this can interfere with voltage sensitive T tubule proteins

  • Decreased glycogen

• Lack of ATP is rarely a reason for fatigue, except in severely stressed muscles

Excess Postexercise Oxygen Consumption

• For a muscle to return to its pre-exercise state:

  • Oxygen reserves are replenished

  • Lactic acid is reconverted to pyruvic acid

  • Glycogen stores are replaced

  • ATP and creatine phosphate reserves are resynthesized

• All replenishing steps require extra oxygen, so this is referred to as excess postexercise oxygen consumption (EPOC)

  • Formerly referred to as “oxygen debt”

Factors of Muscle Contraction

• Force of contraction depends on number of cross bridges attached, which is affected by four factors:

  • Number of muscle fibers stimulated (recruitment): the more motor units recruited, the greater the force.

  • Relative size of fibers: the bulkier the muscle, the more tension it can develop

    • Muscle cells can increase in size (hypertrophy) with regular exercise

  • Frequency of stimulation: the higher the frequency, the greater the force

    • Stimuli are added together

  • Degree of muscle stretch: muscle fibers with sarcomeres that are 80–120% their normal resting length generate more force

    • If sarcomere is less than 80% resting length, filaments overlap too much, and force decreases

    • If sarcomere is greater than 120% of resting length, filaments do not overlap enough so force decreases

Velocity and Duration of Contraction

• How fast a muscle contracts and how long it can stay contracted is influenced by:

  • Muscle fiber type

  • Load

  • Recruitment

Muscle Fiber Type

• Classified according to two characteristics

  • Speed of contraction

    • Slow or fast fibers according to:

      • Speed at which myosin ATPases split ATP

      • Pattern of electrical activity of motor neurons

  • Metabolic pathways used for ATP synthesis

    • Oxidative fibers: use aerobic pathways

    • Glycolytic fibers: use anaerobic glycolysis

• Based on these two criteria, skeletal muscle fibers can be classified into three types:

  • Slow oxidative fibers, fast oxidative fibers, or fast glycolytic fibers

• Most muscles contain mixture of fiber types, resulting in a range of contractile speed and fatigue resistance

  • All fibers in one motor unit are the same type

  • Genetics dictate individual’s percentage of each

• Different muscle types are better suited for different jobs

  • Slow oxidative fibers: low-intensity, endurance activities

    • Example: maintaining posture

  • Fast oxidative fibers: medium-intensity activities

    • Example: sprinting or walking

  • Fast glycolytic fibers: short-term intense or powerful movements

    • Example: hitting a baseball

Load and Recruitment

• Load: muscles contract fastest when no load is added

  • The greater the load, the shorter the duration of contraction

  • The greater the load, the slower the contraction

• Recruitment: the more motor units contracting, the faster and more prolonged the contraction

Adaptation to Exercise

Aerobic (Endurance) Exercise

• Aerobic (endurance) exercise, such as jogging, swimming, biking leads to increased:

  • Muscle capillaries

  • Number of mitochondria

  • Myoglobin synthesis

• Results in greater endurance, strength, and resistance to fatigue

• May convert fast glycolytic fibers into fast oxidative fibers

Resistance Exercise

• Resistance exercise (typically anaerobic), such as weight lifting or isometric exercises, leads to

  • Muscle hypertrophy

    • Due primarily to increase in fiber size

    • Increased mitochondria, myofilaments, glycogen stores, and connective tissue

  • Increased muscle strength and size

Clinical - Homeostatic Imbalance 9.4

• Muscles must be active to remain healthy

• Disuse atrophy (degeneration and loss of mass)

  • Due to immobilization or loss of neural stimulation

  • Can begin almost immediately

• Muscle strength can decline 5% per day

• Paralyzed muscles may atrophy to one-fourth initial size

• Fibrous connective tissue replaces lost muscle tissue

• Rehabilitation is impossible at this point

Smooth Muscle

• Found in walls of most hollow organs:

  • Respiratory, digestive, urinary, reproductive, circulatory (except in smallest of blood vessels)

  • Not found in the heart - heart contains cardiac muscle, not smooth

• Most smooth muscle organized into sheets of tightly packed fibers.

• Most organs contain two layers of sheets with fibers oriented at right angles to each other.

  • Longitudinal layer: fibers run parallel to long axis of organ

    • Contraction causes organ to shorten

  • Circular layer: fibers run around circumference of organ

    • Contraction causes lumen of organ to constrict

• Alternating contractions and relaxations of layers mix and squeeze substances through lumen of hollow organs

Differences between Smooth and Skeletal Muscle Fibers

• Smooth muscle fibers are spindle-shaped fibers

  • Thin and short compared with skeletal muscle fibers which are wider and much longer

  • Only one nucleus, no striations

• Lacks connective tissue sheaths

  • Contains endomysium only

• Contain varicosities (bulbous swellings) of nerve fibers instead of neuromuscular junctions

  • Varicosities store and release neurotransmitters into a wide synaptic cleft referred to as a diffuse junction

  • Innervated by the autonomic nervous system

• Smooth muscle has less elaborate SR, and no T tubules

  • SR is less developed than in skeletal muscle

    • SR does store intracellular Ca^{2+}, but most calcium used for contraction has extracellular origins

• Sarcolemma contains pouchlike infoldings called caveolae

  • Caveolae contain numerous Ca^{2+} channels that open to allow rapid influx of extracellular Ca^{2+}

• Smooth muscle fibers are usually electrically connected via gap junctions whereas skeletal muscle fibers are electrically isolated

  • Gap junctions are specialized cell connections that allow depolarization to spread from cell to cell

• There are no striations and no sarcomeres, but they do contain overlapping thick and thin filaments

• Smooth muscle also differs from skeletal muscle in following ways:

  • Thick filaments are fewer and have myosin heads along entire length

    • Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)

    • Thick filaments have heads along entire length, making smooth muscle as powerful as skeletal muscle

  • No troponin complex

    • Does contain tropomyosin, but not troponin

    • Protein calmodulin binds Ca^{2+}

  • Thick and thin filaments arranged diagonally

    • Myofilaments are spirally arranged, causing smooth muscle to contract in corkscrew manner

  • Intermediate filament–dense body network

    • Contain lattice-like arrangement of non contractile intermediate filaments that resist tension

    • Dense bodies: proteins that anchor filaments to sarcolemma at regular intervals

      • Correspond to Z discs of skeletal muscle

      • During contraction, areas of sarcolemma between dense bodies bulge outward

      • Make muscle cell look puffy

Contraction of Smooth Muscle

• Mechanism of contraction

  • Slow, synchronized contractions

  • Cells electrically coupled by gap junctions

    • Action potentials transmitted from fiber to fiber

  • Some cells are self-excitatory (depolarize without external stimuli)

    • Act as pacemakers for sheets of muscle

    • Rate and intensity of contraction may be modified by neural and chemical stimuli

• Contraction in smooth muscle is similar to skeletal muscle contraction in following ways:

  • Actin and myosin interact by sliding filament mechanism

  • Final trigger is increased intracellular Ca^{2+} level

  • ATP energizes sliding process

  • Contraction stops when Ca^{2+} is no longer available

• Contraction in smooth muscle is different from skeletal muscle in following ways:

  • Some Ca^{2+} still obtained from SR, but mostly comes from extracellular space

  • Ca^{2+} binds to calmodulin, not troponin

  • Activated calmodulin then activates myosin kinase (myosin light chain kinase)

  • Activated myosin kinase phosphorylates myosin head, activating it

    • Leads to crossbridge formation with actin

• Stopping smooth muscle contraction requires more steps than skeletal muscle

  • Relaxation requires:

    • Ca^{2+} detachment from calmodulin

    • Active transport of Ca^{2+} into SR and extracellularly

    • Dephosphorylation of myosin to inactive myosin

Energy Efficiency of Smooth Muscle Contraction

• Slower to contract and relax but maintains contraction for prolonged periods with little energy cost

  • Slower