Muscles and Muscle Tissue

Muscles and Muscle Tissue

P A R T A

Muscle Overview

The three types of muscle tissue are skeletal, cardiac, and smooth

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

Muscle contraction depends on two kinds of myofilaments – actin and myosin

Muscle Similarities

Muscle terminology is similarSarcolemma – muscle plasma

membraneSarcoplasm – cytoplasm of a

muscle cellPrefixes – myo, mys, and sarco

all refer to muscle

Skeletal Muscle Tissue

Packaged in skeletal muscles that attach to and cover the bony skeleton

It is striated Is controlled voluntarily Contracts rapidly but tires easily Is responsible for all locomotion,

posture, joint stabilization and heat generation

Cardiac Muscle Tissue

Occurs only in the heart Is striated like skeletal muscle but

involuntary Contracts at a fairly steady rate set

by the heart’s pacemaker

Smooth Muscle Tissue

Found in the walls of hollow visceral organs

Propels food and other substances through organs

It is not striated and involuntary

Functional Characteristics of Muscle Tissue

Excitability, or irritability – the ability to receive and respond to stimuli

Contractility – the ability to shorten forcibly

Extensibility – the ability to be stretched or extended

Elasticity – the ability to recoil and resume the original resting length

Structure and Organization of Skeletal Muscle

Table 9.1a

Structure and Organization of Skeletal Muscle

Skeletal Muscle

Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

Skeletal Muscle

The three connective tissue sheaths are:Endomysium – fine sheath of

connective tissue surrounding each muscle fiber

Perimysium – fibrous connective tissue that surrounds groups of muscle fibersFascicles

Skeletal Muscle

Epimysium A layer of fibrous connective tissue that surrounds the entire muscle

Skeletal Muscle

Figure 9.2a

Skeletal Muscle: Nerve and Blood Supply

Each muscle is served by one nerve, an artery, and one or more veins

Each skeletal muscle fiber is supplied with a nerve ending that controls contraction

Contracting fibers require continuous delivery of oxygen and nutrients via arteries

Wastes must be removed via veins

Skeletal Muscle: Attachments

Most skeletal muscles span joints and are attached to bone in at least two places

When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone, the muscle’s origin

Skeletal Muscle: Attachments

Muscles attach:Directly – epimysium of the

muscle is fused to the periosteum of a bone

Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

Microscopic Anatomy of a Skeletal Muscle Fiber

Fibers are up to hundreds of centimeters long

Each cell is a syncytium produced by fusion of embryonic cells

Microscopic Anatomy of a Skeletal Muscle Fiber

Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin

Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules

Sarcoplasmic reticulum (modified ER) T-tubules and myofibrils aid in

contractionT-tubules are continuous with the

sarcolemma and filled with extracellular fluid

Skeletal muscle fibers

Sarcoplasmic Reticulum (SR)

SR - smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibrilTerminal cisterna

Terminal enlargement of the SR Functions in the regulation of

intracellular calcium levels

Sarcoplasmic Reticulum (SR)

Figure 9.5

T Tubules

T tubules are continuous with the sarcolemma

T tubule proteins act as voltage sensors

They conduct impulses to the deepest regions of the muscle

These impulses signal for the release of Ca2+ from adjacent terminal cisternae

T Tubules

They penetrate into the cell’s interior at each A band–I band junction

Filled with extracellular fluid Triad

Pair of terminal cisternaT-tubule

Myofibrils

Myofibrils are densely packed, rodlike contractile elements

They make up most of the muscle volume

The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident

Myofibrils

Figure 9.3b

Sarcomeres

The smallest contractile unit of a muscle

It goes from Z disc to Z disc Composed of thin (actin) and thick

(myosin) myofilaments

Sarcomeres

Figure 9.3c

Sarcomere A bands

Formed by thin and thick filamentsM line

Central part of the thick filamentProtein that keeps the thick filaments together

H zoneOn each side of the M lineFormed by thick filament

Zone of overlapThin filament between thick filaments

Sarcomere

I bandFormed by thin filamentsZ line

actinin proteins that anchors the thin filaments

Titin keep the thick and thin filaments aligned and prevents the muscle fiber from overstretching

From Z line to Z line

30

Titin

Keep the thick and thin filaments aligned

Prevents the muscle fiber from overstretching

Helps the sarcomere to restore to resting length after contraction

Myofilaments: Banding Pattern

Figure 9.3c,d

Ultrastructure of Myofilaments: Thick Filaments

Thick filaments are composed of the protein myosin

Each myosin molecule has a tail and two globular headsTails – two interwoven, heavy

polypeptide chainsHeads – two smaller, light

polypeptide chains called cross bridges

Thick Filaments

Figure 9.4a,b

Thin Filaments

Thin filaments are chiefly composed of G actin - globular subunits

The G actin contain the active sites to which myosin heads attach during contraction

Thin Filaments

Tropomyosin Double stranded protein that covers the active sites of the G protein

Troponin – composed of 3 globular subunitsOne subunit binds CaOne subunit binds to tropomyosinOne subunit binds to G actin

Ultrastructure of Myofilaments: Thin Filaments

Figure 9.4c

Arrangement of the Filaments in a Sarcomere

• Longitudinal section within one sarcomere

Figure 9.4d

Sliding Filament Model of Contraction

Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree

In the relaxed state, thin and thick filaments overlap only slightly

Upon stimulation, myosin heads bind to actin and sliding begins

Sliding Filament Model of Contraction

Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere

As this event occurs throughout the sarcomeres, the muscle shortens

Skeletal Muscle Contraction

In order to contract, a skeletal muscle must:Be stimulated by a nerve endingPropagate an electrical current, or

action potential, along its sarcolemma

Have a rise in intracellular Ca2+ levels, the final trigger for contraction

Skeletal Muscle Contraction

Linking the electrical signal to the contraction is excitation-contraction coupling

Nerve Stimulus of Skeletal Muscle

Skeletal muscles are stimulated by motor neurons of the somatic nervous system

Axons of these neurons travel in nerves to muscle cells

Axons of motor neurons branch profusely as they enter muscles

Each axonal branch forms a neuromuscular junction with a single muscle fiber

Neuromuscular Junction

Formed from:Axonal endings whith synaptic

vesicles that contain acetylcholineThe motor end plate of a muscle,

which is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junction


Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft


Figure 9.7 (a-c)


When a nerve impulse reaches the end of an axon at the neuromuscular junction:Voltage-regulated calcium channels

open and allow Ca2+ to enter the axon

Ca2+ inside the axon terminal causes axonal vesicles to fuse with the axonal membrane


This fusion releases ACh into the synaptic cleft via exocytosis

ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma

Binding of ACh to its receptors initiates an action potential in the muscle

Role of Acetylcholine (Ach)

ACh binds its receptors at the motor end plate

Binding opens chemically (ligand) gated channels that allows Na+ and K+ movements

Na+ diffuses in, K+ diffuse out and the interior of the sarcolemma becomes less negative

Depolarization

Initially, this is a local electrical event called end plate potential

Later, it ignites an action potential that spreads in all directions across the sarcolemma

Destruction of Acetylcholine

ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase

This destruction prevents continued muscle fiber contraction in the absence of additional stimuli


P A R T B

Excitation-Contraction Coupling

Once generated, the action potential:Is propagated along the

sarcolemmaTravels down the T tubulesTriggers Ca2+ release from

terminal cisternae


Ca2+ binds to troponin and causes Tropomyosin to expose the active

site of G protein


Myosin cross bridges alternately attach and detach

Thin filaments move toward the center of the sarcomere

Hydrolysis of ATP powers this cycling process

Ca2+ is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes

Figure 9.10

ADP

Pi

Net entry of Na+ Initiatesan action potential whichis propagated along thesarcolemma and downthe T tubules.

T tubuleSarcolemma

SR tubules (cut)

SynapticcleftSynaptic

vesicle

Axon terminal

ACh ACh ACh

Neurotransmitter released diffusesacross the synaptic cleft and attachesto ACh receptors on the sarcolemma.

Action potential inT tubule activatesvoltage-sensitive receptors,which in turn trigger Ca2+

release from terminalcisternae of SRinto cytosol.

Calcium ions bind to troponin;troponin changes shape, removingthe blocking action of tropomyosin;actin active sites exposed.

Contraction; myosin heads alternately attach toactin and detach, pulling the actin filaments towardthe center of the sarcomere; release of energy byATP hydrolysis powers the cycling process.

Removal of Ca2+ by active transportinto the SR after the actionpotential ends.

SR

Tropomyosin blockage restored,blocking myosin binding sites onactin; contraction ends andmuscle fiber relaxes.

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

1

2

3

4

5

6

Sequential Events of Contraction

Cross bridge formation – myosin cross bridge attaches to actin filament

Working (power) stroke – myosin head pivots and pulls actin filament toward M line

Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches

“Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state

Figure 9.12

ATP

ADP

ADPATPhydrolysis

ADP

ATP

Pi

Pi

Myosin head(high-energyconfiguration)

Myosin head attaches to the actinmyofilament, forming a cross bridge.

Thin filament

As ATP is split into ADP and Pi, the myosinhead is energized (cocked into the high-energyconformation).

Inorganic phosphate (Pi) generated in theprevious contraction cycle is released, initiatingthe power (working) stroke. The myosin headpivots and bends as it pulls on the actin filament,sliding it toward the M line. Then ADP is released.

Myosin head(low-energyconfiguration)

As new ATP attaches to the myosin head, the link betweenmyosin and actin weakens, and the cross bridge detaches.

Thick filament

1

4 2

3

Contraction of Skeletal Muscle Fibers

Contraction – refers to the activation of myosin’s cross bridges (force-generating sites)

Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening

Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced

Motor Unit: The Nerve-Muscle Functional Unit

A motor unit is a motor neuron and all the muscle fibers it supplies

The number of muscle fibers per motor unit can vary from four to several hundred

Muscles that control fine movements (fingers, eyes) have small motor units


Figure 9.13a


Large weight-bearing muscles (thighs, hips) have large motor units

Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

Muscle Twitch

Cycle of contraction, relaxation produced by a single brief threshold stimulus

There are three phases to a muscle twitchLatent periodPeriod of contractionPeriod of relaxation

Phases of a Muscle Twitch• Latent period – first few

msec after stimulus; EC coupling taking place

• Period of contraction – cross bridges from; muscle shortens

• Period of relaxation – Ca2+ reabsorbed; muscle tension goes to zero

Figure 9.14a

Muscle Twitch Comparisons

Figure 9.14b

Treppe: The Staircase Effect

Treppe Repeated stimulation of the same

strength after relaxation phase has been completed

It causes increased contraction There is increasing availability of Ca2+

in the sarcoplasmMuscle enzyme systems become more

efficient because heat is increased as muscle contracts

Treppe: The Staircase Effect

Figure 9.18

Graded Muscle Responses

Graded muscle responses are:Variations in the degree of muscle

contraction according to the demand imposed on the muscle

Responses are graded by:Changing the frequency of

stimulationChanging the strength of the

stimulus

Changing the Frequency of Stimulation

Repeated stimulation before relaxation phase has been completed will cause:

Wave summationFrequently delivered stimuli where

muscle does not have time to completely relax

It increases contractile force

Changing the Frequency of Stimulation

Incomplete tetanusMore rapidly delivered stimuli than

in wave summation. Only some muscle relaxation allowed

Complete tetanusIf stimuli are given even more

rapidly than in incomplete tetanus. No muscle relaxation allowed

Figure 9.15

Frequency of Stimulation

Changing the Strength of Stimulation

Threshold stimulus – the stimulus strength at which the first muscle contraction occurs

Multiple motor unit summation or recruitment

Muscle contracts more vigorously as stimulus strength is increased

It brings more and more muscle fibers into play

Threshold and Recruitment

Figure 9.16

Size Principle

Figure 9.17

Muscle Tone

Muscle tone:Is the constant, slightly contracted

state of all muscles, which does not produce active movements

Keeps the muscles firm, healthy, and ready to respond to stimulus

Keeps postureStabilizes bones and joints

Muscle Tone

Spinal reflexes account for muscle tone by:Activating one motor unit and then

anotherResponding to activation of stretch

receptors in muscles and tendons

Isotonic Contractions

Isotonic contractionsThe muscle changes in length Once tension is sufficient to move

the load it will stay the same Types of isotonic contractions

Concentric contractions – the muscle shortens because the muscle tension is greater than the load


Eccentric contractions – the muscle elongates because the peak tension developed is less than the load

The muscle elongates because the contraction of opposing muscles or

the gravity


Figure 9.19a

Isometric Contractions

Isometric ContractionsTension increases to the muscle’s

capacity, but the muscle neither shortens nor lengthens

Occurs if the load is greater than the tension the muscle is able to develop

Isometric Contractions

Figure 9.19b

Muscle Metabolism: Energy for Contraction

ATP is the only source used directly for contractile activity

As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are regenerated by:The interaction of ADP with

creatine phosphate (CP) Aerobic respirationAnaerobic glycolysis

Muscle Contraction requires large amounts of energy

Immediate energyATP + Creatine = ADP + creatine

phosphateCreatine phosphate releases stored

energy to convert ADP to ATP Aerobic metabolism provides most

ATP needed for contractionLong-term energyKrebs cycle in mitochondria

Muscle Metabolism: Energy for Contraction

Figure 9.20


Anaerobic metabolism When muscle contractile activity

reaches 70% of maximum:Bulging muscles compress blood

vesselsOxygen delivery is impairedPyruvic acid is converted into

lactic acid


The lactic acid:Diffuses into the bloodstreamIs picked up and used as fuel by

the liver, kidneys, and heartIs converted back into pyruvic

acid by the liver Short-term energy

Muscle Fatigue

The muscle is in a state of physiological inability to contract

Muscle fatigue occurs when:ATP production fails to keep pace with

ATP useThere is a relative deficit of ATP,

causing contracturesLactic acid accumulates in the muscleIonic imbalances are present

Muscle Fatigue

Intense exercise produces rapid muscle fatigue (with rapid recovery)

Na+-K+ pumps cannot restore ionic balances quickly enough

Low-intensity exercise produces slow-developing fatigue

SR is damaged and Ca2+ regulation is disrupted


P A R T C

Oxygen Debt

Oxygen debt – the extra amount of O2 needed for the above restorative processes

Oxygen Debt

Vigorous exercise causes dramatic changes in muscle chemistry

For a muscle to return to a resting state:Oxygen reserves must be replenishedLactic acid must be converted to

pyruvic acidGlycogen stores must be replacedATP and CP reserves must be

resynthesized

Heat Production During Muscle Activity

Only 40% of the energy released in muscle activity is useful as work

The remaining 60% is given off as heat

Dangerous heat levels are prevented by radiation of heat from the skin and sweating

Force of Muscle Contraction

The force of contraction is affected by:The number of muscle fibers

contracting – the more motor fibers in a muscle, the stronger the contraction

The relative size of the muscle – the bulkier the muscle, the greater its strength


Degree of muscle stretch – muscles contract strongest when muscle fibers are 80-120% of their normal resting length


Figure 9.21a–c

Length Tension Relationships

Figure 9.22

Muscle Fiber Type: Functional Characteristics

Speed of contraction – determined by speed in which ATPases split ATP

ATP-forming pathwaysOxidative fibers – use aerobic

pathwaysGlycolytic fibers – use anaerobic

glycolysis These two criteria define three

categories – slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers

Fast versus Slow Fibers

Figure 10.21

Half the diameter of fast fibers Take three times as long to contract after

stimulation Abundant mitochondria

aerobic metabolism Extensive capillary supply High concentrations of myoglobin Fatigue resistant Red color

Slow oxidative fibers

Large in diameter Contain densely packed myofibrils Large glycogen reserves Relatively few mitochondria Produce rapid, powerful contractions of

short duration Fast fatigue White color

Fast glycolytic fibers

Similar to fast fibers Greater resistance to fatigue Pink color Intermediate glycogen storage many mitochondria Many capillaries Uses mainly aerobic metabolism

Fast oxidative or Intermediate fibers

Load and Contraction

Figure 9.23

Effects of Aerobic Exercise

Aerobic exercise results in an increase of:Muscle capillariesNumber of mitochondriaMyoglobin synthesisMainly in slow oxidative fibersNot significant muscle

hypertrophy

Effects of Resistance Exercise

Resistance exercise (typically anaerobic) results in:Muscle hypertrophyIncreased mitochondria,

myofilaments, and glycogen stores

Increases muscle fibers

The Overload Principle

Forcing a muscle to work promotes increased muscular strength

Muscles adapt to increased demands

Muscles must be overloaded to produce further gains

Smooth Muscle

Composed of spindle-shaped fibers with a diameter of 2-10 m and lengths of several hundred m

Lack the coarse connective tissue sheaths of skeletal muscle, but have fine endomysium

Organized into two layers (longitudinal and circular) of closely apposed fibers

Smooth Muscle

Found in walls of hollow organs (except the heart)

Have essentially the same contractile mechanisms as skeletal muscle

Smooth Muscle

Figure 9.24

Peristalsis

When the longitudinal layer contracts, the organ dilates and shorten

When the circular layer contracts, the organ elongates and constrict

Peristalsis – alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs

Innervation of Smooth Muscle

Smooth muscle lacks neuromuscular junctions

Innervating nerves have bulbous swellings called varicosities

Varicosities release neurotransmitters into wide synaptic clefts called diffuse junctions

Innervation of Smooth Muscle

Figure 9.25

Microscopic Anatomy of Smooth Muscle

SR is less developed than in skeletal muscle and lacks a specific pattern

T tubules are absent Plasma membranes have pouchlike

infoldings called caveoli

Microscopic Anatomy of Smooth Muscle

Ca2+ is sequestered in the extracellular space near the caveoli, allowing rapid influx when channels are opened

There are no visible striations and no sarcomeres

Thin and thick filaments are present

Proportion and Organization of Myofilaments in Smooth Muscle

Ratio of thick to thin filaments is much lower than in skeletal muscle

Thick filaments have heads along their entire length

There is no troponin complex


Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner

Noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs) at regular intervals

Smooth Muscle


Figure 9.26

Contraction of Smooth Muscle

Whole sheets of smooth muscle exhibit slow, synchronized contraction

They contract in unison, reflecting their electrical coupling with gap junctions

Action potentials are transmitted from cell to cell

Contraction of Smooth Muscle

Some smooth muscle cells: Act as pacemakers and set the

contractile pace for whole sheets of muscle

Are self-excitatory and depolarize without external stimuli

Contraction Mechanism

Actin and myosin interact according to the sliding filament mechanism

The final trigger for contractions is a rise in intracellular Ca2+

Ca2+ is released from the SR and from the extracellular space

Ca2+ interacts with calmodulin and myosin light chain kinase to activate myosin


Ca2+ binds to calmodulin and activates it Activated calmodulin activates the kinase

enzyme Activated kinase transfers phosphate

from ATP to myosin cross bridges Phosphorylated myosin heads form cross

bridges with actin


Power stroke is executed Smooth muscle relaxes when

intracellular Ca2+ levels drop

Figure 9.27

Special Features of Smooth Muscle Contraction

Unique characteristics of smooth muscle include:Smooth muscle toneSlow, prolonged contractile activityLow energy requirementsResponse to stretch

Response to Stretch Smooth muscle exhibits a phenomenon

called stress-relaxation response in which: Smooth muscle responds to stretch

only briefly, and then adapts to its new length

The new length, however, retains its ability to contract

This enables organs such as the stomach and bladder to temporarily store contents

Hyperplasia Certain smooth muscles can divide and

increase their numbers by undergoing hyperplasia

This is shown by estrogen’s effect on the uterusAt puberty, estrogen stimulates the

synthesis of more smooth muscle, causing the uterus to grow to adult size

Types of Smooth Muscle: Single Unit

The cells of single-unit smooth muscle, commonly called visceral muscle:Contract rhythmically as a unitAre electrically coupled to one

another via gap junctionsOften exhibit spontaneous action

potentialsAre arranged in opposing sheets and

exhibit stress-relaxation response

Types of Smooth Muscle: Multiunit

Multiunit smooth muscles are found:In large airways to the lungsIn large arteriesIn arrector pili musclesIn the intrinsic eye muscles

Types of Smooth Muscle: Multiunit

Their characteristics include:Rare gap junctionsInfrequent spontaneous depolarizationsStructurally independent muscle fibers A rich nerve supply, which, with a

number of muscle fibers, forms motor units

Graded contractions in response to neural stimuli - recruitment

Table 9.3.3

Table 9.3.4

Developmental Aspects

Muscle tissue develops from embryonic mesoderm called myoblasts

Multinucleated skeletal muscle cells form by fusion of myoblastsSyncytium

The growth factor agrin stimulates the clustering of ACh receptors at newly forming motor end plates

Developmental Aspects

As muscles are brought under the control of the somatic nervous system, the numbers of fast and slow fibers are also determined

Cardiac and smooth muscle myoblasts do not fuse but develop gap junctions at an early embryonic stage

Developmental Aspects: Regeneration

Cardiac and skeletal muscle become amitotic, but can lengthen and thicken

Myoblastlike satellite cells show very limited regenerative ability

Cardiac cells lack satellite cells Smooth muscle has good

regenerative ability

Developmental Aspects: After Birth

Muscular development reflects neuromuscular coordination

Development occurs head-to-toe, and proximal-to-distal

Peak natural neural control of muscles is achieved by midadolescence

Athletics and training can improve neuromuscular control

Developmental Aspects: Male and Female

There is a biological basis for greater strength in men than in women

Women’s skeletal muscle makes up 36% of their body mass

Men’s skeletal muscle makes up 42% of their body mass

Developmental Aspects: Male and Female

These differences are due primarily to the male sex hormone testosterone

With more muscle mass, men are generally stronger than women

Developmental Aspects: Age Related

With age, connective tissue increases and muscle fibers decrease

Muscles become stringier and more sinewy

By age 80, 50% of muscle mass is lost (sarcopenia)

Developmental Aspects: Age Related

Regular exercise reverses sarcopenia

Aging of the cardiovascular system affects every organ in the body

Atherosclerosis may block distal arteries, leading to intermittent claudication and causing severe pain in leg muscles

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Muscles and Muscle Tissue