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Muscles and Muscle Tissue. Chapter 9. http://graphics8.nytimes.com/images/2012/05/09/health/09Physed/09Physed-tmagArticle.jpg. Three Types of Muscle Tissue. Skeletal muscle tissue: Attached to bones and skin Striated Voluntary (i.e., conscious control) Powerful Fibers = cells.

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muscles and muscle tissue

Muscles and Muscle Tissue

Chapter 9

http://graphics8.nytimes.com/images/2012/05/09/health/09Physed/09Physed-tmagArticle.jpg

three types of muscle tissue
Three Types of Muscle Tissue
  • Skeletal muscle tissue:
    • Attached to bones and skin
    • Striated
    • Voluntary (i.e., conscious control)
    • Powerful
    • Fibers = cells
three types of muscle tissue1
Three Types of Muscle Tissue
  • Cardiac muscle tissue:
    • Only in the heart
    • Striated
    • Involuntary
    • Branched cells
    • Intercalated disks
three types of muscle tissue2
Three Types of Muscle Tissue
  • Smooth muscle tissue:
    • In the walls of hollow organs (e.g., stomach, urinary bladder, and airways)
    • Not striated
    • Involuntary
    • Fibers = cells
special characteristics of muscle tissue
Special Characteristics of Muscle Tissue
  • Excitability (responsiveness or irritability):ability to receive and respond to stimuli
  • Contractility: ability to shorten when stimulated
  • Extensibility: ability to be stretched
  • Elasticity: ability to recoil to resting length
muscle functions
Muscle Functions
  • Movement of bones or fluids (e.g., blood)
  • Maintaining posture and body position
  • Stabilizing joints
  • Heat generation (especially skeletal muscle)
skeletal muscle
Skeletal Muscle
  • Each muscle is served by one artery, one nerve, and one or more veins
    • Enter near center, branch extensively through connective tissue sheaths
    • Each muscle fiber has a nerve ending
  • Connective Tissue sheaths
    • Epimysium
    • Perimysium
    • Endomysium
slide8

Epimysium

Epimysium

Bone

Perimysium

Endomysium

Tendon

Muscle fiber

in middle of

a fascicle

(b)

Blood vessel

Fascicle

(wrapped by perimysium)

Endomysium

(between individual

muscle fibers)

Perimysium

Fascicle

Muscle fiber

(a)

Figure 9.1

skeletal muscle attachments
Skeletal Muscle: Attachments
  • Muscles attach:
    • Directly—epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage
    • Indirectly—connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlikeaponeurosis
microscopic anatomy of a skeletal muscle fiber
Microscopic Anatomy of a Skeletal Muscle Fiber
  • Cylindrical cell 10 to 100 m in diameter, up to 30 cm long
  • Multiple peripheral nuclei
  • Many mitochondria
  • Glycosomes for glycogen storage, myoglobin for O2 storage
  • Also contain myofibrils, sarcoplasmic reticulum, and T tubules
myofibrils
Myofibrils
  • Densely packed, rodlike elements
  • ~80% of cell volume
  • Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands
slide13

Sarcolemma

Mitochondrion

Myofibril

Dark A band

Light I band

Nucleus

(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended afrom the cut end of the fiber.

sarcomere
Sarcomere
  • Smallest contractile unit (functional unit) of a muscle fiber
  • The region of a myofibril between two successive Z discs
  • Composed of thick and thin myofilaments made of contractile proteins
features of a sarcomere
Features of a Sarcomere
  • Thick filaments
  • Thin filaments
  • Z disc
  • H zone
  • M line
ultrastructure of thick filament
Ultrastructure of Thick Filament
  • Composed of the protein myosin
    • Myosin tails contain:
      • 2 interwoven, heavy polypeptide chains
    • Myosin heads contain:
      • 2 smaller, light polypeptide chains that act as cross bridges during contraction
      • Binding sites for actin of thin filaments
      • Binding sites for ATP
      • ATPase enzymes
ultrastructure of thin filament
Ultrastructure of Thin Filament
  • Twisted double strand of fibrous protein F actin
  • F actin consists of G (globular) actin subunits
  • G actin bears active sites for myosin head attachment during contraction
  • Tropomyosin and troponin: regulatory proteins bound to actin
sarcoplasmic reticulum sr
Sarcoplasmic Reticulum (SR)
  • Network of smooth endoplasmic reticulum surrounding each myofibril
  • Pairs of terminal cisternae form perpendicular cross channels
  • Functions in the regulation of intracellular Ca2+ levels
t tubules
T Tubules
  • Continuous with the sarcolemma
  • Penetrate the cell’s interior at each A band–I band junction
  • Associate with the paired terminal cisternae to form triads that encircle each sarcomere
slide23

Part of a skeletal

muscle fiber (cell)

I band

A band

I band

Z disc

H zone

Z disc

Myofibril

M line

Sarcolemma

Triad:

T tubule

Terminal

cisternae

of the SR (2)

Sarcolemma

Tubules of

the SR

Myofibrils

Mitochondria

Figure 9.5

triad relationships
Triad Relationships
  • T tubules conduct impulses deep into muscle fiber
  • Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes
  • T tubule proteins: voltage sensors
  • SR foot proteins: gated channels that regulate Ca2+ release from the SR cisternae
contraction
Contraction
  • The generation of force
  • Does not necessarily cause shortening of the fiber
  • Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening
sliding filament model of contraction
Sliding Filament Model of Contraction
  • In the relaxed state, thin and thick filaments overlap only slightly
  • During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line
  • As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens
slide27

Z

Z

H

A

I

I

Fully relaxed sarcomere of a muscle fiber

Z

Z

I

A

I

Fully contracted sarcomere of a muscle fiber

Figure 9.6

requirements for skeletal muscle contraction
Requirements for Skeletal Muscle Contraction
  • Activation: neural stimulation at aneuromuscular junction
  • Excitation-contraction coupling:
    • Generation and propagation of an action potential along the sarcolemma
    • Final trigger: a brief rise in intracellular Ca2+ levels
slide31

Myelinated axon

of motor neuron

Action

potential (AP)

Axon terminal of

neuromuscular

junction

Nucleus

Sarcolemma of

the muscle fiber

1

Action potential arrives at

axon terminal of motor neuron.

Ca2+

Synaptic vesicle

containing ACh

Ca2+

2

Voltage-gated Ca2+ channels

open and Ca2+ enters the axon

terminal.

Mitochondrion

Synaptic

cleft

Axon terminal

of motor neuron

3

Ca2+ entry causes some

synaptic vesicles to release

their contents (acetylcholine)

by exocytosis.

Fusing synaptic

vesicles

Junctional

folds of

sarcolemma

ACh

4

Acetylcholine, a

neurotransmitter, diffuses across

the synaptic cleft and binds to

receptors in the sarcolemma.

Sarcoplasm of

muscle fiber

Postsynaptic membrane

ion channel opens;

ions pass.

5

ACh binding opens ion

channels that allow simultaneous

passage of Na+ into the muscle

fiber and K+ out of the muscle

fiber.

K+

Na+

Degraded ACh

6

ACh effects are terminated

by its enzymatic breakdown in

the synaptic cleft by

acetylcholinesterase.

Ach–

Postsynaptic membrane

ion channel closed;

ions cannot pass.

Na+

Acetyl-

cholinesterase

K+

Figure 9.8

events in generation of an action potential
Events in Generation of an Action Potential
  • Local depolarization (end plate potential):
    • ACh binding opens chemically (ligand) gated ion channels
    • Simultaneous diffusion of Na+ (inward) and K+ (outward)
    • More Na+ diffuses, so the interior of the sarcolemma becomes less negative
slide33

Axon terminal

Open Na+

Channel

Closed K+

Channel

Na+

Synaptic

cleft

ACh

K+

Na+

K+

+

+

+

+

ACh

+

+

+

+

+

+

Action potential

n

+

+

o

i

t

Na+

K+

a

z

i

r

a

l

o

p

e

d

f

o

e

v

a

W

1

1

Local depolarization: generation of the

end plate potential on the sarcolemma

Sarcoplasm of muscle fiber

Figure 9.9, step 1

events in generation of an action potential1
Events in Generation of an Action Potential
  • Generation and propagation of an action potential:
    • End plate potential spreads to adjacent membrane areas
    • Voltage-gated Na+ channels open
    • Na+ influx decreases the membrane voltage toward a critical threshold
    • If threshold is reached, an action potential is generated
slide35

Axon terminal

Open Na+

Channel

Closed K+

Channel

Na+

Synaptic

cleft

ACh

K+

Na+

K+

+

+

+

+

ACh

+

+

+

+

+

+

Action potential

n

+

+

o

i

t

Na+

K+

a

z

2

i

r

Generation and propagation of the

action potential (AP)

a

l

o

p

e

d

f

o

e

v

a

W

1

1

Local depolarization: generation of the

end plate potential on the sarcolemma

Sarcoplasm of muscle fiber

Figure 9.9, step 2

events in generation of an action potential2
Events in Generation of an Action Potential
  • Local depolarization wave continues to spread, changing the permeability of the sarcolemma
  • Voltage-regulated Na+ channels open in the adjacent patch, causing it to depolarize to threshold
events in generation of an action potential3
Events in Generation of an Action Potential
  • Repolarization:
  • Na+ channels close and voltage-gated K+ channels open
  • K+ efflux rapidly restores the resting polarity
  • Fiber cannot be stimulated and is in a refractory period until repolarization is complete
  • Ionic conditions of the resting state are restored by the Na+-K+ pump
slide38

Axon terminal

Open Na+

Channel

Closed K+

Channel

Synaptic

cleft

Na+

ACh

K+

Na+

K+

+

+

+

+

ACh

+

+

+

+

+

+

Action potential

n

+

+

o

i

Na+

K+

t

a

2

Generation and propagation of

the action potential (AP)

z

i

r

a

l

o

p

e

d

f

o

e

v

Closed Na+

Channel

Open K+

Channel

a

W

1

Local depolarization:

generation of the end

plate potential on the

sarcolemma

Na+

K+

3

Repolarization

Sarcoplasm of muscle fiber

Figure 9.9

slide39

Na+ channels

close, K+ channels

open

Depolarization

due to Na+ entry

Repolarization

due to K+ exit

Na+

channels

open

Threshold

K+ channels

close

Figure 9.10

excitation contraction e c coupling
Excitation-Contraction (E-C) Coupling
  • Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments
  • Latent period:
    • Time when E-C coupling events occur
    • Time between AP initiation and the beginning of contraction
events of excitation contraction e c coupling
Events of Excitation-Contraction (E-C) Coupling
  • AP is propagated along sarcomere to T tubules
  • Voltage-sensitive proteins stimulate Ca2+ release from SR
    • Ca2+ is necessary for contraction
slide42

1

Action potential is

propagated along the

sarcolemma and down

the T tubules.

Steps in

E-C Coupling:

Sarcolemma

Voltage-sensitive

tubule protein

T tubule

Ca2+

release

channel

Terminal

cisterna

of SR

Ca2+

Figure 9.11, step 3

slide43

1

Action potential is

propagated along the

sarcolemma and down

the T tubules.

Steps in

E-C Coupling:

Sarcolemma

Voltage-sensitive

tubule protein

T tubule

Ca2+

release

channel

2

Calcium

ions are

released.

Terminal

cisterna

of SR

Ca2+

Figure 9.11, step 4

slide44

Actin

Troponin

Tropomyosin

blocking active sites

Ca2+

Myosin

The aftermath

Figure 9.11, step 5

slide45

Actin

Troponin

Tropomyosin

blocking active sites

Ca2+

Myosin

3

Calcium binds to

troponin and removes

the blocking action of

tropomyosin.

Active sites exposed and

ready for myosin binding

The aftermath

Figure 9.11, step 6

slide46

Actin

Troponin

Tropomyosin

blocking active sites

Ca2+

Myosin

3

Calcium binds to

troponin and removes

the blocking action of

tropomyosin.

Active sites exposed and

ready for myosin binding

Contraction begins

4

Myosin

cross

bridge

The aftermath

Figure 9.11, step 7

role of calcium ca 2 in contraction
Role of Calcium (Ca2+) in Contraction
  • At low intracellular Ca2+ concentration:
    • Tropomyosin blocks the active sites on actin
    • Myosin heads cannot attach to actin
    • Muscle fiber relaxes
role of calcium ca 2 in contraction1
Role of Calcium (Ca2+) in Contraction
  • At higher intracellular Ca2+ concentrations:
    • Ca2+ binds to troponin
    • Troponin changes shape and moves tropomyosin away from active sites
    • Events of the cross bridge cycle occur
    • When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends
cross bridge cycle
Cross Bridge Cycle
  • Continues as long as the Ca2+ signal and adequate ATP are present
  • Cross bridge formation—high-energy myosin head attaches to thin filament
  • Working (power) stroke—myosin head pivots and pulls thin filament toward M line
cross bridge cycle1
Cross Bridge Cycle
  • 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
principles of muscle mechanics
Principles of Muscle Mechanics
  • Same principles apply to contraction of a single fiber and a whole muscle
  • Contraction produces tension, the force exerted on the load or object to be moved
  • Contraction does not always shorten a muscle:
    • Isometric contraction: no shortening; muscle tension increases but does not exceed the load
    • Isotonic contraction: muscle shortens because muscle tension exceeds the load
principles of muscle mechanics1
Principles of Muscle Mechanics
  • Force and duration of contraction vary in response to stimuli of different frequencies and intensities
motor unit
Motor Unit
  • Motor unit = a motor neuron and all muscle fibers it supplies
    • Small motor units in muscles that control fine movements (fingers, eyes)
    • Large motor units in large weight-bearing muscles (thighs, hips)
  • Muscle fibers from a motor unit are spread throughout the muscle
  • Motor units in a muscle usually contract asynchronously
slide55

Spinal cord

Axon terminals at

neuromuscular junctions

Motor

unit 1

Motor

unit 2

Nerve

Motor neuron

cell body

Motor

neuron

axon

Muscle

Muscle

fibers

Axons of motor neurons extend from the spinal cord to the

muscle. There each axon divides into a number of axon

terminals that form neuromuscular junctions with muscle

fibers scattered throughout the muscle.

Figure 9.13a

muscle twitch
Muscle Twitch
  • Response of a muscle to a single, brief threshold stimulus
  • Three phases of a twitch:
    • Latent period: events of excitation-contraction coupling
    • Period of contraction: cross bridge formation; tension increases
    • Period of relaxation: Ca2+re-entry into the SR; tension declines to zero
slide57

Latent

period

Period of

contraction

Period of

relaxation

Single

stimulus

(a) Myogram showing the three phases of an isometric twitch

Figure 9.14a

muscle twitch comparisons
Muscle Twitch Comparisons

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

graded muscle responses
Graded Muscle Responses
  • Variations in the degree of muscle contraction
  • Required for proper control of skeletal movement

Responses are graded by:

    • Changing the frequency of stimulation
    • Changing the strength of the stimulus
response to change in stimulus frequency
Response to Change in Stimulus Frequency
  • A single stimulus results in a single contractile response—a muscle twitch
response to change in stimulus frequency1
Response to Change in Stimulus Frequency
  • Increase frequency of stimulus (muscle does not have time to completely relax between stimuli)
  • Ca2+ release stimulates further contraction temporal (wave) summation
  • Further increase in stimulus frequency unfused (incomplete) tetanus
slide62

Low stimulation frequency

unfused (incomplete) tetanus

Partial relaxation

Stimuli

(b) If another stimulus is applied before the muscle

relaxes completely, then more tension results.

This is temporal (or wave) summation and results

in unfused (or incomplete) tetanus.

Figure 9.15b

response to change in stimulus frequency2
Response to Change in Stimulus Frequency
  • If stimuli are given quickly enough, fused (complete) tetany results
response to change in stimulus strength
Response to Change in Stimulus Strength
  • Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs
  • Muscle contracts more vigorously as stimulus strength is increased above threshold
  • Contraction force is precisely controlled by recruitment (multiple motor unit summation)
    • Increase in the number of active motor units
slide65

Stimulus strength

Maximal

stimulus

Threshold

stimulus

Proportion of motor units excited

Strength of muscle contraction

Maximal contraction

Figure 9.16

response to change in stimulus strength1
Response to Change in Stimulus Strength
  • Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases
muscle tone
Muscle Tone
  • Constant, slightly contracted state of all muscles
  • Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles
  • Keeps muscles firm, healthy, and ready to respond
isotonic contractions
Isotonic Contractions
  • Muscle changes in length and moves the load
  • Isotonic contractions are either concentric or eccentric:
    • Concentric contractions—the muscle shortens and does work
    • Eccentric contractions—the muscle contracts as it lengthens
isometric contractions
Isometric Contractions
  • The load is greater than the tension the muscle is able to develop
  • Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens
muscle metabolism energy for contraction
Muscle Metabolism: Energy for Contraction
  • ATP is the only source used directly for contractile activities
  • Available stores of ATP are depleted in 4–6 seconds
muscle metabolism energy for contraction1
Muscle Metabolism: Energy for Contraction
  • ATP is regenerated by:
    • Direct phosphorylation of ADP by creatine phosphate (CP)
    • Anaerobic pathway (glycolysis)
    • Aerobic respiration
slide73

(a) Direct phosphorylation

Coupled reaction of creatine

phosphate (CP) and ADP

Energy source: CP

CP

ADP

Creatine

kinase

Creatine

ATP

Oxygen use: None

Products: 1 ATP per CP, creatine

Duration of energy provision:

15 seconds

Figure 9.19a

anaerobic pathway
Anaerobic Pathway
  • At 70% of maximum contractile activity:
    • Bulging muscles compress blood vessels
    • Oxygen delivery is impaired
    • Pyruvic acid is converted into lactic acid
anaerobic pathway1
Anaerobic Pathway
  • Lactic acid:
    • Diffuses into the bloodstream
    • Used as fuel by the liver, kidneys, and heart
    • Converted back into pyruvic acid by the liver
aerobic pathway
Aerobic Pathway
  • Produces 95% of ATP during rest and light to moderate exercise
  • Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids
muscle fatigue
Muscle Fatigue
  • Physiological inability to contract
  • Occurs when:
    • Ionic imbalances (K+, Ca2+, Pi) interfere with E-C coupling
    • Prolonged exercise damages the SR and interferes with Ca2+ regulation and release
  • Total lack of ATP occurs rarely, during states of continuous contraction, and causes contractures (continuous contractions)
oxygen deficit
Oxygen Deficit

Extra O2 needed after exercise for:

  • Replenishment of
    • Oxygen reserves
    • Glycogen stores
    • ATP and CP reserves
  • Conversion of lactic acid to pyruvic acid, glucose, and glycogen
heat production during muscle activity
Heat Production During Muscle Activity
  • ~ 40% of the energy released in muscle activity is useful as work
  • Remaining energy (60%) given off as heat
  • Dangerous heat levels are prevented by radiation of heat from the skin and sweating
force of muscle contraction
Force of Muscle Contraction
  • The force of contraction is affected by:
    • Number of muscle fibers stimulated (recruitment)
    • Relative size of the fibers—hypertrophy of cells increases strength
    • Frequency of stimulation— frequency allows time for more effective transfer of tension to noncontractile components
    • Length-tension relationship—muscles contract most strongly when muscle fibers are 80–120% of their normal resting length
slide81

Sarcomeres

greatly

shortened

Sarcomeres at

resting length

Sarcomeres excessively

stretched

75%

100%

170%

Optimal sarcomere

operating length

(80%–120% of

resting length)

Figure 9.22

velocity and duration of contraction
Velocity and Duration of Contraction

Influenced by:

  • Muscle fiber type
  • Load
  • Recruitment
muscle fiber type
Muscle Fiber Type

Classified according to two characteristics:

  • Speed of contraction: slow or fast, according to:
    • Speed at which myosin ATPases split ATP
    • Pattern of electrical activity of the motor neurons
  • Metabolic pathways for ATP synthesis:
    • Oxidative fibers—use aerobic pathways
    • Glycolytic fibers—use anaerobic glycolysis
muscle fiber type1
Muscle Fiber Type

Three types:

  • Slow oxidative fibers
  • Fast oxidative fibers
  • Fast glycolytic fibers
slide86

FO

SO

FG

Figure 9.24

influence of load
Influence of Load

 load  latent period,  contraction, and  duration of contraction

influence of recruitment
Influence of Recruitment

Recruitment  faster contraction and  duration of contraction

effects of exercise
Effects of Exercise

Aerobic (endurance) exercise:

  • 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
effects of resistance exercise
Effects of Resistance Exercise
  • Resistance exercise (typically anaerobic) results in:
    • Muscle hypertrophy (due to increase in fiber size)
    • Increased mitochondria, myofilaments, glycogen stores, and connective tissue
smooth muscle
Smooth Muscle
  • Found in walls of most hollow organs(except heart)
  • Usually in two layers (longitudinal and circular)
peristalsis
Peristalsis
  • Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs
    • Longitudinal layer contracts; organ dilates and shortens
    • Circular layer contracts; organ constricts and elongates
microscopic structure
Microscopic Structure
  • Spindle-shaped fibers: thin and short compared with skeletal muscle fibers
  • Connective tissue: endomysium only
  • SR: less developed than in skeletal muscle
  • Pouchlikeinfoldings (caveolae) of sarcolemma sequester Ca2+
  • No sarcomeres, myofibrils, or T tubules
innervation of smooth muscle
Innervation of Smooth Muscle
  • Autonomic nerve fibers innervate smooth muscle at diffuse junctions
  • Varicosities (bulbous swellings) of nerve fibers store and release neurotransmitters
myofilaments in smooth muscle
Myofilaments in Smooth Muscle
  • Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)
  • Thick filaments have heads along their entire length
  • No troponin complex; protein calmodulin binds Ca2+
  • Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner
  • Dense bodies: proteins that anchor noncontractile intermediate filaments to sarcolemma at regular intervals
contraction of smooth muscle
Contraction of Smooth Muscle
  • Slow, synchronized contractions
  • Cells are electrically coupled by gap junctions
  • 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 of smooth muscle1
Contraction of Smooth Muscle
  • Sliding filament mechanism
  • Final trigger is  intracellular Ca2+
  • Ca2+ is obtained from the SR and extracellular space
  • Ca2+ binds to and activates calmodulin
  • Activated calmodulin activates myosin (light chain) kinase
  • Activated kinase phosphorylates and activates myosin
  • Cross bridges interact with actin
slide99

Extracellular fluid (ECF)

Ca2+

Plasma membrane

Cytoplasm

1

Calcium ions (Ca2+)

enter the cytosol from

the ECF via voltage-

dependent or voltage-

independent Ca2+

channels, or from

the scant SR.

Ca2+

Sarcoplasmic

reticulum

Figure 9.29, step 1

contraction of smooth muscle2
Contraction of Smooth Muscle
  • Very energy efficient (slow ATPases)
  • Myofilaments may maintain a latch state for prolonged contractions

Relaxation requires:

  • Ca2+ detachment from calmodulin
  • Active transport of Ca2+ into SR and ECF
  • Dephosphorylation of myosin to reduce myosin ATPase activity
regulation of contraction
Regulation of Contraction

Neural regulation:

  • Neurotransmitter binding  [Ca2+] in sarcoplasm; either graded (local) potential or action potential
  • Response depends on neurotransmitter released and type of receptor molecules

Hormones and local chemicals:

    • May bind to G protein–linked receptors
    • May either enhance or inhibit Ca2+ entry
special features of smooth muscle contraction
Special Features of Smooth Muscle Contraction

Stress-relaxation response:

  • Responds to stretch only briefly, then adapts to new length
  • Retains ability to contract on demand
  • Enables organs such as the stomach and bladder to temporarily store contents

Length and tension changes:

  • Can contract when between half and twice its resting length
special features of smooth muscle contraction1
Special Features of Smooth Muscle Contraction

Hyperplasia:

  • Smooth muscle cells can divide and increase their numbers
  • Example:
    • estrogen effects on uterus at puberty and during pregnancy
types of smooth muscle
Types of Smooth Muscle

Single-unit (visceral) smooth muscle:

  • Sheets contract rhythmically as a unit (gap junctions)
  • Often exhibit spontaneous action potentials
  • Arranged in opposing sheets and exhibit stress-relaxation response

Multiunit smooth muscle:

  • Located in large airways, large arteries, arrectorpili muscles, and iris of eye
  • Gap junctions are rare
  • Arranged in motor units
  • Graded contractions occur in response to neural stimuli
other interesting points
Other interesting points..
  • Athletics and training can improve neuromuscular control
  • Female skeletal muscle makes up 36% of body mass, male 42% (testosterone!)
    • Body strength per unit muscle mass is the same in both sexes
  • With age, connective tissue increases and muscle fibers decrease
  • By age 30, loss of muscle mass (sarcopenia) begins
    • Regular exercise reverses sarcopenia
muscular dystrophy
Muscular Dystrophy

Duchenne muscular dystrophy (DMD):

  • Most common and severe type
  • Inherited, sex-linked, carried by females and expressed in males (1/3500) as lack of dystrophin
  • Victims become clumsy and fall frequently; usually die of respiratory failure in their 20s
  • No cure, but viral gene therapy or infusion of stem cells with correct dystrophin genes show promise