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Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
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 sheetlike aponeurosis
The Muscular System Muscles are responsible for all types of body
movement
Three basic muscle types are found in the body
Characteristics of Muscles Muscle cells are elongated (muscle cell =
muscle fiber)
Contraction of muscles is due to the movement of microfilaments within fiber cells
All muscles share some terminology
• Prefix myo refers to muscle
• Prefix mys refers to muscle
• Prefix sarco refers to flesh
Skeletal Muscle Characteristics Most are attached by tendons to bones
Cells are multinucleate
Striated – have visible banding
Voluntary – subject to conscious control
Cells are cylindrical
Cells are surrounded
and bundled by
connective tissue
Plasma/cell membrane called a sarcolemma
Glycosomes for glycogen storage, myoglobin for O2 storage
Also contain myofibrils, sarcoplasmic reticulum (modified ER), and T tubules
Smooth Muscle Characteristics Has no striations
Spindle-shaped cells
Single nucleus
Involuntary – no conscious control
Found mainly in the walls of hollow organs
Cardiac Muscle Characteristics Has striations
Usually has a single nucleus
Joined to another muscle cell at an intercalated disc
Involuntary
Found only in the heart
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
Figure 9.1
Bone
Perimysium
Endomysium(between individualmuscle fibers)
Muscle fiber
Fascicle(wrapped by perimysium)
Epimysium
Tendon
Epimysium
Muscle fiberin middle ofa fascicle
Blood vessel
Perimysium
Endomysium
Fascicle
(b)
• Each fascicle is composed of muscle fibers (cells), surrounded by a perimysium• Each muscle fiber is surrounded by endomysium ( & then the sarcolemma)• Muscle fibers (cells) contain several myofibrils
• Whole muscle is surrounded by an epimysium and is composed of wrapped fascicles
Nested Structures in a Muscle Fib-Endo-Fas-Per-Ep
NucleusLight I bandDark A band
Sarcolemma
Mitochondrion
(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended afrom the cut end of the fiber.
Myofibril
Figure 9.5
Myofibril
Myofibrils
Triad:
Tubules ofthe SR
Sarcolemma (muscle fiber
plasma membrane)
Sarcolemma
Mitochondria
I band I bandA band
H zone Z discZ disc
Part of a skeletalmuscle fiber (cell)
• T tubule• Terminal
cisternaeof the SR (2)
M line
Sarcoplasmic Reticulum is Modified Endoplasmic Reticulum(Storage Depot for Calcium Ions)
T tubules are continuous with the sarcolemma
They penetrate the cell’s interior at each A band–I band junction
They’re associated with the paired terminal cisternae to form triads that encircle each sarcomere
T tubules conduct impulses deep into muscle fiber; contains gated channels that regulate Ca 2+ release
Figure 9.2c, d
I band I bandA bandSarcomere
Thick (myosin)filament
M line
Z disc Z discM line
Sarcomere
Thin (actin) filament
Thick (myosin) filament
Elastic (titin) filaments
H zoneThin (actin)filament Z disc Z disc
Patterns Visible in the Sarcomere (Smallest Contractile Unit of a Myofibril)
Microscopic Muscle Anatomy (online)
Figure 9.3
Flexible hinge region
Tail
Tropomyosin Troponin Actin
Myosin head
ATP-bindingsite
Heads Active sitesfor myosinattachment
Actinsubunits
Actin-binding sites
Thick filamentEach thick filament consists of manymyosin molecules whose heads protrude at opposite ends of the filament.
Thin filamentA thin filament consists of two strandsof actin subunits twisted into a helix plus two types of regulatory proteins(troponin and tropomyosin).
Thin filamentThick filament
In the center of the sarcomere, the thickfilaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap.
Longitudinal section of filamentswithin one sarcomere of a myofibril
Portion of a thick filamentPortion of a thin filament
Myosin molecule Actin subunits
Tropomyosin and troponin: regulatory proteins bound to actin
Thin and Thick Filament Composition
Figure 9.6
I
Fully relaxed sarcomere of a muscle fiber
Fully contracted sarcomere of a muscle fiber
IA
Z ZH
I IA
Z Z
1
2
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
Nucleus
Actionpotential (AP)
Myelinated axonof motor neuron
Axon terminal ofneuromuscular junction
Sarcolemma ofthe muscle fiber
Ca2+Ca2+
Axon terminalof motor neuron
Synaptic vesiclecontaining ACh
MitochondrionSynapticcleft
Fusing synaptic vesicles
1 Action potential arrives ataxon terminal of motor neuron.
2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.
Figure 9.8
Skeletal muscles are stimulated by the axon termini of somatic motor neuronsAssociation of one axon to a particular group of fibers = 1 motor unit
How Muscle Contracts: 1) Events at the Neuromuscular Junction
Figure 9.8
Nucleus
Actionpotential (AP)
Myelinated axonof motor neuron
Axon terminal ofneuromuscular junction
Sarcolemma ofthe muscle fiber
Ca2+Ca2+
Axon terminalof motor neuron
Synaptic vesiclecontaining AChMitochondrionSynapticcleft
Junctionalfolds ofsarcolemma
Fusing synaptic vesicles
ACh
Sarcoplasm ofmuscle fiber
Postsynaptic membraneion channel opens;ions pass.
Na+ K+
Ach–
Na+
K+
Degraded ACh
Acetyl-cholinesterase
Postsynaptic membraneion channel closed;ions cannot pass.
1 Action potential arrives ataxon terminal of motor neuron.
2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.
3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.
4 Acetylcholine, aneurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.
5 ACh binding opens ionchannels that allow simultaneous passage of Na+ into the musclefiber and K+ out of the muscle fiber.
6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.
How Muscle Contracts: 1) Events at the Neuromuscular Junction
Events at neuro-muscular junction movie
Nerve impulse causes muscular contraction: excitation-contraction (E-C) coupling
Figure 9.9
Na+
Na+
Open Na+
Channel
Closed Na+Channel
Closed K+
Channel
Open K+ Channel
Action potential++++++
+++++
+
Axon terminal
Synapticcleft
ACh
ACh
Sarcoplasm of muscle fiber
K+
2 Generation and propagation ofthe action potential (AP)
3 Repolarization
1 Local depolarization: generation of the end plate potential on the sarcolemma
K+
K+Na+
K+Na+
Wave ofde
po
lari
zatio
n
How Muscle Contracts: 2) Initiation of an Action Potential
Figure 9.10
Na+ channelsclose, K+ channelsopen
K+ channelsclose
Repolarizationdue to K+ exit
Threshold
Na+
channelsopen
Depolarizationdue to Na+ entry
How Muscle Contracts: 3) How the Sarcolemma Resets
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
Axon terminalof motor neuron
Muscle fiberTriad
One sarcomere
Synaptic cleft
Sarcolemma
Action potentialis generated
Terminal cisterna of SR ACh
Ca2+
How Muscle Contracts: 4) Action Potential Causes Ca++ Release
Figure 9.11, step 1
Figure 9.11, step 4
Steps inE-C Coupling:
Terminal cisterna of SR
Voltage-sensitivetubule protein
T tubule
Ca2+
releasechannel
Ca2+
Sarcolemma
Action potential ispropagated along thesarcolemma and downthe T tubules.
Calciumions arereleased.
1
2
How Muscle Contracts: 4) Action Potential Causes Ca++ Release
Figure 9.11, step 5
Troponin Tropomyosinblocking active sitesMyosin
Actin
Ca2+
The aftermath
How Muscle Contracts: 5) Ca++ Binds to Troponin
Figure 9.11, step 6
Troponin Tropomyosinblocking active sitesMyosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Calcium binds totroponin and removesthe blocking action oftropomyosin.
The aftermath
3
How Muscle Contracts: 5) Ca++ Binds to Troponin
Figure 9.11, step 7
Troponin Tropomyosinblocking active sitesMyosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Myosincross bridge
Calcium binds totroponin and removesthe blocking action oftropomyosin.
Contraction begins
The aftermath
3
4
How Muscle Contracts: 6) Troponin Slides Tropomyosin Off Myosin Binding Sites
Calcium is sequestered again by the SR, lowering Ca++ levels, and causing muscle to relax as tropomyo. covers binding sites
Figure 9.12
1
Actin
Cross bridge formation.
Cocking of myosin head. The power (working) stroke.
Cross bridge detachment.
Ca2+
Myosincross bridge
Thick filament
Thin filament
ADP
Myosin
Pi
ATPhydrolysis
ATP
ATP
24
3
ADP
Pi
ADPPi
Four Step Power Cycle or “Cross Bridge Cycle”
Figure 9.12, step 1
Actin
Cross bridge formation.
Ca2+
Myosincross bridge
Thick filament
Thin filament
ADP
Myosin
Pi
1
Step One of the Cross Bridge Cycle
Figure 9.12, step 5
Cocking of myosin head.
ATPhydrolysis
ADPPi
4
Step Four of the Cross Bridge Cycle
Figure 9.12
1
Actin
Cross bridge formation.
Cocking of myosin head. The power (working) stroke.
Cross bridge detachment.
Ca2+
Myosincross bridge
Thick filament
Thin filament
ADP
Myosin
Pi
ATPhydrolysis
ATP
ATP
24
3
ADP
Pi
ADPPi
Sliding Filament Theory
Summary of the Cross Bridge Cycle
The Power CycleA. Masking protein complex (tropomyosin) binds Ca++ released from the
SR moves aside to expose head-binding sitesB. Steps of the Power Cycle
1. "Cocked" myosin head binds to actin myofilament site 2. Head bends towards the M line (sarcomere center-line), pulling
thin filament along and releasing ADP and P (broken ATP) for the power stroke
3. ATP binds to the myosin head, causing it to detach4. Myosin head “recocks” as ATP broken down to ADP and P
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
2. Contraction produces tension, the force exerted on the load or object to be moved
Principles of Muscle Mechanics
3. 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
4. Force and duration of contraction vary in response to stimuli of different frequencies and intensities
1. The same principles apply to contraction of a single fiber and a whole muscle
Muscle Twitch Response of a muscle to a single, brief threshold
stimulus
Simplest contraction observable in the lab (recorded as a myogram)
Three phases of a twitch:
• Latent period: events of excitation-contraction coupling
• Period of contraction: cross bridge formation; tension increases
• Period of relaxation: Ca2+ reentry into the SR; tension declines to zero
Figure 9.14a
Latentperiod
Singlestimulus
Period ofcontraction
Period ofrelaxation
(a) Myogram showing the three phases of an isometric twitch
Excitation-contraction coupling
Cross bridge formation; tension
increases
Ca2+ reentry into the SR; tension declines to zero
Graded Muscle ResponsesVariations in the degree of muscle contraction
Required for proper control of skeletal movement
Responses are graded by:
1. Changing the frequency of stimulation
2. Changing the strength of the stimulus
Response to Change in Stimulus Frequency
A single stimulus results in a single contractile response—a muscle twitch
Contraction
Relaxation
Stimulus
Single stimulus single twitch
A single stimulus is delivered. The muscle contracts and relaxes
Response to Change in Stimulus Frequency Increases frequency of stimulus (muscle does not have time to
completely relax between stimuli). Distinct peaks are still seen in the myogram.
Figure 9.15b
Stimuli
Partial relaxation
Low stimulation frequency -->unfused (incomplete) tetanus
(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.
Response to Change in Stimulus Frequency Ca2+ release stimulates further contraction temporal (wave) summation
Further increase in stimulus frequency unfused (incomplete) tetanus If stimuli are given quickly enough, fused (complete) tetany results
Figure 9.15c
Stimuli
High stimulation frequency fused (complete) tetanus
(c) At higher stimulus frequencies, there is no relaxation at all between stimuli. This is fused (complete) tetanus.
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), which brings more and more muscle fibers into action
Figure 9.16
Stimulus strength
Proportion of motor units excited
Strength of muscle contraction
Maximal contraction
Maximalstimulus
Thresholdstimulus
Response to Change in Stimulus Strength
Response to Change in Stimulus Strength Size principle: motor units with larger and larger fibers are
recruited as stimulus intensity increases
Figure 9.17
Motorunit 1Recruited(smallfibers)
Motorunit 2recruited(mediumfibers)
Motorunit 3recruited(largefibers)
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
Figure 9.20
Short-duration exerciseProlonged-durationexercise
ATP stored inmuscles isused first.
ATP is formedfrom creatinePhosphateand ADP.
Glycogen stored in muscles is brokendown to glucose, which is oxidized togenerate ATP.
ATP is generated bybreakdown of severalnutrient energy fuels byaerobic pathway. Thispathway uses oxygenreleased from myoglobinor delivered in the bloodby hemoglobin. When itends, the oxygen deficit ispaid back.
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
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 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
Figure 9.21
Largenumber of
musclefibers
activated
Contractile force
Highfrequency ofstimulation
Largemusclefibers
Muscle andsarcomere
stretched to slightly over 100%of resting length
Summary of Factors Increasing Contractile Force
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
Effects of Exercise on Muscle
Results of increased muscle use
• Increase in muscle size
• Increase in muscle strength
• Increase in muscle efficiency
• Muscle becomes more fatigue resistant
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Muscle Fiber Types
Developmental Aspects
Muscular Dystrophy
Muscle Fiber TypeFibers classified according to two characteristics:
1. Speed of contraction: slow or fast, according to:
• Speed at which myosin ATPases split ATP
• Pattern of electrical activity of the motor neurons
2. Metabolic pathways for ATP synthesis:
• Slow and fast oxidative fibers—use aerobic pathways (fast oxidative fibers = red meat in birds)
• Glycolytic fibers—use anaerobic glycolysis (these fibers are “fast twitch” white meat in birds)
Figure 9.23
Predominanceof fast glycolytic(fatigable) fibers:“fast twitch” or“white meat”
Predominanceof slow oxidative(fatigue-resistant)
fibers: “slow twitch” ordark meat
Small load
Contractilevelocity
Contractileduration
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy
Developmental Aspects Cardiac and skeletal muscle become amitotic, but can
lengthen and thicken
Injured heart muscle is mostly replaced by connective tissue
Smooth muscle regenerates throughout life
Myoblast-like skeletal muscle satellite cells have limited regenerative ability; are responsible for generating more fibers and in muscle repair
Muscular development reflects neuromuscular coordination
• Development occurs head to toe, and proximal to distal
• Peak natural neural control occurs by midadolescence
• Athletics and training can improve neuromuscular control
Diseases and Medical Conditions (Myopathies) of the Muscular System
• Myasthenia gravis (autoimmumity; destruction of ACh receptors so neuromuscular junctions don’t work)
• Poliomyelitis (viral infection of muscle nerves)
• Muscle strains cause myalgia (pain) and sometimes myositis (inflammation). Inflamed tendons are fibromyositis.
• Fibromyalgia- (muscle pain) causes widespread pain in the muscles accompanied by fatigue and sleep disorders. Thought to be neurologically, blood flow, based.
• Cramps (muscle spasms)
• Contusion (muscle bruise)
• Crush injury (severe trauma releasing myoglobin)
• Muscular dystrophy (e.g. Duchenne's; degeneration & atrophy of muscles)
Muscular Dystrophy Group of inherited muscle-destroying diseases
Muscles enlarge due to fat and connective tissue deposits
Muscle fibers atrophy
Duchenne muscular dystrophy (DMD):
• Most common and severe type
• Inherited, sex-linked, carried by females and expressed in males (1/3500) as a lack of dystrophin, a protein that links muscle fibers together
• 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
Muscle Types and Physiology Types and Characteristics of Muscle
Muscle Function and Types
Microscopic Anatomy of Muscle
Muscular Stimulation
Muscular Contraction Mechanism
Muscular Response Based on Stimulus
Energy Sources for Muscular Contraction
Types of Muscular Contractions
Effects of Exercise on Muscles
Developmental Aspects
Muscular Dystrophy