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
Function of Muscles
 Produce movement
 Maintain posture
 Stabilize joints
 Generate heat
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
Table 9.3
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
Table 9.1
Fib-Endo-Fas-Per-Ep
Nested Structures in a Muscle
Epimysium
Bone Epimysium
Perimysium
Endomysium
Tendon
(b)
Perimysium Fascicle
Muscle fiber
in middle of
a fascicle
Blood vessel
Fascicle
(wrapped by perimysium)
Endomysium
(between individual
muscle fibers)
Muscle fiber
• Whole muscle is surrounded by an epimysium and is composed of wrapped fascicles
• Each fascicle is composed of muscle fibers (cells), surrounded by a perimysium
Figure 9.1
• Each muscle fiber is surrounded by endomysium ( & then the sarcolemma)
• Muscle fibers (cells) contain several myofibrils
Sarcolemma
Mitochondrion
Myofibril
Dark A band Light I band Nucleus
(b) Diagram of part of a muscle fiber showing the myofibrils. One
myofibril is extended afrom the cut end of the fiber.
Sarcoplasmic Reticulum is Modified Endoplasmic Reticulum
(Storage Depot for Calcium Ions)
Part of a skeletal
muscle fiber (cell)
I band
A band
I band
Z disc
H zone
Z disc
Myofibril
M line
Sarcolemma
Sarcolemma
(muscle fiber
plasma
membrane)
Triad:
• T tubule
• Terminal
cisternae
of the SR (2)
Tubules of
the SR
Myofibrils
Mitochondria

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 9.5
Patterns Visible in the Sarcomere
(Smallest Contractile Unit of a Myofibril)
Thin (actin)
filament
Thick (myosin)
filament
Z disc
I band
H zone
A band
Sarcomere
Z disc
I band
Sarcomere
Z disc
M line
Z disc
M line
Microscopic
Muscle
Anatomy
(online)
Thin (actin) filament
Elastic (titin) filaments
Thick (myosin) filament
Figure 9.2c, d
Thin and Thick Filament Composition
Longitudinal section of filaments
within one sarcomere of a myofibril
Thick filament
Thin filament
In the center of the sarcomere, the thick
filaments lack myosin heads. Myosin heads
are present only in areas of myosin-actin overlap.
Thick filament
Thin filament
Each thick filament consists of many
A thin filament consists of two strands
myosin molecules whose heads protrude of actin subunits twisted into a helix
at opposite ends of the filament.
plus two types of regulatory proteins
(troponin and tropomyosin).
Portion of a thick filament
Portion of a thin filament
Myosin head
Tropomyosin
Troponin
Actin
Actin-binding sites
ATPbinding
site
Heads
Tail
Flexible hinge region
Myosin molecule
Active sites
for myosin
attachment
Actin
subunits
Actin subunits
Tropomyosin and troponin: regulatory proteins bound to actin
Figure 9.3
Z
Z
H
A
I
I
1 Fully relaxed sarcomere of a muscle fiber
Z
I
Z
A
I
2 Fully contracted sarcomere of a muscle fiber
Figure 9.6
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
How Muscle Contracts: 1) Events at the Neuromuscular Junction
Action
potential (AP)
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Nucleus
Sarcolemma of
the muscle fiber
1 Action potential arrives at
axon terminal of motor neuron.
2 Voltage-gated Ca2+ channels
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Axon terminal
of motor neuron
Synaptic vesicle
containing ACh
Mitochondrion
Synaptic
cleft
Fusing synaptic
vesicles
Skeletal muscles are stimulated by the axon termini of somatic motor neurons
Association of one axon to a particular group of fibers = 1 motor unit
Figure 9.8
How Muscle Contracts: 1) Events at the Neuromuscular Junction
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
Action
potential (AP)
Nucleus
1 Action potential arrives at
axon terminal of motor neuron.
2 Voltage-gated Ca2+ channels
Ca2+
Ca2+
open and Ca2+ enters the axon
terminal.
Axon terminal
of motor neuron
3 Ca2+ entry causes some
synaptic vesicles to release
their contents (acetylcholine)
by exocytosis.
Synaptic vesicle
containing ACh
Mitochondrion
Synaptic
cleft
Fusing synaptic
vesicles
ACh
4 Acetylcholine, a
neurotransmitter, diffuses across
the synaptic cleft and binds to
receptors in the sarcolemma.
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
5 ACh binding opens ion
channels that allow simultaneous
passage of Na+ into the muscle
fiber and K+ out of the muscle
fiber.
6 ACh effects are terminated
by its enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase.
Nerve impulse causes
muscular contraction:
excitation-contraction
(E-C) coupling
Na+
Ach–
K+
Degraded ACh
Na+
Acetylcholinesterase
K+
Postsynaptic membrane
ion channel opens;
ions pass.
Events at
neuromuscular
junction
movie
Postsynaptic membrane
ion channel closed;
ions cannot pass.
Figure 9.8
How Muscle Contracts: 2) Initiation of an Action Potential
Axon terminal
Open Na+
Channel
Na+
Synaptic
cleft
Closed K+
Channel
ACh
ACh
Na+ K+
Na+ K+
K+
++
++ +
+
Action potential
+
+ +++
+
2 Generation and propagation of
the action potential (AP)
Closed Na+Channel
1 Local depolarization:
generation of the end
plate potential on the
sarcolemma
Sarcoplasm of muscle fiber
Open K+ Channel
Na+
K+
3 Repolarization
Figure 9.9
How Muscle Contracts: 3) How the Sarcolemma Resets
Depolarization
due to Na+ entry
Na+ channels
close, K+ channels
open
Repolarization
due to K+ exit
Na+
channels
open
Threshold
K+ channels
close
Figure 9.10
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
How Muscle Contracts: 4) Action Potential Causes Ca++ Release
Axon terminal
of motor neuron
Action potential
Synaptic cleft
is generated
ACh
Sarcolemma
Terminal cisterna of SR
Muscle fiber Ca2+
Triad
One sarcomere
Figure 9.11, step 1
How Muscle Contracts: 4) Action Potential Causes Ca++ Release
1 Action potential is
Steps in
E-C Coupling:
propagated along the
sarcolemma and down
the T tubules.
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
Ca2+
release
channel
Terminal
cisterna
of SR
2 Calcium
ions are
released.
Ca2+
Figure 9.11, step 4
How Muscle Contracts: 5) Ca++ Binds to Troponin
Actin
Ca2+
Troponin
Tropomyosin
blocking active sites
Myosin
The aftermath
Figure 9.11, step 5
How Muscle Contracts: 5) Ca++ Binds to Troponin
Actin
Ca2+
Troponin
Tropomyosin
blocking active sites
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
How Muscle Contracts: 6) Troponin Slides Tropomyosin Off Myosin Binding Sites
Actin
Ca2+
Troponin
Tropomyosin
blocking active sites
Myosin
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin.
Active sites exposed and
ready for myosin binding
4 Contraction begins
Myosin
cross
bridge
The aftermath Calcium is sequestered again by the SR, lowering Ca++ levels,
and causing muscle to relax as tropomyo. covers binding sites
Figure 9.11, step 7
Four Step Power Cycle or “Cross Bridge Cycle”
Thin filament
Actin
Ca2+
Myosin
cross bridge
ADP
Pi
Thick
filament
Myosin
Cross
bridge
formation.
1
ADP
ADP
Pi
Pi
ATP
hydrolysis
2 The power (working)
stroke.
4 Cocking of myosin head.
ATP
ATP
3 Cross bridge
detachment.
Figure 9.12
Step One of the Cross Bridge Cycle
Actin
Ca2+
Myosin
cross bridge
Thin filament
ADP
Pi
Thick filament
Myosin
1 Cross bridge formation.
Figure 9.12, step 1
Step Two of the Cross Bridge Cycle
ADP
Pi
2 The power (working) stroke.
Figure 9.12, step 3
Step Three of the Cross Bridge Cycle
ATP
3 Cross bridge detachment.
Figure 9.12, step 4
Step Four of the Cross Bridge Cycle
ADP
ATP
Pi
hydrolysis
4 Cocking of myosin head.
Figure 9.12, step 5
Summary of the Cross Bridge Cycle
Thin filament
Actin
Ca2+
Myosin
cross bridge
ADP
Pi
Thick
filament
Myosin
Cross
bridge
formation.
1
ADP
ADP
Pi
Pi
ATP
hydrolysis
2 The power (working)
stroke.
4 Cocking of myosin head.
Sliding
Filament
Theory
ATP
ATP
3 Cross bridge
detachment.
Figure 9.12
The Power Cycle
A. Masking protein complex (tropomyosin) binds Ca++ released from the
SR moves aside to expose head-binding sites
B. 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 detach
4. 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
Principles of Muscle Mechanics
1. The same principles apply to contraction of a single fiber
and a whole muscle
2. Contraction produces tension, the force exerted on the load
or object to be moved
3.
4.
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
Force and duration of contraction vary in response to stimuli
of different frequencies and intensities
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
ExcitationCross bridge
contraction formation; tension
coupling
increases
Latent Period of
period contraction
Ca2+ reentry into the SR;
tension declines to zero
Period of
relaxation
Single
stimulus
(a) Myogram showing the three phases of an isometric twitch
Figure 9.14a
Graded Muscle Responses
 Variations 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
Single stimulus
single twitch
Contraction
Relaxation
Stimulus
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.
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 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
High stimulation frequency fused (complete) tetanus
Stimuli
Figure 9.15c
(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
Response to Change in Stimulus Strength
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 Strength
 Size principle: motor units with larger and larger fibers are
recruited as stimulus intensity increases
Motor
Motor
Motor
unit 1
unit 2
unit 3
Recruitedrecruitedrecruited
(small (medium (large
fibers) fibers) fibers)
Figure 9.17
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
Energy for Muscle Contraction
Short-duration exercise
ATP stored in
muscles is
used first.
ATP is formed
from creatine
Phosphate
and ADP.
Glycogen stored in muscles is broken
down to glucose, which is oxidized to
generate ATP.
Prolonged-duration
exercise
ATP is generated by
breakdown of several
nutrient energy fuels by
aerobic pathway. This
pathway uses oxygen
released from myoglobin
or delivered in the blood
by hemoglobin. When it
ends, the oxygen deficit is
paid back.
Figure 9.20
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
Summary of Factors Increasing Contractile Force
Large
number of
muscle
fibers
activated
Large
muscle
fibers
High
frequency of
stimulation
Muscle and
sarcomere
stretched to
slightly over 100%
of resting length
Contractile force
Figure 9.21
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 Type
Fibers 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)
Predominance
of fast glycolytic
(fatigable) fibers:
“fast twitch” or
“white meat”
Contractile
velocity
Small load
Predominance
of slow oxidative
(fatigue-resistant)
fibers: “slow twitch” or
dark meat
Contractile
duration
Figure 9.23
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