Muscle
Three types of muscle:
• smooth
• cardiac
• skeletal
All muscles require ATP to produce movement.
Thus, muscles are chemotransducers
Skeletal Muscle
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•
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Muscle organization
Muscle innervation
Architecture and structure
Excitation-contraction
Fiber type characteristics
Training adaptations
Exam 1 (Feb 8)
Skeletal muscle organization
Connective tissue
layers
• Epimysium
• Perimeysium
• Endomysium
Muscle fiber covering
• Sarcolemma
– basement membrane
– plasma membrane
• Plasma membrane has
– membrane receptors
– ion channels
– integrins
– satellite cells
– multinuclei
Muscle Architecture
Effect on force output and
shortening velocity
Muscle Architecture
Muscle
architecture
Muscle Architecture
Parallel
Unipennation
Multipennation
Pennation: Effect on Physiological Crosssectional Area (PCSA)
Greater PCSA when fiber is at angle to
line of force
B
A
A
Pennation: Effect on Force and
Shortening Distance/Velocity
Fiber B
Fiber A
Equal number of sarcomeres in both examples, but Fiber A has
longer fiber and smaller PSFA than Fiber B, which allows for
greater shortening distance/velocity at sacrifice of force.
Identify which muscles
are best suited for
force; for speed
A
B
C
D
Muscle Architecture
• quadriceps and planter flexors designed
for force production
– larger pennation angles
– large PCSAs
• hamstrings and dorsiflexors designed
for velocity
– smaller pennation angles
– intermediate PCSAs
Muscle Architecture
Summary
– Muscles designed to fit purpose of joint
• Muscles designed for velocity have longer fiber
length and small pennation angle
• Muscles designed for force have shorter fiber
length and larger pennation angle
Review questions
1. Describe the difference between a muscle with
a fusiform architecture and one with a uni- or
multipennate architecture. Identify a muscle
for each type of architecture.
2. Discuss how muscle architecture affects force
output and shortening velocity. Provide a
general explanation as to why some muscles
are designed more for rapid shortening velocity
(e.g. hamstrings) or higher force output (e.g.
quadriceps muscles).
Muscle Innervation
Motoneurons, neuromuscular
junctions, motor units
Motoneurons
• muscle fibers
innervated by large
(alpha) myelinated
nerves
• motoneurons originate
from spinal cord
• nerve ending ends at
neuromuscular
junction
• motor unit composed
of motor neuron and
all the fibers it
innervates
Action Potential
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depolarization – influx of Na+
repolarization – efflux of K+
refractory period – hyperpolarization
threshold level – minimal stimulus required
to elicit response
• muscle and nerve follow “all or nothing
principle”
Membrane potential (mV)
+20
0
-20
-40
-60
-80
Time (ms)
Na+
K+
Na+
Na+
Na+
Na+
K+
Na+
Na+
Na+
Na+
channel
K+
K+
K+
intracellular
K+
K+
K+
Na+
Na+
Na+
Na+-K+
exchange pump
K+
Na+
Na+
K+
K+
Na+
ATPase
K+
ADP
Pi
K+
Na+
Na+
K+
channel
K+
K+
Na+
ATP
Na+
K+
K+
K+
K+
Na+
K+
Neuromuscular Junction
Electromyography (EMG)
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Describe the relative weights being lifted
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Review questions
1. Define the motor unit.
2. Describe the events that occur as an action
potential approaches the nerve terminal.
3. Explain the purpose of acetylcholinesterase
and the consequences of its absence.
4. A common agent found in flea powders is a
low dose of an antiacetylcholinesterase
inhibitor. Explain the effects that the flea
powder would have on fleas.
5. Explain the interpretation of an EMG tracing.
Sarcomere Structure
Skeletal Muscle Structure
Cross-Sectional View of Skeletal Muscle (X40)
Skeletal Muscle Structure
• sarcomeres (smallest functional unit) are
linked end-to-end to form myofibrils
• myofibrils are bunched to form fibers
• sarcomeres are composed of thick and
thin filaments
Scanning EM
1
4
2
3
5
Thick Filament
• composed of numerous myosin protein
strands
• flexible “heads” protrude outward all
around filament (except center)
• myosin heads attach to “active” sites on
actin (thin) filament
• myosin heads contain ATPase to break
down ATP
Myosin
filament
Myosin Filament
Thin Filament
Composed of three proteins
• actin - two protein strands twisted around
each other, contain “active sites”
• tropomyosin - thin strand laying in actin
groove that covers active sites
• troponin - attached to actin and
tropomyosin strands; has strong affinity
for Ca2+
Thin Filament
Cytoskeleton (structural) proteins
• M-band – located in middle of thick filament;
provides structural support to myosin filaments;
contains creatine kinase (CK)
• Titan –connects myosin filament to Z-disk;
stabilizes myosin in middle of sarcomere.
• Z-disk –thin filaments attachment; composed of
several cytoskeletal proteins
Actin-myosin
orientation
Transverse Tubule
• in human skeletal muscle, each
sarcomere has two transverse tubules
running perpendicular to fiber
• T-tubules extend through fiber and have
openings at sarcolemma allowing
communication with plasma
• cardiac fibers have only one T-tubule
which lies at Z-line
Sarcoplasmic Reticulum (SR)
• made up of terminal cisternae and
longitudinal tubules
• serves as a storage depot for Ca2+
• terminal cisternae abut T-tubules
• longitudinal tubules cover myofibrils and
connect terminal cisternae
1. On what component does Ca2+ bind to?
a.
b.
c.
d.
Sarcoplasmic reticulum
Myosin heads
Troponin
Tropomyosin
2. What protein returns Ca2+ to the sarcoplasmic
reticulum?
a.
b.
c.
d.
Myosin head
Ca2+ pump
Ca2+ channels
tropomyosin
Review questions
1. Describe the myosin filament of a skeletal
muscle fiber. Include a detailed description
and function of the myosin head.
2. Describe the thin filament of a skeletal muscle
fiber.
3. Describe the cytoskeleton proteins and their
functions in the sarcomere.
4. Describe the sarcoplasmic reticulum and its
role in excitation-contraction.
Excitation-Contraction
How muscle contracts
Excitation-Contraction Coupling
• action potentials, generated at neuromuscular
junction travel around sarcolemma and
through T-tubules
• T-tubules signal SR to release Ca2+ into
sarcoplasm (cytosol)
• Ca2+ saturates troponin (in non-fatigued state)
• troponin undergoes conformational change
that lifts tropomyosin away from actin filament
E-C Coupling (cont.)
• myosin head attaches to active site on actin
filament
• after attaching to actin, myosin head moves actinmyosin complex forward and releases ADP and Pi
• ATP binds with myosin head, which releases actin,
and returns to original position
• in resting state, myosin head contains partially
hydrolyzed ATP (ADP and Pi)
E-C Coupling Schematic
E-C Coupling (cont.)
• entire cycle takes ~50 ms although myosin
heads are attached for ~2 ms
• a single cross-bridge produces 3-4 pN and
shortens 10 nm
• as long as action potentials continue, Ca2+
will continue to be released
• when action potentials cease, SR Ca2+
pumps return Ca2+ ceasing contractions
• skeletal motor units follow “all or nothing”
principle
Excitation-Contraction
1. AP causes vesicles to
release Ach
2. Muscle AP travels down ttubules
3. SR releases Ca2+ into
sarcoplasm
4. Ca2+ binds to troponin
5. Myosin heads bind to actin;
mysoin ATPase splits ATP
6. ATP binds to myosin heat;
releases from actin
7. Crossbridge action continues
while Ca2+ is present
8. When AP stops, Ca2+
pumped back to SR
9. Tropomyosin covers active
sites
EC Coupling
QuickTime Movie of sliding filaments
• http://www.sci.sdsu.edu/movies/actin_myosin.html
• Click on Link
• Click on Actin Myosin Crossbridge 3D Animation
3. What will happen if ATP is depleted in muscle?
a.
b.
c.
Nothing
Muscle will relax
Muscle will not relax
4. What will happen if sarcoplasmic reticulum of
fiber is enhanced?
a.
b.
c.
d.
Fiber will develop tension more quickly
Fiber will relax more quickly
Nothing
Both a and b will occur
Review questions
1. Discuss the signaling process of the T-tubules
that leads to Ca2+ release by the sarcoplasmic
reticulum.
2. Describe ATP hydrolysis by the myosin
filament.
3. Discuss factors that could affect the rate of ATP
hydrolysis by the myosin head as well as
factors that affect tension development.
Skeletal Muscle Fiber Types
• generally categorized by histochemical criteria
• innervating nerve is primary determinant of fiber
type
• motor units composed of homogenous fibers
• all human muscles contain mixture of three
general fiber types
– slow twitch (ST, oxidative, red, Type I)
– fast twitch (FTa, fast-oxidative, white, Type IIa)
– fast twitch (FTb, glycolytic, white, Type IIx [often called
IIb])
stained for myosin ATPase (pH = 10.3)
(dark stained)
Type I
Type IIa
stained for myosin ATPase (pH = 4.3) (light
stained)
stained for SDH
(dark stained)
Type IIx
Muscle Twitch Characteristics
 frontalis/orbicularis oculi
(15% ST)
 first dorsal interosseous
(57% ST)
 soleus (80% ST)
 extensor digitorum brevis
(60% ST)
Fiber Type Characteristics
Performance characteristics affected by:
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size of motoneuron
size of muscle fibers
amount of SR
Ca2+-ATPase
myosin ATPase
aerobic capacity (amount of mitochondria)
anaerobic capacity (amount of glycolytic
enzymes)
Be able to explain the differences in the force responses
between motor units.
5. Which fiber reaches peak tension most quickly?
a.
b.
c.
Type I
Type IIa
Type IIx
6. What is the reasoning for your response to Q5?
a.
b.
c.
d.
e.
faster myosin ATPase
more Ca2+ channels
more Ca2+ pumps
faster action potentials
none of the above are correct
Exam 1 – Thu, Feb 8
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Begin preparing for exam NOW!
Use posted learning objectives as basis for studying
Read text to clarify material
Initially, study by self, then study with classmates
“Teach” each other course material; question
accuracy/completeness of other’s explanations
• See me if questions remain
• You may start the exam at 7:45 am
– Bring the medium-sized RED scoring sheet (sheet that enables
you to bubble in your name)
Motor Unit
Recruitment
Pattern – Size
Principle
Muscle Movements
• isotonic – develops tension while changing
length
• isokinetic – resistance to muscle changes with
muscle length to ensure equal tension
development
• isometric (static) – develops tension but no
length change
• concentric – develops tension while shortening
• eccentric – develops tension while lengthening
Muscle Performance Characteristics
Force and power development dependent on:
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number of muscle fibers recruited
muscle architecture
angle of pull
length of fiber
velocity of shortening
load place on muscle
Length-Tension Relationship
Length-Tension Relationship
How sarcomere
length affects force
output
This explains the
length-tension
relationship
At which length would force output by the
biceps muscle be greatest?
a.
b.
c.
d.
When the arm is in full extension
When the arm is flexed at 90-100º
When the arm is at full flexion
Strength (force) would be the same throughout the
entire range of motion
Force-Velocity
Relationship
How would the EMG activity to a leg squat during
the lowering (eccentric) phase compare to the
upward (concentric) phase.
a. EMG activity would be the same for both phases.
b. EMG activity would be greater for the concentric phase.
c. EMG activity would be greater for the eccentric phase.
EMG comparison of concentric
and eccentric actions
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Muscle Spindles (sensitive to stretch)
Golgi
tendon
organs
(sensitive to
strain)
Resistance Training Adaptations
• dependent on neural and physiological
adaptations
• training specificity determines adaptations
Strength
Training
Adaptations
Neural Adaptations
• increased motor unit recruitment
• decreased neural inhibition of motor unit
recruitment
• decreased antagonist muscle recruitment
• increased neural coordination
Muscle Fiber Adaptations
• increased fiber size (both types)
– increased hypertrophy (1º)
– increased hyperplasia (2º)
– occurs more to FT fibers than ST
• little or no change of fiber types
• testosterone explains only part of larger
muscle mass in males
How does
1. an untrained individual increase strength?
2. a trained individual further increase
strength?
a. neuromuscular adaptations
b. hypertrophy
c. both neuromuscular adaptations and hypertrophy
Exercise-Induced Muscle Damage
and Soreness
Unaccustomed exercise stimulates sequence
of events that:
• diminishes performance
• causes ultrastructure damage
• initiates inflammatory reaction
• causes delayed-onset muscular soreness
(DOMS)
Muscle Damage/Repair Overview
• damage occurs during lengthening
(eccentric) movements
• damage commonly occurs to sarcolemma, Zdisk (streaming), T-tubules/SR, myofibrils,
cytoskeleton
• initial muscle damage followed by
inflammatory-induced damage
• produces muscle swelling
• affects FT fibers more than ST fibers
• repair begins ~3 d post-exercise
Z-line streaming
Muscle Fiber
Damage –
Sarcolemma damage
Exercise-Induced Muscle Damage
• extent of injury more related to length than
force or velocity
• weaker fibers become overstretched, which
become damaged (Morgan, 1990)
Stages of Muscle Damage
1. During exercise:
Mechanical (strain) damage results in:
– sarcolemma damage
– SR damage
– myofibrillar damage
– Ca2+ influx
2. After exercise:
Inflammatory response
causes:
Effects of Elevated intracellular Ca2+
• activates proteases
– damages cytoskeleton
proteins
• activates
phospholipases
– generates free radicals
– damages plasma
membranes
Acute Phase Response
Promotes clearance of damaged tissue and
initiates repair
•  circulating neutrophils (w/in 1-12 h) and
monocytes (w/in 1-3 d)
– enters injury site and phagocytizes damaged
tissue
– release cytotoxic factors (e.g., oxygen
radicals)
Typical Times of Peak Effects
• Ultrastructural damage  3-d postexercise
• DOMS  1-2 d postexercise
CK from 60-min Downhill Running
1000
CK (IU)
800
600
400
200
0
pre
0
24
48
72
Postexercise sampling time (h)
Kolkhorst, unpublished observations
Effects on Performance/Soreness
• greater damage to FT fibers
• prolonged strength loss
– primary cause  failure of SR-Ca2+ release
– ultrastructure damage secondary cause of
strength loss
• muscle swelling/DOMS
– DOMS caused by tissue breakdown
products that sensitize pain receptors
Muscle Repair
• macrophage infiltration required for activation
of satellite cells
• satellite cells located between basement
membrane and plasma membrane
• in response to signal from injury site, satellite
cells migrate to injury
• differentiate into myoblasts, which fuse into
myotubes
Muscle repair
Immediately after crush injury
2 days
• At 2 d, damaged fibers have
undergone necrosis, with
digestion/removal by
macrophages.
• At 5 d, several newly formed
myotubes are visible.
5 days
• At 10 d, myotubes have
transformed into fibers, many of
which have linked up with fibers
stumps on either side.
Which type of activity would likely cause the
most severe DOMS or muscle damage?
a. level running (involves about half concentric
and half eccentric movements)
b. rowing exercise (involves mostly pulling
motion, a concentric movement)
c. running down stadium stairs (involves more
eccentric than concentric movements)
d. cycling (entirely concentric movements)
e. none of the above would cause DOMS
Eccentric exercise
a. causes the greatest damage at the shortest
muscle lengths.
b. causes the greatest damage to ST fibers.
c. initiates an inflammatory response that causes
further myofibril damage.
d. stimulates macrophage infiltration to the
damaged area, which is essential for muscle
repair.
e. both c and d are correct
The greater the load placed on a muscle during a
shortening movement, the _____ it can
shorten. This illustrates the _____ relationship
of skeletal muscle mechanics.
a. slower; power-load
b. slower; length-tension
c. faster; length-tension
d. slower; force-velocity
e. faster; force-tension
According to the Force-Velocity relationship, how
does force output of a fiber when shortening
compare to when it is forced to lengthen?
a.
b.
c.
force output is greater when it is allowed to shorten
force output is equal regardless of shortening or
lengthening
force output is less when it is allowed to shorten