Chapter 10: An Introduction to Muscle Tissue
Learning Outcomes
• 10-1 List and describe the functions of skeletal muscle tissue.
• 10-2 Describe the organization of muscle at the tissue level. Identify and describe the three
layers of connective tissues.
• 10-3 Identify the structures of skeletal muscles fibers along with the function of each. Identify
the structural components of a sarcomere and the f(x) of each.
• 10-4 Identify the components of the neuromuscular junction, and summarize the events
involved in the neural control of skeletal muscle contraction and relaxation. Know the steps of
muscle contraction in order (neuron dance)
• 10-6 Describe the mechanisms by which muscle fibers produce ATP to power contractions
(aerobic vs. anaerobic). Describe how muscle fibers store energy. Describe how muscles
become fatigued and what occurs during the recovery period in terms of lactic acid removal and
oxygen debt. Describe DOMS and explain what may cause DOMS after exercise. Describe the
Cori cycle.
• Discuss the role of hormones in muscle metabolism.
• 10-7 Relate the types of muscle fibers to muscle performance, and compare aerobic and
anaerobic endurance. Describe how exercise/lack of exercise influences muscle fibers. Describe
what can cause muscle atrophy or death of muscle fibers. Describe hypertrophy and atrophy.
Discuss the role of hormones in muscle metabolism.
• 10-8/10-9 Compare and contrast skeletal, cardiac and smooth muscle
• Be able to answer questions similar to clinical applications completed in class.
Chapter 10: An Introduction to Muscle Tissue
I.
An Introduction to Muscle Tissue
A. Muscle Tissue is:
1. A primary tissue type, divided into 3 groups:
a. skeletal muscle
b. smooth muscle
c. cardiac muscle
B. Muscular system = skeletal muscle only + CT, nerves and blood vessels
II.
10-1 Functions of Skeletal Muscle Tissue
A. Produce skeletal movement: pull on tendons and moves bones of skeleton
B. Maintain posture and body position: constant tension in skeletal muscles maintains
body posture
C. Support organs: support weight of visceral organs and shield internal tissues from injury
D. Guard entrances and exits
1. Encircle openings of digestive and urinary tracts
2. Provides voluntary control over swallowing, defecation and urination
E. Maintain body temperature
1. Muscle contractions use energy, some of which is converted to heat
2. Heat released by working muscles keeps body temperature in normal range
F. Store energy
1. Proteins in skeletal muscles can be broken down, releasing energy
2. Amino acids travel to liver where they can be converted into glucose or broken
down to provide energy
III.
10-2 Organization of Muscle
A. Skeletal Muscle contains:
1. Muscle tissue (muscle cells or muscle fibers)
2. Connective tissues
3. Nerves
4. Blood vessels
B. Organization of Connective Tissues
1. Muscles have three layers of connective tissues
a. Epimysium
i.
Dense layer of collagen fibers surrounding entire muscle
ii.
Separates muscle from nearby tissues/organs
iii.
Connects to deep fascia
Chapter 10: An Introduction to Muscle Tissue
b. Perimysium
i.
Divides muscle into compartments (fascicle)
ii.
Fascicle: bundle of muscle fibers
iii.
Contains collagen/elastic fibers, blood vessels and nerves
c. Endomysium
i.
Surrounds individual skeletal muscle cells (aka muscle
fibers)
ii.
Contains capillary network, myosatellite cells (stem cells)
and nerve fibers
d. At end of muscle, collagen fibers of epimysium, perimysium and
endomysium come together to form
IV.
i.
tendon: bundle
ii.
aponeurosis: broad sheet
10-3 Characteristics of Skeletal Muscle Fibers
A. Skeletal muscle cells differ from typical cell structure in
1. Size: muscle cells are larger/longer
2. Number of nuclei: muscle cells contain hundreds of nuclei
B. Formation of skeletal muscle fibers
1. During embryonic development, cells called myoblasts fuse to form skeletal
muscle fibers
2. More than one cell fusing = multiple nuclei
3. Unfused myoblasts = Myosatellite cells (muscle stem cells)
C. Skeletal Muscle Fiber Structure
1. The sarcolemma (plasma membrane of muscle fiber)
a. has characteristic transmembrane potential
b. Change in transmembrane potential = first step in muscle contraction
c. All regions of cell must contract simultaneously
2. How is signal to contract quickly distributed through cell? Transverse or T
tubules
a. Narrow tubes continuous with sarcolemma
b. Extend into sarcoplasm (cytoplasm of muscle fiber)
c. Electrical impulses conducted by sarcolemma travel along T tubules to
cell interior
d. Action potentials (impulses) trigger muscle fiber contraction
Chapter 10: An Introduction to Muscle Tissue
3. Myofibrils
a. Protein filaments
b. Bundle of myofilaments make up myofibrils
c. Function: shorten to cause contractions
d. Both thick and thin filaments
i.
Thick filaments composed mainly of protein myosin
ii.
Thin filaments composed mainly of protein actin
4. Sarcoplasmic Reticulum (SR)
a. Membranous structure surrounding each myofibril
b. Helps transmit action potential to myofibril
c. Similar in structure to smooth ER
d. Terminal cisternae
i.
ii.
Fusion of SR tubules on either side of T tubules
Function: concentration calcium ions and release into
sarcomeres to begin muscle contraction
e. triad: 2 terminal cisternae + 1 T tubule
5. sarcomere
a. Organization of thin/thick myofilaments into repeating functional units.
b. Interactions between thin and thick fibers responsible for muscle
contraction
c. A band
i.
ii.
iii.
Dark bands
Composed of thick and thin filaments
Three subdivisions
 M line: connects thick filaments to help stabilize their
position
 H band: regions on either side of M line
 Zone of overlap: thin and thick filaments overlap
d. I band
i.
Composed only of thin filaments
ii.
Z lines: mark boundary between sarcomeres
iii.
titin: strands of elastic protein
 Keep filaments in proper alignment
 Aids in restoring resting sarcomere length after
contraction
Chapter 10: An Introduction to Muscle Tissue
6. Structure of Thin Filaments
a. Contains four proteins
i.
filamentous actin (F-actin): twisted strand of G-actin (globular
actin)
ii.
nebulin: holds F-actin strands together
iii.
tropomyosin: covers active sites on G-actin so myosin cannot
bind
iv.
troponin
 Has Ca ion receptors
 Rotates tropomyosin away from active sites
7. Structure of Thick Filaments
a. Each contains:
i.
300 myosin molecules
ii.
titin (elastic myofilament)
b. Tail: Binds to other myosin molecules
c. Head
V.
i.
Made of two globular protein subunits
ii.
Head pivots to interact/bind with thin filament = cross-bridge
8. Sliding Filament Theory
a. During contraction, thin filaments slide toward center of sarcomere
alongside thick filaments
b. Results in myofibril getting shorter = muscle fiber getting shorter
10-4 Skeletal Muscle Contraction
A. Communication between nervous system and skeletal fiber occurs at the
Neuromuscular Junction (NMJ)
B. At NMJ synaptic terminal of neuron lies near motor end plate of muscle fiber
C. Synaptic terminal of neuron has vesicle filled with acetylcholine (ACh)
1. ACh is a neurotransmitter released by neuron to change permeability of another
cell’s plasma membrane
2. Enzyme acetylcholinesterase (AChE) also present which breaks down ACh
Chapter 10: An Introduction to Muscle Tissue
D. Steps of Skeletal Muscle Contraction
1. Action potential reaches synaptic terminal
2. Ca2+ channels open allowing Ca2+ to move into synaptic terminal
3. Permeability changes in the membrane due to Ca2+ entry triggers the
exocytosis of acetylcholine into the synaptic cleft
4. ACh molecules diffuse across synaptic cleft and bind to ACh receptors on motor
end plate
5. Binding of ACh alters membrane permeability to Na+, allowing these ions to
rush into the sarcoplasm while K+ leave, changing the transmembrane potential
6. Sudden influx of Na+ generates action potential in sarcolemma. ACh is broken
down and sarcolemma receptors are deactivated
7. Action potential reaches triad via T tubules and triggers release of Ca2+ from
terminal cisternae of SR
8. Ca2+ binds to troponin which changes shape of troponin and position of
tropomyosin to unlock active site on actin where myosin head will bind.
9. ATP supplies myosin with energy to bind with exposed active site on actin
forming cross-bridge
10. Energy is release causing myosin head to pivot and pull thin filaments toward
M line
11. New ATP binds to myosin which releases head from active site on actin
VI.
12. Myosin now reactivated and process is repeated.
10-4 Skeletal Muscle Relaxation
A. Contraction Duration depends on:
1. Duration of neural stimulus
2. Number of free Ca2+ ions in sarcoplasm
3. Availability of ATP
B. Steps in muscle relaxation
1. ACh broken down by AChE
2. Terminal cisternae recaptures Ca2+
3. Active sites on actin are again covered by tropomyosin
4. Contraction ends
5. Relaxation occurs returning sarcomere to resting position
Chapter 10: An Introduction to Muscle Tissue
VII.
10-6 Energy to Power Contractions
A. ATP Provides Energy For Muscle Contraction
1. Sustained muscle contraction uses a lot of ATP energy
2. Muscles store enough energy to start contraction
3. Muscle fibers must manufacture more ATP as needed
B. How is energy stored by muscle fibers?
1. Creatine phosphate (CP): the storage molecule for excess ATP energy in resting
muscle
ATP + creatine  ADP + creatine phosphate
2. If ATP is needed the following reaction occurs to release the energy stored in
creatine phosphate
ADP + creatine phosphate ATP + creatine
3. Creatine kinase (CK) facilitates creatine reactions
C. How is ATP generated by muscle fibers? Cells produce ATP in two ways
1. aerobic metabolism of fatty acids in the mitochondria
a. Used by resting skeletal muscle fibers
b. When muscle begin contracting glucose is used instead of fatty acids
c. Provides 95% of ATP demands of resting cell
d. Requires oxygen
2. anaerobic glycolysis in the cytoplasm
a. Does not require oxygen
b. Occurs when oxygen supply is limited and energy demands are high
c. Produces lactic acid as a byproduct which can lower pH within cell
d. Eventually, muscle fiber will no longer be able to contract due to low pH
D. When muscles can no longer perform a required activity, they are fatigued
1. Results of Muscle Fatigue include
a. Depletion of metabolic reserves
b. Damage to sarcolemma and sarcoplasmic reticulum
c. Low pH = decrease in Ca2+ binding to troponin
d. All leads to muscle exhaustion and pain
Chapter 10: An Introduction to Muscle Tissue
2. DOMS: Delayed-onset muscle soreness
a. Soreness experienced after exercise
b. Increased levels of CK and myoglobin indicate damage to sarcolemma
c. Three possible explanations for DOMS:
i.
Damage to sarcolemma = release of CK and myoglobin =
stimulate pain receptors
ii.
Muscle spasms = pain (alleviate by stretching)
iii.
Tears in CT/tendons = pain
E. The Recovery Period: time required after exertion for muscles to return to normal
1. The Cori Cycle
a. Removal/recycling of lactic acid by liver
b. Liver converts lactate to pyruvate
c. Glucose released to recharge muscle glycogen reserves
2. The Oxygen Debt
a. After exercise or other exertion body needs more oxygen
b. Resulting in heavy breathing
c. Also called excess post exercise oxygen consumption (EPOC)
F. Hormones and Muscle Metabolism
1. Growth hormone and testosterone: stimulate synthesis of contractile proteins
and enlargement of skeletal muscle
2. thyroid hormones: increase rate of energy consumption
3. epinephrine: stimulate muscle metabolism and increase duration of stimulation
and force of contraction
VII.
10-7Muscle performance depend on fiber type and physical conditioning
A. Types of Skeletal Muscle Fibers
1. Fast fibers
a. Contract quickly and fatigue quickly
b. Large diameter with few mitochondria
c. Muscles dominated by fast fibers appear pale; “white meat”
2. Slow fibers
a. Contract slowly and fatigue slowly
b. Small diameter with many mitochondria
c. Also contain myoglobin which can bind oxygen and hold oxygen reserves
d. Muscles dominated by slow fibers appear dark reddish color; “dark meat”
3. Intermediate fibers
Chapter 10: An Introduction to Muscle Tissue
B. Types of Muscles Fibers and Endurance
1. Hypertrophy
a. Muscle growth from heavy training
b. Increases:
i.
diameter of muscle fibers
ii.
number of myofibrils
iii.
Mitochondria & glycogen reserves
2. Atrophy
a. Lack of muscle activity
b. Reduces muscle size, tone, and power
c. Can occur after individual has worn a cast, suffered paralysis, loss of signal from
nerve cells (ALS)
d. Extreme cases = muscle cell death and loss of function
3. Physical Conditioning improves both power and endurance
a. Anaerobic activities
i.
e.g. 50m dash, weightlifting
ii.
Use fast fibers = quick fatigue
iii.
Improved by frequent, brief, intensive workouts
iv.
Causes hypertrophy
b. Aerobic activities
i.
e.g. marathon
ii.
Increases:

Endurance by training fast fibers to be more like intermediate
fibers

Cardiovascular performance
Chapter 10: An Introduction to Muscle Tissue
VIII. Comparison of Muscle Tissue
Property
Skeletal
Cardiac
Smooth
Cell size
Large
Small
Smallest
Nuclei
Multinucleate
Generally singe
Single
Filament
Organization
Sarcomeres
Sarcomeres
Scattered throughout sarcoplasm
SR
Triads
No triads
No T tubules
Control
Voluntary via nervous
system
Involuntary via
pacemaker cells
Involuntary via pacesetter cells
and nervous system/endocrine
system
Ca2+ source
Release from SR
SR and extracellular
fluid
SR and extracellular fluid
Ca2+ regulation
Troponin
Troponin
Calmodulin on myosin heads
Contraction
Rapid onset; rapid
fatigue
Slow onset; slow to
fatigue
Slow onset; resistant to fatigue
Other properties
Striated
Striated; contains
intercalated discs
No striations
Location
Muscular system
Heart
Walls of BVs, digestive,
reproductive, respiratory and
urinary organs
Energy source
Aerobic/anaerobic
Aerobic only
Primarily aerobic