Chapter 10:
Muscle Tissue
Muscle Tissue
• A primary tissue type, divided into:
• Cardiac muscle
• Smooth muscle
• Skeletal muscle
• Attached to bones
• Allows us to move
• Contains CT, nerves and blood vessels
Functions of Skeletal Muscles
1.
2.
3.
4.
5.
CT Organization – 3 layers
1.



Epimysium
Surrounds entire muscle
Separates muscle from
surrounding tissues
Connected to deep fascia
1. Perimysium
 Divides the skeletal
muscle into a series of
compartments
 Each compartment
contains a bundle of
muscle fibers:
1. Endomysium
 Surrounds individual
skeletal muscle fibers
 Interconnects adjacent
muscle fibers
 Satellite Cells -
At the end of a muscle:
 All 3 layers come together
to form a:
 Both attach skeletal
muscles to bones
 Tendon fibers extend into
the bone matrix
Microanatomy of Skeletal Muscle Fibers
•
•
•
•
Skeletal muscle cells are called fibers
Enormous
Multinucleate
Myoblasts fuse during development to form individual
skeletal muscle fibers
Microanatomy of Skeletal Muscle Fibers
• Sarcolemma – cell membrane of
muscle fiber
▫ Surround sarcoplasm
▫ Change in the transmembrane
potential is the start of a
contraction
• Transverse Tubules –
continuous with sarcolemma
and extends into the sarcoplasm
▫ form passageways through
muscle fibers
▫ Filled with extracellular fluid
▫ Action potentials
Microanatomy of Skeletal Muscle Fibers
• Myofibrils – cylindrical
structures encircled by T tubules
▫ As long as the cell
▫ Made of myofilaments
 Thin filaments - actin
 Thick filaments – myosin
▫ Responsible for muscle fiber
contraction
▫ Mitochondria and glycogen
Microanatomy of Skeletal Muscle Fibers
• Sarcoplasmic Reticulum – similar to
ER of other cells
▫ Forms network around each
myofibril
• Terminal cisternae – expanded
chambers of SR on either side of a T
tubule
▫ Ca+2 ions storage
• Triad – pair of terminal cisternae
plus a T tubule
▫ Separate fluids
Microanatomy of Skeletal Muscle Fibers
• Sarcomere – repeating contractile units that make up myofibrils
▫ Smallest functional unit in muscle fibers
▫ Muscle contraction
▫ Made up of: thick and thin filaments, stabilizing proteins and
regulating proteins
▫ Striated
Microanatomy of Skeletal Muscle Fibers
• A bands – dark bands at center
of sarcomere
▫ Thick filaments (myosin)
▫ Contains:
 M line – center of A band,
connects each thick filament
together
 H zone – lighter region on
either side of M line, contains
thick filaments
 Zone of overlap – thick and
thin filaments overlap one
another
Microanatomy of Skeletal Muscle Fibers
• I bands – light bands on both
sides of A band
▫ Thin filaments (actin)
▫ Contains:
 Z lines – boundary between
adjacent sarcomeres
 Titin – protein that aligns
thick and thin filaments
▫ Extends from thick
filaments
Level 1: Skeletal Muscle
Level 4: Myofibril
Level 2: Muscle Fascicle
Level 5: Sarcomere
Level 3: Muscle Fiber
Muscle Contraction
• Sliding Filament Theory
▫ Caused by interactions of thick and thin filaments
▫ Triggered by free Ca2+ in sarcoplasm
Muscle Contraction
• Thin Filaments – made of 4 proteins:
▫ F actin – 2 twisted strands of G actin, contain active sites for the
binding of myosin
▫ Nebulin – holds 2 strands of G actin together
▫ Tropomyosin – covers G actin active sites to prevent actin/myosin
interactions
▫ Troponin – holds tropomyosin to G actin AND contains a site for the
binding of Ca2+
 Holds until Ca2+ binds to the active site
 Contraction can only occur if position changes
Muscle Contraction
• Thick Filaments – consist of a pair of myosin subunits wrapped
around each other
▫ Tail – binds to other myosin molecules
▫ Head – 2 subunits, project towards nearest thin filament
▫ During muscle contractions myosin heads pivot towards thin
filaments, forming cross-bridges with G actin active sites
Muscle Contraction
• Sliding Filament Theory
▫ Thin filaments slide towards
M line – shortening
▫ A band remains the same,
but the Z lines move closer
together
Muscle Contraction
• Neuromuscular Junction - NMJ
▫ Where the action potential starts
▫ Each branch ends at a synaptic terminal, which contains mitochondria
and Acetylcholine
 Neurotransmitter that alters the permeability of the sarcolemma
Muscle Contraction
• Synaptic cleft –
• Motor end plate –
▫ Both contain AChE – breaks down
Ach
• Action potential travels along the nerve
axon and ends at the synaptic terminal,
which changes the permeability
• ACh is released
Muscle Contraction
• ACh diffuses across the synaptic cleft and
binds to ACh receptors on motor end plate
• Increase in membrane permeability to
sodium ions that rush into the sarcoplasm
▫ Keeps going until AChE removes all ACh
• Travels along sarcolemma to T tubules and
leads to excitation-contraction coupling ▫ Action potential leads to contraction
▫ Triads release Ca2+
▫ Triggers muscle contractions
Muscle Contraction at Sarcomere
1. Exposure of active sites
▫ Calcium ions bind to troponin,
changing its position and
shifting tropomyosin away
from active sites
2. Attachment of cross-bridges
▫ Myosin heads bind to active
sites
Muscle Contraction at Sarcomere
3. Pivoting
▫ Power stroke
4. Detachment of cross-bridges
▫ ATP binds to myosin head, link is
broken
▫ Attach to another active site
Muscle Contraction at Sarcomere
5. Reactivation of myosin
Muscle
Contraction at
Sarcomere
▫
▫
ATP to ADP and phosphate
Cycle is repeated
•
All sarcomeres contract at the
same time
•
Contraction duration depends
on:
▫
▫
▫
Duration of neural stimulus
Amount of free Ca2+ ions in
sarcoplasm
Availability of ATP
Muscle Contraction
• 1. At NMJ, ACh is released and binds to
receptors on sarcolemma
• 2. Change in transmembrane potential
results in action potential that spreads
across entire surface of cell and T tubules
• 3. SR releases stored calcium ions,
increasing Ca2+ around sarcomeres
• 4. Calcium ions bind to troponin, which
exposes active sites on thin filaments and
cross-bridges form
• 5. Contraction begins as repeated cycles
of cross-bridge formation and
detachment happen
Muscle Contraction
• 6. ACh is broken down by AChE and
action potential ends
• 7. SR reabsorbs calcium ions and
concentration in sarcoplasm decreases
• 8. Active sites are re-covered
• 9. Contraction ends
• 10. Muscle relaxation – sarcomeres
remain uncontracted
Rigor Mortis
• Stop in blood circulation causes skeletal muscles to be deprived
of oxygen and nutrients –
• SR becomes unable to pump calcium ions out of sarcoplasm
• Extra calcium ions trigger a sustained contraction
▫ Cross-bridges form, but cannot detach
• Lasts 15-25 hours after death
2 Types of Muscle Tension
• Isotonic Contraction
▫ Skeletal muscle changes length resulting in motion
▫ If muscle tension > resistance: muscle shortens (concentric contraction)
▫ If muscle tension < resistance: muscle lengthens (eccentric contraction)
2 Types of Muscle Contraction
• Isometric Contraction
▫ Muscle develops tension, but does not shorten
Resistance and Speed of Contraction
• Inversely related
• The heavier the resistance on a muscle:
▫ the longer it takes for shortening to begin
▫ the less the muscle will shorten
Muscle Relaxation
• After contraction, a muscle fiber returns to resting length by:
▫ Elastic forces
 The pull of elastic elements (tendons and ligaments)
 Expands the sarcomeres to resting length
▫ Opposing muscle contractions
 Reverse the direction of the original motion
 The work of opposing skeletal muscle pairs
▫ Gravity
 Can take the place of opposing muscle contraction to return a
muscle to its resting state
ATP and Muscle Contraction
• Muscle contraction uses a lot of ATP
• Muscles store enough energy to start contraction, but must
manufacture more ATP
▫ Generates ATP at the same rate that it is used
• ATP and CP
▫ ATP – active energy model (aerobic and anaerobic)
▫ Creatine Phosphate (CP) – storage molecule for excess ATP in
resting muscle
▫
▫
▫
▫
ATP – 2 seconds
CP – 15 seconds
Glycogen – 130 seconds (anaerobic) and 40 mins (aerobic)
Fats
ATP and Muscle Contraction
• At rest:
▫ Cells use fatty acids to create CP, ATP and glycogen –
rebuilding their storages (beta oxidation)
• Moderate Activity:
▫ Cells use fatty acids or glucose and oxygen to produce
ATP (aerobic respiration)
 Muscle wont fatigue until all energy is used up
 Marathon runners
• Peak Activity
▫ Cells use oxygen faster than it is supplied
 Aerobic resp only provides 1/3 of needed ATP
 Anaerobic resp provides the rest – lactic acid
Muscle Metabolism
Muscle Fatigue
• When muscles can no longer perform a required activity, they
are fatigued
• Results of Muscle Fatigue:
Depletion of metabolic reserves
Damage to sarcolemma and SR
Low pH (lactic acid)
Muscle exhaustion and pain
• The Recovery Period



The time required after exertion for muscles to return to normal
Oxygen becomes available
Mitochondrial activity resumes
Muscle Fatigue
• The Cori Cycle



The removal and recycling of lactic acid by the liver
Liver converts lactic acid to pyruvic acid
Glucose is released to recharge muscle glycogen reserves
Oxygen Debt – after exercise:
Body needs more oxygen than usual to normalize metabolic activity
Heavy breathing
3 Types of Skeletal Fibers
• Fast Fibers:
▫
▫
▫
▫
▫
Contract quickly
High CP
Large diameter, huge glycogen reserves and few mitochondria
Strong contractions, but fatigue quickly
White meat – chicken breast
• Slow Fibers
▫
▫
▫
▫
▫
▫
Slow to contract and slow to fatigue
Low CP
Small diameter, but a lot of mitochondria
High oxygen supply
Contain myoglobin (red pigment, binds to oxygen)
Dark meat – chicken legs
3 Types of Muscle Fibers
• Intermediate Fibers
▫
▫
▫
▫
▫
Mid-sized
Low myoglobin
More capillaries than fast fibers, slower to fatigue
Table 10-3, page 298
Human Muscles
• Muscle Hypertrophy - muscle
Growth from heavy training
▫ increases diameter of muscle
fibers
▫ increases number of myofibrils
▫ increases mitochondria,
glycogen reserves
• Muscle Atrophy – lack of
muscle activity
▫ Reduced in muscle size, tone
and power
Physical Conditioning
• Anaerobic Endurance
▫ Uses fast fibers, fatigues quickly with strenuous activities
 50 m dash, weightlifting
▫ Improved by frequent, brief, intensive workouts – interval
training
• Aerobic Endurance – supported by mitochondria
▫ Prolonged activity – uses a lot of oxygen and nutrients
 Marathon running
▫ Improved by repetitive and cardiovascular training
Cardiac Muscle Tissue
• Striated tissue
• Smaller cells with single nucleus
• Short T-tubules and sarcoplasm
▫ No triads or terminal cisternae
• All aerobic
▫ High in myoglobin and
mitochondria
• Intercalated discs
Smooth Muscle
• Blood vessels, reproductive and digestive systems, etc
• Different arrangement of actin and myosin
• Non-striated
Characteristics of Skeletal, Cardiac, and
Smooth Muscle