Download Muscle Tissue

Chapter 9
Muscles and Muscle Tissue
J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.
Some Muscle Terminology
Myology: the scientific study of muscle
muscle fibers = muscle cells
myo, mys & sarco: word roots referring to muscle
Three Types of Muscle
Skeletal, cardiac, and smooth muscle differ in:
 Microscopic anatomy
 Location
 Regulation by the endocrine system and the
nervous system
Characteristics of Skeletal Muscle
• Attached primarily to
bones
• Voluntary (conscious)
control (usually)
• Contracts quickly, tires
easily (fatigable)
• Allows for wide range
of forces to be generated
Skeletal
Muscle Cells
• Long, cylindrical cells
• Striated (banded)
• Multinucleate
Characteristics of Cardiac Muscle
• Forms most of heart wall
(myocardium)
• Involuntary (unconscious)
• Autorhythmicity (contracts without
external stimuli)
• Fast contraction, non-fatigable
• Beats at constant rhythm that can be
modified by neural and hormonal
signals
Cardiac Muscle
Cells
• Branched cells
• Uninucleate (may
occasionally be
binucleate)
• Striated
• Intercalated discs
Characteristics of Smooth Muscle
• Found in the walls of hollow
internal structures (digestive,
respiratory, reproductive
tracts, blood vessels)
• Arrector pili, pupil of the
eye, etc.
• Involuntary (unconscious)
• Long, slow contractions,
non-fatigable
Smooth Muscle
Cells
• Nonstriated =
smooth
• Uninucleate
Functions of Muscle Tissue
• Motion: external (walking, running, talking, looking)
and internal (heartbeat, blood pressure, digestion,
elimination) body part movements
• Posture: maintain body posture
• Stabilization: stabilize joints – muscles have tone even
at rest
• Thermogenesis: generating heat by normal contractions
and by shivering
Functional Characteristics
• Excitability (irritability)
– the ability to receive and respond to a stimulus (chemical
signal molecules)
• Contractility
– ability of muscle tissue to shorten
• Extensibility
– the ability to be stretched without damage
– most muscles are arranged in functionally opposing pairs – as
one contracts, the other relaxes, which permits the relaxing
muscle to be stretched back
• Elasticity
– the ability to return to its original shape
• Conductivity (impulse transmission)
– the ability to conduct excitation over length of muscle
Connective Tissue Wrappings of Skeletal
Muscle Tissue
• Superficial Fascia:
"hypodermis"
• Deep Fascia: lines body walls
& extremities; binds muscle
together, separating them into
functional groups
• Epimysium: wraps an entire
muscle
• Perimysium: subdivides each
muscle into fascicles, bundles
of 10-100 muscle fibers
• Endomysium: wraps
individual muscle fibers
Nerve and Blood Supply
• Each muscle fiber is supplied by a branch
of a motor nerve
• Each muscle is supplied by its own
arteries and veins
• Blood vessels branch profusely to provide
each muscle fiber with a direct blood
supply
Attachments (to bone)
• Origin: the part of a muscle attached to the stationary
bone (relative to a particular motion)
• Insertion: the part of a muscle attached to the bone that
moves (relative to a particular motion)
• Attachments are extensions of connective tissue sheaths
beyond a muscle, attaching it to other structures
• Direct attachment: epimysium fused to periosteum
Attachment Structure
• Indirect attachment: connective tissue wrappings
gathered into a tendon or aponeurosis which
attaches to an origin or insertion on bone
– Tendon: cord (of dense regular connective tissue)
– Aponeurosis: sheet (of dense regular connective
tissue)
Microscopic Anatomy of A Skeletal
Muscle Fiber
• Muscle fibers (cells): long,
cylindrical, and
multinucleate (individual
muscle cells fuse during
embryonic development)
• Sarcolemma: the cell
membrane of a muscle fiber
• Sarcoplasm: the cytoplasm
of a muscle fiber, rich in
oxygen-storing myoglobin
protein
Myofibrils of A Skeletal Muscle
Fiber
• Myofibrils: bundles of
contractile protein
filaments (myofilaments)
arranged in parallel, fill
most of the cytoplasm of
each muscle fiber; 100’s
to 1000’s per cell
• Sarcomeres: the
repeating unit of
contraction in each
myofibril
Organelles of A Skeletal Muscle Fiber
• Mitochondria: provide the
ATP required for contraction
• Sarcoplasmic reticulum
(smooth ER): stores Ca2+ ions
which serve as second
messengers for contraction
Striations/Sarcomeres
• Z discs (lines): the boundary
between sarcomeres; proteins
anchor the thin filaments;
bisects each I band
• A (anisotropic) band:
overlap of thick (myosin)
filaments & thin filaments
• I (isotropic) band: thin
(actin) filaments only
• H zone: thick filaments only
• M line: proteins anchor the
adjacent thick filaments
Myofilaments
• Thin filaments: actin (plus
some tropomyosin &
troponin)
• Thick filaments: myosin
• Elastic filaments: titin
(connectin) attaches myosin
to the Z discs (very high
mol. wt.)
Sarcomeres
• Components of the muscle fiber with myofilaments
arranged into contractile units
• The functional unit of striated muscle contraction
• Produce the visible banding pattern (striations)
• The myofilaments between two successive z discs
Summary of Muscle Structure
Myosin Protein
• Rod-like tail with two heads
• Each head contains ATPase and an actin-binding site; point to
the Z line
• Tails point to the M line
• Splitting ATP releases energy which causes the head to
“ratchet” and pull on actin fibers
Thick (Myosin) Myofilaments
• Each thick filament contains many myosin
units woven together
Thin (Actin) Myofilaments
Two G actin strands are arranged into helical strands
• Each G actin has a binding site for myosin
• Two tropomyosin filaments spiral around the actin
strands
• Troponin regulatory proteins (“switch molecules”) may
bind to actin and tropomyosin & have Ca2+ binding sites
Muscle Fiber Triads
• Triads: 2 terminal cisternae + 1 T tubule
• Sarcoplasmic reticulum (SER): modified smooth ER, stores
Ca2+ ions
• Terminal cisternae: large flattened sacs of the SER
• Transverse (T) tubules: inward folding of the sarcolemma
Regulation of Contraction &
The Neuromuscular Junction
The Neuromuscular Junction:
• Where motor neurons
communicate with the muscle
fibers
• Composed of an axon terminal
& motor end plate
– Axon terminal: end of the motor
neuron’s branches (axon)
– Motor end plate: the specialized
region of the muscle cell plasma
membrane adjacent to the axon
terminal
The Neuromuscular Junction:
• Synapse: point of
communication is a small
gap
• Synaptic cleft: the space
between axon terminal &
motor end plate
• Synaptic vesicles:
membrane-enclosed sacs
in the axon terminals
containing the
neurotransmitter
The Neuromuscular Junction:
• Neurotransmitter: the
chemical that travels
across the synapse, i.e.,
acetylcholine, ACh)
• Acetylcholine (ACh)
receptors: integral
membrane proteins which
bind ACh
Generation of an Action Potential
(Excitation)
• Binding of
neurotransmitter
(ACh) causes the
ligand-gated Na+
channels to open
axonal terminal
• Opening of the Na+
channels depolarizes
the sarcolemma (cell
membrane)
motor end plate
Generation of an Action Potential
• Initial depolarization
causes adjacent voltagegated Na+ channels to
open; Na+ ions flow in,
beginning an action
potential
• Action potential: a
large transient
depolarization of the
membrane potential
– transmitted over the
entire sarcolemma (and
down the T tubules)
Generation of an Action Potential
• Repolarization: the return to polarization due to the
closing voltage-gated Na+ channels and the opening of
voltage gated K+ channels
• Refractory period: the time during membrane
repolarization when the muscle fiber cannot respond
to a new stimulus (a few milliseconds)
• All-or-none response: once an action potential is
initiated it results in a complete contraction of the
muscle cell
Excitation-Contraction Coupling
• The action potential
(excitation) travels over the
sarcolemma, including Ttubules
• DHP receptors serve as
voltage sensors on the Ttubules and cause ryanodine
receptors on the SR to open
and release Ca2+ ions
• And now, for the interactions
between calcium and the
sarcomere…
The Sliding Filament Model of
Muscle Contraction
• Thin and thick
filaments slide past
each other to
shorten each
sarcomere and,
thus, each
myofibril
• The cumulative
effect is to shorten
the muscle
• This simulation of the sliding filament model can also be
viewed on line at the web site below along with additional
information on muscle tissue
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Muscle/Muscle.htm#SKELETAL
Calcium
2+
(Ca )
The “on-off switch”: allows myosin to bind to actin
off
on
Calcium Movements Inside
Muscle Fibers
Action potential causes release of Ca2+ ions (from
the cisternae of the SR)
Ca2+ combines with troponin, causing a change in
the position of tropomyosin, allowing actin to
bind to myosin and be pulled (“slide”)
Ca2+ pumps on the SR remove calcium ions from the
sarcoplasm when the stimulus ends
The Power Stroke & ATP
1. Cross bridge
attachment. myosin
binds to actin
2. The working stroke.
myosin changes shape
(pulls actin toward it);
releases ADP + Pi
3. Cross bridge
detachment. myosin
binds to new ATP;
releases actin
The Power Stroke & ATP
4. "Cocking" of the
myosin head. ATP
hydrolyzed (split) to
ADP + Pi; provides
potential energy for the
next stroke
The “Ratchet Effect”
Repeat steps 1-4:
The “ratchet
action” repeats the
process,
shortening the
sarcomeres and
myofibrils, until
Ca2+ ions are
removed from the
sarcoplasm or the
ATP supply is
exhausted
Attach
Repeat
Power
Stroke
Release
RATCHET EFFECT ANIMATION
http://www.sci.sdsu.edu/movies/actin_myosin_gif.html
Excitation-Contraction Coupling
1.
The action potential
(excitation) travels over the
sarcolemma, including Ttubules
2.
DHP receptors serve as
voltage sensors on the Ttubules and cause ryanodine
receptors on the SR to open
and release Ca2+ ions
3.
Ca2+ binds to troponin,
causing tropomyosin to move
out of its blocking position
4.
Myosin forms cross bridges to
actin, the power stroke
occurs, filaments slide,
muscle shortens
5.
Calsequestrin and
calmodulin help regulate
Ca2+ levels inside muscle
cells
Destruction of Acetylcholine
• Acetylcholinesterase: an enzyme that rapidly breaks
down acetylcholine is located in the neuromuscular
junction
– Prevents continuous excitation (generation of more action
potentials)
• Many drugs and diseases interfere with events in the
neuromuscular junction
– Myasthenia gravis: loss of function at ACh receptors
(autoimmune disease?)
– Curare (poison arrow toxin): binds irreversibly to and
blocks the ACh receptors
MUSCLE CONTRACTION
• One power stroke shortens a muscle about 1%
• Normal muscle contraction shortens a muscle by about
35%
– Cross bridge (ratchet effect) cycle repeats
• continue repeating power strokes, continue pulling
• increasing overlap of fibers; Z lines come together
– About half the myosin molecules are attached at any time
• Cross bridges are maintained until Ca2+ levels decrease
– Ca2+ released in response to action potential delivered by
motor neuron
– Ca2+ ATPase pumps Ca2+ ions back into the SR
RIGOR MORTIS IN DEATH
•
Ca2+ ions leak from SR causing binding of actin and
myosin and some contraction of the muscles
•
Lasts ~24 hours, then enzymatic tissue disintegration
eliminates it in another 12 hours
Skeletal Muscle Motor Units
• The Motor Unit = Motor Neuron + Muscle Fibers to
which it connects (Synapses)
Skeletal Muscle Motor Units
• The size of Motor Units
varies:
– Small - two muscle
fibers/unit (larynx, eyes)
– Large – hundreds to
thousands/unit (biceps,
gastrocnemius, lower back
muscles)
• The individual muscle
cells/fibers of each unit are
spread throughout the
muscle for smooth efficient
operation of the muscle as a
whole
The Myogram
• Myogram: a recording of
muscle contraction
• Stimulus: nerve impulse or
electrical charge
• Twitch: a single contraction
of all the muscle fibers in a
motor unit (one nerve
signal)
Myogram
1.
Latent period: delay between
stimulus and response
2.
Contraction phase: tension
or shortening occurs
3. Relaxation phase: relaxation
or lengthening
Muscle Twitches
All or none rule:
All the muscle
fibers of a motor
unit contract all
the way when
stimulated
Graded Muscle Responses
• Force of muscle contraction varies
depending on need. How much tension is
needed?
• Twitch does not provide much force
• Contraction force can be altered in 3 ways:
1. changing the frequency of stimulation (temporal
summation)
2. changing the stimulus strength (recruitment)
3. changing the muscle’s length
Temporal Summation
• Temporal (wave) summation: contractions repeated before
complete relaxation, leads to progressively stronger contractions
– unfused (incomplete) tetanus: frequency of stimulation allows only
incomplete relaxation
– fused (complete) tetanus: frequency of stimulation allows no relaxation
Treppe: the staircase effect
“warming up” of a muscle fiber
Multiple Motor Unit Summation
( Recruitment)
The stimulation of
more motor units leads
to more forceful
muscle contraction
The Size Principle
As stimulus
intensity increases,
motor units leads
with larger fibers
are recruited
Stretch: Length-Tension Relationship
• Stretch (sarcomere length)
determines the number of
cross bridges
– extensive overlap of actin
with myosin: less tension
– optimal overlap of actin with
myosin: most tension
– reduced overlap of actin with
myosin: less tension
• Optimal overlap: most cross
bridges available for the
power stroke and least
structural interference
more resistance
most cross bridges/least resistance
fewest cross bridges
Stretch: Length-Tension Relationship
Optimal length - Lo
• maximum number of cross bridges
• no overlap of actin fibers from opposite ends of the sarcomere
• normal working muscle range from 70 - 130% of Lo
Contraction of a Skeletal Muscle
• Isometric Contraction: Muscle does not shorten
• Tension increases
Contraction of a Skeletal Muscle
• Isotonic Contraction: tension does not change
• Muscle (length) shortens
Muscle Tone
Regular small contractions caused by spinal
reflexes
Respond to tendon stretch receptor sensory input
Activate different motor units over time
Provide constant tension development
muscles are firm
but no movement
e.g., neck, back and leg muscles
maintain posture
Muscle Metabolism
• Energy availability
–
–
–
–
Not much ATP is available at any given moment
ATP is needed for cross bridges and Ca2+ removal
Maintaining ATP levels is vital for continued activity
Three ways to replenish ATP:
1. Creatine Phosphate energy storage system
2. Anaerobic Glycolysis -- Lactic Acid system
3. Aerobic Respiration
Direct Phosphorylation –
Creatine Phosphate System
• CrP stored in cell
• Allows for rapid
ATP replenishment
• Only a small amount
available (10-30
seconds worth)
Anaerobic Glycolysis –
Lactic Acid System
• Anaerobic system - no
O2 required
• Very inefficient, does
not create much ATP
• Only useful in short
term situations (30 sec
- 1 min)
• Produces lactic acid as
a by-product
Aerobic System
- Uses oxygen for ATP
production
- Oxygen comes from the
RBCs in the blood and the
myoglobin storage depot
- Uses many substrates:
carbohydrates, lipids, proteins
- Good for long term exercise
- May provide 90-100% of the
needed ATP during these
periods
Summary of Muscle Metabolism
Oxygen Debt
• The amount of oxygen needed to
restore muscle tissue (and the body)
to the pre-exercise state
• Muscle O2, ATP, creatine phosphate, and
glycogen levels, and a normal pH must be
restored after any vigorous exercise
• Circulating lactic acid is converted/recycled
back to glucose by the liver
Factors Affecting the
Force of Contraction
1. Number of muscle fibers contracting (recruitment)
2. Size of the muscle
3. Frequency of stimulation
4. Degree of muscle stretch when the contraction
begins
Muscle Fiber Type: Speed of
Contraction
• Slow oxidative fibers contract slowly, have slow
acting myosin ATPases, and are fatigue resistant
(red)
• Fast oxidative fibers contract quickly, have fast
myosin ATPases, and have moderate resistance to
fatigue
• Fast glycolytic fibers contract quickly, have fast
myosin ATPases, and are easily fatigued (white)
Force, Velocity, and Duration of
Muscle Contraction
Smooth Muscle Tissue
• When the longitudinal
layer contracts, the
organ dilates and
contracts
• When the circular
layer contracts, the
organ elongates
Smooth Muscle Contractions
• Peristalsis – alternating contractions and
relaxations of smooth muscles that squeeze
substances through the lumen of hollow organs
• Segmentation – contractions and relaxations of
smooth muscles that mix substances in the lumen
of hollow organs
Peristalsis Animation
Contraction of Smooth Muscle
• Some smooth muscle cells:
– Act as pacemakers and set the contractile pace for whole sheets of
muscle
– Are self-excitatory and depolarize without external stimuli
• Whole sheets of smooth muscle exhibit slow, synchronized
contraction
– They contract in unison, reflecting their electrical coupling with
gap junctions
• Action potentials are transmitted from cell to cell
Smooth Muscle Tissue
• Contracts under the
influence of:
– Autonomic nerves
– Hormones
– Local factors
Developmental Aspects of the Muscular
System
• Muscle tissue develops from embryonic mesoderm called myoblasts
(except the muscles of the iris of the eye and the arrector pili muscles
in the skin)
• Multinucleated skeletal muscles form by fusion of myoblasts
• The growth factor agrin stimulates the clustering of ACh receptors at
newly forming motor end plates
• As muscles are brought under the control of the somatic nervous
system, the numbers of fast and slow fibers are also determined
• Cardiac and smooth muscle myoblasts do not fuse but develop gap
junctions at an early embryonic stage
Regeneration of Muscle Tissue
• Cardiac and skeletal muscle become amitotic, but
can lengthen and thicken
• Myoblast-like satellite cells show very limited
regenerative ability
• Satellite (stem) cells can fuse to form new skeletal
muscle fibers
• Cardiac cells lack satellite cells
• Smooth muscle has good regenerative ability
Developmental Aspects: After Birth
• Muscular development reflects neuromuscular
coordination
• Development occurs head-to-toe, and proximal-to-distal
• Peak natural neural control of muscles is achieved by
midadolescence
• Athletics and training can improve neuromuscular control
Developmental Aspects: Male and Female
• There is a biological basis for greater
strength in men than in women
• Women’s skeletal muscle makes up 36% of
their body mass
• Men’s skeletal muscle makes up 42% of
their body mass
Developmental Aspects: Male and Female
• These differences are due primarily to the
male sex hormone testosterone
• With more muscle mass, men are generally
stronger than women
• Body strength per unit muscle mass,
however, is the same in both sexes
Developmental Aspects: Age Related
• With age, connective tissue increases and muscle
fibers decrease
• Muscles become stringier and more sinewy
• By age 80, 50% of muscle mass is lost (sarcopenia)
• Regular exercise reverses sarcopenia
• Aging of the cardiovascular system affects every
organ in the body
• Atherosclerosis may block distal arteries, leading to
intermittent claudication and causing severe pain in
leg muscles
Homeostatic Imbalances
Muscular dystrophy – group of inherited muscledestroying diseases where muscles enlarge due
to fat and connective tissue deposits, but
muscle fibers atrophy.
Homeostatic Imbalances
Duchenne Muscular Dystrophy:
• Inherited lack of functional gene
for formation of a protein,
dystrophin, that helps maintain the
integrity of the sarcolemma
• Onset in early childhood, victims
rarely live to adulthood
End Chapter 9
Cardiac Muscle Tissue
• Striated
• Unicellular
• Branched
• Intercalated discs
Intercalated Discs
• Desmosomes
– connect cells
• Gap junctions
– Electrical synapses
– Excitation spreads rapidly
Smooth Muscle
• Composed of spindle-shaped fibers with a diameter of 2-10 m and
lengths of several hundred m
• Lack the coarse connective tissue sheaths of skeletal muscle, but
have fine endomysium
• Organized into two layers (longitudinal and circular) of closely
apposed fibers
• Found in walls of hollow organs (except the heart)
• Have essentially the same contractile mechanisms as skeletal muscle
Smooth Muscle Tissue
• No striations (no sarcomeres)
• Uninucleate
• Spindle-shaped
• Involuntary
• May be autorhythmic
• May have gap junctions
Series Elastic Elements
• All of the noncontractile structures of a
muscle:
– Connective tissue coverings and tendons
– Elastic elements of sarcomeres
Internal load: force generated by myofibrils on the
series elastic elements
External load: force generated by series elastic
elements on load