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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.

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chapter 9

Chapter 9

Muscles and Muscle Tissue

J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.

some muscle terminology
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
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
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
Skeletal Muscle Cells
  • Long, cylindrical cells
  • Striated (banded)
  • Multinucleate
characteristics of cardiac muscle
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
Cardiac Muscle Cells
  • Branched cells
  • Uninucleate (may occasionally be binucleate)
  • Striated
  • Intercalated discs
characteristics of smooth muscle
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
Smooth Muscle Cells
  • Nonstriated = smooth
  • Uninucleate
functions of muscle tissue
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
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
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
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
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
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
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 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
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
  • Thin filaments: actin (plus some tropomyosin & troponin)
  • Thick filaments: myosin
  • Elastic filaments: titin (connectin) attaches myosin to the Z discs (very high mol. wt.)
  • 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
myosin protein
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 actinfibers
thick myosin myofilaments
Thick (Myosin) Myofilaments
  • Each thick filament contains many myosin units woven together
thin actin myofilaments
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
Muscle FiberTriads
  • 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
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
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 junction1
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

axonal terminal

motor end plate

Generation of an Action Potential(Excitation)
  • Binding of neurotransmitter (ACh) causes the ligand-gated Na+ channels to open
  • Opening of the Na+ channels depolarizes the sarcolemma (cell membrane)
generation of an action potential
Generation of an Action Potential
  • Initial depolarization causes adjacent voltage-gated 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 potential1
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
Excitation-Contraction Coupling
  • The action potential (excitation) travels over the sarcolemma, including T-tubules
  • DHP receptors serve as voltage sensors on the T-tubules 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
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
http www lab anhb uwa edu au mb140 corepages muscle muscle htm skeletal
  • 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
calcium ca 2
Calcium (Ca2+)

The “on-off switch”: allows myosin to bind to actin



calcium movements inside muscle fibers
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
The Power Stroke & ATP
  • Cross bridge attachment. myosin binds to actin
  • The working stroke. myosin changes shape (pulls actin toward it); releases ADP + Pi
  • Cross bridge detachment. myosin binds to new ATP; releases actin
the power stroke atp1
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
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



ratchet effect animation

excitation contraction coupling1
Excitation-Contraction Coupling
  • The action potential (excitation) travels over the sarcolemma, including T-tubules
  • DHP receptors serve as voltage sensors on the T-tubules and cause ryanodine receptors on the SR to open and release Ca2+ ions
  • Ca2+ binds to troponin, causing tropomyosin to move out of its blocking position
  • Myosin forms cross bridges to actin, the power stroke occurs, filaments slide, muscle shortens
  • Calsequestrin and calmodulin help regulate Ca2+ levels inside muscle cells
destruction of acetylcholine
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
Skeletal Muscle Motor Units
  • The Motor Unit = Motor Neuron + Muscle Fibers to which it connects (Synapses)
skeletal muscle motor units1
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
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)    
  • Latent period: delay between stimulus and response
  • Contraction phase: tension or shortening occurs
  • Relaxation phase: relaxation or lengthening
muscle twitches
Muscle Twitches

All or none rule: All the muscle fibers of a motor unit contract all the way when stimulated

graded muscle responses
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 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
Treppe: the staircase effect

“warming up” of a muscle fiber

multiple motor unit summation recruitment
Multiple Motor Unit Summation ( Recruitment)

The stimulation of more motor units leads to more forceful muscle contraction

the size principle
The Size Principle

As stimulus intensity increases, motor units leads with larger fibers are recruited

stretch length tension relationship
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 relationship1
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
Contraction of a Skeletal Muscle
  • Isometric Contraction: Muscle does not shorten
  • Tension increases
contraction of a skeletal muscle1
Contraction of a Skeletal Muscle
  • Isotonic Contraction: tension does not change
  • Muscle (length) shortens
muscle tone

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
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
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 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
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
oxygen debt
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
Factors Affecting theForce of Contraction
  • Number of muscle fibers contracting (recruitment)

2. Size of the muscle

  • Frequency of stimulation
  • Degree of muscle stretchwhen the contraction begins
muscle fiber type speed of contraction
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)
smooth muscle tissue
Smooth Muscle Tissue
  • When the longitudinal layer contracts, the organ dilates and contracts
  • When the circular layer contracts, the organ elongates
smooth muscle contractions
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
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 tissue1
Smooth Muscle Tissue
  • Contracts under the influence of:
    • Autonomic nerves
    • Hormones
    • Local factors
developmental aspects of the muscular system
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
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
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
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 female1
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
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
Homeostatic Imbalances

Muscular dystrophy – group of inherited muscle-destroying diseases where muscles enlarge due to fat and connective tissue deposits, but muscle fibers atrophy.

homeostatic imbalances1
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
cardiac muscle tissue
Cardiac Muscle Tissue
  • Striated
  • Unicellular
  • Branched
  • Intercalated discs
intercalated discs
Intercalated Discs
  • Desmosomes
    • connect cells
  • Gap junctions
    • Electrical synapses
    • Excitation spreads rapidly
smooth muscle
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 tissue2
Smooth Muscle Tissue
  • No striations (no sarcomeres)
  • Uninucleate
  • Spindle-shaped
  • Involuntary
  • May be autorhythmic
  • May have gap junctions
series elastic elements
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