Chpt. 9.1 - 9.6 Skeletal Muscle, Smooth Muscle & Cardiac Muscle Flashcards
Muscle
- A bundle of fascicles
- Surrounded by epimysium and fascia
Fascicle
- A bundle of muscle fibers within a muscle
- Surrounded by perimysium
Muscle fiber
- A single multinucleated muscle cell
- -Surrounded by endomysium
Myofibril
-Thick and thin filaments in repeating pattern (sarcomeres)
I-Bands
- Light bands composed of thin actin filaments held together by attachments to Z-lines
- Narrows during contraction
A-bands
- Dark bands composed of thick myosin filaments overlapping thin actin filaments (length = length of myosin filament)
- Does not change size during contraction
H-zone
- consists of only thick myosin filaments
- Narrows during contraction
M-line
-consists of proteins that help hold the thick myosin filaments in place
Z-line
- consists of thin actin filaments
- lines move closer together during contraction, shortening the sarcomere
Stimulus for Contraction
1) Action potential reaches ACh vesicles in distal end of axon
2) Vesicles release acetylcholine into the synaptic cleft
3) Acetylcholine molecules bind to protein-specific receptors in the muscle fiber membrane, increasing membrane permeability to sodium and potassium ions, opens more sodium channels on sarcolemma
4) Action potential conducted spreads throughout the muscle cell which triggers the release of calcium ions from the SR, contraction can occur
Muscle Contraction
1) Ca2+ binds to troponin exposing binding sites on actin molecules
2) Myosin heads bind to actin
3) ADP+P release from myosin and power stroke pulls actin filaments
4) ATPs forms, breaking connection to actin
5) ATP splits into ADP+P, cocks myosin heads and store energy for next power stroke
Muscle Relaxation
1) Action potential stops
2) Acetylcholesterase decomposes remaining ACh in synaptic cleft
3) Calcium ions move back into SR
Summation
Force of individual twitches combines and increases in frequency, before relaxing
Tetanus
Forceful, sustained contraction that lacks partial relaxation
Recruitment
Increase in number of activated motor units, recruitment continues until the intensity of stimulation increases
Isotonic contractions
concentric - muscle contracts with force greaer than resistance (shortens)
eccentric - muscle contracts with force less than resistance and lengthens
Isometric contractions
Tension with muscle increases, but wall does not move
Slow-twitch muscle fiber
- red fibers (contain myoglobin)
- contracts slower
- good blood supply
- lots of mitochondria
- high respiratory capacity
- fatigue-resistant (primarily oxidative)
Fast-twitch muscle fiber
- white fibers (less myoglobin)
- contracts rapidly
- poorer blood supply
- fewer mitochondria
- reduced respiratory capacity
- more extensive SR & faster ATPase
- fatigue easily (primarily glycolytic)
Intermediate muscle fiber
- white fiber
- contracts rapidly
- fatigue-resistant (intermediate oxidative capacity)
Oxygen debt
- Oxygen low due to strenuous activity
- Caused by anaerobic respiration
- Amount of oxygen needed to convert lactic acid to glucose, resynthesize ATP & creatine phosphate
- Amount of oxygen needed to restore blood and tissue oxygen levels to pre-exercise levels
Smooth Muscle Characteristics
- shorter fibers than skeletal
- single, centrally located nuclei
- lack striations and transverse tubules
- SR not well-developed
Multiunit Smooth Muscle
- cells less well-organized and function independently
- contract only by stimulation by neurons or hormones
- ex: iris of the eye and large blood vessels
Visceral Smooth Muscle
- composed of sheets of spindle-shaped cells held together by gap junctions
- respond as single unit
- peristalsis (wavelike motion) due conduction of impulse by nearby cells and rhythmicity
Smooth Muscle Contraction
Similar to skeletal:
- actin and myosin
- both use Ca2+ and ATP
- both are triggered by membrane impulsese
Different to skeletal:
- lacks troponin, has calmodulin
- uses acetylcholine AND norepinephrine
- hormones can stimulate/inhibit smooth muscle
- more resistant to fatigue
- change length w/o changing tautness
Cardiac Muscle Characteristics
- found in the walls of the heart
- striated, uninucleated
- joined together by intercalated discs
- fibers branch and join together at these discs
- contract as unit
- self-exciting and has rhythmicity
- long refractory period
- no sustained or tetanic contractions
agonist
causes an action
antagonist
works against the action
