Muscle tissue pt 1 Flashcards
What is muscle tissue?
- nearly half of body’s mass
- transforms ATP to directed mechanical energy –> exert force
- 3 types: skeletal, cardiac, smooth
- myo, mys, sacro
Skeletal muscles
- elongated cells called muscle fibers
- striated (stripes)
- voluntary (conscious control)
- contract rapidly, tire easily, powerful
- require nervous system stimulation
- each muscle served by one artery, nerve, and one/more veins
- connective tissue sheaths of skeletal muscle:
1. epimysium: dense irregular CT surrounding entire muscle
2. Perimysium: CT surrounding fascicles (groups of muscle fibers)
3. Endomysium: fine areolar CT surrounding muscle fiber
Cardiac + smooth muscles
Cardiac
- only in heart, bulk of heart walls
- striated
- contract w/o nervous system stimulation
- involuntary
Smooth
- walls of hollow organs (stomach, urinary bladder, airways)
- not striated
- can contract w/o nervous system stimulation
- involuntary
4 characteristics of muscle tissues
- Excitability: responsiveness/irritability, ability to recieve/respond to stimuli
- Contractility: ability to shorten forcibly when stimulated
- Extensibility: ability to be stretched
4: elasticity: ability to recoil to resting length
4 muscle functions
- Movement of bones/fluids (blood)
- Maintaining posture + body position
- Stabilizing joints
- Heat generation (skeletal muscle –> shivering)
- additional functions: protects organs, forms valves, controls pupil size, causes goosebumps
Microscopic anatomy of a skeletal muscle fiber cell
- long cylindrical cell (less than diameter of human hair; up to 30 cm long
- multiple peripheral nuclei
- sarcolemma = plasma membrane
- sarcoplasm = cytoplasm (glycosomes for glycogen storage, myoglobin for O2 storage)
- modified structures: myofibrils, sarcoplasmic reticulum, t tubules
Myofibrils
- densely packed rodlike elements
- 80% of cell volume
- contain sarcomeres (contractile units) that contain myofilaments
- exhibit striations: perfectly aligned repeated series of dark A bands and light I bands
Striations
- thick filaments: run entire length of A band (myosin)
- thin filament: run length of I band and partway into A band (actin)
- H zone: lighter region in midsection of dark A band where filaments do not overlap
- Z disc: coin shaped sheet of proteins on midline of light I band that anchors thin filaments and connects myofibrils to one another
- sacromere: region bw 2 successive Z discs
Sarcoplasmic reticulum (SR)
- network of smooth ER surrounding each myofibril
- pairds of terminal cisternae form perpendicular cross channels
- stores + release Ca2+
Transverse T Tubules
- continuations of sarcolemma (plasma membrane)
- penetrate cell’s interior
- associate with paired terminal cisterns (part of SR) to form triads that encircle each sarcomere
- T tubules conduct impulses deep into muscle fiber; every sarcomere
- triad = pair of terminal cistern + T tubule
Sliding filament theory of muscle contraction
- skeletal muscle fiber contracts, myosin heads bind to actin = sliding begins
- cross bridges form + break several times, ratcheting thin filaments toward center of sarcomere
- H bands + I bands get smaller
- Zones of overlap get larger
- Z lines move closer together
- Width of A band remains constant
Nervous system control of skeletal muscle
- skeletal muscle fiber contracts when stimulated by a motor neuron
- neuromuscular junction: site where motor neuron meets midpoint of muscle fiber
- each muscle fiber has only one NMJ
- single neuron may branch to control more than one muscle fiber
Generation of AP at NMJ
Phase 1: motor neuron stimulates muscle fiber
- AP arrives at axon terminal at neuromuscular junction
- ACh released; binds to receptors on sarcolemma
- ion permeability of sarcolemma changes
- local change in membrane voltage (depolarization) occurs
- local depolarization (end plate potential) ignites AP in sarcolemma
Phase 2: Excitation-contraction coupling
1. AP travels across entire sarcolemma
2. AP travels along T tubules
3. SR releases Ca2+ and it binds to troponin, myosin binding sites on actin are exposed
4. myosin heads bind to actin so contraction begins
- events that transmit AP along sarcolemma lead to sliding of myofilaments (excitation: AP traveling, coupling: actual contraction)
- AP brief; ends before contraction (causes rise in intracellular Ca2+ which leads to contraction)
- latent period (time bw excitation + coupling): time bw AP initiation + beginning of contraction
Homeostatic imbalance
rigor mortis: cross bridge detachment requires ATP
- 3-4 hours after death muscles begin to stiffen with weak rigidity at 12 hours post mortem
- dying cells take in calcium –> cross bridge formation
- no ATP generated to break cross bridges
