Question

Anatomy Tutorial: Muscle Anatomy

Answer 1

\({\bf{Functions:}}\) movement, posture/position, support, thermoregulation, entry and exit from orifices

\({\bf{Characteristics:}}\) excitable (can be stimulated), contractible (can shorten), extensible (can extend), elastic (can return to original shape)

\({\bf{Gross~Anatomy:}}\) three layers of connective tissue
- epimysium: dense irregular CT that surrounds whole muscle tissue
- perimysium: collagen/elastic CT that divides muscle into fascicles
- endomysium: reticular CT that surrounds muscle fiber, binds them
- myosatellite cells: stem cells, repair muscle tissue

accessory structures:
- tendon vs aponeuroses (direct vs indirect): collagen cords where the three CT layers come together vs sheets. continuous w/ periosteum.
- (neuromuscular junction) NMJ: axon terminal + synpatic cleft + motor end plate
- blood vessels

Answer 2

\({\bf{Microanatomy:}}\) (skeletal muscles)
- sarcolemma (plasma membrane) and sarcoplasm (cytoplasm)
- myoblasts: embryonic cells, fuse to form muscle fibers (can also potentially become myosatellites)
- multi-nucleate, in contrast to smooth/cardiac which are uni-nucleate
- T-tubules: extensions of the sarcolemma that extend into the cell, conduct impulses
- myofibrils: myofilaments that make up the sarcomere
> actin: (thin filaments)
> myosin: (thick filaments)
- SR (sarcoplasmic reticulum) surrounds the myofibrils. release site for calcium, controls muscle contraction
terminal cisternae (two tubules) + T-tubule = triad

\({\bf{Sarcomere~Organization:}}\) thick filaments near center
> light (I bands) thin filaments only (actin)
- Z discs: boundary between sarcomeres, made of actinin
- titin cores: attach thick filaments to Z-line
> dark (A bands)
- M line: connects/anchors the thick filaments
- H band: thick filaments only (myosin)
- zone of overlap: overlap between thick and thin filaments, where calcium ions are released

Answer 3

\({\bf{Thin~Filaments}}\) actin
- four proteins: F-actin, nebulin, tropomyosin, troponin
- G-actin = sub-unit of F-actin. contains active sites for myosin binding.
- nebulin: holds F-actin together
- tropomyosin: covers the binding site until the impulse arrives
- troponin: binds calcium, stabilizes tropomyosin

\({\bf{Thick~Filaments}}\) myosin
- tail end binds to other myosin molecules
- head binds to thin filament to form cross-bridge
- titin core, attaches to Z-line
- stationary

Answer 4

\({\bf{Sliding~Filament~Theory}}\)
* H and I bands shorten
* zone of overlap increases (to increase the interactions between thick and thin filament)
* A band stays the same size
1. calcium ions released from SR into zone of overlap
2. calcium ions bind to troponin --> tropomyosin is moved away from active sites
3. cross-bridge formation
4. power stroke: movement of myosin heads towards M-line
5. ATP binding --> breaks the cross-bridge
6. return of myosin heads to original position via ATP supply

Answer 5

Adapted from Human Anatomy, Martini, et. al. 9th edition