Skeletal Muscle Structure

Skeletal Muscle Structure

Overview of Muscular Tissue

1. Three Types: skeletal, cardiac, & smooth.

Common Properties:

2. Excitability - ability to send electric action potentials (skeletal muscles completely rely on this, cardiac and smooth respond to hormones and local stimuli as well)

3. Contractility - shorten with force

4. Extensibility - stretch or extend 

5. Elasticity  - return to original length 

Differences: 

6. Cell structure, body location, how they are stimulated to contract.

7. Similarities: Skeletal & Smooth elongated and called fibers, shorten due to microfilaments, “myo”, “mys”, “sarco” reference

Skeletal Muscle

8. Huge, cigar-shaped, multinucleate cells (multiple nuclei per cell)

9. Largest of the muscle fiber cells “striated muscle” due to obvious stripes (arrangement of proteins)

10. Voluntary muscles - as only type subject to conscious control (in addition to reflex action)

11. Only respond to nervous system stimuli

12. “Skeletal, striated, voluntary”

Skeletal Muscle Fiber Structure:

13. Endomysium - connective tissue sheath around a muscle fiber (fiber = ~ cell)

14. Fascicle - a bundle of muscle fibers

15. Perimysium - coarser fibrous membrane sheath around a bundle of fibers

16. Epimysium - an even ‘tougher’ overcoat of connective tissue which covers the entire muscle

17. Tendon - strong cords that bind the muscle via fusing with periosteum 

Abduction is a movement away from the midline – just as abducting someone is to take them away. 

Adduction is a movement towards the midline. 

Extension refers to a movement that increases the angle between two body parts. 

Flexion refers to a movement that decreases the angle between two body parts. 

Lateral rotation is a rotating movement away from the midline. 

Medial rotation is a rotational movement towards the midline

Muscle Functions:

18. Producing movement is a common function of all types of muscles

19. Skeletal muscle provides three addition roles in addition to movement:

Maintaining posture 

Stabilizing joints 

Generating heat

Skeletal Muscle Structure

20. Multinucleate

21. Sarcolemma - plasma membrane that surrounds the muscle cells

22. Myofibrils - long ribbon-like organelles that fill the cytoplasm

Skeletal Muscle Structure

23. Sarcomeres - chains of tiny contractile units lined up end to end making up the myofibril (a portion of an organelle, not a cell)

24. Creates the banding pattern

Sarcomere

25. Alternating light and dark proteins/bands give a striated appearance

I Bands - “light” (no myosin)

A Bands - “dark” (both myosin/actin)

Z Disc - dark mid-line interruption of the light I Band (think end of sarcomere segment)

H Zone - lighter central area on the dark A Band (no actin)

M Line - tiny protein rods in the H zone that hold thick filaments together (middle of sarcomere)

Sarcomere Structure

26. Myosin Filament - “thick filament” made of myosin protein. Contain ATPase which split ATP in order to generate energy for contraction

27. Extend length of Dark A Band. Ends studded with projections called myosin heads heads or cross bridges which link to the thin filament during contraction. Actin Filament - “thin filament” made of actin protein. Anchored to z-disc (disc-like membrane). Light I bands made up of only the thin filaments from two adjacent sarcomeres. Overlap thick filaments but do not connect when muscle is relaxed allowing for a “bare zone”/H-zone which disappears during contraction

Sarcoplasmic Reticulum

28. A specialized smooth endoplasmic reticulum that surrounds each myofibril

Major role is to store and release calcium during contraction stimulation.