Microanatomy of Muscles

I. 3 Types of muscle fibers. A I. 3 Types of muscle fibers A. Skeletal – multinucleate, striated, voluntary 1. Can be controlled consciously or by reflex B. Cardiac – heart, involuntary, striated C. Smooth – single nuclei, not striated, involuntary, walls of organs

II. Muscle Functions A. Movement B. Structure/Posture C. Generate Heat

III. Microscopic Anatomy of Skeletal Muscle. A III. Microscopic Anatomy of Skeletal Muscle A. Sarco = flesh, myo = muscle B. Sarcolemma = plasma membrane of the muscle cell C. Sarcoplasmic Reticulum (SR) – muscle smooth endoplasmic reticulum – calcium storage D. Myofibrils – organelles which basically fill the cytoplasm

IV. Steps of a Muscle Contraction https://www. youtube. com/watch

1. At rest, Na+ is actively pumped out of 1. At rest, Na+ is actively pumped out of the cell and K+ is actively pumped into the cell (requires ATP).

2. Neurotransmitter (chemical messenger in nervous system) call acetylcholine (Ach) is released by exocytosis at the neuromuscular junction (NMJ)

3. Ach diffuses across the synapse and attaches to receptors on the muscle cell, causing sodium gates on the sarcolemma to open and let Na+ into the cell which causes K+ to leave the cell

4. If enough Ach is released, an “action potential” is generated (can’t stop once it has started) 5. At rest, actin and myosin are kept apart by troponin and tropomyosin and Ca++ is being actively transported into the SR

6. Change in Na+/K+ concentrations causes the SR to release Ca++ 7 6. Change in Na+/K+ concentrations causes the SR to release Ca++ 7. Ca++ attaches to troponin and tropomyosin, pulling them away from actin and exposing the myosin binding sites

8. Myosin heads bind to actin and start pulling the actin inward, shortening the sarcomere (a contraction)

9. As this is occurring, Ach is being broken down and its components are returning to the nerve cell 10. Once Ach is broken down, sodium gates and potassium gates close, and Na+ is actively pumped out and K+ is pumped into the muscle cell

11. Return of Na+/K+ concentrations causes SR to start taking Ca++ back in 12. Without Ca++, troponin and tropomyosin block myosin and actin from binding 13. Sarcomere relaxes

https://www. youtube. com/watch. v=GneonFlcZG8 https://www. youtube https://www.youtube.com/watch?v=GneonFlcZG8 https://www.youtube.com/watch?v=2YZJt_Bw3eo https://www.youtube.com/watch?v=xxgcnU9FpAo