Why do muscles shorten? 10/28 . What is the sliding filament theory of contraction? How are myofibers organized? What is the structure and function of actin, myosin and troponin? Why do we “see” striations when we look through a light microscope?
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The “Sliding Filament Theory” describes how two proteins called actin and myosin interact to shorten sarcomeres and the muscle to generate force.
What are the three troublesome “s” words?
2) Sarcoplasmic reticulum:
Intracellular structure or cell?
Myosin heads bind actin fimaments, only if TROPONIN does not cover actin. Normally very little calcium is present and troponin is attached to actin (myosin blocked). Calcium binds troponin causing tropomyosin to move away from actin. Actin is now exposed and the myosin heads can bind to actin and a contraction begins if ATP is available (energy) to flex the myosin heads.
Many actins bind with many myosin heads on the thick filaments during a contraction. One Sarcomere=Z-line to Z-line with actin/mysin/actin in between.
As the sarcomeres shorten, the z-lines (discs) get closer, eventually crunching into myosin. This crunch prevents further shortening and limits the maximum contraction (muscle shortening)!
ACH opens Na channels!
2) Latent period
3) Contraction phase
4) Relaxation phase
How is muscle fiber recruitment effected by: nicotine, temp, or nerve damage?
Myathinia gravis and ACH-receptors?
Cholinesterase inhibitors and warfare
What are the phases of a myofiber depolarization and contraction? Total elapsed time for a single twitch is short and still extremely variable: 7 to 100 msec
Cardiac cells depolarize for long periods and do not have the quick twitches that are characteristic of skeletal muscle
Cardiac cells are linked by Gap Junctions so when Na+ enters a cell it can cross into and depolarize the next cell and so-on.
While each skeletal myofiber requires a depolarization from a synapse, cardiac cells can depolarize (heart beats) independently of the nervous system. (Take it out and watch it beat autorhythmically)
Their membranes become leaky to Na+ and K+ such that it causes “pacemaker” cells to depolarize. We call this pacemaker activity autorhythmicity.
The depolarization is passed on to all neighbors via gap junctions and so on to all the cells of the heart.
The autonomic nervous system serves simply to modify the function of the pacemaker cells (heart rate) and the amount of Ca++ that enters during contraction (force of contraction).
Cardiac cells work 24/7 so they need more ATP and have more mitochondria than the typical skeletal muscle cell.
1) Where do we find smooth muscle under involuntary control?
2) Smooth Muscle usually comes in flat sheets or tubes with cells that may or may not be connected by gap junctions.
3) The gut and artery have both circular and longitudinally arranged sheets of smooth muscle to complement each others function.
Function of circular fibers Function of longitudinal fibers
4) In smooth muscle cells the actin and myosin are not organized into sarcomeres, but more loosely attached to the plasma membrane and sarcoplasm (no striations)
5) Smooth Muscle can generate force that is sustained for a longer time and uses less ATP (its cells typically have fewer mitochondria and rely more on glycolysis)
6) Smooth muscle is stimulated by the autonomic NS at a classic synapse (Multi-unit) or via a series of varicosities from a single axon (Single-Unit) where gap junctions carry depolarization to neighboring cells.
Smooth muscle cells are found in blood vessels, glands, guts,and other places. SMCs contract using calcium entry/calmodulin binding as a signal to activate myosin light chain kinase (MLCK). MLCK phosphorylates myosin letting it bind actin and contract.
Once the myosin in smooth muscle is phosphorylated it binds actin and the cell contracts, contraction ends when Ca++ leaves the cell and MLC-phosphatase removes the phosphate from MYOSIN…leading to SMC relaxation
Asthma: excess constriction of airways
Solution: promote dilation (reduce SMC contraction)
High Blood Pressure: Excess contraction of blood vessel Solution: vasodilator drugs
Low Blood Pressure: Not enough SMC tone in blood vessels. Solution: vasoconstrict blood vessels to push blood back to heart
Peristaltic Waves in intestine: propel chyme, often the contractions are not strong enough (constipation)…what is the solution?
Hypermotile Intestine (Diarrhea): Solution is to reduce intestinal SCM contractile force
Hyperactive Bladder: improve receptive relaxation