01-05-07

Yesterday we began with the physiology of muscle contraction where we learned that the myosin molecules hydrolyze with some tips at junctions to the actin molecules and thus pull the z-lines closer and closer together making the sarcomere known as the unit of contraction. This means that one of the requirements is ATP and water. Remember, myocytes have many mitochondria such that they are able to metabolize nutrients and resources down to the 34 ATP molecules that are available as result of aerobic respiration with the assistance of the mitochondria that are spattered throughout the multiple nuclei of the elongated myocyte (muscle cell).

The actin is attached to the z-lines and the myosin grabs the actin and pulls it so that the whole sarcomere contracts and gets smaller. The cross-bridge is where the myosin and actin is physically touching each other. When the muscle is stimulated to contract, the cross-bridges move, pulling the two filaments past each other. After each cross-bridge has moved as far as it can, it releases the actin filament and the myosin returns to its original position. This is an explanation of what you watched.

The cross-bridge then attaches to the actin filament at another place and the cycle is repeated. This action shortens the length of the sarcomere. The synchronized shortening of sarcomeres along the full length of a muscle fiber causes the whole fiber, and hence the muscle, to contract. An entire fiber contracts at the same time - you do not stimulate individual sacromeres or one half of the entire muscle fiber or not. The z-lines do not go anywhere. It's an overall shortening because they are all attached to each other. At each end of the fiber, it is attached to the bone by tendons. The muscle fibers are attached to a tendon, which is attached to a bone. Since it is attached to the tendons, it is going to move those bones.

The nose is the only location of the body where the nerves are exposed directly to the external environment. There is sour, bitter, sweet, and salty on the tongue, those are the only four different receptors on the tongue, so the subtleness of taste has to do with the food molecules getting into the nose even when you chew. When you have a cold, the mucus covers those little nerve endings and that's why you can't taste much when you have a cold.

When thousands of actin and myosin filaments interact this way, the entire muscle cell shortens. And that is the concept of the sliding filament theory. The proteins are there, but we really have no idea if they move in these very particular ways. There are certainly the available chemical reaction pathways, but we don't know if those are occurring.