01-08-07, Anatomy/Physiology notes again

The muscles test is on next Tuesday. Memorize the muscles by this Friday. Identify the muscles via looking at where the “origin” of the muscle is (anatomically). Look for the “origin” of the muscle (anatomically).

Motor neurons connect the CNS to the skeleton muscle cells (effectors). Impulses (action potentials) are responsible for starting the contraction. The nerve cell and muscle cell comes together at the neuromuscular junction.

Vesicles, or pockets, in the axon terminals of the motor neuron release molecules of the neurotransmitter acetylcholine. These molecules diffuse across the synapse junction, producing an impulse in the cell membrane of the muscle cell (myocyte).

Acetylcholine diffuses from the end of the nerve cell to the muscle cell and produces an impulse in the cell membrane of the muscle cell.

It's basically going to open a whole bunch of gates and let ions flow in and that's what starts the muscle contraction. Acetylcholine (Ach) has been referred to as a lock-and-key situation because it is essentially what opens up the holes to allow Ca²⁺ and other ions to flow through. The calcium ions are needed to work with the molecular complex at the junction between actin and myosin filaments in these long, stretched myocytes with multiple nucleus.

The space between the very end of the nerve cell and muscle cell is the synaptic clef. It's the space between the two. That's where the chemicals are going to dump out at the tip at the synaptic clef and then move into the muscle cell. Acetylcholine and in its space on the membrane, makes the molecule changes its shape and allows space for other molecules and chemicals to come through.

You have more mitochondria in myocytes than epithelial cells because muscles need much more energy. The impulse causes the release of calcium ions within the cell. Calcium ions affect regulatory proteins that allow actin and myosin filaments to interact and form cross-bridges. A muscle cell will remain in a state of contraction until the production of acetylcholine stops.

So the calcium ions are required to allow for the moment of the myosin and actin. That's an important point. The only way to get calcium ions is to get Acetlycholine. ATP is required for the attach and the detachment of the myosin and actin heads at which place hydrolysis takes place. So the rest is calcium that is doing the muscle contractions. ATP is indirectly responsible for making it all shorter. The myosin and actin filaments move towards each other (across one another) with the support of the calcium ions and not ATP. That's important.

Tuesday we will take a test on the muscles of the human body. Then on Friday we will have another test most likely. Monday we have the day off next week. This Friday we will likely be done with these muscle tests. You will have one week to soak all of this in. We will review next Thursday, although we may have a test on Thursday instead. The test on Thursday is going to be a short-answer test. The packet that you were given will be tested on Tuesday and will have no word bank.

Calcium ions affect regulatory proteins that allow actin and myosin filaments to interact and form cross-bridges. A muscle cell will remain in a state of contraction until the production of acetylcholine stops.

ATP is required for the myosin head to be released from the actin filament. There is also an ATP requirement in the beginning as well, where ATP is phosphorylated to ADP and a phosphate ion. Dead people usually have “rigor mortis” (not permanent) where the myosin and actin filaments are bonded together (thanks to a lack of ATP availability). If you pull hard enough then the muscles will return to the more relaxed positions of course.

Muscle contractions require energy which is supplied by ATP. This energy is used to detach the myosin heads from the actin filaments. ATP is used elsewhere as well. Since myosin heads must attach and detach a number of times during a single muscle contraction, muscle cells must have a continuous supply of ATP. Without ATP the myosin heads would stay attached to the actin filaments, keeping muscles permanently contracted.

A muscle contraction, like a nerve impulse, is an “all-or-none” response. It will either contract fully or not at all. The number of muscle fibers that are stimulated resulting from the muscle contraction determines the force of a muscle contraction. The greater the number of muscle fibers are stimulated, the greater the force. Acetylcholine is the neurotransmitter that stimulates skeletal muscles for contraction. Acetylcholine is released at the axon branches of the motor neuron and into the myocyte's nucleus that contains the sarcomere units of myosin and actin filaments. Anyway, there is the spectrum of force. Muscle fibers are all or nothing, but this does not mean that every fiber in the muscle is contracting when stimulated. Certain fibers are stimulated depending on the amount of forced needed for whatever you are about to do.

Some muscles, such as the muscles that hold the body in an upright position and maintain posture, are nearly always at least partially contracted. Glycogen can allow for glycolysis in myocytes for muscle contractions for about one minute. However, aerobic respiration can power muscle contractions for nearly an hour.

When the skeletal muscle is at rest, there is a protein called tropomyosin blocking the receptor site on the actin filament so that myosin and actin cannot bond. The calcium ions regulate the troponin complex and this troponin complex controls the position of tropomyosin. Thus you need a concentration of calcium so that the tropomyosin-troponin complex is rearranged to allow for myosin and actin to allow the sliding-filament theory to take place.

Tropomyosin blocks the myosin binding sites and allows myosin and actin filaments. The test on Tuesday is on muscle structures in the human body and the test on Thursday is on the cellular and chemical basis of muscular contraction. The intersection of a neuron and muscle (neuromuscular junction) is full of mitochondria, synaptic vesicles, etc. etc. and the cells are very close to each other.

Acetylcholine hits the muscle membrane and opens up large gates that allows calcium ions to flow through and that allows the troponin-tropomyosin complexes to reform and and the calcium will come in and it contracts at that point.

Acetylcholine diffuses into the membrane of the myocyte (from the dendrite of the effector neuron) at the neuromuscular junctions and this causes the integral proteins to restructure and allow calcium ions to flow into the cell along new pathways / gradients (“gates” are opened by the acetylcholine).

An enzyme called acetylcholinesterase breaks down acetylcholine. Enzymes work on milk-sugars would be called lactase, because of `lact' and the `ase' ending signifying an enzyme.

An enzyme called acetylcholinesterase produced at the neuromuscular junction, destroys acetylcholine and this permits the re-absorption of calcium ions into the muscle cell and terminates the contraction. Upon retraction the calcium has to be absorbed by the myosin molecules and is moved along in another way, that way the movement is done back to its original state, so calcium ions are also required to terminate contraction, it seems.

You can have a Weak or Strong contraction based on the type of action that you are trying to accomplish. The brain (frontal lobes of the cerebrum) decides what and how many muscle cells need to contract. Blinking your eye would be a weak contraction, but lifting heavy weights, the brain would signal most muscle cells contract.

The brain's ability to know where our muscles are and what they are doing is known as muscle sense. This permits us to perform everyday activities without having to concentrate on muscle position. It's kind of like being in a position while you relax, and forgetting about the position, but as soon as you get up to do something else, your brain understands where you were. You don't have to go to a set position to start from any sort of position, you can be laying on your side and still be able to write and so on, because you can get to different muscle states through sometimes novel pathways and so on.

This is all from page 1113 in the Biology_7ed book on the local hard drive which has some more information the chemical pathways that regulate muscle contraction.

Motor neurons connect the central nervous system to the skeleton muscle cells (effectors). Impulses called action potentials from motor neurons control the contraction of skeleton muscle cells. The point of contact between a motor neuron and a muscle cell is called the neuromuscular junction.