Anatomy/Physiology

Muscle overview test

The three types of muscle tissue are skeletal, cardiac, and smooth muscles. The skeletal muscles are attached to bones or sometimes to epithelial epidermis, the cell shape is single, very long cylindrical, with multiple nuclei and are obviously striated with striations. The cardiac muscles are the walls of the heart and consist of branching chains of cells also with striations, and smooth muscles are singular, and have no striations and function with internal organs largely.

The attachment sites of the muscles are known as the origin and the insertion. The origin is the head of the muscle.

Synergistic muscles are those muscles that aid the action of a prime mover by effecting the same movement or by stabilizing joints across which the prime mover acts to prevent undesirable movements. Synergistic muscles can be found where muscles are not entirely specialized for particular actions. Whereas the brachoradialis may be used to move the forearm, the sternocleidomastoid is not entirely adept at turning the head all the way to the side and thus the abdomen must be mildly twisted as well and in that way the muscles of the body can be synergistic in getting jobs done.

Not all muscles are able to completely move the body back to its original position. Other muscles are required in order to return the skeletal system back to an original position before the antagonistic muscle contracted.

Contraction review questions

The muscle begins at the bone and the tendon and then stretches into the body of the muscle which is comprised of blood capillaries and connective tissue between the packets of muscle fibers known as muscle fascicles. The muscle fascicles have the bundles of mycoytes (“muscle fibers”). The myocytes are elongated and stretched out in length. Myocytes have multiple nuclei and a greater than normal number of mitochondria in order to provide for the aerobic respiration where glucose is broken down into two pyruvate acid molecules and then the mitochondria continues with the Krebs citric acid cycle and embeds the molecules in the folds of the mitochondrial inner membrane to get to the ATP-synthase which creates that proton gradient from inside and outside of the mitochondria that eventually produces the 34 ATP molecules. This ATP is very important and serves in the next unit of muscles on this scale known as the sarcomere which is defined as the structure which can be visually identified from z-line to z-line. The sarcomere can be most generally divided into the myosin and actin filaments that have been experimentally tested for, though the exact interaction of the two molecules is not precisely known.

Actin filaments are stationary. Myosin filaments are the ones that “move”. In the sliding filament theory, scientists suspect that the myosin filaments “grab” (bond!) to the actin and then continue grabbing, as if climbing a rope, which consequently brings the z-lines of the sarcomere closer and closer together.

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 come 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). 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. 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 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). 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. 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. 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 availability of acetylcholine is eliminated. The only way to get calcium ions is to get Acetlycholine. ATP is required for the attachment 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. 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. An enzyme called acetylcholinesterase breaks down acetylcholine. 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.

The unit of contraction in the myocyte is known as the sarcomere and can be visually defined as the molecular components that are stretched (or sometimes compressed/coiled) from one z-line to the next z-line. The sarcomere has stationary actin filaments and moving myosin filaments. Sarcomeres appear in the myocyte in a linear array such that there are thousands of sarcomeres from the origin end of the muscle to the insertion point with some tendon.

ATP is needed to bind the myosin head or tip of the molecule to the actin filament, and later even more ATP is required in order to break that bond at the bonding-site of the two filaments. The use of ATP is done via phosphorylation and the phosphate group is dropped leaving ADP and one inorganic phosphate atom in the open.

A muscle contraction is an “all or nothing” response. You do not have only one or two sarcomeres out of the thousands of the myocyte contracting at different intervals. It happens all at once due to the diffusion of calcium into the system which allows for the myosin filament heads to move along the actin filament and cause the z-lines to move closer and closer together. Diffusion, in general, will be across the entire body of the myocyte because particles will move from an area of higher concentration to an area of lower concentration, which means that there will be a more or less even distribution of the calcium ions throughout the cell body.

The neuroanatomy suggests that the dendrites of the neuron intersect the myocyte at the neuromuscular junction and that it is at thiscite that the nerve impulse causes acetylcholine to be released into the junction and reshape the receptor integral protein molecules expressed on the surface of the myocyte near the synaptic cleft. This means that when these integral proteins come in contact with acetylcholine that they will subsequently cause the membrane to restructure and allow for calcium ions to diffuse into the cytoplasm of the myocyte (a concentration gradient is formed). This calcium influx causes the tropomyosin-troponin complexes to be reshaped and allows the use of calcium atoms (well, ions) to move the myosin filaments over the stationary actin molecules stretched from z-line to z-line defining the sarcomere unit of the myocyte. Sodium, potassium, calcium, phosphate, and oxygen are all required in this process: the sodium and potassium pump that allows the electric signal to pass from neuron to neuron in the dendrimer-like network (not a typo!), the calcium to make the extracellular matrix comprising bones as well as in muscular contraction, phosphate in the energy provision molecule ATP, and even oxygen in the process of aerobic respiration at the many mitochondria in the body of the myocyte.

Acetylcholine is released by the dendrites at the neuromuscular junctions and the ones that reach the surface of the cell cause the integral proteins to change shape (due to chemical reaction) and allow the influx of calcium ions which consequently cause the motion of myosin across actin at the tropomyosin-troponin complex. To close these open gates (the acetylcholine-regulated integral proteins expressed on the membrane of the cell), the dendrites can release acetylcholinesterase, which breaks down the acetylcholine and causes the integral proteins to return to their original (blocking) state. The reason why the calcium ions no longer function in their duties is because their charges are now changed, that they either have one too few electrons or one too many electrons to allow the process to occur once more.

Bryan Bishop Anatomy&Physiology Test Review January 19^th, 02007

Muscular Contraction