Anonymous asked: Hi I don't know if you know this kind of stuff but I have a final tomorrow and forgot my textbook to study and google and my notes have been no help to me. Do you know the functions of these muscular structures? A band, I band, Z line, sarcomere, myosin, actin, ACH, cholinesterase, motor end plate, neuromuscular junction, t-tubules, and sarcoplasmic reticulum. Sorry that that's a lot and it's okay if you don't know their functions thanks anyway
I believe (am doing this from my memory so may not all be correct) that a sarcomere runs from z line to z line, with z lines (or disks) being composed of proteins to anchor the filaments in place, such as Titin. I bands are regions near the Z lines composed only of thin actin filaments, which in turn are composed of actin, troponin and tropomyosin (they are lighter in colour on images). The A band spans the length of the thick myosin filaments, with the H zone in the middle which is exclusively thick myosin, this is lighter than the rest of the A band, but darker than the I band on images. During contraction H zone decreases in size, as the myosin heads in the thick filament bind to, move, and dissociate (a process dependent on calcium) from the thin actin filaments, drawing them into the middle of the sarcomere, shortening the overall length of the muscle fibre.
ACH is acetylcholine, a neurotransmitter, released from a synapse by exocytosis in response to an influx of calcium into the cell. It diffuses across the cleft and activates the post synaptic membrane via ligand-activated channels, which allow a sodium influx, and the continuation of an action potential or activation of a response. Acetyl cholinesterase is an enzyme which degrades ACH in the synaptic cleft to halt the transmission of the signal, so the post-synaptic cells does not continue to be constantly stimulated.
Acetylcholine is the transmitter at neuromuscular junctions, where one neuron can extend processes to many different muscle fibres (as long as they are of the same type) so many fibres are stimulated to contract simultaneously, this can also be called a motor end plate. The sarcoplasmic reticulum is smooth endoplasmic reticulum found in muscle cells (which contains a large store of calcium. T-tubules are mass invaginations of the sarcolemma (plasma membrane of muscle cells) which allow rapid depolarisation of the muscle cell, and the release of calcium, which activates myosin binding (with the hydrolysis of ATP) in the sarcomeres, and therefore contraction.
Apologies for the awfully composed essay as an answer (have to leave for uni in a minute!) I hope I answered in time for your exam! Good luck with it.