• Describe the organisation and roles of the main proteins located within the sarcomere

orginisation of the sarcomere is as shown above the lighter I band indicates the presence of actin and the darker part in the middle indicates the A Band and myosin the Zlines are the sectionings of different sarcomeres from each other and they are pulled closer into the middle of each sarcomere when contraction occurs.

• Explain the interaction of myosin and actin in sarcomere shortening

myosin heads located in the A Band of the sarcomere binds to actin that is located in the I Band and pulls it into the A Band making the I Band shorter, pulling the Z discs closer together and also contracting the muscle, thereby shortening the sarcomere

• Explain the role of regulatory proteins troponin and tropomyosin

tropomyosin works to prevent myosin from binding to actin

troponin works to bind calcium when it enters the cell and move tropomyosin out of the way

Calcium is released from the Sarcoplasmic reticulum and binds to  ...TnC - binds Calcium

2 calcium will bind as it enters the cytoplasm which causes a conformational change in TnI (which usually binds actin and inhibits contraction), TnT rotates Tropomyosin to reveal myosin binding sites on actin and binding can occur

(Basically when Calcium comes into the cytoplasm of the cell it binds to Troponin's subunit TnC 2 calcium per TnC and causes a conformational change and therefore inhibition of TnI which stops actin from being able to bind. This then causes TnT to bind to tropomyosin which usually blocks actin-myosin binding sites. thereby revealing the sites for myosin to bind to and for contraction to occur)

TnT - Binds to Tropomyosin

• Describe events occurring at the neuromuscular junction

as a nerve impulse reaches the bouton of the neuron it causes the opening of calcium channels into the bouton this then triggers vesicles holding NT (acetylcholine) to fuse with the membrane and be released into the synapatic clef the NT then diffuses across the synaptic clef and binds to postsynaptic ligand gated channel receptors on the muscle cell when the NT binds it then causes Na to enter the muscle cell through the ligand gated channel causing depolarisation to occur within the muscle cell. the sarcoplasmic reticulum is in very close proximity to the plasma membrane and once depolarisation has occured it releases calcium into the cell which leads to contraction