Question

A discussion of microscopic muscle contraction should include all activities, chemicals and structures involved in muscle contraction. So a discussion of the depolarization of the sarcolemma, the role of the ACH receptor, sodium and potassium in generating an action potential as well as the role of calcium, T tubules, the sarcoplasmic reticulum and tropomycin is expected. You should describe the myofilaments and the process of the sliding filaments. A discussion of the contraction of a whole muscle should include the macroscopic structures of a muscle and the motor end plates, connective tissue covering and the relationships between these structures.

Answer 1

The skeletal muscle contract only when stimulated by somatic motor unit.This can cause muscle fibre to contract. One somatic motor neurone can innervate 3-1000 muscle Fibres.
The unit of nerve fibre and muscle fibre is called a motor unit.
The muscle contain many muscle fibre units called fascicles.
Within a motor unit that assist the contraction of muscle, there is axon terminal of motor neurone. The end bulb of axon of the motor neurone have some vesicles. These vesicles contain neurotransmitter called acetylcholine. The muscle fibre contain many myofibrils. Within the muscle fibre there is mitochondria which can provide ATP.There are many nucleus around the muscle fibre, because the muscle fibre is multinucleated. The muscle fibre has a membrane called sarcolemma which contains many receptors and Ion channels. Myofibrils contain thin and thick filaments. The portion between one z line and adjacent z line is called a sarcomere. The are T-tubules that run around the myofibril. Surrounding the T-tubule, there are sarcoplasmic reticulum. These sarcoplasmic reticulum contain calcium ions.

T-tubules essentially connect with the outer membrane and wrap around the myofibril. T tubules contain special Ion channels for calcium ions. Surrounding the T tubules we have sarcoplasmic reticulum which contain calcium ions, specifically called terminal cisternae. The terminal cisternae contain calcium ions. But these ions cannot enter into T-tubules because the calcium channels are blocked.

Within the myofibrils there are thin and thick filaments. Thin filaments require calcium in order to initiate muscle contraction.

Action potential is arrived at the neurone terminal.This action potential causes the vesicles containing acetylcholine to release the acetyl choline into the synapse. The acetyl choline bind to the receptors on the sarcolemma.

Outside the sarcolemma in the extracellular fluid, there is high concentration of sodium ions. Inside the sarcolemma, there is high concentration of potassium ions. When the vesicle containing acetylcholine release the acetylcholine out, it will bind to the receptors causing the receptors to open. When the receptors open, the sodium ions come inside, which will open the voltage gated Ion channels and causes action potential. This action potential travels through the sarcolemma and comes down to T-tubules. This causes the calcium channels in the T-tubules which are close ld ; to to open up.this is because the change in the voltage of the T-tubule which cause open the channels. The calcium move out from the terminal cisternae into the T-tubules. Calcium ions travel down and bind to the thin filaments.Calcium and ATP initiate the muscle contraction through sliding filament theory at a molecular level.
The muscle contractions are controlled by the actions of calcium. The thin actin filaments are associated with the regulatory proteins called troponin and trop. When a muscle is relaxed, tropomyosin blocks the crossbridges binding sites on actin. When calcium ion levels are high enough and ATP is present calcium ions bind to the troponin which displaces tropomyosin exposing the myosin binding sites on actin. This allows myosin to attached to a binding site on actin forming a cross bridge.

The contraction begins when a bound ATP is hydrolyzed to ADP and inorganic phosphate. This causes of the myosin head to extend and can attach to a binding site on actin forming a cross bridge.an action called the power stroke is triggered allowing myosin to pull the actin filament towards the M line thereby shortening the sarcomere. ADP and inorganic phosphate are released during the power stroke. The myosin remains attached to actin until a new molecule of ATP binds freeing the myosin to either go through another cycle of binding and more contraction or remains unattached to allow the muscle to relax.
The muscle contract when thick and thin filaments slide past each other. The thick filaments are myosin which are anchored at the centre of sarcomere called the M line. The thin filaments are composed of the protein

in actin which are anchored to the z line on the outer edges of the sarcomere. Sarcomere shortens from both sides when actin filaments slide along the myosin filaments. Although the action between the filaments is described as sliding the myosin filament actually pulls the actin along its length.The cross bridges of the myosin filaments attached to the actin filaments and exert force on them to move. This action is known as the sliding filament mechanism of muscle. In this the sarcomere contracts without shortening the length of thick and thin filaments. Is cause of the muscle to contract.