Question

Describe a muscle contraction, be sure to include all the players (example – sarcoplasmic reticulum, actin, myosin…) and aslo expalin the responsiveness of skeletal,cardiac, and smooth muscle

Answer 1

Muscle Contraction:

Muscle contraction is tightening or shortening of a muscle based on variation in tension or length respectively.

Action potential from nerve impulse to sarcolemma spreads through sarcoplasmic reticulum through T-tubules which results in release of calcium ions (Ca+2) from the sarcoplasmic reticulum.These calcium ions stimulate the regulatory proteins and binds with Troponin C (regulatory protein). The other regulatory proteins Troponin T and Troponin I also plays major role in masking myosin binding site of actin.

As calcium ions binds to Troponin C, the active site of actin gets unmasked and results in interaction of active site of actin with myosin head forming a cross bridge. There by utilising the ATP myosin drags actin filament thus contracting sarcomere, ultimately muscle contraction takes place.

Skeletal Muscle:

• Ca+2-ATPase pumps calcium ions (Ca+2) into longitudinal part of sarcoplasmic reticulum and then Ca+2 is transferred into cisternae and get stored.
• When action potential arrives at T-tubule and gets transmitted deep into the tubule which opens voltage-gated Ca+2 channels, located in dihydropyridine receptor.
• Conformational changes occurs in ryanodine receptors, results in Ca+2 release into sarcoplasm which diffuses to bind with troponin C to cause muscle contraction by forming actin-myosin cross-bridge cycle.
• Auto-activation of Ca+2-ATPase pumps Ca+2 from sarcoplasm to sarcoplasmic reticulum.
• Ca+2 then binds with calsequestrin in cisternae and stored there.
• Ultimately, ends in muscle relaxation due to decrease in sarcoplasmic Ca+2 concentration.

​​​​​​​Smooth Muscle:

• Calcium ions (Ca+2) influx into the smooth muscle cell, makes it bind with calmodulin-dependant myosin light chain kinase.
• This causes phosphorylation of myosin by increasing myosin ATPase activity.
• Myosin binds with actin resulting in cross-bridge formation.
• Later dephosphorylation of myosin by myosin light chain phosphatase occurs resulting in latch-bridge state which means sustained contraction followed by slow relaxation.

​​​​​​​Cardiac Muscle:

• ​​​​​​​Na+ is required for action potential of cardiac muscle and increase in extracellular calcium increases the force of contraction.
• When the cardiac muscle is depolarized, through the T-tubules, Ca+2 is released into sarcoplasmic reticulum.
• Ca+2 entry into the cell from intestinal fluid triggers Ca+2 release from sarcoplasmic reticulum which then binds to Troponin C forming a calcium troponin complex which interacts with tropomyosin that uncovers the active site of myosin and actin.
• This allows cross-bridge cycling, which causes muscle contraction.

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