Smooth-muscles

A i.e. Troponin (+ve)

Ans. is ‘c’ i.e., Troponin is absent

• Troponin is absent in smooth muscle, but required in skeletal muscles for contraction.
• Ca⁺ and Myosin are required by both smooth muscles and skeletal muscles.
• K⁺ has no direct role in contraction of smooth muscle and skeletal muscle.

Smooth muscle contraction

• In smooth muscle there is no troponin. Therefore calcium initiates contraction through a mechanism different from that employed by skeletal muscle. Smooth muscle contains a calcium binding protein called calmodulin. Increase in cytoplasmic (sarcoplasmic) calcium leads to its binding to calmodulin. The calcium-calmodulin complex activates myosin kinase, also called myosin light chain kinase (MLCK). MLCK is a phosphorylase which phosphorylates a light chain belonging to myosin chain, often called cross-bridge phosphorylation. The phosphorylated myosin head interact with actin, i.e., cross-bridging of myosin with actin. The cross-bridging leads to contraction.
• The reversal of contractile response depends on a reduction in sarcoplasmic calcium by calcium pumps in the sarcoplasmic reticulum membrane and sarcolemma which pump calcium into sarcoplasmic reticulum and ex­tracellular fluid respectively. Reduction in cytoplasmic (sarcoplasmic) calcium level activates an enzyme myosin phosphatase. Myosin phosphatase dephosphorylates the myosin head, thereby terminating the actin-myosin interaction and produces relaxation.

So, calcium influx has two functions in smooth muscles : –

i) Generation of action potential

ii) Essential for contraction initiation (Excitation contraction coupeling)

• It is intersting to know that although, dephosphorylation of myosin head is necessary for relaxation, dephos­phorylation of myosin does not necessarily leads to relaxation of the smooth muscle. This is due to the latch effect, i.e., myosin bridges hold on to the actin filament like a latch. Due to this smooth muscle contracts, and it can maintain its contraction for prolonged period (sustained contraction) for a long period of time with minimal expenditure of energy → Characteristic feature of smooth muscle.
• As with skeletal, muscle ATP is required which is hydrolysed by myosin head which has ATPase activity (In a similar way as in skeletal muscle).