SMOOTH MUSCLES
The difference between skeletal an smooth muscle contraction and relaxation is/are:
| A | Troponin (+ve) | |
| B | Myosin light chain kinase | |
| C |
Ca+2 |
|
| D |
Actin |
The difference between skeletal an smooth muscle contraction and relaxation is/are:
| A |
Troponin (+ve) |
|
| B |
Myosin light chain kinase |
|
| C |
Ca+2 |
|
| D |
Actin |
A i.e. Troponin (+ve)
Smooth muscle physiology different from skeletal muscle
| A |
K+ requires for contraction |
|
| B |
Ca2+ required for contraction |
|
| C |
Troponin is absent |
|
| D |
Myosin is required for contraction |
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 the 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. An 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 interacts 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 the 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 : –
* Generation of action potential
* Essential for contraction initiation (Excitation contraction coupling)
– It is interesting to know that although dephosphorylation of myosin head is necessary for relaxation, the dephosphorylation of myosin does not necessarily lead to the relaxation of the smooth muscle. This is due to the latching 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 a prolonged period (sustained contraction) for a long period with minimal expenditure of energy → Characteristic feature of smooth muscle.
– As with skeletal, muscle ATP is required which is hydrolyzed by myosin head which has ATPase activity (In a similar way as in skeletal muscle).
