The mechanism of contraction in smooth muscle is different from skeletal that?

Mechanism of contraction in smooth muscle:

1. Initiation:

- Action potential arrives at the cell membrane.

- Calcium channels open, allowing calcium ions (Ca2+) to enter the cell.

2. Calcium Binding:

- Ca2+ binds to calmodulin, a calcium-binding protein.

- The Ca2+-calmodulin complex activates myosin light-chain kinase (MLCK).

3. Phosphorylation:

- MLCK phosphorylates myosin light chains (MLC).

- Phosphorylated MLCs have higher affinity for actin.

4. Crossbridge Formation:

- Phosphorylated MLCs bind to actin filaments, forming crossbridges.

5. Sliding Filament Mechanism:

- Myosin heads undergo conformational changes, pulling actin filaments toward the center of the cell.

- This causes the smooth muscle cell to contract.

Differences from Skeletal Muscle Contraction:

1. Calcium Source:

- In skeletal muscle, Ca2+ is released from the sarcoplasmic reticulum (SR) upon action potential arrival.

- In smooth muscle, Ca2+ primarily enters from the extracellular space through calcium channels.

2. Role of Calmodulin:

- Smooth muscle contraction involves calmodulin as an intermediary between Ca2+ and MLCK activation.

- Skeletal muscle contraction does not involve calmodulin.

3. MLC Phosphorylation:

- In smooth muscle, MLCK phosphorylates MLCs to initiate contraction.

- In skeletal muscle, a different enzyme, myosin heavy chain kinase, phosphorylates MLCs.

4. Regulation:

- Smooth muscle contraction is regulated by various factors, including hormones, neurotransmitters, and intracellular signaling pathways.

- Skeletal muscle contraction is primarily regulated by the nervous system.

Overall, while both smooth and skeletal muscles use a sliding filament mechanism for contraction, they differ in the source of calcium, the involvement of calmodulin, the enzyme responsible for MLC phosphorylation, and the regulatory mechanisms.

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