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