Understand how contraction occurs in non-striated smooth muscle.

Sure, here's a step-by-step explanation of how contraction occurs in non-striated smooth muscle:

1. The process begins when the smooth muscle cell is stimulated by a signal, such as a neurotransmitter or hormone. This signal causes the cell membrane to depolarize, or become more positive.

2. This depolarization opens voltage-gated calcium channels in the cell membrane, allowing calcium ions to flow into the cell.

3. The increase in intracellular calcium triggers the release of even more calcium from the sarcoplasmic reticulum, a storage organelle within the cell.

4. The calcium ions bind to a protein called calmodulin, which is present in the cytoplasm of the smooth muscle cell.

5. The calcium-calmodulin complex then activates an enzyme called myosin light chain kinase (MLCK).

6. MLCK phosphorylates (adds a phosphate group to) the myosin molecule, changing its shape and allowing it to bind to actin, another protein in the muscle cell.

7. This binding of myosin to actin is what causes the muscle to contract. The myosin pulls on the actin, shortening the muscle cell.

8. The contraction continues as long as the myosin is phosphorylated and can bind to actin. When the signal that started the contraction is removed, calcium is pumped back into the sarcoplasmic reticulum and out of the cell, and the myosin is dephosphorylated, causing the muscle to relax.

This process is different from contraction in striated muscle (like skeletal and cardiac muscle) in several ways, including the role of calcium and the lack of troponin, a protein that regulates contraction in striated muscle.