Smooth muscle contracts

Are these your choices?

A. Fast and rhythmically

B. Slowly and rhythmically

C. With intensity and speed

D. Slowly and sporadically

What is your answer?

Is it b?

Smooth muscle cannot be controlled consciously and thus acts ________.


A. Involuntarily


B. Voluntarily


C. Under the control of the peripheral nervous system


D. None of the above

Smooth muscle contracts through a process called calcium-dependent sliding filament mechanism. This mechanism involves the interaction between actin and myosin proteins within the smooth muscle cells. Here's a step-by-step explanation of how smooth muscle contraction occurs:

1. Action potential: The process begins with an electrical signal, known as an action potential, being generated in the nerve cells or specialized cells called pacemaker cells in the smooth muscle.

2. Calcium influx: The action potential triggers the opening of voltage-gated calcium channels in the smooth muscle cell membrane. Calcium ions (Ca2+) flow into the cell from the extracellular fluid, increasing the intracellular calcium concentration.

3. Calcium-Calmodulin complex: Inside the smooth muscle cell, the increased calcium concentration leads to the binding of calcium ions to a protein called calmodulin. This forms a calcium-calmodulin complex.

4. Activation of myosin light chain kinase (MLCK): The calcium-calmodulin complex then activates an enzyme called myosin light chain kinase (MLCK) within the smooth muscle cell.

5. Phosphorylation of myosin: MLCK phosphorylates (adds a phosphate group to) the myosin light chain, which is a part of the myosin protein within the smooth muscle cell.

6. Cross-bridge formation: Phosphorylation of the myosin light chain induces a conformational change in the myosin protein, exposing the myosin heads. The myosin heads can now bind to actin, a protein filament present in the smooth muscle cell.

7. Sliding filament mechanism: Once the myosin heads bind to actin, they undergo a series of cyclic interactions known as cross-bridge cycling. This cycling involves the myosin heads pulling on the actin filaments, causing them to slide past each other. This results in the contraction of the smooth muscle cell.

8. Relaxation: To initiate relaxation, the calcium ions are actively pumped out of the smooth muscle cell, reducing the intracellular calcium concentration. As a result, the calcium-calmodulin complex dissociates, and MLCK activity decreases, leading to dephosphorylation of the myosin light chain. This allows the myosin heads to detach from actin, and the smooth muscle cell returns to its original relaxed state.

It's important to note that the exact process of smooth muscle contraction can vary depending on the specific type of smooth muscle and the physiological context in which it occurs.