How does smooth muscle contract?

Smooth muscle contraction occurs through a process involving calcium ions and the interaction of actin and myosin filaments.

The Basic Mechanism of Smooth Muscle Contraction

Regardless of the specific stimulus, smooth muscle cells rely on a fundamental mechanism for contraction:

• Cross-bridge cycling: Like other muscle types, smooth muscle cells use the interaction between actin and myosin filaments to generate force. This process is known as cross-bridge cycling.
• Calcium Ion Role: Crucially, calcium ions (Ca2+) serve as the primary initiators of contraction. When the concentration of Ca2+ ions increases within the smooth muscle cell, it triggers a series of events that lead to muscle contraction.

Detailed Steps

Here’s a breakdown of how this process typically unfolds:

1. Stimulus: Various stimuli like nerve signals, hormones, or local factors can initiate smooth muscle contraction.
2. Calcium Influx: These stimuli lead to an increase in intracellular calcium concentration. This can happen through various mechanisms, including:
   • Influx of Ca2+ from outside the cell.
   • Release of Ca2+ from intracellular stores, like the sarcoplasmic reticulum.
3. Calmodulin Binding: The increased Ca2+ binds to a protein called calmodulin.
4. Myosin Light Chain Kinase Activation: The Ca2+-calmodulin complex activates an enzyme called myosin light chain kinase (MLCK).
5. Myosin Phosphorylation: MLCK phosphorylates myosin light chains, which are part of the myosin protein.
6. Cross-bridge Formation: Phosphorylated myosin can now interact with actin, allowing cross-bridges to form.
7. Contraction: The cycling of these cross-bridges leads to the sliding of actin and myosin filaments relative to each other, resulting in smooth muscle contraction.

Relaxation

Relaxation of smooth muscle occurs when the concentration of calcium ions decreases within the cell, which is accomplished by processes such as:

• Calcium pumps actively transporting Ca2+ back out of the cell or into intracellular storage.
• Decreased activity of the MLCK enzyme.
• Increased activity of myosin light chain phosphatase which dephosphorylates the myosin light chains.

Key Differences from Skeletal Muscle

While both smooth and skeletal muscle use actin and myosin cross-bridge cycling, there are differences:

Feature	Smooth Muscle	Skeletal Muscle
Initiator	Calcium (Ca2+)	Calcium (Ca2+)
Regulatory Protein	Calmodulin + MLCK	Troponin + Tropomyosin
Contraction Speed	Slower	Faster