Smooth muscle contraction Flashcards
What is the name of the mechanism of contraction in smooth muscle?
Sliding filament mechanism
What is the size of the smooth muscle cells?
- Spindle shaped
- 2-20 micrometres tall
- 20-200 micrometers long
In comparison to skeletal muscle, how much actin and myosin does smooth muscle have?
- 10%
* Actin to myosin ration 15:1
Describe the structure of smooth muscle
- Relaxed= extended out
- Thick and thin filaments but rather on Z lines, the filaments are anchored onto dense bodies
- Actin filaments are attached to the dense bodies, myosin in the arrangement between the actin filaments
What are Caveoli?
- Can be thought of as the smooth muscle equivalent of T tubules
- Caveoli exist at the plasma membrane
- Ca2+ is sequestered in the extracellular space near caveoli
Describe cross bridge activation in smooth muscle cells
- Only phosphorylated myosin can bind to actin and undergo cross bridge cycling
- Myosin Light Chain Kinase (MLCK) is responsible for the phosphorylation, attachment and cross bridges
- To relax the contracted smooth muscle, myosin must be dephosphorylated because it is then unable to bind to the actin.
- Desphosphorylation is mediated by myosin light chain phosphatase (MLCP)
Describe the stages of smooth muscle contraction
- Initiated by calcium from the extracellular fluid or the sarcoplasmic reticulum
- Calcium binds to calmodulin
- The Ca-calmodulin-MLCK complex leads to the phosphorylation of MLC (requires 1 ATP)
- MLC is part of the myosin head
- Phosphorylated myosin head binds to the actin and a power stroke occurs immediately
- A second ATP is required to release the myosin head from the actin
Describe the stages of smooth muscle relaxation
- Calcium concentration dips below a critical level due to being pumped out of the cell or into the sarcoplasmic reticulum
- Calcium is released from the calmodulin
- Myosin Light Chain Phosphatase removes the phosphate from the MLC, causing detachment of the myosin head from the actin filament, causing relaxation
What are the two sources of cytosolic Ca 2+ in smooth muscle?
- Sarcoplasmic reticulum (less)
* Extracellular Ca2+ entering the cell through plasma membrane Ca 2+ channels
What are the input influencing smooth muscle contractile activity?
- Spontaneous electrical activity in the plasma membrane of the muscle cell
- Neurotransmitter released by autonomic neurones
- Hormones
- Locally induced changes in the chemical composition (paracrine factors, acidity, oxygen, osmolarity, ion concentrations) of the extracellular fluid surrounding the cell
- Stretch
Neurotransmitters released by what influence the contractile activity of smooth muscle?
Autonomic neurone endings
Describe spontaneous electrical activity in smooth muscle
• Some types generate action potentials spontaneously in the absence of any neural or hormonal input
What is the membrane potential change occurring during the spontaneous depolarization to the threshold known as?
Pacemaker potential
What are slow waves?
Periodic fluctuations in the membrane potential due to variation in the ion flux across the membrane
Where are pacemaker cells found?
Throughout the gastrointestinal tract and thus gut smooth muscle tends to contract rhythmically even in the absence of neural input
