Smooth Muscle Contraction Flashcards
Describe the structure of smooth muscle
- Thick and thin filaments are NOT organised into myofibrils so there are no sarcomeres
- Contraction occurs via a sliding filament mechanism
- Fibres can undergo changes in length to accompany the changes of volume in organs
Describe how smooth muscle is activated (contracted)?
A calcium regulated enzyme called Myosin Light Chain Kinase (MLCK), that phosphorylates myosin. This is because only the phosphorylated form of myosin can bind to actin and undergo cross-bridge cycling.
How can smooth muscle be relaxed?
Myosin must be dephosphorylated and this is done my the enzyme Myosin Light-Chain Phosphotase
Describe the ratio of actin to myosin in smooth muscle cells
Actin 15:1 Myosin
Describe the arrangement of actin in smooth muscle cells
It is anchored by dense bodies and the actin criss-crosses across the actin muscle fibre
Describe the scarcoplasmic reticulum in smooth muscle cells
It is rudimentary (basic) and is located close the cell membrane (which has no t-tubules)
What are caveoli?
They are little dips along the sarcolemma that operate in a similar way to t-tubules
Describe the process of smooth muscle contraction
- It is initiated by calcium from the ECF or from the SR
- Calcium binds to calmodulin and froms a complex
- Ca-calmodulin-MLCK leads to the phorphorylation of MLC (requires 1 ATP)
- MLC is part of the myosin head
Phosphorylated mysosin head bins to actin and a power stroke occurs automatically - A second ATP is required to release myosin head from actin
Describe the process of smooth muscle relaxation
1) Calcium concentration dips below critical levels due to it being pumped out of the cell and into the SR
2) Calcium is released from calmodulin
3) MLCP removes phosphate from MLC causing myosin to detach from actin, causing relaxation
Name some of the inputs influence smooth muscle contractile activity
- Spontaneous electrical activity
- Neurotransmitters released from autonomic neurons
- Locally induced changes in the chemical composition (paracrine factors, acidity, oxygen, osmolarity, ion concentrations) of the extracellular fluid surrounding the cell
- Strech
What is a pacemaker potential?
It is the membrane potential change occurring during the spontaneous depolarisation
What is the spontaneous electrical activity seen in the GI?
Slow waves with spikes of action potentials
The swollen regions along autonomic neurons are called what?
Varicosities
Describe some of the features of varicosities
They contain many vesicles filled with neurotransmitters which are released when an action potential passes the varicosity.
Variscosities from a single axon may be located along several muscle cells, therefore it means many smooth muscle cells are influenced by the release of NTs from a single neuron
Do all neurotransmitters enhance contractile activity? and does the same neurotransmitter always produce the same effect?
1) No, some neurotransmitters can also decrease contractile activity
2) No, the same neurotransmitter can produce the opposite effect in some tissues. It depends on the receptors rather than the chemical message
