Lecture 9 - smooth muscle & EC coupling Flashcards
Decribe the structure/characteristics of a smooth muscle cell
- spindle/ rod shaped
- uninucleated
- no strations
- able to proliferate throughout lifetime, unlike skeletal muscle
How is smooth muscle classified?
- classfied by its** patterns of activity**
What are the 2 most common forms of classification of smooth muscle?
- rhythmic smooth muscle
- tonic smooth muscle
Describe **rhythmic smooth muscle **
ie phasic smooth muscle & eg
- active/contracting for discrete/short periods at a time
* phasic smooth muscle spends a fair amount of time in a relaxed state, contracting only from time to time - EG - smooth muscle of the** bladder **only contracts a few times a day & the GIT sm contracts a few times an hour
Describe **tonic smooth muscle **
- tonically active smooth muscle remains contracted for most of the time & only relaxes briefly
- EG - the lower oesophageal sphincter which controls the entry of food into stomach, remains contracted all day long to prevent food going back up …
In what other way are smooth muscle classified?
- based on whether the smooth muscle cells in an organ function together as ‘one’ muscle or whether each muscle cell is independent/acts on it’s own
- ie single unit or multi unit smooth muscle
Describe multi unit smooth muscle
+ example
- the smooth muscle cells in an organ all behave independently of each other - ie each cell contracts & relaxes on it’s own
- has** fewer gap junctions** than the other types of smooth muscle - since it acts on it’s own- there is no need for electrically communicating to other cells
- EG - tonically active smooth muscle - eg vascular smooth muscle
Describe **single unit/visceral **smooth muscle
- all the smooth muscle cells in an organ behave as** one unit ** - multiple fibres contract as a unit or in some cases are self excitable
- they have lots of gap junctions - to allow cells to communicate with eachother electrically - when one cell depolarises, they all do
- eg smooth muscle found in the walls of hollow organs eg smooth muscle in the GIT
What is a dense body?
thin filaments that are anchored to cytoplasmic stuctures
What controls smooth muscle?
- The autonomic system
- hormonal secretions
- spontaneous electrical activity - ie pacemaker cells
Whats the difference between thin & thick filaments in skeletal vs smooth muscle?
- smooth muscle contains** thick myosin filaments & thin actin filaments **
- instead of the troponin protein on actin in skeletal muscle, it contains calmodulin which binds to Ca2+
- actin and myosin** loosely arranged** in the cell & held by dense bodies - unlike sarcomeres in skeletal muscle
Describe the sequence of events of ‘cross bridge cycling’ in smooth muscle
- increase in intracellular Ca2+
- Ca2+ binds to calmodulin
- Calmodulin then binds to the **myosin light chain kinase enzyme ** (MLCK)
- MLCK is activated & it uses ATP to phosphorylate myosin light chains on myosin head
- myosin ATPase enzyme hydrolyses ATP-ADP
- phosphorylated myosins bind to actin - cross bridge
How can the cross bridge cycling in smooth muscle cells be intervened by drugs?
ie what ways ?
- through a Ca2+ channel blocker which will reduce the intracellular Ca2+
*
How can certain drugs increase intracellular Ca2+ in smooth muscles?
- they increase intracellular Ca2+ by agonists activating G protein coupled receptors and hence using the PLC pathway to generate IP3 & DAG which causes a release of stored intracellular Ca+ in the SR
what are the 2 different sources of cytosolic Ca2+?
- from the sarcoplasmic reticulum (Ca2+ stored here)
- extracellular Ca2+ that enters via voltage channels
Why does smooth muscle **have a much slower shortening velocity **in comparision to straited skeletal muscle?
smooth muscle myosin has a** low rate of ATP splitting** (breaking up - adp & pi)
What ion is involved in the rising phase of an action potential of smooth muscle?
- Ca2+ - influx of Ca2+ through open calcium channels
How do some smooth muscle cells generate action potentials in the absence of any external input?
- due to the presence of **pacemaker cells **
- they generate pace maker potentials (graded potentials) that reach threshold & cause the membrane to depolarise
In comparison to skeletal muscle (that use an NMJ), what do smooth muscles have instead?
- instead of an NMJ, smooth muscle cells are innervated by many autonomic neurons & a number of smooth muscle cells may be influenced by neurotransmitters that are released from the varicosities on these neurons
- the neurotransmitters released can have inhibitory or excitatory effects on the smoth muscle
Compare smooth muscle contraction w/ skeletal muscle contraction
What is** rhokinase pathway**?
- this pathway** modulates the level of phosphorylation** of the myosin light chain kinase
- it mainly acts through inhibition of myosin phosphatase and contributes to
what is the difference between skeletal muscle SR and smooth muscle SR?
- skeletal muscles have an organised SR & T tubules
- Smooth muscles dont have an organised SR and T tubules
What channel does EET activate?
note that EET is an EDHF produced by endothelium
- EET activates a transient receptor channel eg TRPV4 in the sarcolemma of smooth muscle
what does the activation of the TRPV4 channel lead to?
- it leads to an influx of calcium which opens the ryanodine channels in the sarcoplasmic reticulum
after the influx of Ca2+, what channel is activated & what happens?
- the large influx / sparks of Ca2+ activate a large conductance K+ channel in the sarcolemma and the smooth muscle cell becomes hyperpolarised
- hyperpolarisation in turn decreases basal Ca2+ influx and therefore **decreases intracellular Ca2+ and hence relaxes the smooth muscle **
