Smooth Muscle Contraction Flashcards
What does smooth muscle not contain that skeletal and cardiac muscle does
the thick and thin filaments are not arranged into myofibrils (no striations = no myofibrillar structure) and there are NO SARCOMERES
How does contraction occur in smooth muscle
sliding-filament mechanism
where is smooth muscle found
surrounds hollow structures and organs that undergo changes in volume with accompanying changes in the lengths of the smooth muscle fibres in their wall
What shape are smooth muscles
spindle shaped
20-200 um in length
2-20 um in diameter
what is the actin:myosin ratio
15:1
Tell me about Caveoli and the sarcoplasmic reticulum in smooth muscle
rudimentary SR
No t-tubules
caveoli at the plasma membrane (little pockets, invaginations)
Where is Ca2+ stored/hidden
in the extracellular space near caveoli
In smooth muscle how is cross-bridge cycling controlled
by Ca2+ regulated enzyme that phosphorylates myosin (Myosin Light Chain Kinase MLCK). As only the phosphorylated form of smooth muscle myosin can bind and undergo cross-bridge cycling
in smooth muscle how does relaxation of the contracted muscle occur
myosin must be dephosphorylated as in this new form it is unable to bind to actin
MEDIATED BY Myosin Light chain Phosphatase MLCP
What are the two sources of Ca2+
- SR
- Extracellular Ca2+ entering the cell through the plasma membrane Ca2+ channels
what has to happen to Ca2+ for the smooth muscle to relax
the ca2+ has to be removed either to the SR or back to the extracellular fluid
What inputs influence smooth muscle contractile activity
spontaneous electrical activity in the plasma membrane of the muscle cell
neurotransmitter released by the autonomic neurones
hormones
locally induced changes in the chemical composition (almost always results in relaxation
stretch
examples of locally induced changes in the chemical composition
paracrine factors, acidity, oxygen, osmolarity, ion concentrations
spontaneous electrical activity in the plasma membrane of the muscle cell
this is in the absence of any neural or hormonal input
what is the pacemaker potential
The membrane potential change occurring during the spontaneous depolarization to threshold
what causes the membrane poetical to drift up and down
regular variation in ion flux across the membrane
THIS IS KNOWN AS SLOW WAVES
why can the GI tract rhythmically
there are pacemaker cells found throughout the GI tract therefore can contract rhythmically even in the absence of neural input
instead of a specilizes motor end plate region what does smooth muscle have
swollen regions known as varicosities, these contain many vesicles filled with neurotransmitters, some of which are released when an AP passes the varicosity
effects of noradrenaline (having opposite effects un different muscle tissues
if released from a postganglionic sympathetic neurone, enhances contraction of most vascular smooth muscle by acting on alpha - adrenergic receptors, but produces relaxation of airway (bronchiolar) smooth muscle by acting in beta-2adrenergic receptors.
What dermines the type of response by neurotransmitters
the receptors the chemical messenger binds to in the membrane on the intracellular signalling mechanisms those receptors have
What are some local factors which can effect smooth muscle tension
paracrine signals acidity O2 and CO2 levels osmolarity ion composition of the extracellular fluid
What produces smooth muscle relaxation acting in a paracrine manner
nitric oxide (NO)
What happens during stretching
opens mechxnosensitive ion channels, leading to membrane depolarisation
the resulting contraction opposes the forces acting to stretch the muscle
Unitary, single unit, visceral
heets of electrically coupled cells which acts in unison - often spontaneously active (a ‘syncytium’ e.g., gut and blood vessels)
Multiunitary, multi unit
tissue made of discrete bundles of cells which are densely innervated and contract only in response to its innervation (e.g., vas deferens, iris, piloerectors)
