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tonic contraction

one of the types of SM contraction - sphincters

phasic contraction

one of the types of SM contraction - GI tract (phasic contractions are rhythmic)

dense bodies

Actin filaments of contractile units are attached to dense bodies. They appear to serve as anchors from which the thin filaments can exert force.

stretch-relaxation response

if smooth muscle is quickly stretched, it contracts, but over time, it gradually relasxes adjusting to a new length.


ability of smooth muscle to be stretched and then relax maintaining constant pressure (ex: bladder filling with urine)

myosin phosphorylation

Myosin can't bind to actin unless it is phosphorylated! (begins contraction)

Calmodulin > (Calcium) > Calcium-Calmodulin
Inactive myosin kinase > (Calcium-Calmodulin) > Active myosin kinase
Inactive myosin > (Active myosin kinase) > Phosphorylated myosin!

how does Ca+2 enter the cell

Smooth muscle doesn't have t-tubules and has small sarcoplasmic reticulum. Calcium enters from the ECF bia Ca+2 channels.

calcium blockers

1. Verapamil
2. Nifedipine
3. Norvasc

antagonists to dihydropyradines -> block receptors
*last line of protection

unitary smooth muscle

function independently from eachother
each smooth muscle innervated independently
neurogenic (from the neuron)
no AP in this type of muscle
found in vessels, airways, eye, erector pili

single unit smooth muscle

most abundant
contracts as a single unit
electrically linked by gap junctions
forms functional synctium
self excitable rather than requiring nervous stimulation
pacemaker cells can depolarize on their own
do not have resting membrane potential
myogenic (from the muscle)
innervated - metabotrophic receptors respond to Ca from 2nd messenger systems
symp and parasymp don't intitiate contraction, but rate and force

spontaneous depolarization

two types:
pacemaker potential (reproductive, urinary tract)
slow wave potential (only found in gut, peristalisis)

smooth muscle innervation

Norepi and ACh can be excitatory or inhibitory. They are usually antagonistic to eachother and is the case for multiunit smooth muscle. Can be Norepi, ACh, oxytocin, serotonin, angiotensin, histamine that alter Ca+2 levels.

Ca+2 channels

2 categories:
V sensitive (dihydropyradine receptors)
Receptor mediated (g-proteins)

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