Exam 2- smooth muscle Flashcards
describe smooth muscle
non-striated: no z-line, so no sarcomeres (actin and myosin form loosely associated bundles with each other, instead of sarcomeres)
- smooth muscle cells are much smaller- thinner and shorter than skeletal muscle
- smooth muscle is circular- forms rings around hollow organs (blood vessels, airways, GI tract)
- when smooth muscle contracts and relaxes, regulates the area of the tube it surrounds (contraction reduces diameter of the tube)
smooth muscle that surrounds arteries
thick layer of smooth muscle surrounding arteries- contracts and relaxes- changes blood flow and blood pressure
- regulates airways that leads to lungs
describe smooth muscle in GI tract
in GI tract, have both circular and longitudinal strands of smooth muscle
- circular contracts and squeezes/mixes the contents
- longitudinal contracts in wave-like form and propels the contents through
describe the variety of contractions by smooth muscle
- depends on organ its associated with and that organs function
- smooth muscle generally maintains a more tonic contraction (a base line level of contraction that maintains muscle tone- more tonic than twitch in skeletal
- some smooth muscle is phasically active- results in a slow, rhythmic contraction (push food through intestines)
- more common is tonic function- maintains constant level of muscle contraction (the muscle that surrounds blood vessels and airways maintain tonic tone, controls diameter of vessels)
- smooth muscle that surrounds sphincters is also tonic
tonic function of smooth muscle occurs here…
blood vessels, airways, sphincters
smooth muscle of esophagus and urinary bladder
mostly relaxed and only contract intermittently/sporatically (eliminate urine)
brief structure of smooth muscle
no sarcomere
- actin and myosin arranged in stair-step arrangement
- actin attaches directly to inside of cell membrane itself at a dense body, anchored by alpha actinin (dense bodies are parts of smooth muscle membrane itself)
- when smooth muscle contracts, does not shorten linearly like skeletal muscle does –> smooth shortens by twisting and bulging (twists in spiral like fashion)
describe actin structure in smooth muscle
actin still double stranded string of beads, binding sites all along globular actins that make up actin fulaments
- tropomyosin wraps around actin (but no troponin –> so have to activate/initiate actin and contraction in a different way)
- small globular proteins called calponin attached to tropomyosin (unsure function)
- caldesmon wraps around actin, caldesmon masks the actin binding sites in smooth muscles at rest
myosin of smooth muscle
stays the same
intermediate filaments of smooth muscle
non-contractile protein filaments that connect the dense bodies all throughout the membrane
- network of ropes that rope through the membrane, when muscle contracts, the ropes tighten up and help to contract the muscle cell
what are the small indentations/invaginations of membrane in smooth muscle called
caveolae (function like T-tubule)
- underneath caveolae is SR (SR much less developed in smooth muscle)
do smooth muscle cells contain gap junctions?
may or may not, some do to communicate directly with one another
smooth muscle is very diff at ___ and ___ level than skeletal muscle in most cases
cellular
molecular
smooth muscle is ___, meaning that the AP that initiates contraction in smooth muscle originates at the membrane of the smooth muscle itself (generates its own AP, instead of from motor neuron)
myogenic
describe how smooth muscle generates its own Ap
funny sodium channel (not voltage or ligand-gated), stays open and leaky all the time- this allows sodium current to slowly diffuse through membrane and slowly and spontaneously depolarize membrane to threshold- doesn’t need any external stimulus
- as it gets near threshold, depolarization recruits calcium channel called T-type (transient type) calcium channel
- funny sodium channel and calcium channel depolarize membrane to threshold –> at threshold, activation of voltage-gated L-type (long lasting) calcium channel, stays open
–> as a result of L-type channel opening, inward directed calcium current –> wave of depolarization of the AP
what ion causes an AP in smooth muscle
calcium (instead of sodium)
describe the transmembrane potential, the overshoot, and wave of repolarization
- maximum transmembrane potential is -60 mV
- wave of depolarization is a little slower and less powerful b/c influx of calcium is a little slower
- AP depolarizes membrane to +10
- once hit overshoot, L-type channel closes and voltage-gated potassium channels repolarize the membrane
- wave of repolarization gets no where near -90 mV b/c funny sodium channels are still open and begin to depolarize the membrane again
- gets to -60, potassium channels begin to close, all the other channels begin to open rhythmically and spontaneously cause another AP
caleolae’s function in contraction
(short, small T-tubule like invaginations of membrane)
- L-type calcium channels most heavily located in caveolae
- initial inward movement of calcium called calcium spark- calcium itself activates ryanodine receptor, results in outward diffusion of calcium which floods the smooth muscle (called calcium-induced-calcium-release) –> build up of all the calcium which initiates contraction of smooth muscle
2 types of smooth muscle: ___ and ___ unit…
single and multi unit
single unit- muscle cells that have gap junctions that connect each other- depolarization in one cell, influx of calcium- calcium signal directly sent to adjacent cell, so immediate contraction, keeps goin on down the line (only need to depolarize one cell, then get contraction in entire muscle)
- smooth muscle still can receive neural signals to speed/slow down (regulate muscle, instead of initiate- only one cell needs the neural signal to act on the whole muscle
multi unit- cells have to each be regulated by the nervous system to speed up/slow down
how is myosin activated to be ready for contraction in smooth muscle?
calcium floods smooth muscle through calcium spark and calcium-induced-calcium-release –> Ca binds to calcium-binding protein called calmodulin –> activates enzyme called myosin light chain kinase (MLCK) –> MLCK phosphorylates the myosin head, this takes myosin from folded up configuration to normal straightened, ready to work configuration to interact with actin
how is actin activated to be ready for contraction?
at rest, actin-tropomodulin-caldesmon complex (caldesmon masks actin binding sites, need to get caldesmon off)
- can do this through calcium-calmodulin complex, which binds directly to caldesmon and pulls it off –> activates actin, actin can interact with myosin –> allows powerstroke
- other way to pull caldesmon off is to activate a MAP kinase, that phosphorylates caldesmon –> pulls it off actin
describe muscle relaxation in smooth muscle
similar to skeletal-
calcium pump ATPase pumps calcium back into SR, also calcium ATPase in muscle itself which pumps it back to interstitial fluid
- also sodium pump: (3 Na out and 2 K in), the 3 sodium work in a sodium-calcium exchange, sodium comes in and pushes calcium back out
the main mechanism for getting calcium out of SR to start depolarization and contraction is calcium-induced-calcium-release through ryanodine receptor), what is the 2nd mechanism?
ligand-gated channel which activates a G protein –> activates enzyme called phospholipase C- converts membrane phospholipids to IP3 –> IP3 interacts with IP3 receptor on membrane of SR, opens another calcium channel, which facilitates release of calcium from SR
contraction of smooth muscle is regulated by nerves…
the nerves do not form classical NMJ with smooth muscle membranes, instead, the terminals just spread out and form webbed-like structure with little swellings all along the terminus (swellings contain the neurotransmitter, which is released and regulates smooth muscle contraction- speeds or slows)
