Smooth muscle physio Flashcards
connexons in smooth muscle
Formed like connexins in cardiac muscle, to form electrical syncytium; 1.5nm in diameter, six subunits
two types of smooth muscle contractions
tonic and phasic; tonic is constant, phasic is more like waveform. Phasic is single unit and tonic is more typically multi-unit
Varicosities in smooth muscle
varicosities are more important in the multi-unit type as each individual muscle cell has to be innervated, whereas for the single unit type of smooth muscle, the gap junctions can propagate an AP contraction (or relaxation) wave
examples of phasic smooth muscle
think GI and GU, except the vas deferens (which is already isolated because it is activated by the sympathetic system); GI tract urinary bladder ureter uterus
examples of tonic smooth muscle
think muscles that contract for effect; sympathetic type innervations; pilomotor ciliary iris arterioles vas deferens
channels in smooth muscle
Present: Na/K pump
Ca pump
Ca channel (L-type)
K channel (Kv)
Notably Absent:
Na(v) - voltage gated sodium channel
T-tubules (not a channel, but important for the spread of action potentials throughout the cell)
messengers embedded in the sarcoplasma of smoooth muscle cells
IP3, DAG, cAMP (adenylyl cyclase)
sarcoplasmic reticulum in smooth muscle cells
located near the sarcolemma
contains Ca release channels that are activated by IP3
has lower capacity than the same in heart or skeletal muscle
tension/relaxation of smooth muscle and concordinant calcium flux
Ca can enter the smooth muscle cell via VGCC and ligand gated Ca channels;
It can then be released from the SR by IP3 mediated channels
These steps lead to contraction
Ca flows back into the SR or out of the cell
That leads to relaxation
p.298
actin/myosin/tropomyosin in smooth muscle
actin and tropomyosin found in greater quantities relative to myosin, compared to skeletal and cardiac muscle
not organized into sarcomeres - more like a spiral conduit with long actin filaments bound to much smaller myosin bands; all bound to dense bodies which is the midpoint of the actin filaments and the equivalent of z-discs
p. 298
three types of membrane depolarizations in smooth muscle
1) action potentials - just like skeletal or cardiac depol.
2) Rhythmical activity - wave-like potential changes that may generate quick bursts of action potentials then recede for some time
3) Graded changes in membrane potential that may be strong enough to increase contractile force
p. 299
action potentials in smooth muscle
generated by VGCC (L-type) and therefore arise much more slowly than Na gated channels
Rhythmical electrical activity in smooth muscle
produced by changes in the Ca-dependent K channels (K-sub-Ca)
p.299
Graded changes in electrical potential of smooth muscle
ligand created changes that alter K permeability. K channels are shut off by activation of PLC by DAG by Gq. depolarization (due to K maintenance) causes opening of Ca channels (L-type) which gives enough influx to cause some contraction
Biochemical coupling of contraction in smooth muscle cells
Ca can be released via IP3 by PLC (on PIP2) by Gq by alpha GPCR by norepinephrine - no action potential or membrane depolarization necessary
