TERM 1
Microscopic Structure of Smooth Muscle
Fibers
DEFINITION 1
Peristalsis: The alternating contracti on and relaxation of these two layers mixes
substances in the lumen and squeezes the m through the origin's internal
pathwayInnervating nerve fibers, which are pa rt of the automatic (involuntary)
nervous system, have numerous bulbous s wellings, called
varicositiesVaricosities:release neurotransm itter into a wide synaptic cleft in
the general area of the smooth muscle cells ; such junctions are called diffuse
junctionsComparing the ne ural input to skeletal and smooth muscles, you could
say the skeletal muscle gets priority mail wh ile smooth muscle gets bulk
mailingsThe SR lacks a specific pattern relativ e to the myofilamentsT Tubules are
absent, but the sarcolemma has multiple caveolae , pouchlike infoldings that
sequester bits of extracellular fluid containing a hgh concentration of Ca close to
the membraneWhen calcium channels in t he caveolae open, Ca influx occurs
rapidlyAlthough the SR does release some o f the calcium that triggers
contraction, most Ca entrers through calcium channels directly from the
extracellular spaceContraction ends when cyt oplasmic calcium is actively
transported into the SR and out of the cellNo striations in smooth muscle
therefore no sarcomeresDo not contain inter digitating thick and thin filaments,
but the myosin filaments are a lot shorter tha n the actin filaments and the type
of myosin contained differs from skeletal mu scle
TERM 2
The proportion and organization of smooth
muscle myofilaments differ from skeletal
muscle in the following ways:
DEFINITION 2
Thick filaments are fewer but have mysoin h eads along their entire length: the
ratio of thick to thin filaments is much lowe r in smooth muscle; However, thick
filaments of smooth muscle contain actin-gr ipping myosin heads along their
entire length. The myosin heads are oriente d in one direction on one side of the
filament and in the opposite direction on the other sideNo troponin complex in
thin filaments: a protein called calmodulin ac ts as the calcium binding siteThick
and thin filaments arranged diagonally: the sm ooth muscles cells contract in a
twisting way so that they look like tiny corks crewsIntermediate filament-dense
body network: contain a lattice-like arrangeme nt of noncontractile intermediate
filaments that resist tension; attach at regular intervals to cytoplasmic structures
called dense bodies- these dense bodies, w hich are also tethered to to the
sarcolemma, act as anchoring points for thin filaments and therefore correspond
to Z discs of skeletal muscle. Forms a stron g, cable-like intracellular cytoskeleton
that harnesses the pull generated by the slid ing of the thick and thin filaments.
TERM 3
Contraction of Smooth Muscle
DEFINITION 3
Most cases, adjacent smooth muscle fiber s exhibit slow, synchronized
contractions, the whole sheet responding to a stimulus in unisonThis
synchronization reflects electrical coupling of smooth muscle cells by gap
junctions, specialized cell connectionsElectrica lly isolated from one
another, each stimulated to contract by its own neuromuscular junctionBy
contrast, gap junctions allow smooth musc les to transmit action
potentials from fiber to fiberSome smooth muscle fibers in the stomach
and small intestine are pacemaker cells: o nce excited, they act as
"drummer" to set the pace of contraction for the entire muscle
sheetPacemakers depolarize spontaneous ly in the absence of external
stimuli
TERM 4
Contraction of Smooth Muscle
DEFINITION 4
Contraction in smooth muscle is like contra ction in skeletal muscle in the
following ways:Actin and myosin interact by the sliding filament mechanismThe
final trigger for contraction is a rise in the intra cellular calcium ion levelATP
energizes the sliding processDuring excitatio n-contraction coupling, the tubules
of the SR release Ca but as mentioned abov e, Ca also moves into the cell from
the extracellular space via membrane channe lsCalcium activates myosin by
interacting with a regulatory molecule called calmodulin, a cytoplasmic calcium-
binding proteinCalmodulin, in turn, interacts with a kinase enzyme called myosin
kinase or myosin light chain kinase, which phosphorylates the myosin,
activating it
TERM 5
Contraction of Smooth Muscle
DEFINITION 5
Smooth muscle relaxes when intracellular C a levels drop- but getting smooth
muscle to stop contracting is more complex Energy Efficiency of Smooth Muscle
Contraction:Can maintain the same contractil e tension for prolonged periods at
less than 1% energy costPart of the striking e nergy economy of smooth muscle is
the sluggishness of its ATPases compared to those in skeletal muscleSmooth
muscle myofilaments may latch together du ring prolonged contractions, saving
energy in that way as wellMay maintain that l atch state even after myosin is
dephosphorylatedThe smooth muscle in sm all arterioles and other visceral
organs routinely maintains a moderate degre e of contraction, called smooth
muscle tone, day in and day out without fatig uingSmooth muscle has low energy
requirements, and as a rule, it makes enou gh ATP via aerobic pathways to keep
up with the deman
