D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
MUSCLE CONTRACTION
Lecture 8: Properties of Smooth Muscle
1.
Smooth Muscle: (chapter 8)
• characteristics: fibers much smaller (up to 1000X shorter and 1/30 diameter
of skeletal muscle); contractile elements are less well organized (no sarcomeres to
line up in register –> no striations)
- two patterns of organization (fig. 8 – 1 & ppt. 1) – multiunit is individual
fibers with own separate nerve supply, e.g. ciliary muscles of eye; contractions are
faster than visceral type (more similar to characteristics of skeletal m. contraction
than is unitary smooth muscle)
- unitary smooth muscle (visceral smooth muscle) is a sheet-like
population of fibers (ppt. 2); nerve supply less discrete, but stimuli pass readily
throughout population by means of gap junctions; contractions are slower, more
sustained (often hours in duration = tonic contractions); found in tubular organs, e.g.
ureter, intestines, blood vessels
• smooth muscle action potentials: spontaneous depolarization
(autostimulation) results from less negative resting potential (-50 to -60 mv.) and
permeability of sarcolemma; sufficient numbers of sodium-calcium (slow) channels
are open at resting potential to provoke depolarization; in some cases, rhythmical
'slow waves' occur (graded potentials); in many cases, threshold is exceeded and
action potentials occur (may be spike potentials or APs with plateau) (figs. 5 - 14, 8 - 5
& ppts. 3 & 4)
- depolarization is largely due to fast sodium & slow calcium influx; the latter
serves double duty as 'switch' to activate contraction
Bio 659
- p. 2 -
- most calcium influx is from ECF, although some comes from
sarcoplasmic reticulum (sparser than skeletal muscle); SR is excited via caveolae that
bring APs in proximity to SR calcium channels (fig. 8 - 6 & ppts. 5 & 6)
Bio 659
- p. 3 -
• excitation of smooth muscle: calcium ions activate contraction as in skeletal
muscle, but not via troponin & tropomyosin, which are absent from smooth m.
- smooth muscle depolarization involves calcium influx (little sodium)
- calcium ions interact with a protein (calmodulin), which activates
phosphorylation of myosin to stimulate contraction
- cycling of contractile mechanism continues as long as calcium remains
high; when calcium level drops, another enzyme (myosin phosphatase) disables
contractile process (fig. 8 - 3 & ppts. 7 & 8)
• contractile mechanism: myosin & actin filaments interact in sliding
filament fashion as in skeletal m.; thin filaments extend from dense bodies (no Z
discs) distributed throughout cytoplasm (part of cytoskeleton, interconnected by
intermediate filaments)
- some dense bodies are anchored to the cell membrane & connected to
dense bodies of neighboring smooth muscle cells; this facilitates transmission of
force amongst muscle cells (fig. 8 - 2 & ppt. 10)
- thin filaments from neighboring dense bodies overlap with myosin
filaments; orientation of contractile units causes more of a twisting or wringing
contraction (as opposed to linear shortening of skeletal m.) (ppt. 11)
• tonic contraction: sustained state of contraction can be maintained at very
little energy cost – myosin cross-bridges can remain attached to thin filament resulting
in slow or arrested cycling of contractile process (latch mechanism)
- stress-relaxation: stretched smooth muscle responds with contraction
(increase pressure in lumen of hollow organ) then returns to the contractile force it
had prior to the stretch (accommodates volume change without prolonged increase
in pressure); facilitates maintenance of constant pressure within hollow organs that
change volume of contents; reverse stress-relaxation also occurs
Bio 659
- p. 4 -
• nerve supply (fig. 8 – 4 & ppts. 12 & 13): nerve fibers do not make discrete
neuromuscular junctions (like motor endplates) in most smooth muscle, but some
multiunit fibers have direct contact with nerve fibers (like motor units in skeletal
muscle)
- most junctions are diffuse junctions involving varicosities along the axon
distributed over the surface of the smooth muscle
- neurotransmitters may be excitatory or inhibitory, making it possible to
increase or decrease rate and strength of contractions; neurotransmitters are mainly
acetylcholine & norepinephrine, which operate in opposition to one another; in some
tissues one is inhibitory & the other excitatory, but in other tissues the reverse is true
• various hormones may stimulate or inhibit smooth muscle: e.g.,
gastrointestinal hormones, histamine, angiotensin, etc.; these regulate contractile
activity (often without action potentials) via receptors operating chemically-gated calcium
channels