8. Smooth Muscle Histology and Physiology Flashcards
• Locations: – Wall of \_\_\_\_ organs, blood vessels, \_\_\_\_ of the eye, \_\_\_\_ muscle of the hair follicles. • Regulation of the internal environment: e.g. movement of fluids, air. • Capable of \_\_\_\_ and \_\_\_\_. • Very economical in energy terms. • Involuntary control: – \_\_\_\_ system. – Circulating \_\_\_\_. – \_\_\_\_ hormones.
• Multiple arrangements:
– ____
– ____
– ____
tubular
iris and ciliary body
piloerector
contraction
relaxation
autonomous
hormones
local
sheets
rings
bundles
III. Smooth Muscle
Unique to smooth: capable of contraction and relaxation; striated muscle is always referred to by contraction, only relaxes once stimulation is eliminated; smooth muscle has ____ (neural/hormonal) that induce a contraction (similar to skeletal muscle), and there are modulators that induce ____
Steady state of smooth muscle = always ____ (mechanisms to induce relaxation smooth muscle, different from skeletal)
Smooth muscle uses little energy when compared to skeletal muscle (binding actin to myosin, and myosin in smooth muscle has a low rate of ATPase-activity [____ lower]) > contraction is much ____ than skeletal, but very ____
regulators
relaxation
contracted
100-fold
slower
cheap
Smooth Muscle Histology
• The smooth muscle cell: • Small: average length = \_\_\_\_ μm. • Fusiform with a \_\_\_\_ nucleus. • not \_\_\_\_. • \_\_\_\_ instead of Z disks. • Ionic connections between cells through gap junctions: \_\_\_\_.
Cell that is nothing like skeletal/striated muscle > these cells can be confused with connective tissue cells = ____; similar phenotype: small (similar to euk cell in mammals)
Cytoplasm has no indication of ____
Highly interconnected (both mechanical and electrical connections) > induced to contract and relax as a single functional unit, in a coordinated manner
Each cell has one nucleus, but the ____ acts sintitially (due to the nature of their mechincal/electrical connections)
50-500 single central striatied dense bodies functional syncytium
fibroblasts
sarcomeres
organ
The Smooth Muscle Cell
- No ____ system.
- Instead of T tubules → ____:
- Invaginations of the sarcolemma:
- ↑ ____ → ↑ Ca2+ transport.
- Associated with the ____.
- Sarcoplasmic reticulum:
- Poorly ____.
- Organized in tubes or sheets.
- Myofilaments:
- Thick ____ (slow myosin) filaments.
- Thin ____ filaments.
- Intermediate filaments (desmin, vimentin).
- Cytoplasmic and plasma membrane dense bodies (α-actinin):
- Functionally similar to ____.
- Attachment of actin and intermediate filaments.
- Cell connections:
- Mechanical: through ____
- Electrical: through gap junctions - ____.
- ____ surrounding each SMC.
T tubules
caveolae
surface area
SR
developed
myosin
actin
Z disks
desmosomes
funtional syncytium
basal lamina
The Smooth Muscle Cell
No sarcomeres
Caveolae > increase ____ > enrichment in receptors involved with modulation, and in calcium channels
SR in smooth is morphologically identical to a ____ (like in fibroblasts)
Myofilaments are not bound to Z-disks > dense bodies (two types): ____ dense bodies, and ____ dense bodies > built by same protein as Z disk, but structure is very different
Electrical connection > flow of ions, ions traffick from one cell to the next thereby transmitting the contractile impulse
Mechanical connection > ensures that cells that are tightly attached will be pushed/pulled > ____ of smooth muscle fiber
____ is produced by smooth muscle cell
SA
regular ER
PM
cytoplasmic
coordinated
basal lamina
Smooth Muscle Histology
Longitudinal > ____ nuclei
Cross > the tissue is not different from connective tissue in what they produce [???] (elastin, etc.) but can uniquely be stimulated
centrally located
Structural Arrangements of SM Fibers: Circular Organization
- Typical of ____ and ____.
- Partially contracted at all times: ____.
- Vasoconstriction: additional contraction, which ____ the diameter of the vessel lumen.
- Vasodilation: relaxation, which ____ the diameter of the vessel lumen.
Circular organization > BV and airways > muscle cells are partially contracted at all times, the vessel is never completely relaxed > the ____: partially contracted, or muscle tone/tonicity
From this basal state > BV can be induced to contract > vasoconstriction > modulators increase contraction and diameter is reduced
Other modulators that induce vasodilation, opening the lumen of vessel/airway by relaxing the SMC
BV
airways
muscle tone
narrows
widens
steady state
Structural Arrangements of SM Fibers: Circular and Longitudinal Organization
• Typical of GI tract organs: e.g. ____ and ____ intestines.
- Two layers:
- Outer: ____.
- SM fibers run parallel to organ’s long axis.
- Contraction: organ ____.
• Inner: ____.
• SM fibers run around circumference of
organ.
• Contraction: organ ____
• Both layers coordinate contractions and relaxations: ____ movements.
small
large
longitudinal
shortening
circular
elongation
peristaltic
The Smooth Muscle Neuromuscular Junctions
• Very different from the skeletal muscle NMJ.
- Two types:
- Unitary NMJ:
- Hundreds to millions of muscle cells act as a functional unit.
• SMCs mechanically connected by desmosomes and electrically connected by gap junctions.
- Autonomic nerve cells do not make ____ with the SMCs: ____ junctions:
- Release of neurotransmitters into the ECM at some ____ from the cells: diffusion to reach target SMCs.
- Neurotransmitter released from ____: enlargements on the axon.
- E.g. ____ wall, ____ and many blood vessels.
Unitary > innervated by a ____ from the autonomous NS (functional unit), but it is not in direct contact with any of the cells > releases NTs from specialized structures > and disperse to the entire FU and they are induced to contract; all these cells are also connected via gap junctions and the diffusion spread via gap junction
SMC REACT TO TWO STIMULI: direct binding to ____, and the ____ between cells > result: contract/relax in a coordinated matter
direct contact diffuse distance varicosities gastrointestinal uterus
single neural termination
receptors
electrical gap junction
The SM Neuromuscular Junctions
Multiunit NMJ:
Each SMC is innervated ____: low cell- to-cell communication.
Nerve cell varicosities make ____ contact with the target SMCs: ____ junctions.
E.g. ____ muscle of the hair follicle, the ____ and ____ muscles of the eye.
The contact between neuron and cell is in direct contact but not as ____ as skeletal muscle
independently direct contact piloerector iris ciliary
intimate
Contraction and Excitation of Smooth Muscle
• SM contracts or relaxes in response to nerves and stimuli (striated muscle can only ____).
• Numerous possible inputs to contract or relax smooth muscle:
a. ____ nerves.
b. Circulating hormones: e.g. angiotensin, vasopressin.
c. ____ hormones (or paracrines): e.g. histamine, somatostatin, and
endothelin [potent vasoconstrictor]).
d. Ions and metabolites: e.g. O2 , CO2.
e. ____ from other cells: e.g. NO.
f. Signals from electrically-coupled SMCs.
g. ____ cues: e.g. stretch.
contract autonomic local signals mechanical
Contraction and Excitation of SM: Neuromuscular Transmission
Neuromuscular transmission in SM:
• ____ process (up to minutes).
• Commonly modulates ____
rhythmic activity.
Contraction/relaxation of smooth muscle can be initiated by:
- Intrinsic activity of ____ cells.
- Neural transmitters.
- Circulating or locally generated hormones or signaling molecules.
Multiunit muscles:
• No ____.
• ____ is most important.
Single unit muscles:
• ____ is propagated throughout the tissue.
Phasic contraction/relaxation > generate peristaltic movements [???}
Smooth muscle present in tissues that contain specialized cells (may not be ____) > that may regulate the pace of contraction
slow
spontaneous
pacemaker
electrical coupling
neural regulation
electrical activity
smooth muscle
Contraction and Excitation of Smooth Muscle: Membrane potentials
Resting Vm: -50 mV, (~ ____ higher than in skeletal muscle).
A. Spike potentials:
• Similar to those in skeletal muscle.
• Triggered by nerve stimulation, stretch or hormones; also
____.
• Single twitch or larger summed ____ responses.
• Characteristic of single-unit SM (e.g. ____).
• Gap junctions permit the spread of action potentials
throughout the tissue.
Much more negative potential in skeletal muscle cells
30
spontaneously
mechanical
visceral SM
Contraction and Excitation of Smooth Muscle: Membrane potentials
B. Slow waves:
• Rhythmic activity that may trigger action potentials.
• Not ____: fluctuations in membrane
potential from the activity of ____
(e.g. ____) in the cell membrane.
• Typical of the intestinal wall: important in ____.
Slow waves > ____, internal change in membrane potential due to variations in pumps working (pumping different rates at different times)
Sometimes the potential can be so depolarized that it is enough to cross the threshold and create an AP (NT has to induce traffic of ions, and if it is above threshold, more ions would enter and the depolarization would be even higher)
Cells contract ____ by themselves; waves are not AP, but sometimes the depolarization is so high that it induces the formation of an AP
action potentials
electrogenic umps
Na+/K+ ATPase
peristalsis
innate
spontaneously
Contraction and Excitation of SM: Membrane potentials
C. Graded changes in membrane potential:
____ contractile activity related to membrane potential in the absence of action potentials. Common in ____ (e.g., vasculature).
Never a creation in ____ > change in membrane potential > some degree of contraction/relaxation > variations in amount of ____, without the need to generate an AP
tonic
multiunit SM
AP
contractile force
