Smooth Muscle Flashcards
Where is smooth muscle located?
lines viscera GI tract, respiratory tract, blood vessels, reproductive system
Describe the structure of smooth muscle?
- non-striated fibers
- spindle-shaped
- tapered
Do smooth muscle have T-tubules?
no
Is smooth muscle contraction voluntary or involuntary?
involuntary (spontaneous)
What is smooth muscle contraction regulated by?
ANS (both sympathetic and parasympathetic systems)
What is the function of smooth muscle in arteries and veins?
control vessel diameter (vasoconstriction/vasodilation) to regulate blood pressure
What is the function of smooth muscle in intestine?
longitudinal and circular layers work together to participate in peristalsis
What is peristalsis?
progressive and rhythmic contractions of smooth muscle that propel food through digestive tract
What are the 2 layers of smooth muscle in intestine?
- longitudinal
- circular
What does the longitudinal layer of intestine smooth muscle do?
constricts to allow food to go down GI tract
What does the circular layer of intestine smooth muscle do?
constricts to prevent food from going up GI tract
What is the function of smooth muscle in stomach?
longitudinal, circular, and oblique layers work together to mix food when it arrives in stomach, and propel it down to intestines
What is the organization of smooth muscle in arteries and veins?
tunica media
What is the tunica media made of?
made of smooth muscle cells and elastic tissue
What does the tunica media surround?
artery and vein walls
Where is the tunica media thicker?
thicker in arteries compared to veins – arteries have to sustain higher blood circulation, therefore thicker layer protects from damage
What is the organization of smooth muscle in intestine?
two main layers of smooth muscle
- longitudinal layer
- circular layer
What is the longitudinal layer of smooth muscle in intestine?
outer layer of muscle fibers that run parallel to long axis of organ
What is the circular layer of smooth muscle in intestine?
inner layer of muscle fibers that run around circumference of organ
What is the organization of smooth muscle in stomach?
3 layers of smooth muscle
- longitudinal layer (upper part of stomach)
- circular layer (middle part of stomach)
- oblique layer (lower part of stomach)
Describe the innervation of smooth muscle by ANS
axon of postganglionic autonomic neuron branches once it hits smooth muscle
- innervates smooth muscle, but there’s no synaptic contact with smooth muscle
What are varicosities?
swellings along axonal branches filled with neurotransmitters that are released on top of plasma membrane of smooth muscle fibers, and bind to their appropriate receptors
What is a single-unit muscle?
one group of smooth muscle, with a single axon over the group of muscle fibers
How are cells connected in single-unit muscle?
by gap junctions
- there are more gap junctions than in multi-unit muscle
How do single-unit muscle contract as a unit?
excitation in one cell is rapidly transmitted to ‘connected’ cells – allows entire unit to contract synergistically at the same time
Where are single-unit muscles found?
- walls of viscera: digestive, urinary, and reproductive tracts
- walls of small blood vessels
What are multi-unit muscle?
groups of smooth muscles separated by each other by branches of ANS neurons
How do multi-unit muscle contract as a unit?
one unit can contract independently of the neighbouring units
Where are multi-unit muscles found?
- walls of large blood vessels
- small airways of lungs
- muscle of eye (lens)
- iris of eye
- base of hair follicles
What is a myosin molecule composed of?
- 2 myosin heads
- 2 essential myosin light chains
- 2 accessory myosin light chains
Which myosin light chains get phosphorylated?
accessory myosin light chains
What are the 2 sites on each of the myosin heads?
- site that binds to actin
- site that hydrolyzes ATP into ADP and Pi
What are caveolae?
plasma membrane invaginations
What do caveolae contain?
dihydropyridine receptors, through which influx of Ca2+ from ECF passes to enter sarcoplasm
What do gap junctions allow?
direct electrical communication between adjacent smooth muscle fibers
Where is gap junction?
membranes of two adjacent muscle fibres are very close to one another
How are thin filaments arranged?
obliquely in cell
What are thin filaments attached to?
- some attach to dense bodies in cytoplasm
- some attach to dense plaque of plasma membrane
What do contractile units of smooth muscle consist of?
myosin and actin filaments attached to dense bodies in sarcoplasm or plasma membrane plaques
Where are contractile units of smooth muscle?
stretch across cell, between plasma membrane plaques
What happens when contractile apparatus is close to plasma membrane?
it binds to membrane via dense plaque on one end, and dense body (in cytoplasm) on other end
What happens to inside contents of the cell when smooth muscle fiber is contracted?
everything gets wrinkled and shrinks
all organelles in cell get caught into mesh of cytoskeletal proteins (including intermediate filaments)
What is the contractile apparatus in skeletal and cardiac muscle?
sarcomere
- organized
- parallel to each other
Pharmacomechanical Coupling Process
- receptors bind to hormones/neurotransmitters
- production of IP3 within cytoplasm
- IP3 binds to IP3R
- induces release of Ca2+ in SR into sarcoplasm
**no need for membrane depolarization – pressure signalling causes depolarization
Smooth Muscle Contraction Process
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