Objective 20 Flashcards
How is smooth muscle arranged?
typically organized into two layers (longitudinal and circular)
Where are smooth muscles found?
in the walls of hollow organs (except the heart)
What is perstalsis?
The movement created by alternating contractions and relaxations of these muscle layers in the GI tract
Smooth Muscle Microscopic Anatomy:
What do the fibers of smooth muscle look like and how are they connected?
They are short spindle-shaped fibers, connected in sheets
Smooth Muscle Microscopic Anatomy:
Are there fibrous middle or outer CT layers in smooth muscle?
No. Smooth muscle has endomysium only – (no fibrous middle or outer CT layers)
Smooth Muscle Microscopic Anatomy:
How does SR of smooth muscle compare to skeletal muscle?
SR is less developed in smooth muscle; no T tubules
Smooth Muscle Microscopic Anatomy:
What are caveolae?
pouchlike infoldings in plasma membranes
Where is Ca2+ sequestered?
In the extracellular space near the caveolae (much less in the limited SR)
Are there sarcomeres in smooth muscle?
No. Thick and thin filaments are present but there are no visible striations; arranged in a spiral so muscle contracts in a corkscrew-like fashion
What protein acts as the binding site in smooth muscle?
Calmodulin acts as the calcium binding site. There is no troponin complex
What resists tension and creates anchoring points for actin?
A dense body-intermediate filament cytoskeleton network
Describe how smooth muscle is innervated.
Autonomic nerve fibers innervate smooth muscle at diffuse junctions
What are varicosities?
Bulbous swellings that release neurotransmitters into wide synaptic clefts
What happens when whole sheets of smooth muscle exhibit slow, synchronized contraction
Contraction of smooth muscle
When smooth muscle contracts, how are cells electrically coupled?
Cells are electrically coupled with gap junctions
Which cells are self-excitatory and depolarize without external stimuli?
smooth muscle cells that act as pacemakers
What is the final trigger for contraction of a smooth muscle contraction?
increased intracellular Ca2+ (from caveolae)
What ultimately activates myosin in smooth muscle contraction?
Activated calmodulin ultimately activates myosin
In smooth muscle contraction, after myosin is activated, what happens next?
Cross-bridges form with actin
In smooth muscle contraction, the activation of myosin and actin is similar to what?
similar to skeletal muscle contraction – the sliding filament mechanism of actin and myosin
What is required in order for smooth muscle to relax?
Relaxation requires Ca2+ to detach from calmodulin (more complex than in skeletal muscle)
Describe the phenomenon of stress-relaxation response
Smooth muscle responds to stretch only briefly, and then adapts to its new length.
- new length retains its ability to contract
What is an example of stress-relaxation response?
When organs such as the stomach and bladder are temporarily storing contents
Within what percentage range of resting length can smooth muscle efficiently contract?
Between 50% (1/2) and 200% (2X) of its resting length
How do smooth muscles grow?
Some (not all) smooth muscle cells can divide and increase their numbers by undergoing hyperplasia – best demonstrated by estrogen’s effect on the uterus.
Give two examples of hyperplasia in the uterus as it responds to estrogen
- At puberty, estrogen stimulates the synthesis of more smooth muscle, causing the uterus to grow to adult size
- during pregnancy, estrogen stimulates uterine smooth muscle growth to accommodate the increasing size of the fetus.
