15 Questions
What is the primary function of tonic smooth muscles?
To provide continuous tonus
What is the role of calponin (CaP) in smooth muscle cells?
Actin binding protein that inhibits actomyosin ATPase activity
How does the diameter of skeletal muscle fibers compare to that of smooth muscle fibers?
Skeletal muscle fibers are up to 30 times greater in diameter than smooth muscle fibers
What is the main factor initiating smooth muscle contraction?
Increase in intracellular calcium ions
What is the role of myosin light chain kinase in smooth muscle contraction?
Activated by calmodulin-calcium complex to phosphorylate myosin
Where do most calcium ions for smooth muscle contraction come from?
Extracellular fluid
What type of potential is the slow wave potential in unitary smooth muscle?
Pacemaker potential
What is the role of calcium ions during the smooth muscle action potential?
They act directly on the smooth muscle contractile mechanism to cause contraction
What is the approximate threshold for eliciting action potentials in most visceral smooth muscle?
$-35$ millivolts
What initiates approximately half of all smooth muscle contractions?
Local tissue chemical factors
Which hormone inhibits contraction of smooth muscle in the intestine but stimulates contraction of smooth muscle in blood vessels?
Norepinephrine
In what way are smooth muscles different from skeletal and cardiac muscles in terms of contraction and relaxation?
They are the slowest to contract and to relax
What type of smooth muscle cells are connected by gap junctions and contract as a single unit?
Unitary smooth muscle
Which type of smooth muscle fibers can contract independently and are controlled mainly by nerve signals?
Multi-unit smooth muscle
What is the major difference in the initiation of contraction between smooth muscle and skeletal muscle?
The initiation of contraction in response to calcium ions is much slower in smooth muscle compared to skeletal muscle
Study Notes
Smooth Muscle Physiology
- Smooth muscles serve as the cellular skeleton and contain proteins such as desmin, filamin, crystallin, and vimentin.
- They do not directly participate in contraction but form part of the cytoskeletal framework that supports cell shape.
- There are two major types of smooth muscles: multi-unit smooth muscle and unitary (or single-unit) smooth muscle.
- Unitary smooth muscle cells are connected by gap junctions and contract as a single unit, found primarily in the walls of viscera.
- Multi-unit smooth muscle fibers can contract independently and are controlled mainly by nerve signals.
- Smooth muscles are innervated by the autonomic nervous system (parasympathetic and sympathetic) and have impulse-generating pacemaker cells in many visceral organs.
- Unitary smooth muscles are also called syncytial or visceral smooth muscles and are found in the gastrointestinal tract, bile ducts, ureters, uterus, and blood vessels.
- Smooth muscle contraction is activated by calcium ions, and ATP is degraded to ADP to provide energy for contraction.
- Unlike striated muscle, smooth muscle does not contain the troponin complex and has different mechanisms for the control of contraction.
- Smooth muscle lacks the striated arrangement of actin and myosin filaments found in skeletal muscle, and has large numbers of actin filaments attached to dense bodies.
- Smooth muscle contraction is sustained due to the slow cycling of myosin cross-bridges, low energy requirement, and slowness of onset and relaxation.
- The initiation of contraction in response to calcium ions is much slower in smooth muscle compared to skeletal muscle.
Test your knowledge of smooth muscle physiology with this quiz. Learn about the different types of smooth muscles, their cellular structure, contraction mechanisms, and innervation by the autonomic nervous system. Understand the unique features of smooth muscle compared to skeletal muscle and its role in various visceral organs.
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