Smooth Muscle Contraction Flashcards
How does the sympathetic nerve cause arterial narrowing?
The sympathetic nerve is not myelinated and branches out in a vine-like or “beads on a string” pattern. The bulges in the nerve fibres, known as varicosities, contain vesicles with neurotransmitters. When these neurotransmitters, such as noradrenaline, are released onto smooth muscle cells in arteries, they cause the muscle fibres to contract, leading to arterial narrowing.
What is the role of varicosities in the sympathetic nervous system?
Varicosities are bulges along the sympathetic nerve fibers that contain vesicles filled with neurotransmitters like noradrenaline. These neurotransmitters are released at varicosities and act on smooth muscle cells, causing contraction.
What neurotransmitter is involved in the sympathetic nerve-mediated contraction of smooth muscle?
The neurotransmitter involved is noradrenaline, which is released from varicosities and acts on smooth muscle cells to trigger contraction.
What is the structure of smooth muscle?
Smooth muscle cells are spindle-shaped and contain a single nucleus. They lack the striations found in skeletal and cardiac muscle, as their actin and myosin filaments are not arranged in a regular pattern.
How does smooth muscle contract?
When noradrenaline is released, it binds to an alpha-1 G protein-coupled receptor on the smooth muscle cell. This activates phospholipase C, which generates inositol trisphosphate (IP3). IP3 releases calcium ions from the sarcoplasmic reticulum, leading to an influx of calcium. This calcium activates calmodulin, which then activates myosin light chain kinase (MLCK), ultimately causing muscle contraction.
What receptor is involved in smooth muscle contraction due to noradrenaline?
The receptor involved is the alpha-1 G protein-coupled receptor, which, when activated by noradrenaline, leads to the release of calcium ions and muscle contraction.
What happens when calcium ions enter smooth muscle cells?
When calcium ions enter smooth muscle cells, they bind to calmodulin, which then activates myosin light chain kinase (MLCK). This enzyme phosphorylates the myosin light chain, increasing myosin’s ATPase activity and leading to muscle contraction.
What is the role of Myosin Light Chain Phosphatase (MLCP) in smooth muscle contraction?
MLCP removes phosphate groups from the myosin light chain, which reduces myosin’s ATPase activity and leads to muscle relaxation. This is part of the off mechanism for smooth muscle contraction.
What is the long-term mechanism of force generation in smooth muscle?
In smooth muscle, dephosphorylated myosin has low affinity for ATP and thus remains attached to actin for extended periods. This allows smooth muscle to maintain a degree of contraction or shortening over long periods without the continuous use of ATP.
How does smooth muscle maintain contraction for long periods?
Smooth muscle can maintain contraction for extended periods because dephosphorylated myosin has a low ATP affinity, allowing it to stay bound to actin without needing to repeatedly detach and reattach, which conserves energy.
How does Protein Kinase A (PKA) affect smooth muscle contraction?
PKA (activated by cyclic AMP from beta-adrenergic stimulation) inhibits myosin light chain kinase (MLCK). As a result, even if calcium levels rise, MLCK is unable to activate myosin, leading to relaxation of the smooth muscle.
What causes smooth muscle relaxation in the presence of calcium ions?
Even in the presence of calcium, PKA can inhibit MLCK, preventing the phosphorylation of myosin and leading to relaxation of the smooth muscle. This mechanism is important for controlling smooth muscle tone, especially in response to beta-adrenergic signaling.
What is the role of IP3 in smooth muscle contraction?
IP3 (inositol trisphosphate) is produced by the activation of phospholipase C in response to the activation of alpha-1 G protein-coupled receptors. IP3 triggers the release of calcium ions from the sarcoplasmic reticulum, which is crucial for smooth muscle contraction.
What differentiates smooth muscle from skeletal and cardiac muscle in terms of relaxation?
Smooth muscle relaxes more slowly than skeletal and cardiac muscle. This slower relaxation is partly due to the sustained interaction of myosin and actin, which can remain attached for long periods due to the low ATP affinity of smooth muscle myosin.
