Smooth muscle Flashcards
Is smooth muscle voluntary?
No, involuntary
Difference between skeletal muscle and smooth muscle (contractions)
Skeletal muscle specialise for forceful contraction of short duration and under fine voluntary control
Smooth muscle specialised for continuous contractions of relatively low force.
Locations of smooth muscle in the body
Walls of hollow organs (e.g urinary bladder, uterus, stomach, intestines)
Walls of passageways (e.g arteries and veins of circulatory system, tracts of respiratory, urinary and reproductive systems)
Present in eyes (functions to change the size of iris and alter the shape of the lens)
Located in skin (causes hair to stand erect in response to cold temperature or fear)
Smooth muscle histology
“spindle shape” (wide in the middle, tapered at both ends)
Have a single nucleus
Don’t have striations and sarcomeres but have actin and myosin contractile proteins and thick and thin filaments
These thin filaments are anchored by dense bodies (analogous to the Z-discs of skeletal and cardiac muscle fibres)
No T tubules
What is calmodulin? (CAM)
Stands for Calcium-modulated protein
It’s an intracellular target of the second messenger Ca^2+
Structurally, CaM is similar to troponin
Explain how smooth muscle contraction works
- Calcium comes from the outside of the endoplasmic reticulum and binds to Calmodulin where calmodulin then goes through a conformational change
- Ca-Calmodulin complex binds to MLCK (Myosin like chain kinase) where the MLCK goes through a conformational change
- The kinase then phosphorylates the myosin light chains.
- This changes the myosin head group structure which makes it activate ATPase
- A cross bridge is able to form and then cycling happens which then results in contraction
How to stop smooth muscle contraction
Need a phosphatase to dephosphorylate the myosin light chain head group which results in smooth muscle relaxation.
What do latch-bridges do?
Keep thin and thick filaments linked together for a prolonged period and without the need for ATP.
Allows for maintaining of muscle “tone” in smooth muscle that lines visceral organs with very little energy expenditure
What can stimulate smooth muscle to contract?
- Hormones
- Stretch
- Metabolic state - CO2, low pH, O2 deficiency
- Autonomic nerve fibres stimulate multiple myocytes at diffuse junctions
What are varicosities?
Unlike skeletal muscle fibres, smooth muscle cells don’t have specialised motor end-plate regions (axon terminals?). They have swollen regions known as varicosities
Each varicosity contain many vesicles filled with neurotransmitter (some released when action potential passes varicosity)
How is smooth muscle organised?
Organised in 2 ways; single unit smooth muscle and multi unit smooth muscle
What is a single unit smooth muscle?
Has its muscle fibres joined by gap junctions (channels between cells) so that the muscle contracts as a single unit.
Found in the walls of all visceral organs (except the heart). Therefore commonly called the visceral muscle
Has a stress-relaxation response.
Produces slow, steady contractions that allow substances, such as food in digestive track, to move through the body.
What is the Stress-Relaxation response in a visceral smooth muscle?
As the muscle of a hollow organ is stretched when it fills, the mechanical stress of the stretching will trigger contraction. But, this is immediately followed by relaxation so that the organ doesn’t empty its contents prematurely
What is a multi-unit smooth muscle?
Rarely possess gap junctions so not electrically coupled (don’t move as single unit)
as a result, contraction doesn’t spread from one cell to the next (instead confined to cell that was originally stimulated)
Stimuli come from autonomic nerves or hormones (not from stretching)
Found around large blood vessels, in the respiratory airways, and in the eyes
Electrical isolation of cells allow finer motor control
Does smooth muscle respond to hormones?
Yes
