smooth muscle cells Flashcards
Smooth Muscle- where are they found
Found in blood vessels and hollow organs in the body
Examples of locations of smooth muscle:
Intestine
Uterus
Airways
Attached to hairs in the skin
Eye
what do smooth muscle cells look like
Each smooth muscle cell is spindle shaped with a diameter between 2-10 mm and 50 – 400 mm in length
Individual cells tend to interconnect to form a sheet of smooth muscle
Smooth muscle cells have a single nucleus
SMOOTH MUSCLE CELL
Different arrangement of actin, myosin (compared to skeletal)
Regulatory protein troponin is absent
smooth muscle cells properties
Involuntary muscle
Dense bodies instead of Z discs
Smooth appearance without striations
No troponin
Slow myosin ATPase compared to skeletal muscle
Less well developed sarcoplasmic reticulum compared to skeletal muscle
What is the mechanism of smooth muscle contraction?
Calcium dependent mechanism of contraction
Two sources of calcium (1) internal calcium stores and (2) plasma membrane calcium ion channels
Calmodulin binds calcium to start contraction in smooth muscle
Smooth muscle does not express calcium binding troponin
Two sources of calcium
Sarcoplasmic Reticulum
Less extensive compared to skeletal muscle
No T tubules
Some located near plasma membrane where action potentials can release calcium stores
or second messengers can release Ca2+ stores
2. Extracellular Ca2+ through voltage gated Ca2+channels in the plasma membrane
Sarcoplasmic Reticulum: internal Ca2+ stores
In the resting state – a trimeric G protein
Activation of the GPCR leads to activation of the alpha subunit of the trimeric G protein (the GDP/GTP exchange)
This causes dissociation of the alpha subunit bound to GTP from the beta-gamma subunit
The GTP bound alpha subunit activates phospholipase C
This leads to the cleavage of PIP2 into IP3 and DAG
IP3 binds to IP3 receptors expressed by the sarcoplasmic reticulum
IP3 receptors contain a calcium permeable ion channel
Activation of the IP3 receptor leads to the release of calcium from the sarcoplasmic reticulum
PIP2 = phosphatidyl inositol 4,5 bisphosphate
IP3 = inositol 1,4,5 trisphosphate
DAG = diacylglyercol
Cross Bridge activation in smooth muscle
RESTING MUSCLE:
- Dephosphorylated myosin head group held close to the myosin filament
- ACTIVATION OF CONTRACTION:
- Muscle stimulated and cytosolic calcium levels increase
- The calcium binds to calmodulin
- The calcium-calmodulin complex binds to myosin light chain kinase
- Active myosin-light chain kinase (MLCK) uses ATP to phosphorylate the myosin light chains in the myosin head group
- phosphorylation of myosin drives the cross bridge away from the thick filament into a position where it can bind to actin
- ATP dependent cross bridge cycling
- TERMINATING CROSS BRIDGE:
- Myosin is dephosphorylated by myosin light chain phosphatase
- when calcium goes up then the activity of the MLCK is greater than myosin light chain phosphatase activity and myosin becomes phosphorylated
When calcium decreases then the phosphatase activity is greater than kinase activity and the myosin becomes dephosphorylated
Factors that influence smooth muscle contraction
Spontaneous electrical activity of the muscle cell
Neurotransmitter release from autonomic neurons
Circulating hormones
Local environmental changes (e.g. oxygen, pH, temperature) in the fluid surrounding the cells
Mechanical stretch
The family of acetylcholine receptors
acetylcholine: muscarine GPCR- Muscarinic acetylcholine receptor- smooth muscle cells
acetylcholine: nicotine- ion channel- nicotinic acetylcholine receptor- Skeletal muscle NMJ
Importance of receptors for neurotransmitter action at smooth muscle
alpha-adrenergic receptors mediate constriction in most vascular smooth muscle
beta-adrenergic receptors mediate dilation of vascular smooth muscle and lung airway smooth muscle
Response depends on the receptor expressed as the same neurotransmitter is acts at different receptors
Single unit smooth muscle
Fibres aggregated into sheets or bundles
Connected by gap/nexus junctions
Synchronous activity, contracts as a syncytium (ie. whole muscle responds to a single stimulation)
e.g. intestine, uterus, small diameter blood vessels
Multi-unit smooth muscle
Few or no gap junctions
Not activated by stretch
Resemble striated muscle where there is no electrical coupling, allows fine control and gradual responses
Neural regulation is important
e.g. large arteries, large airways in lungs, pilomotor muscles
Five key points from this lecture
Smaller cells with a less organized actin-myosin arrangement compared to skeletal muscle
Smooth muscle forms visceral organs and blood vessels
Smooth muscle is an involuntary muscle
Contraction is controlled by calcium binding to calmodulin
Smooth muscle is arranged in single unit or multi unit muscle
