L27 - smooth muscle Flashcards
describe smooth muscle
- involuntary
- myocytes with fusiform shape
is skeletal or smooth muscle smaller?
smooth
what is smooth muscle associated with?
visceral organs (tubular and hollow organs) and blood vessels
what is contraction of smooth muscle controlled by?
calcium biding to calmodulin
vasodilation
- relaxation of smooth muscle
- increases blood supply to tissues
vasoconstriction
- contraction of vascular smooth muscle
- decreases blood supply to tissues
describe smooth muscle in intestine
- longitudinal layer of muscle on outside
- circular layer on inside
smooth muscle in stomach
- inner oblique layer of smooth muscle: chunking + mechanical digestion
- circular layer: forms pyloric sphincter that regulates flow of stomach contents into duodenum
- longitudinal: moves food towards pylorus
smooth muscle in lung airways (trachea and bronchia)
- peristalsis for exhalation and mucus propulsion
- regional distribution of airflow during ventilation
- stabilisation of airway wall
- enhancing effectiveness of coughing
describe a single unit of smooth muscle
- fibres aggregated into sheets or bundles
- connected by gap/nexus junctions
- synchronous activity contracts as a syncytium (e.g whole muscle responds to a single stimulation)
- associated with intestine, stomach, uterus, small blood vessels
multi unit smooth muscle
- few or no gap junctions
- allows fine grain control
- activated by neural or hormonal signals
- associated with big arteries, airways, hair folllice muscles
smooth muscle properties
- involuntary
- no striations
- no sarcomere
- calcium binding by CALMODULIN
- slow myosin ATPase
- less extensive SR
- no t tubules
one does one smooth muscle cell look like?
- single nucleus
- spindle shaped
- 2-10um diameter
- 50-400 um long
- interconnect to form sheet
what does it mean by smooth muscle cells are dense bodies?
- lots of alpha actinin
- attached to sarcolemma by intermediate filaments
- anchor actin filaments allowing them to exert a force
mechanism of smooth muscle contraction
- initiated by calcium regulated phosphorylation of mysoin
- calmodulin binds to ca2+
2 stores of calcium
- internal calcium stores
- external influx through ion channels
smooth muscle cross bridge cycling
- resting muscle
- activation of contraction
- terminating cross bridge
describe resting muscle stage of cross bridge cycling
dephosphorylated myosin head held close to myosin filament
describe activation of contraction stage of cross bridge cycling
- muscle stimulated
- calcium levels in cytosol increase
- calcium binds to calmodulin
- calcium-calmodulin complex binds to myosin light chain kinase (MLCK)
- active MLCK uses ATP to phosphorylate myosin light chains in myosin head group
- this causes cross bridge to move from thick filament so it can bind to actin
describe the terminating cross bridge stage of cross bridge cycling
- myosin is dephosporylated by myosin light chain phosphate
- if ca2+ levels high it means MLCK is bigger than myosin light chain phosphate activity so myosin is phosphorylated
- when ca2+ decreases MLCP bigger than MLCK activity so myosin is dephos
describe the internal stores of calcium
sarcoplasmic reticulum
- a.p releases ca from sr
- second messengers release ca from sr
describe extracellular source of calcium
influx through voltage gated ca channels in plasma membrane
factors regulating smooth muscle contraction
- spontaneous electrical activity of muscle cell
- NT release from ANS
- hormones
- environmental changes in fluid around cells
- mechanical stretch
gastric motility
- peristalsis
- contractions in gi tract by slow waves
- waves generated by pacemaker cells called interstitial cells of cajal
- act as intermediates between nerves and smooth muscle cells
- slow waves generated by these interstitial cells spread to smooth muscle via gap junctions
neural stimualtion
- variosities
- swellings of autonomic neurones form motor units
- release NT that cause contraction or relaxation
2 types of ACh receptors
- nicotinic: ion channel
- muscarinin: GPCR
release of internal calcium stores from SR by GPCR activation
- in resting state: a trimeric g protein
- activation of GPCR leads to activation of alpha subunit of trimeric G protein (gdp/gtp) exchange
- causes dissociation of alpha subunit bound to GTP from beta gamma sub unit
- this activates phospholipase C
- leads to cleavage PIP2 into 1P3 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
how receptors help NT action at smooth muscle
- alpha-1 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 acts on different receptors
smooth muscle + lung disease
- asthma and COPD
- M3 muscarinic receptors mediate bronchioconstriction (smooth muscle contraction)
- b-adrenergic receptors mediate smooth muscle relaxation and bronchiodilation
