L26 - Smooth Muscle Flashcards
What are examples of tubular and hollow organs? (10)
- Airways
- Aorta
- Arteries and veins
- Bladder
- Eye
- Intestine
- Prostate
- Skin (hair follicles, nipples)
- Uterus
- vagina
How does vasodilation occur?
Relaxation of vascular smooth muscle
- increases the blood supply to tissue
How does vasoconstriction occur?
Contraction of vascular smooth muscle
- decreasing the blood supply to tissues
What are examples of visceral organs?
- intestine
- stomach
What is the smooth muscle like in the intestine?
- longitudinal layer of smooth muscle outside
- circular layer of smooth muscle inside
What are the layers of smooth muscle in the stomach?
- inner oblique
- circular layer
- longitudinal
What is the purpose of the inner oblique layer in the stomach?
Churning and mechanical digestion
What is the purpose of the circular layer in the stomach?
Forms pyloric sphincter
- regulates flow of stomach contents into duodenum
What is the purpose of the longitudinal layer of the stomach?
Moves food towards the pylorus
What are possible functions of the trachea and bronchia’s smooth muscle? (3)
- peristalsis, assists exhalation and mucus propulsion
- distribution of airflow during ventilation
- stabalises the airway wall, enhances effectiveness of cough/ejection of foreign material
What are single unit smooth muscles like? (3)
- fibres aggregated into sheets/bundles
- connected by gap/nexus junctions
- contracts as a syncytium
What are examples of single unit smooth muscle? (5)
- intestine
- stomach
- bladder
- uterus
- small diameter blood vessels
What are multi-unit smooth muscles like? (3)
- few/no gap junctions (no electrical coupling)
- fine grain control
- activateed by neural/hormonal signals
What are examples of multi-unit smooth muscle? (3)
- large arteries
- large airways in lungs
- pilomotor muscles (hair follicle)
What are properties of smooth muscle? (7)
- involuntary
- smooth appearance
- no sarcomeres (dense bodies)
- Ca2+ binding to calmodulin (not troponin)
- slow myosin ATPase (compared to skeletal muscle)
- less extensive SR (compared to skeletal muscle)
- no T-tubules
What are smooth muscle cells like? (5)
- single nucleus
- spindle shaped (2-10mcm diam, 50-400mcm long)
- interconnect to form sheet of smooth muscle
- no sarcomeres
- no troponin
How are smooth muscle cells dense bodies? (4)
- analogous to Z-discs of striated muscle sarcomeres
- rich in a-actinin
- attached to sarcolemme by intermediate filaments
- anchor actin filament = exert force
What is the mechanism of smooth muslce contractions? (2)
- intiated by Ca2+ regulated phosphorylation of myosin (not Ca2+ activated troponin)
- Calmodulin binds to Ca2+ = starts contraction
What are the 2 sources of Ca2+? (2)
- internal calcium stores
- external influx through calcium ion channels
What is calmodulin?
The calcium binding protein in smooth muscle
What are the steps in the smooth muscle cross bridge cycling? (3)
- resting muscle
- activation of contraction
- terminating cross bridge
What happens during resting muscle in the smooth muscle cross bridge cycling?
Dephosphorylated myosin head group held close to myosin filament
What happens during activation of contraction in the smooth muscle cross bridge cycling? (4)
- muscle stimulated Ca2+ inc
- Ca2+ binds to calmodulin
- Ca2+-Calmodulin complex binds to myosin light chain kinase (MLCK)
- Active MLCK uses ATP to add Pi to MLC in myosin head group
= drives cross bridge away from thick filament, binds to actin
What type of cross bridge cycling happens in smooth muscle?
ATP dependent
What happens during the terminating of cross bridge in the smooth muscle cross bridge cycling? (3)
- myosin - Pi my MLC phophatase
- Ca2+ high = MLCK activity greather than MLCP
- Ca2+ decreases = MLCP activity greater than MLCK = myosin - Pi
How is Ca2+ released from internal stores of Ca2+?
SR
- AP releases from SR near plasa membrane
- 2nd messenger releases froms SR
How is Ca2+ released from extracellular stores of Ca2+?
Influx through VGCC in plasma membrane
What are factors regulating smooth muscle contraction? (5)
- spontaneous electrical activity
- neurotransmitter release of autonomic neurons
- circulating hormones
- local environmental changes in the fluid around cells
- mechanical stretch
What is peristalsis?
Rhythmic contractions in the GI tract
- initiated by slow waves
How are slow waves genereated and propegated? (2)
- by pacemaker cells (interstitial cells of Cajal)
- spread to the surrounding smooth muscle cells via gap-junctions
What are varicosities?
Neural stimulation
- axon ilke sweelings of autonomin neurons (form units in smooth muscle)
What do varicosities do?
Release neurotransmitters that cause smooth muscle to contract or relax
What are the acetylcholine receptor families in smooth muscle?
GPCR - muscarine
- muscarinin acetylcholine receptor
How is the alpha subunit activated?
GDP -> GTP
Association and exchange of GDP for Gtp (GDP/GTP exchange)
What are the steps in the release of internal Ca2+ stores from SR by GPCR activation? (6) (2)
- resting state trimeric G protein
- GPCR activated = activation of alpha subunit of trimeric G protein
= dissociationg of alpha subunit bound to GTP form beta-gamma subunit - GTP activates phospholipase C
= cleavage of PIP2 into IP3 and DAG - IP3 binds to receptor on SR
- receptors contain Ca2+ permeable ion channels
- activation of IP3 = release of Ca2+ from SR
What is PIP2?
Phosphatidyl inositol 4,5 biphosphate
What is IP3?
Inositol 1,4,5 triphosphate
What is DAG?
Diacylglycerol
What do the receptors for neurotransmitters do at smooth muscle? (2)
- alpha-1 adrenergic receptors - mediate constriction in vascular sm
- beta-adrenergic receptors - mediae dilation of vascular sm and lung airway sm
What does response depend on?
Receptor expressed as the same neurotransmitter acts on different receptors
What are asthma and COPD associated with?
Bronchial smooth muscle hypertrophy
What do M3 muscarinin receptors mediate?
Bronchioconstriction
What do B-adrenergic receptors mediate? (2)
- smooth muscle relaxation
- bronchiodilation
What do vascular smooth muscle cells do?
Invade early atherosclerotic lesion expanding lesions
- implicated in early and late stage atherosclerosis
- undergo phenotypic switching - adopting alt phenotypes
What do VSMCs form?
Protective fibrous cap rich
- in extracellular matric to cover the necrotic core - stabilise plaques
