Smooth Muscle Flashcards
What as the functional roles of smooth muscle?
Regulate flow by varying tube diameter ; eg blood vessels during exercise - constriction and dilation
Control flow by occlude get tube eg sphincters
Walls of storage organ e.g bladder - expand and expel
Movement of large bulk e.g oesophagus, intestine - swallowing and peristalsis
Does smooth muscle act on structures e.g. Bone ?
Only skeletal muscle acts on structures such as bone not smooth muscle
What is the structure of smooth muscle?
Small spindle shaped cells
- uni nucleate
- not banded, no straitions I.e. “Smooth”
- no Z bands but dense bodies
Actin and myosin filaments are present
- actin filaments anchored to “dense bodies”
Intracellular cytoskeleton harnesses pull
- contract inwards, bulges
- intercellular connections harness pull between cells
How does smooth muscle contract?
Latch phenomenon / sliding filament theory
How does innervation of smooth muscle come about?
Over range of cells on axon neurotransmitter is released - receptors over whole surface
Contract in unison
Does smooth muscle have neuromuscular junctions?
Only skeletal muscle does - smooth muscle has varicosities (swellings along the nerve the fibre)
What are gap junctions and what purpose do they serve?
Couple smooth muscle cells to one another
- non selective channels allow intracellular communication
-signalling propagate between cells
-fibres act in unison : synchronised contraction and relaxation
E.g uniform, co ordinated contraction of uterus during labour
Mechanism of smooth muscle - contraction and relaxation
What is required?
Contraction requires Ca2+ increase
What sources of calcium for smooth muscle control?
Extra cellular - influx across plasma membrane (voltage gated ca2+ channels in pm)
Calcium stores within cell - activation of second messenger
What does increasing Ca2+ do to tone?
Increases tone by increasing the contraction of SM cells
What are the roles of smooth muscle?
Regulate flow
- vascular system e.g capillaries arteries veins –> hypertension
- airways e.g. trachea bronchioles –> asthma
- gastrointestinal tract e.g stomach sphincter –> gastric spasticity
- urinogenital tract e.g bladder uterus vas deferens –> infertility and incontinence
Contraction by sliding filament theory
- interaction of actin with myosin filament to form X bridges
- Ca2+ binds to calmodulin - biochemical effect
- Ca2+ calmodulin activates myosin light chain kinase (MLCK)
- MLCK phosphorylates the myosin light chain (MLC)
What do B adrenoceptors do in the smooth muscle pathway?
Increase cAMP in the smooth muscle leading to relaxation of airways
What is tone and how is tone regulated?
Tone depends on the amount of contraction and relaxation there is
SM tone is regulated by contractile and relaxatory agents released from neurons, endothelium and blood borne e.g platelets
