RESP - E. SMOOTH MUSCLE STRUCTURE AND FUNCTION-COVERED

Question 1

functional roles of smooth muscle

Answer 1

• regulate flow by varying tube dimension
  eg: contraction and dilatation
• control flow by occulting tube
  eg: sphincters
• walls of storage organ
  eg: bladder - expand and expel
• movement of large bulk
  eg: oesophagus, intestine
  swallowing - peristalsis

Question 2

smooth muscle structural organisation

Answer 2

• supported by and contains connective tissue
  (unlike SM, no tendons)
• small cells - don’t extend full length of muscle
  (unlike skeletal)
• group of cells arranged in sheets
• single sheet eg - arterioles and airways
  circularly orientated
  maintains vessel diameter
  controls blood flow
• multiple sheets eg - ileum
  2 sheets perpendicular
  longitudinal and circular layers
  peristalsis - vary diameter and length

Question 3

smooth muscle cell structure

Answer 3

• small, spindle-shaped cells
• uni-nucleate
• no striations but banded
• no z-bands but dense bodies
• actin and myosin filaments present
• actin anchored to dense bodies
• intracellular cytoskeleton harnesses pull
  ie: myosin pulls itself along actin filament when then pulls cell

Question 4

varicosities

Answer 4

• total muscle surface contains NT receptors
• varicosities are where NT are located in the axon

Question 5

gap junctions

Answer 5

• non-selective channels allow intracellular communication
• signalling propagate between cells
• fibres act in unison: synchronised contraction and relaxation
  *we want all smooth muscle cells to contract at same time

Question 6

smooth muscle relaxation and contraction

Answer 6

• contraction requires increased Ca2+ conc from:
  1. Ca2+ influx across plasma membrane (voltage gated channels)
  2. Ca2+ mobilised from intracellular stores within cell (activation of 2nd messenger)
• increasing Ca2+ increases level of tone by increasing the contraction of the SM cells

Question 7

contractile mechanism

Answer 7

• actin with myosin to form X-bridges
• Ca2+ binds to calmodulin
• Ca2+-calmodulin activates myosin light chain kinase (MLCK)
• MLCK phosphorylates myosin light chain (MLC)

*MLC must first be phosphorylated for actin to bind

Question 8

what is myosin light chain kinase

Answer 8

an enzyme that puts a phosphate group onto a protein (the MLCs)

Question 9

Beta-adrenoceptors

Answer 9

• increase cAMP in smooth muscle
• leads to relaxation
• nitrates cause relaxation of vascular smooth muscle (used in angina)

Question 10

controlling SM tone

Answer 10

SM tone regulated by contractile and relaxatory agents released from:

• neurones: NTs act at receptors to stimulate contraction/relaxation
• endothelium within vasculature: localised effect
• blood-borne ie platelets: Adr released into blood and flows around to activate adrenoceptors on smooth muscle