5. Smooth muscle

Question 1

Structure of the sheets of SM

Answer 1

Single:
circularly orientated
tonically maintains vessel diameter and pressure
vary diameter to control flow and pressure

Multiple (ileum)
two sheets perpendicular
longitudinal and circular
vary diameter and length- peristalsis

Question 2

Does smooth muscle contain connective tissue

Answer 2

yes, to prevent overstretching

SM do not act on structures

Question 3

Single unit SM characteristics

Answer 3

EC gap junctions
non-selective channels 
AP propagates between cells 
fibres act in unison/ syncytium 
uniform and slow contraction

Question 4

multi-unit characteristics

Answer 4

no EC gap junctions
each unit stimulated separately
fine control and graded contraction
no NMJ but diffuse distribution of NT receptors

Question 5

how are SM cells controlled

Answer 5

nerves

Question 6

function of SM

Answer 6

allows hollow organs to be filled with maintained muscle tension

Question 7

unique to SM

Answer 7

stretch-relaxation response

- tension develops rapidly on stretch and expansion then relaxes back to original

Question 8

contractile properties of SM

Answer 8

length tension relationships
SM decrease in 2/3 length when contracted
and increase in 2.5x fold in variation (can stretch)

Question 9

Contraction in SM by sliding filament theory

Answer 9

Depolarisation of membrane potential causes Cav channels to open to release IC ions
calcium binds to calmodulin
ca- calmodulin activates myosin light chain kinase
MCLK phosphorylates myosin light chain
MLC allows actin to bind to myosin to form cross bridge

Question 10

Excitation- contraction coupling

Answer 10

requires calcium from intracellular calcium stores
and calcium influxed across plasma membrane (higher conc gradient outside vs insides)
graduation not by recruitment and summation- cannot increase no. of fibres excited

Question 11

Mobilisation of IC Ca stores

Answer 11

NT binds to metaboreceptors
(adreno and muscurinic receptors)
G protein activated-> increases IP3-> IC Ca stores

Question 12

rate of contraction in SM

Answer 12

slow, little energy, prolonged maintained contraction

Question 13

factors influencing neural control

Answer 13

1. type of innervation and transmitter
2. receptor of NT
   anatomical arrangement of nerve in relation to muscle fibres

Question 14

Innervation

Answer 14

extrinsic- ANS, symp, para 
intrinsic- nerves within SM 
afferent sensory neurones- indirect reflex activation 
local tissue factors and hormones
lack of oxygen (vasodilation)

Question 15

differences compared to Skm

Answer 15

no troponin
2x actin and tropomyosin
1/4 of myosin present