smooth muscle

Question 1

single smooth muscle

Answer 1

is unitary, syncytial

usually arranged in sheets/bundles. Force generated in one is transferred to next by gap junction.
No distinct NMJ and NTs pass from varicosity of ANS neuron causing cell to contract as a single unit.

No reserve units left once the contraction is initiated i.e. all will contract once signal is received.

contractions are graded depending on intracellular calcium concentration.

Question 2

multi unit smooth muscle (allows fine motor control)

Answer 2

single, discrete and separate smooth muscle fibres operating independently.

innervated by varicosities of single ANS nerve ending.
there is no specialised or defined NMJ. No gap junctions between cells.

all cells are electrically insulated from one another.

Question 3

single vs multi unit smooth muscle

Answer 3

single
> established tone
> gap junctions
> stretch reflex

multi
> receives its own individual innervation
> no tone
> recruitment is possible

Question 4

actin myosin ratio

Answer 4

10 - 15 : 1

Question 5

actin and myosin in smooth muscle

Answer 5

much longer
arranged diagonally
myosin has hinge heads along its length
actin filaments anchored to dense bodies (analogous to Z lines)
dense bodies are bound to an other cells dense bodies via adherens junctions

Question 6

smooth muscle cells can contract to 80% of their length

Answer 6

this is because there is no sarcomere and no Z lines

Question 7

calcium entry and increase occurs by

Answer 7

voltage gated calcium channels
receptor operated channels (e.g. drugs)
store operated calcium channels which allow extracellular calcium to enter the cell
mechanically gated calcium channels (GI tissue stretching)

Question 8

calcium release from SR

Answer 8

is tied to voltage gated calcium channel opening and receptor operated calcium release

Question 9

drugs acting on receptors

Answer 9

specific drugs will target the IP3 receptors to cause calcium release from SR

Question 10

contraction of smooth muscle

Answer 10

initiated by increased intracellular calcium, AP is not necessity.

Question 11

calmodulin

Answer 11

regulates cross bridge cycling in absence of troponin/tropomyosin

calmodulin is closely related to troponin C and is calcium binding protein

Question 12

calcium bound calmodulin

Answer 12

triggers conformational change in myosin light chain kinase

MLCK can then phosphorylate other substances by hydrolysing one molecule of ATP into ADP

this activates myosin

Question 13

tonic contractions (latch state)

Answer 13

contraction is maintained at a high level and this level can be maintained for hours/days

tonic force can be maintained with only 20 - 30% of cross bridges phosphorylated

ATP consumption is reduced = latch state

Question 14

latch state

Answer 14

tonic contractions where force is maintained at low energy expenditure (ATP consumption is reduced).

Question 15

contraction termination

Answer 15

contraction is terminated when calcium levels are lowered

closure of calcium channels, calcium - ATPase and NCX cotransporter

myosin heads are dephosphorylated by myosin phosphatase

Question 16

important note: myosin phosphatase

Answer 16

myosin phosphatase is continually active but when intracellular calcium rises, activation of MLCK activity exceeds phosphatase activity