Session 4 Flashcards
Why is it desirable to have a long cardic action potential?
It ensures that once the AP has begun in any part of the heart it is long enough for the cell to still be depolarised when the last cell in the myocardium has begun its AP. This produces a single heartbeat.
What does the force of contraction of the heart for any given stretch depend on?
The concentration of Ca2+ in the cell - dependent on the balance between the rate of Ca2+ enters and removal
Describe the membrane potential of pacemaker cells during diastole
It is not stable, it depolarises steadily. This change is the pacemaker potential. It is due to Na+ channels that activate more with hyperpolarisation - called a funny current.mallows natural automaticity.
Why are pacemaker cells in the bundle of His usually overridden?
Their natural rate is slower than the SA node
Outline the regulation of contraction of vascular smooth muscle
Depolarisation opens V-gated Ca2+ channels
Ca2+ binds to calmodulin to form Ca2+-calmodulin complex, which then activates myosin light chain kinase
MLCK phosphorylates the myosin light chain to permit interaction with actin
Relaxation as Ca2+ levels decline. Myosin light chain phosphatase dephosphorylates the myosin light chain.
MLCK can itself be phosphorylated by PKA - inhibiting its action
Why is the sympathetic stimulation of sweat glands unique?
Postganglionic fibres release ACh
Where are alpha and beta adrenoreceptors broadly found?
Alpha on vascular smooth muscle (mostly alpha 1)
Beta on the heart (beta 1), smooth muscle in airways (beta 2, causing dilation), adipose tissue and some blood vessels (particularly in skeletal muscle - beta 2, causing vasoDILATION)
What are the modified post ganglionic fibres in the adrenal medulla?
Chromaffin cells
What allows for vasodilation to occur in vascular smooth muscle?
There is a constant sympathetic vasomotor tone
How can parasympathetic nerves indirectly cause vasodilation?
Stimulates the gut which in turn releases vasodilator metabolites
What receptors on the heart does the ANS act on and what effect do they have?
Parasympathetic - M2. Negative chronotropic effect
Sympathetic - B1. Positive chronotropic and ionotropic effects
What nerves give sympathetic and parasympathetic control to the heart?
Sympathetic - cardiac nerves
Parasympathetic - vagus nerve
