Pacemaker of Heart
SA Node
SA Nodal cells …
depolarize by increased Na+ conductance, followed by increase in Ca++ conductance
What happens when threshold becomes more negative
becomes more excitable bc it doesn’t have to go as far to reach threshold
Conductance
ability of ion to move across a semipermeable membrane
- if conductance is lower it is harder to move across semi membrane
SA Node is influenced by
autonomic nervous system
parasympathetic input produces a ….. of heart
slowing (bradycardia)
sympathetic input produces a ….. of heart rate
increase (tachycardia)
Acetylcholine (Ach)
(parasympathetic input)
Ach: produces slowing of heart
- increasing K+ conductance out of cell
- decreasing Ca++ conductance into cell
- decreasing Na+ conductance into cell
Sympathetic input releases …
norepinephrine
- to the atria + ventricles
- acts on both SA + AV nodes
in order for sympathetic nervous system to work, parasympathetic must be
inhibited
Norepinephrine (NE)
(sympathetic input)
NE: produces increase in heart rate
- decreasing K+ conductance out of cell
- increasing Ca++ conductance into cell
- increasing Na+ conductance into cell
Thyroid hormone …. HR, contractility, cardiac output
increase
circulating norepinephrine + epinephrine from …. do what to HR, contractility, cardiac output
from adrenal medulla
increase
Hyperkalemia
causes bradycardia + may block SA Node function
Hypokalemia
causes tachycardia
Hypoxia
decreased O2 in tissues
- causes bradycardia
Ischemia
decrease in blood flow to localized area, cause of hypoxia in heart
Adrenergic(B1) receptors
excite conduction system
Cholinergic muscarinic(2)
decrease excitatory effect
Adrenergic(a)
constrict blood vessels
Adrenergic B1 agonists
- SA Node increase rate
- Atria increase force of contraction
- Ventricles increase force of contraction
Muscarinic Receptors
- SA Node decrease rate
- Atrial muscle decrease force
- AV Node decrease conduction velocity
- Ventricular muscle no effect
Calcium channel blockers …
decrease conductance of Ca++
(slows heart)
Beta Blockers …
block action of norepinephrine on heart by blocking B1 receptor
