BSCI450: Pacemaker Conduction

EKG direction

depol moves toward positive electrode - deflection up

depol moves away from positive electrode - deflection down

key diff between pacemaker cells and cardiomyocytes

pacemakers have no VGSC or sarcomeres

how can you increase rate of depolarization in pacemaker cells

increase HCN current and decrease K leak

how does the parasympathetic system affect heart rate

decrease

how does the sympathetic system affect heart rate

increase

autonomic regulation of SA node / heart rate

parasympathetic NT and receptor

sympathetic NT and receptor

ACh, M

NE, B1

compare muscarinic vs adrenergic regulation of adenylyl cyclase

antagonistic

how does parasympathetic activity decrease heart rate

lowers baseline

receptor activity on SA pacemakers

B1

M1

M2

B1 - Gs stimulates adenylyl cyclase - enhances funny current

M1 - Gi inhibits adenylyl cyclase - inhibits funny current

M2 - activates more K leak channels - further from threshold

what is the natural SA rate without autonomic regulation

90-100 BPM

beta blocker: propanolol

what does it block

which ANS uses this

what NT does it block

effect

B receptors

sympathetic

NE

decrease heart rate

beta blocker: atropine

what does it block

which ANS uses this

what NT does it block

effect

muscarinic

parasympathetic

ACh

increase heart rate

typical heart rate

60 bpm

naturally, is the heart more regulated by para or symp

para, tonic activity

resting membrane potential

skeletal

contractile myocyte

pacemaker

-70 mV

-90 mV

n/a

reaching threshold

skeletal

contractile myocyte

pacemaker

Na entry via ACh-operated channels

depol via gap junctions

Na entry through HCN channels + Ca entry

AP rising phase control

skeletal

contractile myocyte

pacemaker

Na entry

Na entry

Ca entry

repolarization

skeletal

contractile myocyte

pacemaker

rapid, K efflux

plateau due to Ca entry then rapid K efflux

rapid, K efflux

hyperpolarization

skeletal

contractile myocyte

pacemaker

excessive K efflux

none

none

duration of action potential

skeletal

contractile myocyte

pacemaker

1-2 ms

200 ms

variable

duration of refractory period

skeletal

contractile myocyte

pacemaker

brief

long (since Na chanels can’t reset until end of AP)

n/a

apex

bottom

base

top

conduction pathway

SA node -> internodal pathway -> AV node -> bundle of His -> purkinje fibers

stereotypical EKG set up

negative on right arm, positive on left arm

what does an EKG show

electrical activity of whole heart

which portion of an EKG shows systole

P-T waves

which portion of an EKG shows diastole

flat segment between T and P

parts of EKG

P wave: atrial depol

PR interval: AV slow + conduction through bundle of His

QRS complex: ventricular depol

QT interval: ventricles contract

T wave: ventricular repol

what does a flat line on an EKG mean

something is contracting

where is atrial repolarization masked in EKG

QRS complex

pacemaker rates (BPM)

SA node - 65

AV node - 50

purkinje fibers - 40