heart physiology: electrical events

electrical events

heart depolarizes & contracts w/out nervous system stimulation; basic rhythm can be modified by autonomic NS (sympathetic/parasympathetic); Intrinsic conduction system

setting the basic rhythm

coordinated heartbeat is a function of: presence of gap junctions & intrinsic conduction system

intrinsic conduction system

network of noncontractile (autorhythmic) cells; specialized to initiate & distribute impulses > coordinated depolarization & contraction of heart

pacemaker (autorhythmic) cells (step 1)

unstable resting membrane potentials due to opening of slow Na+ channels (continuously depolarize);

pacemaker (autorhythmic) cells (step 2)

at threshold, Ca2+ channels open; explosive Ca2+ influx prosuce the rising phase of AP (depolarization)

pacemaker (autorhythmic) cells (step 3)

repolarization results from inactivation of Ca2+ channels & opening of voltage-gated K+ channels

three parts of action potential for pacemaker cells

1. pacemaker potential

2. depolarization

3. repolarization

pacemaker potential (pacemaker cells)

slow depolarization is due to both opening of Na+ channels & closing of K+ channels; membrane potential is never a flat line

depolarization (pacemaker cells)

action potential begins when the pacemaker potential reaches threshold; depolarization is die to Ca2+ influx through Ca2+ channels

repolarization (pacemaker cells)

due to Ca2+ channels inactivating & K+ channels opening; allows K+ efflux, which brings the membrane potential back to its most negative voltage

sequence of excitation

cardiac pacemaker cells pass impulses, in order, across heart in ~220 ms

Pathway of sequence of excitation

1. sinoatrial (SA) nodes

2. atrioventricular (AV) node

3. atrioventricular (AV) bundle

4. right & left bindle branches

5. subendocardial conducting network (purkinje fibers)

intrinsic conduction system sequence of electrical excitation

1. sinoatrial (SA) node (pacemaker) generates impulses

2. impulses pause (0.1s) at atrioventricular (AV) node

3. Atrioventriculat (AV) bundle connects atria to ventricles

4. bundle branches conduct impulses theough interventriculat septum

5. purkinje fibers depolarize contractile cells of both ventricles

sinoatrial (SA) node

pacemaker of heart in right atrium (primary pacemaker) depolarizes faster than rest off myocardium

sinoatrial (SA) node generation of impulse

ab 75X/min (sinus rhythm); inherent rate of 100X/min modified by extrinsic factors (parasympathetic NS)

sinoatrial (SA) node spreading impulse

through atria (via gap junctions), & to AV node (via internodal pathway

picture of intrinsic conduction system

Atrioventricular (AV) node location

in inferior interatrial septum

atrioventricular (AV) node

delays impulses approximately 0.1 sec, allows atria to finish their contraction before the ventricles begin to contract

atrioventricular (AV) node generation of impulse

inherent rate of 50X/min in absence of SA node input (secondary pacemaker)

Atrioventricular (AV) bundle (bundle of His) location

in superior interventricular septum

Atrioventricular (AV) bundle (bundle of His)

only electrical connection between atrial & ventricles; atria & ventricles not connected via gap junctions

right & left bundle branches

2 pathways in interventricular septum; carry impulses toward apex of heart

subendocardial conducting network (purkinje fibers) location

complete pathway through interventricular septum to apex & up into ventricular walls; more elaborate on left side of heart

subendocardial conducting network (purkinje fibers)

AV bindles & this depolarize 30X/min in absence of AV node input; ventricular contraction immediately follows from apex upward toward atria

timing of conduction events

extrinsic innervation (description)

modifies the basic rhythm; HR modified by ANS via cardiac centers in medulla oblongata

extrinsic innervation types

1. sympathetic NS

2. parasympathetic NS

3. cardioacceleratory center

4. cardioinhibitory center

sympathetic NS effect (extrinsic innervation)

higher heart rate & force of contraction

parasympathetic NS effect (extrinsic innervation)

lower heart rate; no effect on contraction bc it does not innervate myocardium

cardioacceleratory center

sympathetic output; to pacemaker & myocardium

cardioinhibitory center

parasympathetic output; via vagus nerves; only pacemaker cells

extrinsic innervation

Action potential of contractile cardiac muscle cells

has characteristic 'hump' or plateau

contraction differences from skeletal muscle (AP) pt.1

depolarization wave ALSO opens slow Ca2+ channels in sacrolemma > SR release its Ca2+

contraction differences from skeletal muscle (AP) pt.2

Ca2+ surge prolongs the depolarization phase (plateau)

contraction differences from skeletal muscle (AP) pt.3

Action potential & contractile phase last much longer; allow blood ejection from heart

contraction differences from skeletal muscle (AP) pt.4

repolarization result of inactivation of Ca2+ channels & opening of voltage-gated K+ channels; Ca2+ pumped back to SR & extracellularly

3 parts of AP for contractile cells

1. depolarization

2. plateau phase

3. repolarization

pacemaker cell action potential

depolarization (contractile cells)

due to Na+ influx through fast voltage-gated Na2+ channels; positive feedback cycle rapidly opens many Na+ channels reversing membrane potential; channel inactivation ends this phase

plateau phase (contractile cells)

due to Ca2+ influx through slow Ca2+ channels; keeps cell depolarized bc few K+ channels are open

repolarization (contractile cells)

due to Ca2+ channels inactivating & K+ channels opening; allows K+ efflux, which brings membrane potential back to resting voltage

AP for contractile cells

AP for contractile cells skeletal vs cardiac

electrocardiography

conduction of impulses through heart generates electrical currents; impulses can be detected on body's surface & recorded

electrocardiogram (ECG or EKG)

composite of ALL AP's generated by intrinsic conduction system & contractile cells at given time

3 waves

1. P wave

2. QRS complex

3. T wave

p wave

atrial depolarization (starts w/SA node)

QRS complex

ventricular depolarization

T wave

ventricular repolarization

What do EKGs show

electrical events ONLY; electrical events come before mechanical events

EKG

Atria AP on EKG

1. atrial depolarization - p wave

2. atrial plateau - P-Q segment

3. Atrial repolarization - not shown on EKG

ventricles AP on EKG

1. ventricular depolarization - QRS complex

2. ventricular plateau - S-T segment

3. ventricular repolarization - t wave

steps of electriocardiogram (EKG)

1. atrial depolarization, initiated by SA node, causes P wave

2. W/atrial depolarization complete (plateau of AP) impulse is delayed at AV node P-G segment

3. ventricular depolarization begins at apex causing QRS; atrial repolarization occurs

4. ventricular depolarization is complete (plateau of AP) S-T segment

5. ventricular repolarization begins at apex, T wave

6. ventricular repolarization complete T-P segment