electrical activity of the heart

cells of the heart include

conducting and contracting cells

Conducting cells, Cells of Conduction System of Heart, function to generate and deliver action potentials to the myocardium, include nodes

the wires of the heart

contractile cells  that make up the myocardium, Contract in response to arrival of electrical signal from conduction system or neighboring cardiac muscle cell

the muscle of the heart

Electrical activity (arrival of an action potential) triggers —— (contraction) of the myocardium

mechanical activity

Electrical activity spreads from cell to cell via

gap junctions

—of one cell will result in rapid spread of positive charge to adjacent cell through gap junctions

depolarization

Rate at which a cell/tissue fires action potentials ( # action potentials/min)

firing frequency

speed at which action potential travels from one cell to the next (m/sec)

conduction velocity

normal pacemaker of heart, determines heart rate, most important, sends action potentials to internodal pathways

SA node

send the action potential from the SA node (wires) to atrial muscle, Triggers propagation of the action potentials resulting in atrial contraction

internodal pathways

delays action potentials to bundle of His, How heart remains synchronized, Will speed up again once it leaves here

AV node

conducts to left and right bundle branches, receives delayed action potentials from AV node

bundle of His

conducts action potential to purkinje fibers, receives action potentials from bundle of His

left and right bundle branches

rapid spread of action potentials to ventricular muscle, causes ventricular contraction

purkinje fibers

order of the heart’s conduction system

SA node, internodal pathways, AV node, bundle of HIs, left and right bundle branches, purkinje fibers

what happens if SA node loses function

pacemaker can be taken over by AV node and purkinje fibers but needs to be fixed fast

all have the ability to spontaneously  fire action potentials, but it is the SA node that is the normal pacemaker of the heart - feature called =

autorhythmicity

action potentials are initiated at

SA node

APs spread throughout heart to trigger contraction of

myocytes

sequence of electrical activity initiated from SA node

normal sinus rhythm

SA node is pacemaker because it has fastest AP firing frequency. This prevents the other tissues from acting as pacemakers (normal heart)

overdrive suppression

what has the slowest conduction speed in the conduction signal

AV node

what has the fastest conduction speed in the conduction signal

purkinje fibers

electrical activity spreads

left to right (from apex, up)

Once action potential arrives to ventricular muscle via purkinje fibers: 

spreads through all muscle cells via gap junctions

values always changing, No resting potential, initial rising on membrane potential chart for the nodes

pacemaker potential

Depolarization of SA node and AV node involves an influx of—-, after pacemaker potential builds, surpasses threshold

calcium

Repolarization of SA node and AV node involves an efflux of —-, falling of membrane potential on the chart, dropping back down to threshold

potassium

voltage gated ion channels present in SA node and AV node

calcium channels, potassium channels, HCN channel

T-type channel: transient, L-Type channel: long lasting, opened by depolarization

voltage gated calcium channel

multiple types, opened by depolarization but delayed response to depolarization compared to calcium channels in cardiac myocytes

voltage gated potassium channels

“non selective” – permeable to any cation. BUT mostly lets sodium into cell, opened by hyperpolarization, “funny” current of positive charge

HCN channel

hyperpolarization opens HCN channel which lets sodium in, then depolarization opens calcium channels letting calcium in, closes potassium channels at the same time to keep it inside, all these make the cell more positive causing the membrane to depolarize to threshold

pacemaker potential phase of SA node

once the membrane depolarizes to threshold the voltage gated L-type calcium channels open allowing more calcium to enter, continuation of depolarization of action potential, upstroke

depolarization phase of SA node

with the membrane potential reaching threshold, the potassium channels open letting potassium exit the cell, while also closing the L-type calcium channels so the influx stops coming into the cell, with these two happening it means that there’s more positive charge leaving the cell = repolarization of cell to negative interior, downstroke

repolarization phase of SA node

1.Depolarization: FAST!

2.Transient repolarization: brief repolarization

3.PLATEAU PHASE: extended time of  staying depolarized

Repolarization: return to RMP

action potential of cardiac myocytes

voltage gated ion channels present in cardiac myocytes

sodium channels, potassium channels, calcium channels

activation gate opened by depolarization to allow sodium into cell QUICKLY, in cardiac myocytes

voltage gated sodium channels

opened by depolarization, but takes a little longer to open compared to other channels, there are a lot of these in cardiac myocytes

voltage gated potassium channels

opened by depolarization, but takes a little longer to open than Na+ channels, L type: long lasting, in cardiac myocytes

voltage gated calcium channels

depolarization to threshold causes the opening of the sodium channels letting sodium into the cell, BIG AND FAST increase in entrance of positive charge: rapid depolarization (upstroke)

depolarization phase of cardiac myocytes

after reaching threshold, some types of potassium channels open letting potassium leave the cell which means there’s an increase in positive charge leaving the cell, right after depolarization in cardiac myocytes

transient repolarization of cardiac myocytes

with potassium and the positive charge decreasing in the cell, it triggers the opening of L type calcium influx and then an overall less potassium efflux, creating a balance of positive charge leaving and entering the cell, experiences effective refractory period

plateau phase of cardiac myocytes

Period of time heart muscle cannot be stimulated by another action potential, happens during plateau phase

effective refractory period

after plateau phase, the calcium channels close so it doesn’t let anymore calcium in and the potassium channels open and let out more potassium causing repolarization

repolarization phase of cardiac myoctyes

Exit of more positive charge than entry of positive charge, downstroke

repolarization

which ion channel is responsible for the upstroke phase of the pacemaker potential in nodal cells

HCN channels

“summed electrical potential” recorded by surface electrodes

ECG

PR interval of an ECG shows —-, with the end of the hump being the completion of this

atrial excitation

atrial relaxation begins in which segment of the ECG

Q

ventricular excitation is shown where in the ECG

QRS complex

ventricular relaxation is shown in which segment of the ECG

T

the QRS complex on the ECG record shows

ventricular depolarization