Module 5: section 3 - electrical activity of the heart

what type of organ is the heart

electro-mechanical

what does electro-mechanical mean

it has both electrical activity and large muscle to contract

what electrical activity is the heart experiencing

it generates and conducts action potentials like nerves

what types of cells does the heart have to accommodate its electro-mechanical

a type to contract and do the mechanical work and another type that is electrical that generate and propagate action potentials to the contractile cells

how can the SNS and/or PNS affect heart rate

• physical and emotional stress can cause changes in HR

• Epinephrine being released to increase rate (SNS)

• illness/injury can cause an increase in blood flow to peripheral tissue, increases HR (SNS)

• exercise increases a need for oxygen to muscles which brings more blood around the body

what are autorhythmic cells

specialized cardiac muscle cells that can produce action potential

what makes autorhythmics difference

they have ion channels that cause the membrane potential to slowly depolarize untl the threshold potential is reached and an action potential is fired instead of having a flat, constant resting membrane potential

what type channels do autorhythmic cells contain

I_f channels

what happens when I_f channels are activated

both sodium and potassium can enter the cell to cause depolarization of the membrane potential

what causes I_f channels to activate

either from the hyperpolarization-activated cyclic nucleotide-gated channel family, or that a type of calcium channel is involved (t-type)

what is unique about autorhythmic cells

the upstroke of the action potential is due to another type of calcium channel, the L-type channel, instead of sodium channels

where are autorhythmic cells found

sinoatrial node, atrioventricular node, bundle of His, Purkinje fibres

what is the sinoatrial node

a small area foind in the right atrial wall near the opening of the vanae cavae

what is the atrioventricular node

a small area found in the right atrium where the right atria and right ventricle come together
*often said to be located in theinteratrical septum since its around the centre of the heart

what is the bundle of His

a bundle that has specialized cells that arise from the AV node. divides into two bindle branches that go down each side of the septum to the bottom of the heart where they curve around and travel towards the atria

what are purkinje ribres

small fibres that branch off the bundle of His and spread along the inner surface of the ventricles

which autorhythmic cells have the fastest depolarization

the ones in the SA node; are considered the pacemaker cells

why are SA node autorhythmic cells considered the pacemaker cells

because they control heart rate and keep it at 70-80 beats/min (without an action potential present).

what happens when an action potential is generated via SA node

it conducts through the rest of the cardiac conduction system, overriding the pacemaker activity of other autorhythmic cells

what needs to happen for the ventricles to fill up wiwth blood completely

the blood from the atria must be removed into the ventricles; atrial excitation and contraction should be complete before the onset of ventricular contractions

why do the AV valves open during relaxation of the heart

because the pressure in the ventricles is lower than the atria pressure

what does opening of the AV vales help with

allowing the passive flow of blood from the atria to the ventricles

what does passive blood flow account for

~80% of total ventricular filling with the remaining 20% coming from when the atria contract

what happens during a normal heartbeat

the atria contract about 160msec before the ventricles

what criteria needs to be satisfied for efficient cardiac contraction

1. atrial excitation and contraction should be complete before onset of ventricular contractions

2. excitation of the cardiac muscle fibres needs to be coordinated

3. the pair of atria and the pair of ventricles must be functionally coordinated

why do the chambers need to have a coordinated contraction

because if different regions of a ventricular wall were to depolarize and contract at different times, then it would not be possible for ventricles to eject blood

what is ventricular fibrillation

when uncoordinated depolarization happens

how can the blood move throughout the body

the atria need to contract together and the ventricles need to contract together

what happens if the ventricles contracted out of sync

stress on the ventricular walls will happen without any reason and a pacemaker may be needed to re-coordinate the contraction
*is not normally dangerous, but can lead to more problems that are

how does the action potential fired by the SA node travel throughout the atria

by gap junctions and pathways

how do gap junctions help

by sending the depolarization signal between atrial cells

what pathways help with the spreading of action potential

interatrial pathway and internodal pathway

how do the interatrial and internodal pathways work

act like nervous tissue by moving the excitation wave faster than possible by gap junctions

where does the interatrial pathway extend

extends from right atrium to the left atrium and ensures that the wave of excitation spreads across both atria

what does the internodal pathway do

connects SA node to the AV node

how can the electrical signal move from the atria to the ventricles

only by the AV node and bundle of His due to there being a dense region of connective tissues between the atria and ventricles

what is the purpose of the AV nodal delay

to make sure the atria have has a chance to contract before the ventricles (160msec)

to maximize the atrial emptying of blood into the ventricles

what are the ventricles

a larger mass of muscle, hollow organs

why are gap junctions needed

to innervate the cells that the purkinje fibres to not reach

what is the "resting” membrane potential for a cardiac myocyte

~-80 mV

why do cardiac myocytes have a resting potential

since these cells have no pacemaker currents

what do cardiac myocytes have

well defined t-tubule system

what are the steps on how a myocyte is activated to initiate contraction

1. action potential in cardiac contractile cell cause L-type calcium channels to open

2. release of calcium

3. either the calcium directly interats with the contractile apparatus or interacts with ryanodine receptors on SR membrane to trigger more calcium to be released (CICR)

4. the influx of calcium initiates contraction of cardiac muscle

when does a cardiac muscle contraction end

when calcium is removed from the cytosol

*either by moving it across the plasma membrane or pumping it back into SR

why cant cardiac muscle not undergo summation and tenanus

since it would lead to inefficient, life-threatening contractile patterns

what is tachycardia

a heart rate that is faster than the normal resting heart rate

what can tachycardia do

decrease the cardiac output due to reduced ventricular filling

what does tachycardia look like

what is extrasystole

when a heartbeat is started by purkinje fibres rather than the SA node

what can extrasystole be a sign of

reduced oxygenation in healthy hearts

how does extrasystole work

ventricles contract before atria and therefore are not optimally filled with blood which leads to reduced cardiac output

is extrasystole dangerous

only if it is frequent

what is ventricular fibrillation

when the heart is quivering rather than pumping due to abnormal electrical activity in the ventricles

what can ventricular fibrillation cause

cardiac arrest with loss of consciousness and no pulse

what does extrasystole look like

what does ventricular fibrillation look like

what is complete heart block

third degree atrioventricular block

the impulse generated at the SA node does not travel to the ventricles forcing the pacemaker cells in the AV node to independently activate the ventricles

what independent rhythms does complete heart block allow to be seen on an ECG

• p wave with regular P to P intervals

• a QRS complex which does not always follow a P wave

what does a complete heart block look like

how does complete heart block present

abnormally low heart rate and blood pressure