Electrical conduction in the heart Flashcards by Lydia Brown

what connects cardiac myocytes

intercalated discs - end to end - desmosomes and gap junctions

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describe the structure of cardiac myocytes

branched tubular discs
high mitochondrial density
central nucleus
striated
contain myofibrils

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myofibril definition

contractile unit of sarcomere - arranged in regular array of thick and thin filaments

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what is the a band

area of sarcomere occupied by myosin

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what is the I band

actin - extent to centre of sarcomere from z lines - contain tropomyosin and troponins, shared between 2 sarcomeres

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describe the z line

bisects each I band - actin attached to z line by titin

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describe the h zone

contains only myosin

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describe the m line

centre of h zone, composed entirely if myosin

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what is the definition of sarcomere

functional unit of contractile apparatus; defined as region between pair of z lines

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what forms sarcoplasmic reticuum

sarcomtubular network at centre and subsatcolemmal cisternae

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what does the lumen of t tubules do

carries extracellular space towards centre of myocardial cell

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what do gap junctions do in cardiac myocytes

electrically connect cells so helps AP spread rapidly

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which depolarises faster, SAN or AV node

SAN

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where is the SAN

RA near entrance of SVC

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what does the SAN do

initiates AP and determines rate of HB

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what is the resting membrane potential of the SAN

-55-60mV; closer to threshold so depolarises 1st - slow Na+ inflow

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what is special about the resting potential of the SAN

Doesn’t have steady resting potential - undergoes slow depolarisation which is called pacemaker potential

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describe the pacemaker potential in the SAN

progressive reduction in K+ permeability
K+ channels gradually close
pacemaker cells have unique F type channels
T type Ca channels are transient - contribute to inward Ca current and provide final depolarisation boost to pacemaker potential

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when do F type channels open

when membrane potential is negative

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where are F type channels found

pacemaker cells

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what do F type channels do

conduct mainly inward Na current

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how do nodal cells depolarise

Ca influx through L type channels

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why is there a slow transmission of cardiac excitation through AV node

AP propagated more slowly along nodal cells than cardiac cells - because L type channels (Ca) are slower than VG Na channels

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what is the purpose of pacemaker potential

makes SAN automatic - spontaneous, rhythmic self excitation

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how is depolarisation carried from SAN to AV node

backman's bundles

how are the atria and ventricles separated and what is the purpose

electrically isolated by fibro-granular rings around AV valves so conduction goes through AV node

pacemaker potential summary

repolarisation then hyperpolarisation causes membrane potential to drop (HCN channels activate at -70mV) na enters causing threshold potential (-50mV0, Ca channels open Ca enters and causes depolarisation

sympathetic control of pacemaker AP

adrenaline/ noradrenaline bind to beta 1 receptor leading to increased Na permeability so threshold reached quicker

parasympathetic control of pacemaker AP

ACh binds to muscarinic receptor caused decreased Na permeability so longer time to reach threshold

Differnences between cardiac myocyte AP and regular AP

Cardiac; has Ca2+ leaving cell to cause a plateau, is 200-300ms instead of <1ms, cardiac has longer refractory period to prevent muscle fatigue

where is the AV node

at base of RA

How does the AV node delay impulses

less gap junctions, fibres are smaller than atrial fibres

what type of cells make up the AV node and what is their purpose

modified cardiac cells that have lost contractile ability but conduct AP with low resistance

why does propagation of AP through AVN have to be slow

enables atria to empty into ventricles so atrial contraction completely ends before ventricular excitation occurs

where does the AP go after leaving AVN

interventricular septum down the bundle of His

what does the bundle of His divide into and where

R and L bundle branches, divides within interventricular septum

what are bundle branches

conducting fibres that separate at bottom of heart and enter walls of both ventricles

what are purkinje fibres

large diameter conducting cells that rapidly distribute the impulse through the ventricles

what do purkinje fibres connect

bundle branches and ventricular myocardial cells

why is conduction from the AVN rapid

to enable coordinated ventricular contraction

what happens during phase 0 of myocyte AP

rapid depolarisation; Na channels open so Na flow IN

what happens during phase 1 of myocyte action potential

partial depolarisation; inward Na current deactivated and outflow of K

what happens during phase 2 of myocyte AP

plateau; slow inwards ca current

what happens during the refractory period of myocyte AP

cannot get another AP

what happens in an absolute refractory period

cannot get another AP as NO channels open

what happens in a relative refractory period

can open ion channels if high enough stimulus

what happens during phase 3 of myocyte AP

repolarisation; K outflow, Ca current deactivated

what happens during phase 4 of myocyte AP

pacemaker potential; slow Na inflow and slowing K outflow

what is necessary for AP propagation

effective cell-cell communication - free flow of positive ions between cardiomyocytes through gap junctions via intercalated discs - allows neighbouring cells to reach threshold and open na channels

what is the key difference between myocardial AP and skeletal AP

plateau phase; calcium channels close at end to allow refractory period, allowing heart to fill contractions last longer than in skeletal

what does a 12 lead ECG show

changes in voltage over time

what time is a small square on a 12 lead ECG

40ms

what time is a big square on a 12 lead ECG

0.2 s

what does an ECG measure

voltage in 1 direction

how can you work out heart rate from ECG

HR = ventricular rate = 300 divided by number of big squares between 2 consecutive QRS complexes

what does an acute anterolateral MI look like on ECG

ST segment raised in anterior (V3-4) and lateral (V5-6) leads

what does an acute inferior MI look like on ECG

ST segments raised in inferior (II, III, aVF) leads

what leads are there on a 12 lead ECG

3 bipolar leads 3 unipolar arm leads 6 unipolar chest leads

how long does a P wave last

120-200ms (3-5 small squares)

what does a P wave on ECG represent

atrial depolarisation

what is the PR interval on ECG

time taken for atria to depolarise and electrical activation to get through AV node

what does a QRS complex in ECG represent

ventricular depolarisation

how long is a QRS complex in ECG

120 ms (3 small squares on ECG)

why is atrial repolarisation not visible on an ECG

occurs at same time as QRS complex

what is the ST segment on ECG

interval between depolarisation and repolarisation

what is the T wave on ECG

ventricular repolarisation

what is the QT interval on ECG

time for depolarisation and repolarisation

what does a high HR mean for the QT interval

is shorter

what do P waves look like on a normal ECG

positive in every lead apart from aVR

what do T waves look like on a normal ECG

positive in every lead except aVR and maybe V1/2

what does sinus rhythm mean

electrical activity originates from sinus node

what is ECG criteria for sinus rhythm

P wave morphology shows origin from SA node - positive in all leads except aVR, every P wave followed by QRS complex

what does tachycardia mean

increased HR

what does bradycardia mean

decreased HR

what does dextrocardia mean

heart on right side of chest, not left

what gives the 1st heart sound and what does it sound like

low pitched lub | AV valve closing

what gives the 2nd heart sound and what does it sound like

louder dub | Closure of aortic and pulmonary valves

what gives the 3rd heart sound

blood rushing into left ventricle

Electrical conduction in the heart Flashcards

(78 cards)