Electrical Activity in the heart

Question 1

What else happens as calcium moves in during ECC?

Answer 1

calcium moves out of cell via Na/Cl pump to ensure a steady state stability over time

Question 2

What are the two challenges in relation to contraction?

Answer 2

1. contraction must be synchronised

2. atria must contract before ventricles

Question 3

What enables the synchronicity of contraction?

Answer 3

gap junctions

Question 4

what regulate the strength of contraction?

Answer 4

number of gap junctions, also the more gap junctions the faster the contraction

Question 5

What initiates the contraction of the heart?

Answer 5

an intrinsic electrical system composed of specialised myocytes

Specifically, the sino atrial node

Question 6

What is the SAN and where is it located?

Answer 6

strip of myocytes roughly 20mm long and 4mm wide , located on the posterior wall of the right atrium, close to the superior vena cava

Question 7

Why is SAN deemed the pacemaker of the cell?

Answer 7

The group of cells discharge spontaneously in regular intervals to initiate a heart beat

Question 8

nerves linked to SAN

Answer 8

does not depend on extrinsic nerve supplies, however the rate of depolarisation can be altered parasympathetically of sympathetically

Question 9

Why are the SAN cells best suited for their role?

Answer 9

the small myocytes have scanty myofibrils and an unstable membrane potential which is capable of spontaneous depolarisation

unstable membrane potential due to the lower number of inwardly delayed rectifying potassium channels

Question 10

Function of AV node

Answer 10

allows conduction of electricity through the otherwise non-conductive annulus fibrosus

delays the impulse by around 0.1s at resting heart rates

Question 11

How does the AVN delay the impulses?

Answer 11

complex local circuitry and the small diameter of nodal cells

Question 12

Explain structure and function of ventricular conducting system

Answer 12

bundle of His- bundle of fast conducting muscle fibres convey the electrical impulse from the AVN to the fibrous part of the interventricular septum . This then splits off into two branches, the left and right into both ventricle bases. End in subendocardial myocytes

purkinje fibres- distribute the impulse rapidly to the subendocardial myocytes

Question 13

relative conduction velocities at each stage

Answer 13

SAN- medium - 1m/s

AVN- low- 0.005m/s

bundle of His- fast

Purkinje- fastest- 3-5/s -

Question 14

why are purkinje fibres the fastest?

Answer 14

have the widest cells in the heart

Question 15

Experiment to prove the myogenic contractility of the heart

Answer 15

place the heart in ringer solution- will contract on its own

Question 16

What happens if the SAN stops working?

Answer 16

Myocytes in the atrium or AV node will take over as the new pacemaker

Question 17

Why does this happen?

Answer 17

There are other potential pacemakers in the heart but they have slower intrinsic frequencies of firing

for example, the bundle of His is less than 40bpm, Purkinje is less than 15bpm

Question 18

What is this called?

Answer 18

gradient of intrinsic pacemaker frequencies

Question 19

Dominance definition

Answer 19

slower lower centres are normally excited by the SA node before they have time to fire spontaneously

Question 20

heart block definition

Answer 20

AV node fails to transmit excitation through the annulus fibrosus so the SA node can no longer dominate the ventricle

pacemaker cells of the bundle of His then drive ventricles at less than 40bmp which is too low

Question 21

what is the pacemaker potential?

Answer 21

the slowly declining membrane potential, which triggers an action potential when it reaches a threshold of -40mv–55mv

Question 22

What is Wolff-Parkinson-White Syndrome?

Answer 22

Believes there is an underlying mechanism involving an accessory electrical conducting pathway between the atria and ventricles which leads to an abnormally fast heartbeat, palpitations, shortness of breath, light headedness and syncope

Question 23

Funny Current definition

Answer 23

electric current in the heart that flows through the pacemaker HCN channel.

A mixed sodium (mainly) and potassium current that activates upon hyperpolarisaton at voltages in diastolic range

determines, at the end of an action potential, the steepness of phase 4 depolarisation, hence the frequency of action potential firing (how long plateau phase is)

Question 24

Electrocardiogram definition

Answer 24

A recording of potential changes - around 1mV- at the skin surface that result from the depolarisation and repolarisation of the heart muscle. Mainly from the atria and ventricles

Question 25

Why can the ECG be measured on the skin?

Answer 25

extracellular currents result from the heart activity (but reversed charges)

Question 26

How is the ECG viewed?

Answer 26

voltmeter is attached to moving paper or a compute screen

Question 27

What does the ECG fail to measure? How is this overcome?

Answer 27

The pacemaker potential changes

use of a cardiac catheter

Question 28

Three main deflections in the human cardiac cycle

Answer 28

P- atrial depolarisation

QRS complex- ventricular depolarisation- spreading outwards

T- Ventricular depolarisation

Question 29

What does the PR interval denote?

Answer 29

The atrioventricular node delay.

The time taken for the impulse to spread across both atria, the AVN, the bundle of His-purkinje system and into the ventricle

Question 30

Why does atrial repolarisation not appear?

Answer 30

Coincident and slow, therefore does not generate sufficient current to be detected

NOT blocked by QRS wave

Question 31

Why is the T wave that particular shape?

Answer 31

ventricular repolarisation is slower and less synchronous than depolarisation so it generates a broad but relatively low magnitude wave

Question 32

Why is the T wave upright, despite showing repolarisation?

Answer 32

Myocytes repolarise in reverse order to depolarisation- subepicardium repolarises before the subendocardium

This is due to the outer cardiac myocytes having shorter action potentials than the inner ones

Question 33

What does the ST segment represent?

Answer 33

depolarised ventricles

Question 34

How is the most basic ECG recorded?

Answer 34

Three limb electrodes are used

one on each arm and one on left leg

wrists, ankles and skins are normally the point of contact to minimise interference from contracting skeletal muscle

Question 35

What are pairs of limb electrodes called?

Answer 35

bipolar limb leads

Question 36

How can they be connected?

Answer 36

Can be connected across a voltmeter in 3 different ways

lead 1- left arm + to right arm -

lead 2- left leg + to right arm -

lead 3- left leg + to left arm -