What else happens as calcium moves in during ECC?
calcium moves out of cell via Na/Cl pump to ensure a steady state stability over time
What are the two challenges in relation to contraction?
- contraction must be synchronised
2. atria must contract before ventricles
What enables the synchronicity of contraction?
what regulate the strength of contraction?
number of gap junctions, also the more gap junctions the faster the contraction
What initiates the contraction of the heart?
an intrinsic electrical system composed of specialised myocytes
Specifically, the sino atrial node
What is the SAN and where is it located?
strip of myocytes roughly 20mm long and 4mm wide , located on the posterior wall of the right atrium, close to the superior vena cava
Why is SAN deemed the pacemaker of the cell?
The group of cells discharge spontaneously in regular intervals to initiate a heart beat
nerves linked to SAN
does not depend on extrinsic nerve supplies, however the rate of depolarisation can be altered parasympathetically of sympathetically
Why are the SAN cells best suited for their role?
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
Function of AV node
allows conduction of electricity through the otherwise non-conductive annulus fibrosus
delays the impulse by around 0.1s at resting heart rates
How does the AVN delay the impulses?
complex local circuitry and the small diameter of nodal cells
Explain structure and function of ventricular conducting system
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
relative conduction velocities at each stage
SAN- medium - 1m/s
AVN- low- 0.005m/s
bundle of His- fast
Purkinje- fastest- 3-5/s -
why are purkinje fibres the fastest?
have the widest cells in the heart
Experiment to prove the myogenic contractility of the heart
place the heart in ringer solution- will contract on its own
What happens if the SAN stops working?
Myocytes in the atrium or AV node will take over as the new pacemaker
Why does this happen?
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
What is this called?
gradient of intrinsic pacemaker frequencies
slower lower centres are normally excited by the SA node before they have time to fire spontaneously
heart block definition
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
what is the pacemaker potential?
the slowly declining membrane potential, which triggers an action potential when it reaches a threshold of -40mv–55mv
What is Wolff-Parkinson-White Syndrome?
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
Funny Current definition
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)
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
Why can the ECG be measured on the skin?
extracellular currents result from the heart activity (but reversed charges)
How is the ECG viewed?
voltmeter is attached to moving paper or a compute screen
What does the ECG fail to measure? How is this overcome?
The pacemaker potential changes
use of a cardiac catheter
Three main deflections in the human cardiac cycle
P- atrial depolarisation
QRS complex- ventricular depolarisation- spreading outwards
T- Ventricular depolarisation
What does the PR interval denote?
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
Why does atrial repolarisation not appear?
Coincident and slow, therefore does not generate sufficient current to be detected
NOT blocked by QRS wave
Why is the T wave that particular shape?
ventricular repolarisation is slower and less synchronous than depolarisation so it generates a broad but relatively low magnitude wave
Why is the T wave upright, despite showing repolarisation?
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
What does the ST segment represent?
How is the most basic ECG recorded?
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
What are pairs of limb electrodes called?
bipolar limb leads
How can they be connected?
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 -