Physiological Properties of the Heart Flashcards
Describe why the heart is a functional syncytium. What happens if one cell in the syncytium depolarises?
- electrical coupling of the muscle cells in the atria and separately in the ventricles
- stimulate one region of that muscle to depolarise, because electrically linked,= entire functional unit contracts in synchrony
What is the principle pacemaker of the heart?
the Sino atrial node
What does the SA node control? What innervates the SA node?
- spontaneous activity that dictates our heart rate comes from
- controlled by autonomic innervation
What is the ionic basis for the membrane potential changes in atrial/ventricular muscle?
- rapid upshot from sodium
- rapid depolarisation by potassium
- plateau, held at positive potential, maintained by calcium
What is key feature of the action potential in nodal tissue?
- never reaches a stable resting membrane potential
- as membrane potential gets negative it drifts up again to the positive
describe the stages 0-4 of the atrial/ventricular action potential. Mention the ions involved and the channels.
0-rapid depolarisation due increased Na permeability, fast Na channels open
1- +20mv=closure of fast Na channels and start of repolarisation, opening of Ca L-type DHP channels
2-influx of Ca, cell can’t repolarise again
3-influx of Ca opens K channels, K floods in to repolarise membrane to resting potential, L-type Ca channels close
4-resting potential, K>Na 50:1
what causes repolarisation in the SA node?
due to potassium influx
How does depolarisation occur in the SA node?
- potassium enters cell repolarising membrane potential to negative
- F-type Na channels open allowing Na influx causing depolarisation to a threshold voltage
- transient Ca channels open and push the action potential a little higher
What 2 components acting together establish a membrane potential?
-F-type channels
-Transient calcium channels
recruit and activate L-type Ca channels, slow, which gradually establish a membrane potential
How do F-type channels operate?
- -sodium channels
- open as membrane potential decreases towards negative as a result of potassium influx
- sodium enters causing depolarisation hence preventing membrane potential reaching a resting voltage
How do the SA node and atrial/ventricular action potentials relate?
depolarisation in nodal tissue, stimulates through electrical connections the cardiac muscle cells to contract during atrial/ventricular depolarisation
What is the neurotransmitter of the sympathetic nervous system? What receptors does it act on?
- Noradrenaline
- beta 1 receptors
What is substance is produced when noradrenaline acts on B1 receptors?
cyclic AMP
what happens as a result of increased cAMP production as a result of sympathetic nervous system?
- recruit protein kinases, phosphorylate certain porteins in the cell
- proteins which form the channels driving pacemaker potential are phosphorylated
what does increased number of phosphorylated channel proteins result in? Why?
- increased conductance of Ca, Na through F-type, hence increased influx
- increased effectiveness of channels or increase in channels recruited
How does sympathetic innervation affect the pacemaker action potential?
increase rate of discharge and increase speed to get to the threshold potential
How does Parasympathetic innervation affect cAMP production? What effect does this have on the ions?
- decrease in production cAMP
- decrease influx of Na and Ca
What effect does decreased cAMP production as a result of parasympathetic innervation have on the pacemaker action potential?
- hyper repolarisation
- decreased protein kinase A due to decreased cAMP decreasing phosphorylation of channel proteins
- increases extent and duration of K channels opening
Where does pacemaker potential spread to? What 2 tracts does it use?
- generated in SA node
- spreads through the atria in interatrial+internodal tracts to AV node
What is the AV node? where is it situated?
- only point of electrical connection between atria and ventricles
- sits at critical point between atria and ventricles in the cardiac skeleton
What happens when the electrical signal reaches the AV node? How does it leave?
- signal to be communicated is held in the AV node for a long time
- rapidly conducted down the R and L bundle of branches of atrioventricular bundle of His
What is the anatomical purpose of holding the electrical signal in the AV node for an extended period of time?
allows atria to contract in a coordinated fashion followed by ventricular contraction afterwards
What is the rate of depolarisation of the SA node?
90 beats per min
What are the rates of depolarisation of the AV node, His, purnkinjie fibres and the ventricles?
AV- 60 beats/min
His- 50 beats/min
purkinjie- 40 beats/min
ventricles- 30 beats/min
what is the time for the electrical impulse to get from SA node to Av node?
0.03 s
what is the impulse delay time in the AV node?
0.09s
What is the time for impulse t/o penetrate the fibres of the cardiac skeleton?
0.04s
what is the speed of the penetrating impulse from AV node to reach the bundle of his?
0.01m/s
what is the speed of conduction down through purkinjie fibres?
1.5 - 4m/s
how many leads are used in a 12 lead ECG?
10
How many limb leads are there in a 12 lead ECG? Which lead is the Earth?
- 4
- right leg is Earth lead
What is the function of the Earth lead?
to eliminate background noise from the ECG trace
What happens if there is depolarisation towards the positive end of the lead?
will show as an upward trace on the ECG
what do the limb leads look at?
total sum of the electrical activity in the heart looking at its:
- size
- direction of movement
How is einhovens triangle formed?
lead 1- L to R arm +ve to -ve
Lead 2- L arm to L leg -ve to +ve
lead 3- L leg to R arm +ve to -ve
How is electrical activity measured by Einhovens triangle?
-measure electrical activity in an area in leads running parallel to that electrical event
