biology

In my a level text book is says with electrocardiograms, ''a bigger wave means more electrical charge, so a bigger wave means more contraction.''

I thought contraction means depolarization so loss of electrical charge?

Reply 1

Jpw1097

Original post by chemquestion

Remember that the membrane is already polarised, the resting membrane potential of a ventricular cardiomyocyte for example is around -90mV (considerably lower than a neurone), this means that the inside of the cell is negatively charged with respect to outside the cell. Therefore, when a membrane depolarises, it becomes less negatively charged (the membrane potential goes from -90mV to, say, +50mV). After the cell depolarises, it then repolarises (i.e. becomes more negatively charged) and the membrane potential returns to its resting value. Another thing to bear in mind is that an ECG does not record the activity of a single cardiomyocyte, therefore it does not record a single action potential. The ECG trace is formed by the combined electrical activity of multiple cardiomyocytes. So the larger the peak, the greater the electrical charge, this means there is more cardiac muscle and therefore greater force will be generated. Also bear in mind that the size of the peak on an ECG is also determined by the position of the electrode with respect to the axis of the heart, therefore different leads have different sized peaks (QRS complexes) and some are even inverted, as different leads record different 'views' of the heart.

Reply 2

2007PSanHa

Polarisation is when the electrical charge inside and outside of a cell becomes further apart.
Depolarisation is when the electrical charge inside and outside of a cell becomes closer together.
The bigger the ECG wave the larger the change in charge between the inside and outside of the cell.

I think this is right. Hope it makes sense! :smile:

Reply 3

macpatgh-Sheldon

Hi,

I think sometimes an attempt at simplification introduces error.

The author of your A level book is probs a biologist, not a doctor. Jpw1097 has provided a completely correct summary of how the ECG works, but might be too detailed for an A level student.

I would simplify it for you thus:-

Think physics for the moment (GCSE level will do): charge refers to STORED electrical energy (measured in coulombs) -electrical activity, effectively current, only occurs when there is change in charge through movement of electrons.

The y axis on an ECG is in mV (millivolts) i.e. potential difference or voltage, which is proportional to current for a fixed system (i.e. if resistance is constant). So it is the voltage level that is manifested by a positive or negative deflection on the ECG, NOT charge.

As JPW says, the total MOVEMENT of electrical activity of all the muscle cells of the heart (Greek: cardio = of heart; myo = muscle; cytos = cell) produces the deflections on the ECG. For A level purposes, just remember that the greater the muscle volume, the taller the R wave (in general ONLY) e.g. a patient with e.g. longstanding essential hypertension (=high blood pressure) will have an enlarged heart and his/her ECG will have taller R waves in certain leads to put it simply (a lead is one recording that "looks" at the heart from a particular direction - there are 12 leads in a standard ECG).

Try and remember only this much - ignore the book on this point!

M (former medical student)