<Tr> <Td> Chloride </Td> <Td> Cl </Td> <Td> 95 - 110 </Td> <Td> 10 - 20 </Td> <Td> 4: 1 </Td> </Tr> <Tr> <Td> Calcium </Td> <Td> Ca </Td> <Td> </Td> <Td> 10 </Td> <Td> 2 x 10: 1 </Td> </Tr> <Tr> <Td_colspan="5"> Although intracellular Ca content is about 2 mM, most of this is bound or sequestered in intracellular organelles (mitochondria and sarcoplasmic reticulum). </Td> </Tr> <P> Similar to skeletal muscle, the resting membrane potential (voltage when the cell is not electrically excited) of ventricular cells, is around - 90 millivolts (mV; 1mV = 0.001 V) i.e. the inside of the membrane is more negative than the outside . The main ions found outside the cell at rest are: sodium (Na), and chloride (Cl), whereas inside the cell it is mainly potassium (K). </P>

Where does the electrical signaling of the heart begin