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Finding numbers for the amount of neurons in the brain is quite easy. Finding them for other human organs is harder. How many neurons are there in the human heart?

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In terms of cell bodies? Zero. There are autonomic projections from the spinal cord (sympathetic) and vagus nerve (parasympathetic) to the sinoatrial node, the atrioventricular node, and at discrete points in the atria and ventricles.

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How can the sinoatrial node fulfill it's function if it doesn't have neurons inside itself? It creates action potentials. Isn't that what being a neuron is about? – Christian Oct 15 '12 at 10:45
@Christian Apologies if my answer was a bit terse. While the tissue of the SA node is excitable (has channels, etc.) I believe it is still considered to be part of the cardiac musculature. Some refer to the activity in heart as action potentials, but really it is the effect of passing the electrical excitation along the membrane (in the case of the heart, the musculature) rather than along an axon. – jonsca Oct 15 '12 at 10:51

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