An upper limit to the number of sodium channels in nerve membrane?

1. A small volume of artificial sea water containing 300 nM tetrodotoxin (TTX) was applied successively to seven lobster nerve trunks and the cumulative uptake of toxin investigated by bio-assay.2. Light and electron microscopy indicated that the nerve trunks had a total axonal area of 0·7 × 10⁴ cm²/g.3. Sodium analysis gave a sodium space for the nerve trunks of 30%.4. The amount of toxin taken up by the cells in 1 g of nerve is less than 1·6 × 10^-11 moles.5. It is argued that there are probably fewer than 13 sodium channels/μ² axon in lobster nerve.