Paula,
Quote,"Can you please explain: "depends on whether the compound has an electrical charge?'"

There is an explanation below, but I would read it then forget it!!
I was only trying to think of how many ways a toxin entering the brain could differ, in order to explain why we get one illness or another.


A molecule is neutral, and has no electric charge. When a molecule can split into 2 parts, and one part takes the electrons they share forming a bond, it means the part which lost the electrons is positively charged, whilst the part which gained the electrons is negatively charged.
So in magnesium chloride, MgCl2, it goes to Mg++ and Cl--.
The Mg++, the magnesium ion is positively gharged and the chlorine ion Cl-- is negatively charged.
Contrast this with say a toxic benzene molecule, C6H6, which is a
neutral molecule, and does not split into charged parts called ions.
The gap in the BBB could have a charge or not, and it would affect what it admitted. I was over complicating things by adding in the possibility of whether the toxin could have a charge.
Ron