Mad as a hatter.

The toxicity of several poisons, toxins and heavy metals result from their action on proteins (most of the time on enzymes). Heavy metals such as lead, cadmium and mercury have teratogenic effects leading to babies being born with malformed limbs.

Mercury nitrate was used by hat makers to soften and shape animal furs, and inevitably some of the chemical was absorbed through the skin. So many in the trade were poisoned in this way that their peculiar manner of behaviour led to the phrase "Mad as a hatter". Mercury poisoning can then, also, affect enzymes in the central nervous system leading to neurological damage.

Acute inhalation of high concentrations causes a wide variety of cognitive, personality, sensory, and motor disturbances. The most prominent symptoms include tremors, emotional lability (characterized by irritability, excessive shyness, confidence loss, and nervousness), insomnia, memory loss, neuromuscular changes, polyneuropathy, and performance deficits in tests of cognitive function.

My interest here, is how emotional lability occurs:

Mercury, because of its effect teratogenic, would act on certain proteins or directly on DNA. It causes disturbances or disruptions of neural networks by destroying neurones responsible for regulating the output of emotions which leads to the disinhibition, or release, of laughing/crying centers, or more generally failure of voluntary control of emotion.

The neurones affected would be then either in the prefrontal cortex (emotional core of the brain), either in the pathway descending from the brain to the brainstem. That is, either the neurones responsible for emotion die or do not work or the ones responsible for the visible response.

How exactly does this happen ?

Mercury modifies DNA of the cell and trigger the synthesis of mutant proteins. Those are either deficient, either unable to play their role. They can be also capable of carrying out new tasks playing against the cell or for instance preventing other proteins from functioning, leading inevitably to the death of the cell.

Mercury could also block other proteins like receptors or empeding them. It impact schemical signaling in the central nervous system.The following response from the next neurone is then modified, inhibited or enhanced (shyness or exhibitionnism, crying or laughing).