If so, what happens to a mono synaptic when it sneezes?

No.
I say no because I doubt that anyone has bothered to even think about trying to measure such an event. There are 100 billion (more or less) neurons in the brain and 100 trillion synapses. You do lose synapses as you age, and you can lose the use of neurons to disease and injury. Neurons don't replicate (with rare exceptions), unlike other cells in our bodies.