Stochastic processes are known in mathematics something like random processes. Such processes have unknown rules of behavior or these rules do not exist at all. Therefore we cannot know precise equations that could describe function of the brain. However, it is possible that we can find equations that are close to the brain function. Physics is the science that describes nature in simplified form.
Physicists always make many compromises to describe processes of nature.
Neuroscience must do it the same way because it is very difficult to find precise differential equations describing function of the brain. Neural tissue is many times more complex than the systems that are typically described mathematically in physics. So we have to account with many simplifications when we want to describe function of the brain. However we cannot oversimplify and therein lies the paradox in mathematical modeling of the brain. On the other hand, even very detailed descriptions of the neuron do not allow us to understand whole brain functioning.