In computer science, neural networks are trained using backpropagation and other methods. Backpropagation heavily relies on mathematical formulas to describe how the weights should be changed throughout the network to effect the final output of the system. Biological neurons need to be able to adjust as well, if I'm not mistaken. What happens within a neuron to cause it to adjust its output? In other words, are biological neurons "trained" and if so, what is the process behind it?