Neurons themselves are classified by how many processes, or extensions, they have. Neurons have extensions on one end usually called dendrites, which act as receiving cables for electrical signals. Dendrites, which mean trees, are like the rabbit ears of an old television set, receiving signals. A neuron is also made up of an axon, down which the action potential, or nerve impulse, travels. Neurons, therefore, are called either unipolar, bipolar, pseudo-unipolar or multipolar, based on the number of processes they have. Unipolar neurons have one branch that serves as the axon. Bipolar neurons, which are predominantly sensory neurons, have two processes, a dendrite and an axon. Pseudo-unipolar neurons are initially bipolar, but then the axon and dendrite fuse into a single, continuous process. The axon actually splits, one part being sensory, the other, motor. Multipolar neurons have one axon and a bunch of dendrites.
Human behavior depends not so much on the different kinds of neurons per se, but rather, more on neuronal organization into circuits that have specific functions. In other words, it is not variety that matters, it is connectivity.
A major organizational principle of the brain is that neurons with similar functions and properties can actually produce different actions based on the way they are interconnected. In other words, one dopaminergic neuron can differ from another dopaminergic neuron based on what connections it has made. Receptors are also a key issue here: what/which neurons connect to what/which receptors is important.
Another key principle of brain function is that the information conveyed by an action potential is based not so much on the signal’s form, but more on the pathway the signals travels along in the brain. Again, connectivity is the key here.
Neurons do not connect randomly with one another within the context of neuronal networks. Rather each neuron makes specific connections with certain neurons, but not with others. The connectivity has specificity, in other words.
Another major principle of the brain is the principle of dynamic polarization. This indicates that electrical signals going through a neuron does so only in one direction. This usually occurs down the axon to the next neuron. Of course, there are exceptions, but most neurons operate in this fashion.