In other posts we have established that the brain is a network of cells, which almost (but not quite) touch each other, which send out pulses of electrochemical activity if there is enough of the right kind of activity around them, and that they have special structures called synapses to manage the communication between them.
Synapses are where the action truly is, and any discussion of how the brain works has to have them at its heart. It is useful to get a primitive idea of what they do split up into three time scales:
On the very short time scale (thousandths of a second), activity in the synapses is dominated by neurotransmitters. These these are chemicals that are produced inside the neurons and are responsible for carrying the wave of activity over the the synaptic cleft to the next neuron. These chemicals are not very 'famous' so we don't really need to know what they are called at this stage. They are either excitatory - that is they increase the likelihood that the post-synaptic neuron will fire - or they are inhibitory. They are released only when the spike reaches the synapse.
On a longer time scale (seconds to hours or even days), the behaviour of the synapses is changed by another, much more famous, group of chemicals called neuromodulators. These are not produced in the synapses they affect, but are usually made in other parts of the brain.
Many neuromodulators are well known because they are linked in the popular imagination to particular behaviours: adrenaline (fight and flight), dopamine (reward and pleasure), histamine (allergic reactions), oxytocin (love and bonding), serotonin (happiness!), melatonin (sleep cycles), and many, many others.
In reality, things are much more complicated than this picture (one modulator - one behaviour) suggests! Things are further complicated by the fact that many neuromodulators are also neurotransmitters (although not necessarily in the brain) and many of them are hormones with wide-ranging effects apart from their effects on synapses. A chemical like oestrogen for example (slightly controversial to include this as a neuromodulator - but justified I think) has hundreds of well-documented effects on pretty much every part of the body.
On the longest time scale (hours, and days, and months), synapses actually appear and disappear, are strengthened and weakened, grow and shrink. (And there may be hundreds of other behaviours yet to be documented.) This is controlled by a vast array of factors including the neuromodulators, genetics and epigenetic control. This is only just beginning to be documented, and is certainly not well understood. The longer ("developmental" some might say) time scale is more or less undiscovered country for neuroscientists at a cellular level, and patterns that emerge on this time scale are still something of a mystery.
Part of: Martin’s Vastly Oversimplified and Woefully Incomplete Guide to Everything in the Brain as featured on the Brainsex website.