Orgasm is a reflex of the autonomic nervous system.

And then the message cascades into the amygdala, and then goes down the autonomic nervous system.

Well it turns out it's these neuromodulators, just like the neuromodulators in that little tiny ganglion in the crabs.

So if the amygdala is excited, and this is something important, the messages then cascade into the autonomic nervous system.

So I really think that, as we learn more, it's going to turn out that the whole fly brain is just like a large version of this stomatogastric ganglion, and that's one of the reasons why it can do so much with so few neurons.

The photoreceptor cells are shown here at the top, and then the signals that are detected by the photoreceptors are transformed by various computations until finally that layer of cells at the bottom, the ganglion cells, relay the information to the brain, where we see that as perception.

The amazing thing about this is that it's controlled by a really tiny set of neurons, about two dozen neurons that can produce a vast variety of different motor patterns, and the reason it can do this is that this little tiny ganglion in the crab is actually inundated by many, many neuromodulators.

Many chemical mediators are involved, including cortisol and adrenalin from the adrenal glands, other hormones and neurotransmitters (such as serotonin and noradrenalin), and responses from the autonomic and immune systems. Depression thus leads to changes in the rest of the body, owing to long term chemical imbalances in the systems that control the heart, the immune system, and metabolism.