Brain genes

The brain is a three-dimensional form, weighing about 3 pounds in adults (less than a pound in newborns).   Each of the 100 billion cells (called "neurons") in our brain connects with thousands of other neurons.  [There are also another 900 billion or so "supporting cells" in the brain, but we'll ignore those for now].  All told, there are around 1 quadrillion (10¹⁵) connections between neurons (called "synapses") in the entire brain, where activity happens to create the mind.

So the development of the brain is largely a problem of how to connect 100 billion neurons using 1 quadrillion synapses, so that the brain can operate, consciousness can emerge, signals can be processed, memories can form, and responses can be effected (free choices can be made?)

About 6,000 of our genes seem to be active only in the brain; genes (or gene-produced proteins) like Robo which induce neurons to grow in specific directions inside the skull, and others that allow them to recognize friendly neurons and cling to them (making a synapse), and then allow signals to be transmitted across those synapses.  Many specialized proteins (such as Reelin) help in the formation of synapses once two neurons find each other and “dock” together.  Reelin also helps the brain develop its characteristic six-layer structure.

Cadherins are sticky molecules that guide neurons as they migrate inside the skull, to find their permanent position.  Think of them like Spiderman climbing a building, using a sticky substance to cling and move against gravity and friction, propelling against other neurons until the right one is found with which to form a more permanent synaptic connection.

The Emx family of genes is involved in establishing the identity of certain regions in the brain.  The brain is full of specialized areas (vision, speech, planning, etc) which are set up in the course of development.  The Eph family of genes help lay out the basic topography map of the brain, by setting up a chemical gradient (like the latitude and longitude on a GPS device) which allows migrating neurons to find their home.  The Hox genes also help to establish basic layouts of the brain and body.

Since genes largely function to create proteins, I’ll use genes and proteins fairly interchangeably.  However, some genes can code for multiple proteins depending on the context, so it’s not as simple as “one gene = one protein”.  The FGF8 gene (fibroblast growth factor 8), for example, can be sliced and diced in different ways, leading to the production of different proteins (depending on context).  Those proteins are also responsible for laying out some of the gross anatomy of the brain.

Once neurons have made their specific connections together in the course of their development, those same synapses can be used not only to send signals from one neuron to other.  Synapses are not truly connections - but gaps - between neurons into which signalling chemicals are injected.  Usually, those chemicals are neurotransmitters (like serotonin) which are used to send signals from one neuron to the next across the synaptic gap.  But hormones and other compounds (like anti-depressants) in the bloodstream are also able to influence the signal of many synapses (and other receptors), at a global level.

For example, vasopressin is a hormone released by the pituitary, that can affect social behavior.  The most promiscuous male prairie voles, for example, have fewer vasopressin (V1a) receptors in a specific region of their forebrain (ventral pallidum region).  Artificially introducing more vasopressin receptors into their brain immediately makes them seek monogamous relationships.

I've often thought that the strategic placement of hormone receptors throughout the brain and body during development (like the placement of troops on a battlefield) is much more interesting than the hormone itself (or "command from the general"), which is really just a molecule (or, to follow the analogy, a single word -- "Charge!").  The signal simply triggers a response that was already planned, practiced and ready.

Other examples of "brain genes" include Pax6, important for the formation of the eye, and NMDA receptors which seem to play an important role in establishing memories when the activity of two neurons coincide closely in time.  Also, in simple organisms like the “sea slug”, entire complex behaviors can be triggered by a single hormone, such as the ELH (egg-laying hormone), due to the way receptors have been set up in advance in the synapses of its neurons.

I've described some of the genes that are active in the development of the brain, but I haven't begun to describe the variants of some of those genes.  Obviously, if two people don't have exactly the same version of the NMDA receptor, perhaps one of them would have a better ability to recall facts.  This is obviously an interesting area for future exploration under the subject of "genetic variation and social fairness".