How the Social Environment Changes the Brain

Source: Flickr: Synapse by Manel Torralba, C.C. by 2.0

A paradox that helps clarify the solution to the ongoing genes vs. environment debate in mental health (described in a previous post) is that one of the things the human brain is genetically programmed to respond more strongly to than just about anything else are social environmental cues. Particularly, we are highly programmed to be highly responsive to our primary attachment figures. Another of the ongoing themes of this blog is the nonsensical practice of some researchers in psychiatry of routinely labeling differences between various diagnostic groups and control subjects in size and activity levels of parts of the brain—as seen on brain scans such as fMRI scans—as abnormalities (See this post).

So what is the physical mechanism within the brain by which the size and shape of various parts of the brain, and the relative strengths of various neural tracts, change in response to environmental cues and experiences? It is a now well-established process within neurons called long-term potentiation (LTP). Yet many researchers in the field seem oblivious to it.

Briefly, if a synapse—the point between two nerve cells at which an electrical impulse passes from one neuron to the other—is stimulated by individuals’ interactions with the environment, this produces a long-lasting increase in the likelihood of signal transmission between the bodies of those two cells. In other words, the power of the connection starts to increase.

Conversely, if such a connection is hardly ever stimulated, its power decreases. In fact, after a while, the synaptic connection may disappear altogether. This is probably the way memory works. Hearing a fact once in a lecture may not lead to its being remembered for long, whereas if someone keeps studying the fact, the memory of it becomes stronger.

It is important to mention that structural changes in the size and shape of the pre- and post-synapse parts of neurons may be mediated by ongoing changes in synaptic efficacy. Stimulation may allow for an increase in the size or number of active zones on both sides of the synapse. The “spines” of the cell can increase in volume after LTP induction. While the degree to which structural re-organization of synapses occurs in adult animals is not yet clear, the process seems to involve a neurotransmitter (a chemical substance that is released at the end of a neuron cell by the arrival of a nerve impulse and, by diffusing across the junction, causes the transfer of the impulse to another neuron) called brain-derived neurotrophic factor (BDNF).

LTP is associated with enhanced recycling of a part of the structure of the synapse, and this process could eventually result in the formation of a new, immature spine.

In other words, the more a part of the brain used for a particular purpose is used, the more likely it is to increase in size due to this process. When many synapses are involved in an individual's interactions with the environment, size differences in those parts of the brain can therefore easily be caused by conditioned responses rather than representing abnormalities.