Behave: The Biology of Humans at Our Best and Worst

Our brains are plastic; they change over time based on the stimulus they’re given. The way events in our life change the nature and structure of our brains—i.e., learning—is the focus of this chapter. As Sapolsky shows, this process is all about the varied ways neurons connect to each other.

“The essence of learning” (139) is the synaptic connection between neurons. In creating synaptic connections, one neuron sends repeated signals containing the neurotransmitter glutamate through the synapse (the gap between the two neurons) to another neuron, which finally activates once its activation threshold is reached. Now neuron A can send information through to the receptive neuron B. This first activational burst of neuron B causes long-term potentiation (LTP), a prolonged increase in the responsiveness of the synapse to subsequent glutamate signals. In a highly complex sequence of events, Sapolsky outlines the basis of learning and memory: strengthened connections between particular neurons.

LTP occurs throughout the nervous system, facilitating our ability not just to remember explicit facts but also conditioned responses like fear, craving, and sensations. Creating and strengthening memory can also occur through the creation of new dendrites—the ends of the neuron receptive to others. As the neuron grows more dendrites, more neurons can connect to it and gather more information. Different hormonal events, stress, and anxiety can further increase or decrease dendritic branches in neurons. In the amygdala, for instance, sustained stress increases dendrites in the basolateral amygdala, which is associated with learned fears, but not the central amygdala, which is associated with innate fears. Therefore, stress and anxiety increase fear and anxiety conditioning but not phobias. This is an example of how understanding the complexities of our brain’s wiring better informs our understanding of human learning capacities.

When we learn, neurons can also produce new axons—the part of the neurons that sends signals to others. Through this process, the neuron can not only connect to local neurons (those in the same cortical region) but also outward to those in other brain regions. This allows plasticity of cortical function, which is particularly evident in damaged nervous systems. In cases of blindness, for example, axons associated with touch project not only to the tactile cortex (which decodes touch information) but also the visual cortex, allowing the blind to “see” letters when reading braille. This can also bring about expansion of cortical regions, such that musicians—even those who have only been practicing for a short time—show expanded auditory cortexes, making them better at detecting sound variations. These are examples that show how neuronally plastic the brain really is.

Growing new neurons is called neurogenesis. Contrary to popular belief, adult brains are capable of neurogenesis. Here, Sapolsky goes into a discussion of the rejection of initial theories on adult neurogenesis from scientific credulity. In this discussion, Sapolsky outlines that even scientists, those who should be the most impartial and reasonable, sometimes behave in the automatic ways that characterize our worst behavior. In the end, adult neurogenesis has become an accepted phenomenon and studies are now abundant in the field.

We can remap our brain in diverse ways through experience. This accounts for the incredible degree of expertise and understanding of nuance we associate with specialists in any field, but it can also cause the crippling results of anxiety, such as when our brain “learns” to increase automatic fear responses. The core takeaway from this chapter is that the way our brains function is neutral; it is what and how we choose to teach our brains that is important in shaping our future behavior. As such, it is our responsibility to be stewards of our own learning and remember to always seek better, more scientific, more humane ways of looking at the world.