Chapter 5: Days to Months Before

Sapolsky explores how neural changes unfolding over days to months shape the behaviors we later enact. The central theme is neuroplasticity—the brain’s ability to remodel synapses, dendrites, axons, and even produce new neurons. Sapolsky begins with synaptic plasticity, showing how repeated stimulation strengthens connections through long-term potentiation (LTP), while long-term depression (LTD) weakens irrelevant signals, sharpening memory. This cellular process underlies learning, fear conditioning in the amygdala, impulse control in the frontal cortex, and reward associations in dopaminergic pathways. Stress powerfully interacts with plasticity: short bursts of stress can enhance hippocampal learning, while chronic stress suppresses hippocampal function, weakens frontal cortex regulation, and strengthens fear circuits in the amygdala. Beyond synapses, Sapolsky highlights activity-dependent synaptogenesis—the creation of new synapses and dendritic branches. Rats in enriched environments grow more synapses; practice reshapes motor and sensory maps, as seen in musicians or jugglers. Stress again shows an inverted-U effect: moderate stress promotes synapse growth, while sustained stress causes dendritic retraction, synapse loss, and cognitive decline. Estrogen also drives cyclical growth in hippocampal dendritic trees, linking hormones to plasticity. Axonal plasticity reveals even more remarkable changes: blind individuals remap tactile or auditory signals into visual cortex, while injury or training causes neurons to sprout new projections, reshaping sensory and motor maps. The chapter then turns to the revolutionary discovery of adult neurogenesis. Long dismissed as impossible, research since the 1960s has confirmed that the adult hippocampus generates new neurons throughout life, influenced by exercise, antidepressants, hormones, learning, and enriched environments, but impaired by chronic stress and depression. These new neurons support “pattern separation”—distinguishing subtle differences in information, such as faces, words, or concepts. Sapolsky also describes how plasticity changes brain region size, such as London taxi drivers’ enlarged hippocampi from mastering spatial maps, or the amygdala’s enlargement in PTSD due to chronic trauma. Crucially, Sapolsky stresses that plasticity is value-free. It enables positive transformations like skill acquisition, resilience, or social change, but also entrenches maladaptive patterns like trauma, addiction, or xenophobia. Neuroplasticity explains both recovery and vulnerability, hope and harm. The chapter concludes by linking biology to social transformation: just as neural connections rewire in response to environment, people and societies change with new experiences, from political revolutions to personal resilience. In the span of months, our brains and behaviors are sculpted by both biology and culture, laying the groundwork for how we act at our best and worst.