This article assumes familiarity with:
· How the brain’s threat-response system is organized as a hierarchy → D-300
· How the prefrontal cortex, limbic system, and brainstem interact → D-325, D-330.
If that is unfamiliar, start there first.
Neurons — The Architecture of the Brain’s Basic Unit
Everything the brain or body does begins here
510 words · 3 min read · Uploaded: 2026-06-29
Every response the brain has ever produced — every threat detection, every memory, every moment of regulation or its failure — began with a neuron firing. Neurons are the basic functional units of the nervous system. Understanding what they are and how they operate is not supplementary knowledge. It is the foundation on which every other concept in this series rests.
The human brain contains approximately 86 billion neurons. They are not uniform. They vary in size, shape, location, and function, but they share a common operating principle: they receive signals, integrate those signals, and transmit an output.
Nothing the brain does occurs without them.
Structure
A neuron has three primary components. The cell body (or soma) contains the nucleus and the machinery required to keep the cell alive. Dendrites extend outward from the soma and receive incoming signals from other neurons. The axon is a single, elongated projection that carries the neuron’s output signal away from the cell body toward other neurons, muscles, or organs.
Many axons are wrapped in myelin, a fatty insulating sheath that dramatically increases the speed and efficiency of signal transmission. Myelination is not fixed at birth. It develops throughout childhood and adolescence and continues into early adulthood. Importantly, it is experience-dependent. Pathways that are activated repeatedly become more heavily myelinated over time, allowing signals to travel faster and with less metabolic cost.
This is one of the mechanisms by which repeated experience — including repeated exposure to threat — becomes embedded in the nervous system’s operating architecture.
How a Neuron Fires
Neurons communicate through an electrochemical process known as the action potential. When incoming signals collectively push the neuron’s internal electrical charge past a critical threshold, the neuron fires, sending an electrical impulse down the axon toward its target.
Firing is all-or-nothing. A neuron either reaches threshold and fires fully, or it does not fire at all. What varies is not the size of the signal, but its frequency and pattern — how often the neuron fires and in what temporal sequence.
Crucially, firing thresholds are not fixed. They are influenced by prior activation, neuromodulators, and the overall physiological state of the nervous system. Under chronic stress, thresholds across multiple pathways shift. Neurons fire more easily in some circuits and less easily in others. This is not damage. It is recalibration.
Neurons and Threat
In the context of threat and trauma, the neuron’s most important property is its capacity to change through activation. Neurons that fire together repeatedly strengthen their connections. Neurons that do not fire together weaken theirs. This principle — often summarized as “cells that fire together wire together” — is the cellular basis of all learning.
It is also the basis of trauma’s persistence.
Threat pathways — the chains of neurons linking sensory input to amygdala activation to PAG-coordinated defensive response — become increasingly efficient with repetition. The more often a response is activated, the lower the threshold for future activation becomes. The system does not deteriorate. It learns.
A neuron in isolation does nothing. What matters is how neurons communicate with one another — and that communication happens at the synapse. The next article examines that junction directly.
Neurons that fire together wire together — the cellular basis of all learning, including trauma responses.