We have walked through the geography of the brain. Now we descend to the level of single cells, where the actual chemical work happens. This is where the metaphors we have built must connect to the molecular machinery, because at the bedside, when you prescribe a drug, you are intervening at this level.
The basic unit of the nervous system is the neuron. A neuron has three parts: a cell body (also called the soma), dendrites that receive signals from other neurons, and an axon that sends signals to other neurons. The end of the axon, the axon terminal, releases neurotransmitter into a tiny gap called the synapse. The receiving neuron has receptors on its dendrites that detect the neurotransmitter.
The cell body contains the nucleus and most of the metabolic machinery. The dendrites form a branching tree that maximizes surface area for receiving incoming signals — a single neuron may receive input from thousands of other neurons. The axon is typically a single long process that can extend from a few hundred micrometers (in local interneurons) to a meter or more (in spinal motor neurons that reach distal limb muscles).
Most axons are wrapped in myelin, a fatty insulating sheath produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Myelin dramatically speeds the propagation of electrical signals — action potentials jump from one node of Ranvier (gaps in the myelin) to the next, rather than propagating continuously down the axon. Loss of myelin, as in multiple sclerosis, slows or blocks signal propagation and produces the corresponding clinical deficits.
Neurons communicate in two main ways. Electrical signaling happens within a neuron — the action potential travels down the axon as a wave of voltage change. Chemical signaling happens between neurons — when the action potential reaches the axon terminal, it triggers release of neurotransmitter into the synapse, where the chemical message crosses to the next neuron.
The synapse is where every psychotropic medication ultimately acts. The five-step synaptic cycle — synthesis, release, receptor binding, reuptake, breakdown — gives us five drug-target points. We will visit each step in the next concepts. By the end of Stage 9, you will be able to map any psychiatric medication onto a specific molecular intervention at a specific step of this cycle.
Hold this baseline. The neuron, the dendrites, the axon, the synapse. Every clinical phenomenon we have discussed — Parkinson's, depression, addiction, psychosis, PTSD — ultimately comes back to signaling at this scale.