Cellular and Molecular Neuroscience

At the cellular level, a NEURON works as an electrochemical device. Its membrane (cell wall) keeps DIFFERENT concentrations of ions inside vs outside — more sodium (Na⁺) outside, more potassium (K⁺) inside. This creates a voltage difference (about −70 millivolts at rest). When the neuron is stimulated enough, this voltage flips dramatically — an ACTION POTENTIAL (the "firing" of the neuron) shoots down the axon.

How an action potential works. (1) STIMULUS pushes voltage past a threshold. (2) Sodium channels open — Na⁺ rushes in. Voltage spikes positive. (3) Sodium channels close; potassium channels open — K⁺ rushes out. Voltage falls back. (4) Pumps restore the original ion balance. This wave travels down the axon at up to 120 m/s (~270 mph). It's digital — either fully fires or doesn't (the "all-or-none" principle).

Synaptic biology. At the axon terminal, voltage-gated calcium channels open during an action potential. Calcium triggers VESICLES (tiny sacs) to fuse with the membrane, releasing neurotransmitters into the synaptic cleft. Receptors on the next neuron detect them. Many psychiatric medications (Prozac, Adderall) modify how these molecular steps work — preventing reuptake, increasing synthesis, or blocking certain receptors.

At the molecular level, the brain is a chemistry experiment running 86 billion times in parallel. Every thought, dream, and decision is rooted in this beautiful machinery.