Nervous System

The nervous system is how you sense, decide, and act in real time. Signals move as electrical pulses down neurons and cross synapses as chemicals. Where the endocrine system takes minutes to days to change behavior, the nervous system takes milliseconds.

At a glance

What it does

Senses the world, runs the body, builds models of both. Pain, vision, temperature, limb position, heart rate, gut motility, memory, language, planning, mood — all of it is nervous system output. Some of it is conscious and some is not; most of it is not.

The nervous system is also a learning machine. Synapses strengthen or weaken based on use (Hebbian plasticity). This is how memory forms, how skills consolidate, how trauma carves in grooves that take years to sand flat.

How it works

Anatomically it splits into the central nervous system (brain and spinal cord) and the peripheral nervous system (everything else — nerves running to muscles, glands, and viscera). Functionally, the peripheral divides into somatic (voluntary muscle control, sensation) and autonomic (involuntary, runs organs).

Autonomic has two arms that mostly oppose each other. The sympathetic arm pushes fight-or-flight: heart rate up, pupils dilate, gut shuts down, glucose dumps into blood, adrenaline surges. The parasympathetic arm handles rest-and-digest: heart rate down, salivation, gut motility, sexual arousal, recovery. Neither is on or off; you are running a weighted balance every second of the day, and chronic tilt in one direction is what most stress pathology actually is.

Key neurotransmitters

• Glutamate — the main excitatory transmitter. About 80% of cortical signaling.
• GABA — the main inhibitory transmitter. Tones down excitation. Benzodiazepines and alcohol both hit it.
• Dopamine — drives motivation, reward prediction, motor control. Not the "pleasure molecule" — it is the want molecule.
• Serotonin — wide-ranging role in mood, gut function, satiety. 90% of body serotonin is in the gut, not the brain.
• Norepinephrine — alertness, vigilance, sympathetic drive.
• Acetylcholine — motor neurons use it at muscles; brain uses it for attention and memory encoding.

When it goes wrong

Stroke kills or damages neurons by cutting blood supply; outcome depends heavily on where the damage lands. Traumatic brain injury is under-diagnosed, especially in contact sports and post-concussion syndromes. Alzheimer's and other dementias involve protein misfolding (beta-amyloid, tau, alpha-synuclein) and eventual neuronal death.

Depression, anxiety, and addiction sit downstream of neurotransmitter regulation, though the chemistry story is more complicated than "low serotonin" (which is mostly wrong). Parkinson's is specifically a loss of dopamine neurons in the substantia nigra. Epilepsy is an excess of synchronized firing.

Interactions

Hormones modulate the nervous system constantly. Cortisol alters prefrontal cortex function and hippocampal memory. Estrogen affects serotonin synthesis. Testosterone changes dopamine tone. Sleep is when most neural housekeeping (glymphatic clearance, memory consolidation) happens, which is why a ruined sleep schedule wrecks mood and cognition faster than any supplement will fix.

Honest take

Sources

• Kandel et al., Principles of Neural Science, 6th ed. — the reference textbook on neuronal and circuit biology.
• Moncrieff et al. (2022), Molecular Psychiatry — the serotonin-depression review that upended the chemical imbalance story.
• Raichle, The Brain's Default Mode Network, Annu Rev Neurosci — shifted thinking toward network-level understanding.