Individual neurons are organized into a vast communication network. This lesson zooms out to the whole nervous system — its branching divisions — and then to the slower chemical network of the endocrine system, which floods the bloodstream with hormones.
Divisions of the nervous system
The nervous system splits into two halves. The central nervous system (CNS) is the brain and spinal cord — the decision-maker. The peripheral nervous system (PNS) is every nerve outside the CNS, connecting it to the rest of the body.
The PNS divides again. The somatic nervous system controls voluntary skeletal muscles. The autonomic nervous system controls involuntary functions (heartbeat, digestion) and itself splits into the sympathetic division (arousal — the “fight-or-flight” accelerator) and the parasympathetic division (calming — the “rest-and-digest” brake).
| Division | Controls | Remember |
|---|---|---|
| Central (CNS) | Brain + spinal cord | The command center |
| Peripheral (PNS) | All other nerves | The wiring to the body |
| Somatic | Voluntary muscles | “Soma” = body you move on purpose |
| Autonomic | Involuntary organs | “Automatic” |
| Sympathetic | Arouses: fight-or-flight | Speeds heart, dilates pupils |
| Parasympathetic | Calms: rest-and-digest | Slows heart, restores |
The nervous system tree. Sympathetic accelerates; parasympathetic brakes.
Reflexes and the spinal cord
Some responses skip the brain entirely. In a reflex arc, a sensory neuron carries a signal to the spinal cord, an interneuron passes it directly to a motor neuron, and you pull your hand off a hot stove before your brain registers pain. This built-in shortcut shows the spinal cord is not just a cable — it can process simple responses on its own.
[Diagram — see the figure in the print workbook.]
Afferent (sensory) neurons carry information IN to the CNS; efferent (motor) neurons carry commands OUT. Mnemonic: Afferent = Arrive, Efferent = Exit.
The endocrine system
Alongside the fast electrical nervous system runs a slower chemical one: the endocrine system, a set of glands that release hormones into the bloodstream. Hormones act more slowly than neurotransmitters but their effects last longer. The pituitary gland is the “master gland” — directed by the brain’s hypothalamus, it controls other glands. The adrenal glands release adrenaline (epinephrine) during stress, triggering the same arousal the sympathetic system does.
Neurotransmitters vs. hormones: both are chemical messengers, but neurotransmitters cross a tiny synapse in milliseconds, while hormones travel the bloodstream — slower to start, slower to fade.
Worked example 1
You slam on the brakes to avoid a crash. Your heart races, your pupils widen, and your palms sweat. Which system produced this, and what returns your body to normal afterward?
The racing heart, dilated pupils, and sweat are the fight-or-flight response, produced by the sympathetic division of the autonomic nervous system, aided by adrenaline from the adrenal glands.
Once the danger passes, the parasympathetic division takes over — slowing the heart and restoring digestion — returning the body to its resting “rest-and-digest” state.
Common trap: Sympathetic = stress, even though it sounds soothing
“Sympathetic” sounds gentle, but it is the arousal system: it speeds your heart, dilates pupils, and dumps adrenaline for fight-or-flight. The parasympathetic system is the calming one. Do not let the friendly-sounding name fool you.
Quick check: pounding heart before a test = sympathetic; the slow calm afterward = parasympathetic.
Hack: Sympathetic = Spend, Parasympathetic = Preserve
The Sympathetic system Spends energy (fight-or-flight); the Parasympathetic system Preserves and restores it (rest-and-digest). One letter, one job.
For the whole nervous system, drill the tree top-down: CNS vs. PNS; within PNS, somatic (voluntary) vs. autonomic (involuntary); within autonomic, sympathetic vs. parasympathetic. If you can redraw that tree, you own the lesson.
