Beyond the Brain and Cord
The Peripheral Nervous System (PNS) is everything outside the brain and spinal cord — the network of nerves linking the CNS to the rest of the body. It splits into two functional divisions. The somatic nervous system handles voluntary control: afferent (sensory) nerves carry information inward from skin, muscles, and joints, while efferent (motor) nerves carry commands outward to skeletal muscle.
The autonomic nervous system (ANS) governs involuntary functions — heart rate, digestion, glandular secretion — and itself splits into two branches that typically oppose each other on the same organs.
| Branch | Nickname | Typical effects |
|---|---|---|
| Sympathetic | "Fight-or-flight" | Heart rate up, pupils dilate, digestion inhibited, adrenaline released |
| Parasympathetic | "Rest-and-digest" | Heart rate down, pupils constrict, digestion stimulated, saliva flows |
The sympathetic branch mobilizes the body for an emergency: the adrenal medulla releases adrenaline into the bloodstream, amplifying and prolonging the nervous signal. The parasympathetic branch, dominated by the vagus nerve, restores and conserves resources once the threat has passed. Most internal organs receive input from both branches, so organ state at any moment reflects the balance between the two — not one branch simply switching off while the other switches on.
Common pitfall: treating "sympathetic" and "parasympathetic" as switches for good and bad states. Both are essential and constantly active to different degrees; chronic sympathetic dominance from unrelieved stress — not sympathetic activation itself — is what damages health over time.