NG-313

⚡ CNS vs PNS

Where the nervous system divides, why it matters clinically, and how one fact predicts whether damage will heal.

🧬 The two divisions

The brain and cord picked out as the central nervous system with nerves branching out as the peripheral, and the same nerve cut twice - the peripheral one regrowing slowly along its sheath, the central one not regrowing at all. — Central does not regenerate and peripheral does. That single fact predicts more of neurology than any list. Swipe it sideways if it is cut off, or tap to open it full size.

Central = brain and spinal cord. Peripheral = everything else. One rule follows from that split and explains most of neurology:

CNS does not regenerate. PNS can. A severed peripheral nerve may slowly recover. A spinal cord injury does not.

	Central (CNS)	Peripheral (PNS)
Contains	Brain, spinal cord	Cranial & spinal nerves, ganglia
Myelin made by	Oligodendrocytes	Schwann cells
Regenerates?	Essentially no	Yes, slowly
Protected by	Skull, vertebrae, meninges, blood–brain barrier	Little protection
Demyelinating disease	Multiple sclerosis	Guillain–Barré

That last row is a whole exam question. MS and GBS are both demyelination — the difference is where. MS relapses and remits in the CNS; GBS ascends through peripheral nerves and usually recovers.

🧠 Splitting the peripheral system

The PNS divides again by what it controls:

SomaticVoluntary — skeletal muscle, conscious control

AutonomicInvoluntary — organs, glands, smooth muscle

	SYMPATHETIC — “fight or flight”	PARASYMPATHETIC — “rest and digest”
Heart rate	Up	Down
Pupils	Dilate	Constrict
Airways	Dilate	Constrict
Digestion	Slows, mouth dry	Speeds up, salivation
Bladder	Retains	Empties
Transmitter	Norepinephrine	Acetylcholine

✅ Why this predicts drug side effects

Anticholinergics block the parasympathetic system, so you get the sympathetic picture:

"Can't see, can't pee, can't spit, can't poop" — blurred vision, urinary retention, dry mouth, constipation. Plus tachycardia and flushing.

Beta blockers block the sympathetic system → slow heart rate, lower blood pressure, and bronchoconstriction — which is why non-selective ones are avoided in asthma.

🩺 Why the split matters at the bedside

🚨 The clinical consequences of “CNS does not regenerate”

Spinal cord injury is permanent — care focuses on preventing complications, not recovery
Stroke damage is permanent — rehabilitation retrains other areas rather than repairing dead tissue
Prevention outranks treatment everywhere in CNS disease

🩸 The blood–brain barrier

It protects the brain from toxins — and also keeps most drugs out.

That is why CNS infections need antibiotics that cross the barrier, at high doses, and why inflammation (as in meningitis) actually helps drugs get in.

🎯 NCLEX traps

MS = CNS demyelination. GBS = PNS demyelination.
Anticholinergic effects = the four “can’ts”
Peripheral nerve injury may recover; cord injury does not
Sympathetic dilates pupils and airways; parasympathetic constricts both

Sources. Written from MedlinePlus, NINDS, CDC and OpenStax A&P 2e (CC BY 4.0).