Chapter 5: The Nervous System — CNS, PNS, and the Neuron

The nervous system is the master control system of the body, integrating sensory input, processing information, and generating motor output — all within milliseconds.

Neuron Structure and Function

The neuron is the fundamental functional unit of the nervous system.

• Dendrites: branching processes that receive incoming signals
• Cell body (soma): contains the nucleus and organelles; metabolic hub
• Axon: single long process that conducts signals away from the cell body
• Myelin sheath: lipid insulation formed by Schwann cells (PNS) or oligodendrocytes (CNS); enables saltatory conduction (action potential “jumps” between nodes of Ranvier)
• Axon terminal / synaptic bouton: releases neurotransmitters into the synapse

Action potential: A graded depolarization reaching threshold (~-55 mV) triggers an all-or-none action potential. Voltage-gated Na⁺ channels open → rapid depolarization (+40 mV) → inactivation → K⁺ channels open → repolarization → hyperpolarization (refractory period).

CNS vs PNS

Feature	Central Nervous System (CNS)	Peripheral Nervous System (PNS)
Components	Brain and spinal cord	Cranial nerves, spinal nerves, ganglia
Support cells	Astrocytes, oligodendrocytes, microglia	Schwann cells, satellite cells
Regeneration	Very limited	Better (Schwann cells guide regrowth)
Protection	Blood-brain barrier, meninges, CSF	Epineurium

Autonomic Nervous System: Sympathetic vs Parasympathetic

Feature	Sympathetic (“fight or flight”)	Parasympathetic (“rest and digest”)
Pre-ganglionic NT	Acetylcholine	Acetylcholine
Post-ganglionic NT	Norepinephrine	Acetylcholine
Heart rate	↑	↓
Pupil size	Dilated (mydriasis)	Constricted (miosis)
GI motility	↓	↑
Bladder	Relaxes detrusor	Contracts detrusor

Major Neurotransmitters

Neurotransmitter	Function	Clinical Relevance
Acetylcholine	Neuromuscular junction, parasympathetic	Myasthenia gravis, Alzheimer’s disease
Dopamine	Reward, motor control	Parkinson’s disease, schizophrenia
Serotonin	Mood, sleep, appetite	Depression, anxiety (SSRIs target this)
Norepinephrine	Arousal, sympathetic tone	PTSD, ADHD
GABA	Primary inhibitory NT	Epilepsy, anxiety (benzodiazepines)
Glutamate	Primary excitatory NT	Excitotoxicity in stroke/TBI

Stroke: Ischemic vs Hemorrhagic

Stroke is a sudden interruption of blood flow to the brain (or rupture of a vessel).

Feature	Ischemic Stroke (87%)	Hemorrhagic Stroke (13%)
Cause	Thrombosis or embolism	Vessel rupture (hypertension, aneurysm)
CT appearance	Initially normal; hypodensity later	Hyperdense (bright) immediately
Treatment	IV tPA within 4.5 h; thrombectomy	BP control, surgical evacuation
Anticoagulants	Yes (after hemorrhage excluded)	Contraindicated

FAST acronym: Facial drooping, Arm weakness, Speech difficulty, Time to call emergency services.

Key Checklist

• Describes neuron anatomy and explains saltatory conduction in myelinated fibers
• Contrasts sympathetic and parasympathetic effects on major organ systems
• Differentiates ischemic from hemorrhagic stroke and outlines acute management