Memory tricks for the most complex system

Each word starts with the same letter as the cranial nerve it represents. I=Olfactory (smell), II=Optic (vision), III=Oculomotor (eye movement), IV=Trochlear (superior oblique), V=Trigeminal (face sensation/chewing), VI=Abducens (lateral eye), VII=Facial (expression/taste), VIII=Auditory/Vestibulocochlear (hearing/balance), IX=Glossopharyngeal (throat/taste), X=Vagus (parasympathetic), XI=Spinal Accessory (SCM/trapezius), XII=Hypoglossal (tongue).

CNS = brain and spinal cord — protected by bone (skull and vertebral column) and meninges. PNS = all nervous tissue outside the CNS — cranial nerves, spinal nerves, ganglia. Key difference: CNS uses oligodendrocytes for myelination. PNS uses Schwann cells. PNS can regenerate after injury — CNS generally cannot.

Sympathetic (fight or flight): increases heart rate, dilates pupils, inhibits digestion, increases blood pressure, dilates bronchioles. Uses norepinephrine (adrenergic). Parasympathetic (rest and digest): decreases heart rate, constricts pupils, stimulates digestion, decreases blood pressure, constricts bronchioles. Uses acetylcholine (cholinergic). Remember: SLUDD for parasympathetic — Salivation, Lacrimation, Urination, Digestion, Defecation.

Sensory (afferent) neurons carry signals TOWARD the CNS — from receptors. Interneurons (association neurons) are entirely within the CNS — they process and integrate information. Motor (efferent) neurons carry signals AWAY from the CNS — to effectors (muscles and glands). Memory trick: Afferent = Arrive (toward CNS). Efferent = Exit (away from CNS).

Dorsal Column-Medial Lemniscal (DCML): fine touch, vibration, proprioception — travels ipsilateral up to medulla then crosses. Spinothalamic: pain and temperature — crosses immediately at spinal level then travels up contralateral. Corticospinal: voluntary motor — descends from cortex, crosses in medulla (pyramidal decussation). This is why a brain lesion on the LEFT causes weakness on the RIGHT.

The three meninges surround and protect the CNS. Dura mater = tough outer layer (dura = tough). Arachnoid mater = middle spider-web layer — CSF flows in the subarachnoid space beneath it. Pia mater = thin delicate layer directly on brain surface (pia = delicate). Meningitis = inflammation of meninges. Epidural hematoma = arterial blood between skull and dura. Subdural hematoma = venous blood between dura and arachnoid.

Frontal lobe: voluntary motor control (primary motor cortex), executive function, personality, Broca's area (speech production). Temporal lobe: hearing, Wernicke's area (speech comprehension), memory (hippocampus). Occipital lobe: vision — all visual processing. Parietal lobe: somatosensory (touch, pressure, pain, temperature), spatial awareness. Damage to Broca's = can't speak. Damage to Wernicke's = can't understand speech.

A reflex arc is the neural pathway for a reflex — it does NOT require conscious thought because it is processed in the spinal cord, not the brain. The five components in order: Receptor detects stimulus → Afferent neuron carries signal to spinal cord → Integration center (spinal cord) → Efferent neuron carries signal to → Effector (muscle or gland) responds. The knee-jerk reflex is a monosynaptic reflex — only one synapse between afferent and efferent.

Glial cells outnumber neurons 10:1 and are responsible for support, protection, and myelination. Astrocytes: blood-brain barrier, nutrient support. Microglia: CNS immune cells — phagocytose debris. Oligodendrocytes: myelinate CNS axons (one cell myelinates multiple axons). Ependymal cells: produce CSF, line ventricles. Schwann cells: myelinate PNS axons (one cell per axon). Satellite cells: support neurons in PNS ganglia.