📝 STANDARD DESCRIPTION:
Two structural divisions:
• Central Nervous System (CNS): Brain + Spinal cord.
• Peripheral Nervous System (PNS): All cranial nerves (12 pairs) + Spinal nerves + ganglia.
Two functional divisions of PNS:
• Somatic (Voluntary) NS: controls skeletal muscle.
• Autonomic (Visceral) NS: controls smooth muscle, cardiac muscle, glands. Two arms:
– Sympathetic — 'fight or flight'; thoracolumbar.
– Parasympathetic — 'rest and digest'; craniosacral.
Cell of nervous tissue = NEURON
• Cell body (soma) + dendrites (input) + axon (output).
• Supported by glia (astrocytes, oligodendrocytes, microglia, Schwann cells).
📚 DETAILED DESCRIPTION:
NERVOUS SYSTEM ORGANIZATION
STRUCTURAL DIVISIONS:
• Central Nervous System (CNS): Brain + Spinal cord. Encased in the bony skull and vertebral canal, wrapped by meninges, bathed in CSF.
• Peripheral Nervous System (PNS): All neural tissue outside the CNS — 12 pairs of cranial nerves + spinal nerves + ganglia + plexuses + receptors.
FUNCTIONAL DIVISIONS:
• Somatic NS — voluntary; somatic afferents (sensation) + somatic efferents (skeletal muscle).
• Visceral NS = Autonomic NS — involuntary; visceral afferents + visceral efferents:
– Sympathetic ('thoracolumbar' outflow, T1-L3) — fight, flight, fright. Long postganglionic fibres releasing noradrenaline.
– Parasympathetic ('craniosacral' outflow, CN III, VII, IX, X + S2-S4) — rest, digest. Short postganglionic fibres releasing acetylcholine.
– Enteric NS — local 'mini-brain' in gut wall (myenteric + submucosal plexuses).
NEURON STRUCTURE:
• Soma (cell body) — contains nucleus + Nissl bodies (RER).
• Dendrites — multiple, short, branched; receive signals.
• Axon — single long process; carries signal away (action potential).
• Axon hillock — initial segment; trigger zone for AP.
• Myelin sheath — fatty insulation; speeds conduction.
– CNS: by oligodendrocytes (1 cell wraps many axons).
– PNS: by Schwann cells (1 cell = 1 internode).
• Nodes of Ranvier — gaps in myelin → saltatory conduction.
• Synapse — junction between neurons (chemical or electrical).
NEUROGLIA (support cells):
CNS: Astrocytes (BBB, support), Oligodendrocytes (myelin), Microglia (immune), Ependymal cells (line ventricles, make CSF).
PNS: Schwann cells (myelin), Satellite cells (around ganglion neurons).
NEURON TYPES (by function):
• Sensory (afferent) — toward CNS.
• Motor (efferent) — away from CNS.
• Interneurons — within CNS, connect the two.
NEURON TYPES (by structure):
• Multipolar (most CNS neurons).
• Bipolar (retina, olfactory).
• Unipolar (pseudounipolar — dorsal root ganglion).
// COMPARATIVE ANALYSIS
| ALL MAMMALS |
Same plan; differences mostly in size and gyrification (cortical folding) — primates and dolphins highly gyrencephalic; rodents lissencephalic (smooth). |
| BIRD |
Highly developed cerebellum and corpus striatum (replaces mammalian cortex for complex behaviour). |
CLINICAL RELEVANCE
Sympathetic vs Parasympathetic dominance drives clinical signs — e.g., dilated pupils (sympathetic) vs miotic pupils (parasympathetic). Horner's syndrome = loss of sympathetic supply to head → ptosis, miosis, enophthalmos, third-eyelid prolapse.
