Core Neural Concepts

• Neuron structure: Cell body $\frac{soma}{cyton}$ + dendrites (receive) + axon (transmit). Nissl granules = rough ER in soma for protein synthesis. Absent in axon.
• Myelin: Schwann cells (PNS) and oligodendrocytes (CNS) form myelin. Nodes of Ranvier = unmyelinated gaps between Schwann cells.
• Resting potential: -70 mV (inside negative). Maintained by Na+/K+ pump (3 Na+ out, 2 K+ in) + K+ leak channels.
• Action potential: Na+ in → depolarization (+30 mV) → Na+ channels inactivate → K+ out → repolarization → hyperpolarization → resting state. All-or-none principle.
• Saltatory conduction: Myelinated neurons (15-120 m/s). Unmyelinated: 0.5-2 m/s. AP jumps node to node.
• Synapse: Ca2+ influx → NT exocytosis into 20 nm cleft → receptor binding → new AP. ACh terminated by acetylcholinesterase.

Brain Regions

Critical Distinctions

• Cerebrum vs Cerebellum: Cerebrum INITIATES voluntary movement. Cerebellum COORDINATES movement. Cerebellum damage → ataxia (not paralysis).
• Sympathetic vs Parasympathetic: Sympathetic = $\frac{fight}{flight}$ (↑HR, dilates pupils, NE postgang). Parasympathetic = $\frac{rest}{digest}$ (↓HR, constricts pupils, ACh postgang).

Photoreceptors

• Rods: 120 million, rhodopsin (Vitamin A + opsin), scotopic (dim-light) vision, peripheral retina.
• Cones: 6-7 million, iodopsin, photopic (colour) vision, concentrated at fovea.

Eye Defects Correction

• Myopia → concave (−) lens. Hypermetropia → convex (+) lens. Presbyopia → bifocal. Astigmatism → cylindrical.

Ear

• Ossicles order: Malleus → Incus → Stapes → oval window. Stapes = smallest bone in body. 20x amplification.
• Cochlea (hearing: organ of Corti). Vestibular apparatus (balance: 3 semicircular canals + utricle + saccule).

Key Numbers

• Resting potential: -70 mV | AP peak: +30 mV | Synaptic cleft: 20 nm
• Na+/K+ pump: 3 out, 2 in | Spinal nerves: 31 pairs | Cranial nerves: 12 pairs
• Rods: 120 million | Cones: 6-7 million | Ossicular gain: ~20-fold