The nervous system — comprising the CNS (brain + spinal cord) and PNS (12 cranial + 31 spinal nerve pairs) — uses specialized neurons consisting of a soma (with Nissl granules for protein synthesis), dendrites, and a single axon myelinated by Schwann cells (PNS) or oligodendrocytes (CNS), with nodes of Ranvier at unmyelinated gaps.

At rest, a neuron maintains -70 mV (inside negative) via the Na+/K+ ATPase pump (3 Na+ OUT, 2 K+ IN) and K+ leak channels; a threshold stimulus opens voltage-gated Na+ channels, Na+ influx depolarizes the membrane to +30 mV (action potential), then K+ efflux restores resting potential (repolarization).

The action potential obeys the all-or-none principle, propagates by saltatory conduction in myelinated fibres (node to node, 15-120 m/s) versus slow continuous conduction in unmyelinated fibres (0.5-2 m/s), and travels in one direction due to the absolute refractory period.

At the synapse, AP arrival → Ca2+ influx → exocytosis of neurotransmitters into the 20 nm synaptic cleft → receptor binding → new impulse; ACh (degraded by acetylcholinesterase) operates at the NMJ and all ANS preganglionic synapses; norepinephrine operates at most sympathetic postganglionic synapses.

The forebrain contains the cerebrum (highest functions; INITIATES voluntary movement), thalamus (sensory relay for all senses except smell), and hypothalamus (homeostasis; links nervous and endocrine systems by controlling the pituitary).

The hindbrain contains the cerebellum (COORDINATES voluntary movement and balance, does NOT initiate), pons (relay + pneumotaxic centre), and medulla oblongata (vital centres: cardiovascular, respiratory, vomiting, swallowing).

The PNS's autonomic nervous system divides into sympathetic ("fight or flight": ↑HR, dilates pupils and bronchi, inhibits digestion, norepinephrine postganglionic) and parasympathetic ("rest and digest": ↓HR, constricts pupils, stimulates digestion, ACh postganglionic).

In the eye, light refracts through the cornea $\frac{2}{3 of power}$, focuses through the lens (accommodation via ciliary muscle contraction for near vision), and forms an inverted image on the retina where 120 million rods (rhodopsin, dim light) and 6-7 million cones (iodopsin, colour, at fovea) transduce light to nerve impulses.

Eye defects: myopia (image in front of retina, concave lens correction), hypermetropia (image behind retina, convex lens), presbyopia (aging lens, bifocal), astigmatism (irregular cornea, cylindrical lens), cataract (cloudy lens, surgery), glaucoma (raised IOP, optic nerve damage, medication/surgery).

In the ear, sound travels from the tympanic membrane → malleus → incus → stapes (smallest bone in the body) → oval window → cochlea → organ of Corti (hair cells generate impulses to auditory nerve); balance is maintained by the vestibular apparatus (3 semicircular canals + utricle + saccule).