The special senses differ from the general (somatic and visceral) senses in three ways: their receptors are concentrated in complex sensory organs rather than scattered through tissues, they are confined to the head, and their afferent fibres travel in cranial nerves — olfactory (I), optic (II), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX) and vagus (X). Two of them (photoreceptors and olfactory receptor neurons) are true neurons; the others (hair cells, taste receptor cells) are modified epithelial cells that synapse onto primary afferent neurons.
Vision. The eyeball (≈24 mm in axial length) has three concentric tunics — fibrous (sclera and cornea), vascular (choroid, ciliary body, iris: the uvea) and neural (retina). Light is refracted chiefly by the cornea (≈43 of the eye's ≈60 dioptres) and fine-focused by the lens, whose shape is altered during accommodation. Rods (≈92 million) and cones (≈4.6 million) transduce light by a G-protein cascade that hyperpolarises the cell. Bipolar and ganglion cells process the signal within the retina, and ≈1.2 million ganglion-cell axons form the optic nerve. At the optic chiasm nasal fibres cross, so each cerebral hemisphere receives the contralateral visual hemifield via the lateral geniculate nucleus and optic radiation to the primary visual cortex (V1, Brodmann area 17).
Hearing and balance. The ear has three parts. The external ear collects and funnels sound to the tympanic membrane; the air-filled middle ear uses the ossicular chain (malleus, incus, stapes) to match the low impedance of air to the high impedance of the cochlear fluids (≈25–30 dB gain); the inner ear, a fluid-filled labyrinth in the petrous temporal bone, contains the cochlea (hearing) and the vestibular apparatus (utricle, saccule and three semicircular canals). All of these receptors are hair cells, which open mechanically gated cation channels when their stereocilia are deflected. The potassium-rich endolymph (+80 mV endocochlear potential) gives hair cells an unusually large driving force. The basilar membrane is tonotopically tuned (base ≈20 kHz, apex ≈20 Hz).
Smell and taste are chemical senses. Olfactory receptor neurons in the roof of the nasal cavity express one of ≈350–400 functional odorant receptor genes; their axons pass through the cribriform plate to glomeruli in the olfactory bulb. Olfaction is unique in reaching the (paleo)cortex — piriform cortex, amygdala and entorhinal cortex — without a first thalamic relay. Taste buds, located mainly on fungiform, foliate and circumvallate papillae, detect five basic qualities (sweet, umami, bitter, sour, salty); taste is carried by cranial nerves VII, IX and X to the nucleus of the solitary tract, then via the VPM thalamus to the insular–opercular gustatory cortex. Much of what we call "flavour" is actually retronasal smell.
Clinically, the special senses account for an enormous burden of disability: uncorrected refractive error and cataract are the leading causes of visual impairment worldwide, glaucoma is the leading cause of irreversible blindness, and hearing loss affects more than 1.5 billion people. Bedside examination — visual acuity, fields, pupils, fundoscopy, otoscopy, tuning-fork tests, Dix–Hallpike, smell and taste testing — localises lesions along each pathway with remarkable precision.
Development
Eye. The eye forms from neuroectoderm, surface ectoderm and mesenchyme. In week 4 the optic vesicle evaginates from the diencephalon and induces the overlying surface ectoderm to form the lens placode. The vesicle invaginates into a double-layered optic cup: the inner layer becomes the neural retina and the outer layer the retinal pigment epithelium; the potential space between them persists (and is the plane of retinal detachment). The optic stalk becomes the optic nerve. The inferior choroid (optic) fissure carries the hyaloid artery and normally closes in week 7 — failure causes an inferonasal coloboma. The lens vesicle detaches from the surface ectoderm, which forms the corneal epithelium; neural crest mesenchyme forms the corneal stroma and endothelium, sclera, choroid and iris stroma. The sphincter and dilator pupillae, unusually, are neuroectodermal muscles. The eyelids fuse at ≈10 weeks and reopen at ≈26–28 weeks.
Ear. The otic placode (surface ectoderm beside the hindbrain) invaginates to form the otic vesicle (otocyst), which gives rise to the entire membranous labyrinth and the sensory epithelia; the surrounding mesenchyme chondrifies and ossifies as the bony otic capsule. The first pharyngeal pouch (tubotympanic recess) forms the tympanic cavity and auditory tube; the first pharyngeal cleft forms the external acoustic meatus; the tympanic membrane has contributions from all three germ layers. The malleus and incus derive from the first arch (Meckel cartilage; their muscle, tensor tympani, is supplied by V3), the stapes from the second arch (Reichert cartilage; stapedius is supplied by VII). The auricle forms from six auricular hillocks around the first cleft — low-set or malformed ears accompany many chromosomal and first/second-arch syndromes.
Smell and taste. The nasal (olfactory) placodes give rise to the olfactory epithelium; GnRH neurons migrate from the placode into the forebrain alongside olfactory axons, explaining Kallmann syndrome (anosmia + hypogonadotropic hypogonadism). Taste buds appear from ≈7–8 weeks, induced by contact with gustatory nerve fibres, and are functional before birth.