Sensory Organs: Eye, Ear, Nose, Taste Buds
Complete anatomy of the sensory organs - eye (globe, retina, visual pathway), ear (external, middle, inner, auditory pathway), nose (olfaction), and taste buds (gustation).
This content is for informational purposes only. Always consult a healthcare professional.
The Eye (Visual System)
Orbit
| Feature |
Description |
| Shape |
Pyramid (4-sided) |
| Volume |
~30 mL |
| Walls |
Roof (frontal bone), floor (maxilla), medial (ethmoid), lateral (zygomatic, sphenoid) |
| Apex |
Optic canal (CN II, ophthalmic artery) |
| Fissures |
Superior orbital (CN III, IV, V1, VI), Inferior orbital (V2, infraorbital nerve) |
Eyeball (Globe)
| Feature |
Description |
| Diameter |
24 mm (anterior-posterior) |
| Weight |
7.5 g |
| Volume |
6.5 mL |
Layers:
| Layer |
Parts |
Function |
| Fibrous (outer) |
Sclera (posterior 5/6), Cornea (anterior 1/6) |
Protection, refraction |
| Vascular (middle) |
Choroid, Ciliary body, Iris |
Blood supply, accommodation, light control |
| Neural (inner) |
Retina |
Photoreception |
Anterior segment:
| Structure |
Description |
| Anterior chamber |
Between cornea and iris |
| Posterior chamber |
Between iris and lens |
| Aqueous humor |
Produced by ciliary processes (trabecular meshwork drainage) |
| Lens |
Biconvex, crystalline (accommodation) |
| Zonules |
Suspensory ligaments (ciliary body to lens) |
Posterior segment:
| Structure |
Description |
| Vitreous humor |
Gel (99% water, hyaluronic acid, collagen) |
| Hyaloid canal |
Remnant of embryonic hyaloid artery |
Retina
| Region |
Description |
| Optic disc |
Blind spot (no photoreceptors, where axons exit) |
| Macula lutea |
Central 5.5 mm (high acuity) |
| Fovea centralis |
Center of macula (only cones, highest acuity) |
| Ora serrata |
Anterior border (junction with ciliary body) |
| Peripheral retina |
Rod-rich, dim light vision |
Layers (10 layers from pigment epithelium to inner limiting membrane):
| Cell type |
Function |
Distribution |
| Rods |
Dim light (scotopic), no color |
Peripheral (120 million) |
| Cones |
Bright light (photopic), color |
Macula (6 million) |
| Bipolar cells |
Connect photoreceptors → ganglion cells |
Intermediate |
| Ganglion cells |
Output to brain (optic nerve) |
Inner retina |
| Horizontal cells |
Lateral inhibition |
Outer plexiform |
| Amacrine cells |
Modulation |
Inner plexiform |
Blood supply:
- Central retinal artery (inner retina) - branch of ophthalmic artery
- Choriocapillaris (outer retina, photoreceptors) - from choroid
Visual Pathway
- Retina (photoreceptors → bipolar → ganglion cells)
- Optic nerve (CN II) - 1.2 million axons
- Optic chiasm (nasal fibers cross)
- Optic tract (temporal fibers stay ipsilateral)
- Lateral geniculate nucleus (LGN) of thalamus
- Optic radiation (Meyer loop for inferior field)
- Primary visual cortex (V1, calcarine sulcus, Brodmann area 17)
| Muscle |
CN |
Action |
| Medial rectus |
III |
Adduction |
| Lateral rectus |
VI |
Abduction |
| Superior rectus |
III |
Elevation, adduction, intorsion |
| Inferior rectus |
III |
Depression, adduction, extorsion |
| Superior oblique |
IV |
Intorsion, depression, abduction |
| Inferior oblique |
III |
Extorsion, elevation, abduction |
Lacrimal System
| Part |
Description |
| Lacrimal gland |
Upper outer orbit (CN VII parasympathetic) |
| Puncta |
Upper and lower lids |
| Canaliculi |
Drain to lacrimal sac |
| Nasolacrimal duct |
→ Inferior meatus of nose |
The Ear (Auditory and Vestibular System)
External Ear
| Part |
Description |
| Auricle (pinna) |
Elastic cartilage |
| External auditory meatus |
2.5 cm (bony 1/3 inner, cartilaginous 2/3 outer) |
| Cerumen (earwax) |
Glands in cartilaginous part (protection) |
Middle Ear (Tympanic Cavity)
| Feature |
Description |
| Tympanic membrane (eardrum) |
9-10 mm (pars tensa, pars flaccida) |
| Ossicles |
Malleus (hammer), Incus (anvil), Stapes (stirrup) |
| Muscles |
Tensor tympani (V3), Stapedius (VII) - dampen loud sounds |
| Windows |
Oval window (footplate of stapes), Round window (pressure release) |
| Eustachian tube |
Connects to nasopharynx (pressure equalization) |
Ossicular chain amplification: ~22× (pressure amplification from TM to oval window)
Inner Ear (Labyrinth)
Bony labyrinth:
| Structure |
Contents |
| Cochlea |
Scala vestibuli, scala media, scala tympani |
| Vestibule |
Utricle, saccule |
| Semicircular canals |
3 (anterior, posterior, lateral) |
Membranous labyrinth:
| Structure |
Function |
| Organ of Corti (cochlea) |
Hearing |
| Utricle, saccule |
Linear acceleration, gravity (otolith organs) |
| Semicircular ducts |
Angular acceleration (head rotation) |
Cochlea
| Feature |
Description |
| Turns |
2.5-2.75 |
| Length |
30-35 mm (uncoiled) |
| Scala media (cochlear duct) |
Endolymph (high K⁺) |
| Scala vestibuli + tympani |
Perilymph (high Na⁺) |
Organ of Corti:
| Cell |
Description |
| Inner hair cells |
3,500 (primary auditory receptors) |
| Outer hair cells |
12,000 (amplification - electromotility) |
| Tectorial membrane |
Overlies hair cells |
| Basilar membrane |
Tonotopic organization (high frequencies at base) |
Auditory Pathway
- Cochlea (hair cells)
- Spiral ganglion
- Cochlear nerve (CN VIII)
- Cochlear nuclei (brainstem)
- Superior olive (sound localization)
- Lateral lemniscus
- Inferior colliculus (midbrain)
- Medial geniculate nucleus (thalamus)
- Primary auditory cortex (Heschl gyrus, BA 41/42)
Vestibular System
| Structure |
Stimulus |
Response |
| Anterior canal |
Nodding (sagittal) |
Vertical eye movements |
| Posterior canal |
Head tilt (coronal) |
Torsional movements |
| Lateral (horizontal) canal |
Head rotation (transverse) |
Horizontal nystagmus |
| Utricle |
Horizontal linear acceleration |
Tilt perception |
| Saccule |
Vertical linear acceleration |
Gravity detection |
Olfaction (Nose)
| Feature |
Description |
| Olfactory epithelium |
Roof of nasal cavity (superior turbinate, 2.5 cm²) |
| Olfactory receptor cells |
~10 million (bipolar neurons) |
| Bowman glands |
Mucus secretion (odorant binding) |
| Olfactory bulb |
Cribriform plate of ethmoid (CN I) |
| Olfactory tract |
→ Primary olfactory cortex (piriform cortex) |
Odorant receptors: ~350 functional types (humans), combinatorial coding
Nasal Cavity
| Structure |
Function |
| Nasal vestibule |
Filter (vibrissae) |
| Nasal turbinates (conchae) |
Airflow, humidification, warming |
| Paranasal sinuses |
Lighten skull, humidify, resonance |
| Olfactory cleft |
Superior turbinate (smell) |
Gustation (Taste)
| Feature |
Description |
| Taste buds |
~5,000 (tongue, palate, epiglottis, pharynx) |
| Taste cells |
50-100 per taste bud (Type I, II, III cells) |
| Taste pores |
Opening to surface |
| Microvilli |
Taste receptors (taste hairs) |
Taste papillae:
| Type |
Location |
Number |
Taste buds |
| Fungiform |
Anterior 2/3 |
200 |
1-20 each |
| Circumvallate |
Posterior (V-shaped) |
8-12 |
250 each |
| Foliate |
Lateral edges |
4-6 |
1,300 total |
Taste qualities:
| Quality |
Receptor |
Mechanism |
| Sweet |
T1R2/T1R3 |
GPCR (cAMP, IP3) |
| Salty |
ENaC (Na⁺ channel) |
Ion channel |
| Sour |
PKD2L1 (H⁺ channel) |
Ion channel |
| Bitter |
T2R (25 types) |
GPCR (gustducin) |
| Umami |
T1R1/T1R3 (glutamate) |
GPCR (mGluR) |
Taste pathways:
| Region |
Nerve |
Nucleus |
Cortex |
| Anterior 2/3 of tongue |
Chorda tympani (CN VII) |
Solitary tract (NST) |
Insula, frontal operculum |
| Posterior 1/3 |
Glossopharyngeal (CN IX) |
Solitary tract |
Insula, frontal operculum |
| Epiglottis/pharynx |
Vagus (CN X) |
Solitary tract |
Insula, frontal operculum |