Control & communication · Topic 9 of 18

The Special Senses

The Eye & Vision

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Anatomy · special senses: vision

The Eye & Vision

Light passes cornea, pupil and lens to the retina; rods and cones turn it into signals the optic nerve carries to the occipital lobe.

01 · Light pathCornea → pupil → lens → retina

02 · Rods vs conesRods = dim light; cones = color

03 · To the brainOptic nerve → thalamus → occipital lobe

Parts & jobs

Retina = inner layer whose photoreceptors turn light into nerve signals must know

3 layers: fibrous (sclera, cornea) · vascular (choroid, ciliary body, iris) · retina

Sclera = tough white coat; cornea = clear front that lets in and bends light

Iris (colored muscle) sizes the pupil: small in bright light, large in dim

Lens focuses light; ciliary muscles reshape it for near vs far (accommodation)

Choroid = the eye's blood-vessel layer

Fovea = center of the retina, packed with cones → sharpest vision

Optic disc (where the optic nerve exits) = blind spot, no photoreceptors

Path of light

Step by step

Light path: cornea → aqueous humor → pupil → lens → vitreous humor → retina

Visual pathway

Step by step

Retina → optic nerve (CN II) → optic chiasm → thalamus → occipital lobe

Don't confuse

Rods = dim light, no color; cones = bright light, color and detail must know

Aqueous humor = watery, in front of the lens; vitreous = gel, behind it

Rods or cones?

Commonly confused

	Rods	Cones
Best light	Dim light, night vision	Bright light, daytime
Color	No — shades of gray only	Yes — three types (red, green, blue)
Detail	Low; peripheral vision	Sharp; central vision
Where most	Outer edges of the retina	Center, packed in the fovea
Pigment	Rhodopsin	Three cone opsins

More detail

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Retina chain: photoreceptor → bipolar cell → ganglion cell → optic nerve

Optic chiasm: inner-retina fibers cross → each brain side sees the opposite field

Myopia = far things blur (focus in front); hyperopia = near things blur (behind)

Presbyopia = aging lens stiffens → near focus fails

Rods: rhodopsin; 3 cone types (red, green, blue) give color

Lacrimal gland = tears; conjunctiva lines the lids and covers the white

6 extraocular muscles move each eye (4 rectus, 2 oblique)

Pearl

Rods for the dark, cones for color.

Sources · OpenStax Anatomy & Physiology 2e §14.1 Sensory Perception (vision) · OpenStax Anatomy & Physiology 2e §14.2 Central Processing (optic chiasm) · OpenStax Biology 2e §36.5 Vision · NurseHub — HESI A2 Anatomy & Physiology review course (nervous system unit: Body Senses quiz) · Union Test Prep — HESI A2 Anatomy & Physiology study guide, p. 3: occipital lobe and sensory organs · NurseHub — free HESI A2 Anatomy & Physiology diagnostic test (item on the eye structure for color vision) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

The Ear (Hearing & Balance), Smell & Taste

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Anatomy · special senses: ear, smell, taste

The Ear (Hearing & Balance), Smell & Taste

Sound shakes the eardrum, the ossicles pass it to the cochlea; the vestibule and canals sense balance; smell and taste detect dissolved chemicals.

01 · Sound pathEardrum → ossicles → cochlea

02 · Inner earCochlea hears; canals + vestibule balance

03 · Chemical sensesSmell + taste = chemoreceptors

What it is

Smell and taste = chemical senses (chemoreceptors in nose and taste buds) must know

Parts & jobs

Outer ear (to eardrum) · middle ear (ossicles) · inner ear (cochlea, balance organs) must know

Ossicles: malleus (hammer), incus (anvil), stapes (stirrup) — amplify sound

Eustachian tube: middle ear ↔ throat; equalizes pressure (swallow, yawn)

CN VIII (vestibulocochlear) carries hearing + balance; hearing → temporal lobe

Taste buds sit in the tongue's papillae and signal cranial nerves

Path of sound

Step by step

Hearing: eardrum → malleus → incus → stapes → oval window → cochlear fluid

Organ of Corti: fluid bends hair cells → nerve impulses (mechanoreceptors)

Path of smell

Step by step

Smell: nasal receptors → olfactory nerve (CN I) → olfactory bulb → temporal lobe

Know by heart

5 tastes: sweet, salty, sour, bitter, umami (savory)

The rules

Smell skips the thalamus and links to memory and emotion

Don't confuse

Cochlea = hearing; vestibule + semicircular canals = balance must know

Utricle + saccule = head position/tilt; semicircular canals = rotation

Hearing or balance?

Commonly confused

	Hearing	Balance (equilibrium)
Inner-ear part	Cochlea	Vestibule (utricle, saccule) + semicircular canals
Detects	Sound vibrations	Head position and movement
Receptor	Hair cells in the organ of Corti	Hair cells in the vestibule and canals
Nerve	Vestibulocochlear nerve (CN VIII)	Vestibulocochlear nerve (CN VIII)
Brain	Auditory cortex, temporal lobe	Brainstem and cerebellum

More detail

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Cochlea base = high pitch, tip = low pitch; hearing range ≈ 20–20,000 Hz

Otoliths (calcium carbonate) let gravity bend hair cells when the head tilts

Taste nerves: facial (front ⅔), glossopharyngeal (back ⅓), vagus (throat)

Salty = Na⁺, sour = H⁺ (acid); sweet, bitter, umami bind receptors

Anosmia (loss of smell) makes food taste bland

Olfactory neurons are replaced through life (less so with age)

Pearl

The same inner ear does two jobs: the snail-shaped cochlea hears, the canals and vestibule keep you balanced.

Sources · OpenStax Anatomy & Physiology 2e §14.1 Sensory Perception (taste, smell, hearing, balance) · OpenStax Anatomy & Physiology 2e §13.2 The Central Nervous System (temporal lobe, thalamus) · NurseHub — HESI A2 Anatomy & Physiology review course (nervous system unit: Body Senses quiz) · Union Test Prep — HESI A2 Anatomy & Physiology study guide, p. 3: temporal lobe hearing · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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