Anatomy: Special Senses

External ear

Auricle (pinna)- “funnels” sound into external acoustic meatus
Tympanic membrane (eardrum)- vibrated by sound

Middle ear

An air-filled space
Ossicles:
Malleus-receives vibration from eardrum
Incus
Stapes: transmits vibration to oval window
Oval window- transmits to internal ear
Round window= where pressure in internal ear pushes back out
Pharyngotympanic tube (eustachian tube)- connects middle ear to pharynx, allows middle ear pressure to equalize with atmosphere

Internal ear

Bony labyrinth
Membranous labyrinth
Semicircular ducts
Saccule and utricle
Anatomy of chochlea
Pathway of sound sensation
Pathway of sound vibrations

Bony labyrinth

Cavities in temporal bone- filled (partly) with perilymph
Semicircular canal (Balance/equilibrium only)
Vestibule (balance/equilibrium only)
Cochlea (hearing only)

Membranous labyrinth

Sacs/ ducts within bony labyrinth
Filled with endolymph
Semicircular ducts (in semicircular canals)
Saccule and utricle (in vestibule)
Cochlear duct (in cochlea)

Semicircular ducts

Sense rotation only
Cristae ampullares= sensory epithelia- stimulated by flow of endolymph

Saccule and utricle

Sense linear acceleration (e.g. gravity) only
Maculae= sensory epithelia
Sensory pathways from maculae and cristae ampullares:
Vestibular nerve (branch of CN VIII) to brain

Anatomy of cochlea

Membranous labyrinth= cochlear duct (Scala media)

Remaining space divided into: Scala vestibuli= "Superior" to cochlear duct Scala tympani= "inferior" to cochlear duct Both filled with perilymph

Cochlear ducts details

Basilar membrane= the “floor”
Spiral organ (of Corti)= sensory epithelium
-hair cells with stereocilia (sensory receptor)
-Tectorial membrane

Pathway of sound sensation

Oval window vibrates
Perilymph of scala vestibuli vibrates
Endolymph of cochlear duct vibrates
Basilar vibrates
Hair cell vibrates
Stereocilia bend against tectorial membrane
Hair cell releases neurotransmitter
Cochlear nerve (branch of cranial nerve VIII) excited
Carry information to thalamus, then to temporal lobe of brain

Pathway of sound vibrations

Oval window vibrates
Perilymph of scala vestibuli vibrates
Endolymph of cochlear duct vibrates
Basilar membrane vibrates ->
Perilymph of scala tympani vibrates->
Round window vibrates

Layers of the eye

Fibrous layer
Vascular layer
-Choroid
-ciliary body
-iris
Inner layer
-pigmented layer
-neural layer

Fibrous layer

Outermost
Sclera- the “white” of the eye
Cornea- transparent, preforms most of the light bending

Vascular layer

Middle layer
Choroid- provides nutrients and oxygen to inner layer, has melanin to absorb scattered light

Vascular layer: Ciliary body

mostly smooth muscle
attached to lens by ciliary zonule (fibers)
For accommodations (change shape of lens to focus light)
Accommodation: at rest, lens is stretched flat by ciliary zone-> distant focused
For near focus: contract ciliary body-> less tension in ciliary zonule -> lens recoils to more spherical shape

Vascular layer: Iris

Smooth muscle- regulates size of pupil (opening) (e.g. smaller for bright light)

Inner layer

Retina
Pigmented layer (outer)
Melanin- absorbs light to prevent scattering
Phagocytizes outer part of photoreceptors

Inner layer: neural layer

Inner layer of retina
Photoreceptors are on the side away from light
Rod cells- do not sense color, can sense dim light
Cone cells- sense color, cannot sense dim light
Fovea centralis= pit in retina corresponding to center of visual field, sharpest visual acuity, 100% cones
Optic disc= where optic nerve leaves the eye, no photoreceptors=blind spot

Path of nervous signal in retina

Photoreceptors
Bipolar cells
Ganglion cells
-axons of ganglion cells leave eye as optic nerve (cranial nerve II)

Segments of the eye: anterior segment

Between cornea and lens
Filled with clear liquid=aqueous humor
secreted by ciliary body into posterior chamber (=space between iris and lens)
Flows through anterior chamber (=space between cornea and iris)
And drains out via scleral venous sinus

Segments of the eye: posterior segment

Between lens and retina
Filled with jelly-like substance= vitreous humor (Supports the retina, not circulated or renewed)

Neural pathway

Photoreceptors
Bipolar cells
Ganglion cells
Optic nerve
Optic chiasma
Optic tract
Thalamus
Occipital lobe