lecture 24, the special sense organs

the eyeball has 6 primary components

1. fibrous tunic

2. the vascular tunic

3. the nervous tunic

4. the lens

5. the anterior chamber

6. the posterior segment

eyeball: 1. fibrous tunic

• Consists of the sclera (white of the eye), the cornea (transparent portion of the eye)

  • Both are avascular connective tissue

• Also consists of the conjunctiva which covers the anterior surface of the sclera

  • A vascular mucous membrane

  • Dilation of blood vessels in this layer results in blood shot eyes

eyeball: 2. the vascular tunic

• The iris (the colored part of the eye)

• The choroid is highly vascular

  • Contains melanin which prevents the reflection and the scattering of light

• Also contains the ciliary body

  • Consists of the ciliary muscle and the ciliary processes which control the shape of the lens and secrete aqueous humor

    • holds eye shape

    • chamber behind cornea

eyeball 3. the nervous tunica

• The retina

  • Sensory portion of the eye

• Consists of the outer pigmented layer and an inner neural layer consisting of three layers of neurons

  • i Photoreceptor neurons:

    • Rods: for black and white vision

    • Cones: for colored vision

    • more rods than cones

  • ii. Bipolar cells

  • iii. Ganglion cells:

    • Axons of these cells form cranial nerve II: the optic nerve

• The fovea centralis is where light focuses

  • The greatest visual acuity is achieved here

• The optic disc is the blind spot

  • This is where the optic nerve and blood vessels exit the eye (bundles of neurons)

eyeball: 4 . the lens

• a transparent, avascular layer

• cataracts occur when the lens becomes cloudy

  • can be caused by a number of different things

eyeball: 5. the anterior chamber

• located anterior to the lens

• contains the aqueous humor

  • very similar to blood plasma (take a sample)

  • replaced every 90 minuets (replenish)

    • drains into a sinus located at the junction between the sclera and the cornea

eyeball: 6. posterior segment

• located posterior to the lens

• contains the gel-like vitreous humor

  • this humor is not replace

• big bubble of the eye

• high viscosity

the ear

1. the external ear

2. the middle ear

3. the inner ear

the external ear

• consists of the

  • auditory canal (meatus) (shunt sound in)

  • tympanic membrane (eardrum) (can rupture)

  • auricle: elastic cartilage covered in skin (heals poorly)

• functions to conduct sound

the middle ear

• consists of the auditory ossicles

  • smallest bone in the body

  • malleus: located against the tympanic membrane of the external ear

  • incus

  • stapes: against the oval window of the inner ear

  • “mis”

• also contains the eustachian tube (auditory tube)

  • connects to the nasopharynx

  • portal of entry for bacteria

• functions to conduct sound

the inner ear

• contains the vestibular nerve

• also contains the bony (hard) and the membranous (soft) labyrinth

  • surrounds and protects the ear

• a. the bony labyrinth:

  • contains the

    • semi-circular canals (not all in same orientation → do different things)

    • cochlea (“snail shell”

    • vestibule

  • lined with periosteum and contains perilymph (inside)

• b. the membranous labyrinth:

  • contains the

    • semi-circular ducts within the canals: connect with the utricle

    • cochlear duct within the cochlea

    • utricle and the saccule within the vestibule

  • contains endolymph (hairs swish due to this)

the cochlea

• this is coiled structure → there are 2.5 coils

• there are three channels within each of coils

  1. the scala vestibuli

  2. the scala tympani

     • both 1 and 2 contain perilymph/are part of the boney labryinth

  3. the scala media: the cochlear duct

the cochlea: the scala vestibuli

• located above the cochlear duct

the cochlea: the scala tympani

• located below the cochlear duct

the cochlae: the scala media: the cochlear duct

• A continuation of the membranous labyrinth: contains endolymph

• The vestibular membrane: separates the cochlear duct from the scala vestibuli

• The basilar membrane: separates the cochlear duct from the scala tympani

• The tectorial membrane: covers hair cells of the spiral organ

• All of the three components contain endolymph

• the receptors are the hair cells that synapse with the

• neurons → like cilia, extend into the endolymph

functions of the ear include

• conduction of sound → the middle and external ear

• hearing and equilibrium → the inner ear

• the ear directs impulses to the brain via cranial nerve VIII → the vestibulocochlear nerve

  • branches into positional and hearing