Download ALH 3205: Special sense Professor Cohen 8/19/09 Ear [plate 92

ALH 3205: Special sense
Professor Cohen
8/19/09
Ear [plate 92-96]:
 The external ear consists of an auricle and an external acoustic meatus
 The auricle is the par that is visible from the outside
o Consists of a single piece of elastic cartilage except the most inferior
portion [lobe] which is composed of fibroaerolar cartilage
o The auricular elastic cartilage is continuous with the cartilage of the
external acoustic meatus
 The sensory supply to the skin of the ear is of significance
o Neurological exams of the ear can reveal information about the upper
spinal cord (C2 & C3), also the medulla (vagus), and about the pons
(where trigeminal nerve exits) because of all the different sensory
supplies to the ear
 The superior portion of the ear is innervated by a branch of CN V3
(mandibular- specifically the auriclotemporal nerve)
 The inferior portion of the ear, especially the ear lobe, is innervated by the
greater auricular nerve which is a branch of the cervical plexus specifically C
2 and C 3
 The external acoustic meatus, especially the skin surrounding the concha
[beginning of the external acoustic meatus], is innervated primarily by
cranial nerve X [vagas]
 Movement of the ears by posterior, superior, and anterior auricular muscles
 The meatus:
o Extends from the concha to the tympanic membrane
o The outer 1/3 is cartilage and the inner 1/3 is bone (temporal bone)
o The cartilaginous of the meatus contains glands that S&S [synthesize
and secrete] cerumen [ear wax]
o By pulling the ear upward, backward, and laterally the meatus
straightens out and the ear drum can be seen by an otoscope
 The middle ear:
o Main function is to amplify sound which occurs as air hits the
tympanic membrane or eardrum causing displacement of the ear
ossicles which in turn compresses the oval window leading to the
inner ear where sound waves are changed into neural impulses
o Air sinus in the temporal bone
o Communicated with the mastoid air cells and with the nasopharynx
through the auditory/eustachian tube
 Repeated middle ear infections in children are due to the
position of the auditory tube parallel to the floor so there is no
drainage of any fluids. When we grow our head grows and the
tube is angled downward to drain fluid.
o Tympanic cavity:
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Divided into a thin roof,
A space of a cavity that allows for the articulation of the head
of the malleous and the head of the incus
 Floor that rests on the jugular bulb of the temporal floor
 The ICA and the IJV diverge from the carotid sheath at the
floor of the tympanic cavity.
 The artery goes anteriorly and the vein goes posteriorly
 The posterior wall of the tympanic cavity has a small tunnel
called the aditus
 Connects the middle ear with the mastoid air cells
 Allows secretions from these cells to drain into the
auditory tube
 Round window/finestra cochlea also found in middle ear
 Can see inflammation of the tympanic membrane to diagnose
middle ear infections
 The tympanic membrane is a thin double layer epithelial
partition bw the meatus and the middle ear
 About 1cm in diameter
 Concave on outside and convex on inside
 Extremely sensitive to pain
 Sensory nerves include branches of the vagus and
trigeminal
 Three ear ossicles in the middle ear that connect the tympanic
membrane to the oval window
 Malleous (hammer)
 Incus (anvil)
 Stapes (stirrups)
 Attached to the walls of the middle ear by ligaments
 From a functional standpoint, the function of these
bones is to transmit and amplify sound waves as they
are transmitted from the tympanic membrane to the
oval window
o Very efficient at this job: increase intensity and
force of the sound waves by 20 fold
 There are two small skeletal muscles called the tensor
tympani and the stapedus that attach the malleous and
the stapes to the wall of the middle ear and protect the
oval window from very loud noises
The inner ear [labyrinth]:
o Contains the functional units for hearing and equilibrium
o The labyrinth contains an outer bony portion and a membranous
portion within the bony portion
o The space within the labyrinth is filled with a fluid called perilymph
o Within the tubular chambers of the membranous labyrinth is another
fluid called endolymph
 These fluids provide a conducting medium for the vibrations
of hearing an equilibrium
o The bony labyrinth divided into three areas:
 Vestibule [first part]- central part of the bony labyrinth
 Contains the oval window onto which the stapes fit and
the round window on the other end
 The membranous labyrinth within the vestibule
contains two cavities: the utricle and the saccule
o Both of these cavities contain receptors that are
sensitive to gravity and linear motion of the
head
 Semicircular canal: come off the vestibule
 There are three of them, which are at right angles to
each other and posterior to the vestibule.
 Receptors that are in the ducts of the semicircular
canals are sensitive to the angular acceleration and
deceleration of the head
 Cochlea [plate 96]: snail shaped structure
 Contains three chambers
o Upper chamber: scala vestibuli
 Begins at the oval window and it is
continuous with the vestibule
o lower chaber: scala tympani
 terminates at the round window
o cochlear duct: middle chamber
 roof is called the vestibular membrane
and the floor is called the basilar
membrane
 scala tympani and scala vestibuli are
separated by the cochlear duct except at
the very end is a duct called the
heilicotrema which is also where the
cochlear duct ends
 the organ of corti is located within the
cochlear duct
 the bases of hair cells in the organ
of corti are anchored to the basilar
membrane and their tips are
embedded in a tectorial
membrane forming a roof over
them
 the sound receptors that transform
mechanical vibrations into nerve

impulses are located along the basilar
membrane so the basilar membrane in
the cochlear duct is the functional unit
of hearing
Physiology of hearing
o Sound waves travel in all directions from their source.
o The waves are measured frequency of the wave which is Hertz and
the greater the hertz, the higher the pitch
o Also measured by intensity called the amplitude of the wave which is
measured in decibels
 Every 10 decibel increase is a 10 fold increase in sound
intensity
 10 decibels is 10 times louder than zero decibels but 20
decibels is 100 x louder than 10 decibels
o The human ear can hear over a frequency range of 20, 000-30,000
hertz
o Sound waves produced during speech are about 60 decibels
o Sound waves passing through the ossicles are amplified as they reach
the oval window movement of the oval window causes pressure
waves through the fluid of the scala vestibuli those pressure waves
travel along the scala vestibuli around the heilicotrema down the
scala tympani displaces the round window back into the middle ear
 When the frequency is low, there is adequate time for the
pressure waves to travel all the way around the heilicotrema
and arrive at the round window.
 As the frequency increases, the waves take a short cut through
the vestibular membrane and then through the basilar
membrane decreasing the distance that the waves travelas
frequency increases the distance traveled decreases
o Displacement of the basilar membrane and the hair cells causes
microvillus to bend which excites sensory endings of the cochlear
nerve (cranial nerve VIII)
o Neural pathways: cochlear sensory neurons in CN VIII synapse with
neurons in the medulla which travel to the thalamus [central relay
station] and then to the auditory cortex of the temporal lobe of the
cerebrum where they are interpreted as sound
Eyes [Plate 83-91]:
 Organs that refract [bend] and focus incoming light waves onto the
photoreceptors at the back of each eye
 These nervous impulses are conveyed through visual pathways in the brain
to the occipital lobes of the cerebrum where the sense of vision is perceived
 The orbit is a bony depression where the eye is located
o Formed from parts of seven bones-very complex
 Frontal
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 Lacrimal
 Ethmoid
 Sphenoid
 Zygomatic
 Maxilla
 Palatine
 Contains superior/inferior orbital fissure and the optic canal
The eyebrows are short transversely positioned rows of hair on the superior
orbital ridge
o Functionally they shade the eye and protect it from sweat and falling
objects
The palpebrae [eyelids] reflexly close once every 7 second to keep the eyes
moist [plate 82]
o The palpebral fissure bw the upper and lower eyelids meet at the
median and lateral commisures [corners of the eyes]
o Medial commisure is larger and has a red elevation which is called the
lacrimal caruncle [gland]
Each eyelid contains plates made of fibrous connective tissue that are called
tarsal plates
o Tarsal meibomian glands on the tarsal plates secrete an oily secretion
o Chalazion is a tumor or a cyst due to an infected tarsal gland
o Ciliary glands [modified sweat glands] are also in eyelid and
 When they become infected or inflamed it is called a sty
Conjunctiva- thin mucus secreting membrane on the interior surface of each
eyelid and the exterior surface of each exposed eyeball
o Conjunctival sac exists when the eyelid is closed which prevents
objects from going behind the eyeball
Lacrimal apparatus- includes a gland and a series of ducts that produce and
drain lacrimal fluid [tears] into the nasal cavity
o The lacrimal gland is located superiolateral in the orbit
o Lacrimal fluid contains the bacteriocidal agent lysozyme which
reduces the incidence of infections
Extrinsic eye muscles [plate84]: 6 total
o Each muscle originates in the orbit and attaches to the tough outer
tunic of the eyeball via a tendon
o 4 muscles called the recti:
 Superior inferior lateral and medial
o 2 oblique muscles:
 Superior and inferior
 Rotate the eye on its axis
o Neural innervations: [SO4] superior oblique by cranial nerve IV, [LR6]
lateral rectus by cranial nerve VI and all the rest innervated by cranial
nerve III
o motor units within the 6 extrinsic eye muscles are the smallest in the
body
Approx one motor neuron to 10 fibers each which gives us
extremely precise movement of the eyes
Structure of the eyeball [plate 87]:Consists of three basic tunics or layers:
o Fibrous tunic [outermost]: divided into two regions
 Inner 5/6 is called the sclera [part you cant see]
 Composed of tightly bound elastic and collagen fibers
which give shape to the eyeballs
 A vascular [no blood vessels] but contains sensory
nerves
o Optic nerve enters our the back
 Outer 1/6 is the cornea [part you can see]
o Transparent [bc its avascular], convex and
permits the passage and refraction [bending] of
incoming light waves
o The circumphrential edge of the cornea is
continuous with the sclera
 Vascular tunic [middle]: also called uvea
 Consists of the
o Choroid: thin, highly vascular and contains many
melanocytes which gives it is dark brown color
which prevents light waves from bouncing out
o Ciliary body: thickened region in the anterior
portion of the vascular tunic that forms an
internal muscular ring in the front of the eyeball
 three planes of smooth muscle fibers
called the ciliary muscles
 numerous extensions of the ciliary body
which attach to the lens of the eyeballcollectively called the suspensory
ligaments
 Contraction and relaxation of these
muscles changes the shape of the lens
 when the muscles are tense there is less
tension on the lens and it is more
spherical which is for near vision
 when they are relaxed, the lens becomes
flatter which is necessary for distance
vision
o Iris: is viewed from the outside as the colored
portion of the eyeball
 Consists of smooth muscle fibers
arranged in a circular radial pattern
 Contraction or relaxation of these fibers
regulated the diameter of the pupil which
is in the center if the iris
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Retina [internal tunic]: covers the choroid as the innermost
layer of the eye. Consists of a thin pigmented layer in contact
with the choroid and a thick inner nervous layer [visual
portion of the retina]
 Visual layer of the retinal terminated in a jagged margin
near the ciliary body called the ora serrate
 Nervous layer of the retina is composed of three layers
of neurons.
 In the order that these neurons conduct light they are
the ganglion neurons, the bipolar neurons, and the
rods/cones
 In the order that they conduct impulses it is exactly
reversed
 Rods: positioned on the peripheral parts of the retina
and respond to dim light for black and white vision but
provide poor visual acuity
 Cones: concentrated in the center of the retina in an
area called the fovea centralis, which is the area where
we have the sharpest and keenest vision. Associated
with the color vision.
 There are NO PHOTORECEPTORS where the optic nerve
passes through and attaches to the retina [blind spot]
called the optic disc
Two ciliary bodies enter the eye independently and anastamois with the
choroid
The central artery which is a branch of the ophthalmic artery enters the
eyeball in contact with the optic nerve
****Be familiar with anterior and posterior chambers of the eye in the
interior of the eyeball [look up in book]
o Anterior chamber is in front of the iris and behind the cornea
o Posterior chamber is bw the iris and the suspensory ligaments in the
lens
o These two chambers communicate with each other and contain a
watery fluid called the aqueous humor
 Constantly produced throughout life and maintains the
intraocular pressure which is critical in providing the
avascular lens and cornea with nutrition and oxygen
o Between the lens and the retina is the large vitreous chamber which is
filled with a jelly like substance called the vitreous humor
 Helps to maintain the shape of the eyeball but also holds the
retina against the choroid
 NOT produced throughout life- produced prenatally
5 basic properties associated with vision:
o 1. Transmission of light waves
o 2. Refraction of light waves
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o 3. Accommodation of the lens
o 4. Constriction of the pupil
o 5. Convergence of the eyeball when viewing close object
Optic nerves converge at the optic chiasma and diverge back to the visual
cortex
The lateral fibers in the optic canal remain on the same side but the medial
fibers cross.
o Medial eye receives and sends information to both sides of the visual
field in the occipital lobe