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The Special Senses
1. Why they are called special senses?
--They have special cortical areas or centers.
--Stimlus originate outside the body and not on body
surface as in somatic senses.
--Receptors are special receptors cells embedded in
epithelial tissues within the special sense organs.
--rods and cones in the eye ball
--Organ of Corti in the inner ear.
 We shall concentrate on the structures and functions of
the special sense organs (eye & ear) and the pathways
involved in conveying impulses from the sense organ
to the CNS.
 Ophthalmology: study of the science that deals with
the eye and its disorders.
 Otolaryngology: Study of the science that deals with
the ear & its disorders.
2. Vision:
i. Introduction:
--More than half the sensory receptors in human body
are located in the eyes.
--A large part of the cerebral cortex is devoted to
processing visual information.
ii. Anatomy of the eye ball:
Eye is constructed of three layers.
A. Outer fibrous tunic. It is again divided into two
regions: Posterior Sclera
Anterior Cornea.
At the junction between these two is the Canal of
Schlemm.
a. Sclera: The white of the eye ball
--it gives shape to the eye ball.
--protects inner parts.
--optic nerve pierces its posterior surface.
b. Cornea: is non-vascular transparent fibrous coat
--iris can be seen through this.
--acts in refraction of light.
--Corneal transplant is most successful
because
cornea is avascular & antibodies do not circulate there.
B. The vascular tunic: The middle layer
--Composed of three portions: Choroid
Ciliary body
Iris
 Choroid: absorbs light rays and so they are not
reflected. This coat provides nutrients to the posterior
surface of the retina.
 Ciliary body: consists of Ciliary processes &
Ciliary muscles
Ciliary processes consists of protrusions or folds on
the internal surface of the ciliary body, where it
secretes aqueous humor
Ciliary muscles are smooth muscles that alters the
shape of the lens for near or far vision.
 The iris: Colored portion seen through the cornea.
--consists of radial & circular muscles arranged
to form a doughnut shaped structure.
--black hole in the center is pupil.
--principal function is to regulate the amount
of light entering the eye ball.
C. Inner coat: the retina (nervous tunic)
--Lines the posterior three-quarter of the eye ball.
--this is the only place where blood vessels can be
viewed directly for pathological changes.
--optic disc: site where optic nerve exit from the eye
ball. There is no rods and cones in the optic disc.(blind
spot)
 Retina: Structure:
i. Pigment epithelium adjacent to choroid. Help
absorb light rays.
ii. There are 3 sets of neurons named in order they
conduct impulses.
--Photoreceptor neurons.
--bipolar neurons.
--ganglion neurons.
D. Photoreceptors: Rods and cones are called
photoreceptors due to the shape of their outer segment.
i.
Rods: they are specialised receptors for dim light
vision(black & white)
 rods also allow to discriminate between shades,
to see shape & movements
 they are concentrated at the periphery of the
retina.
ii. Cones: specialised for color vision & acuity of
vision (sharpness of vision in bright light ).
--they are mostly concentrated in the fovea
centralis, a small depression in the center of
Macula lutea, which is situated in the exact center
in the posterior portion of the retina.
E. The lens:
--this is a non-vascular structure just behind the pupil
and iris.
--It changes shape to focus light rays for clear vision.
--Cataract is a condition when the transparency of the
lens is lost.
--Cataract can occur: due to aging, injury, exposure to
ultra-violet light, medication and as complication of
systemic diseases (diabetes).
F. Interior of the eye ball:
Divided into two chambers –anterior cavity
Vitreous chamber.
i. Anterior cavity: is subvivided into:
 anterior chamber(between cornea and iris)
 posterior chamber(between iris and suspensory
ligament of lens.
 both chambers are filled with aqueous humor which
is continuously secreted by the ciliary process.
 Aqueous humor: drains in the canal of Schlemm
and then into the blood.
 Intraoccular pressure: it is the pressure exerted by
the aqueous humor. The shape of the eye is
maintained by aqueous humor and vitreous body.
 Glaucoma is a condition produced by raised
intraoccular pressure and results in degeneration of
the retina & blindness.
ii. Vitreous chambers(posterior cavity):
--lies between the lens and the retina.
--contains a gel called vitreous body.
G. Refraction abnormalities:
i. Myopia(nearsightedness). Patient has difficulty in
seeing distant object. It is usually due to big eye ball
where image is formed infront of the retina.
ii. Hyperopia (farsightedness): here the image is
formed behind the retina due to small eye ball or
defects in refraction.
iii. Astigmatism: is due to an irregular curvature of
either lens or cornea.
iv. Presbyopia: results from lack of accommodation
reaction and patient has difficulty in reading from
close range. Usually common after 40 years of age.
I.
Accommodation reaction:
Changes occur inside the eye ball to focus near object.
Three changes occur:
--convergence of the eye ball.
--increased curvature of the anterior surface of the lens
--pupillary constriction
J. Physiology of Vision:.
a. Vision transduction:
i. light rays from objects enter the eye ball
through the cornea and lens to fall on the retina
where the light rays are absorbed by the
photoreceptors (rods& cones).
ii. Photoreceptors are stimulated and lead to the
generation of potential which then is
transmitted through the bipolar neuron to the
ganglion cells.
iii. Stimulation of ganglion cells lead to the
generation of action potential to be
transmitteed through the optic nerve to the
visual cortex where vision is perceived.
iv. Photopigment of rods is rhodopsin which
undergo structural changes upon light
absorption.
v. Photopigment in cones: There are three kinds
of pigments –leading to 3 types of cones.
vi. Color blindness results from an inherited
absence of one of the three cone pigments. It is
more common in males.
vii. Rhodopsin when bleached by light is converted
to opsin & retinine. In the dark, opsin again
combine with retinine to regenerate
Rhodopsin.
b. Light and dark adaptation:
i. Dark adaptation: When a person is suddenly
exposed to a dark room from a brightly lighted
area, at first he/she will not be able to see any
thing, but gradually will be able to see objects
around him/her. This is due to regeneration of
Rhodopsin (visual purple) in the dark. This is
called Dark adaptation.
Opsin + Retinine- rhodopsin
Eye become very sensitive to light because of
high concentration of rhodopsin in the rods.
ii.
Light adaptation: This is just opposite of dark
adaptation. This is a function of cones.
c. Visual pathway
--visual impulse originated at the ganglion cells, now
travel by the optic nerve and reaches optic chiasma. ----Here nasal fibers cross to the opposite side and optic
tract is formed to end in LGB(lateral geniculate body)
--From here new neuron arises (geniculocalcarine
tract) and end in the primary visual cortex in the
occipital lobe.
Sense of Hearing
A. Anatomy of the ear:
i.
ii.
The external ear:
--collect sound waves and pass them inwards.
--It consists of pinna, external auditory meatus &
tympanic membrane.
The middle ear (Tympanic cavity) is a small airfilled cavity in the temporal bone.
--it is lined by epithelium & contain auditory
ossicles(malleus, incus & stapes) and auditory
tube(Eustatian tube), the oval window and the
round window.
iii. the internal ear(inner ear):
--also called labyrinth because of its complicated
canals.
--structurally it consists of two main divisions:
--outer bony labyrinth & inner membranous
labyrinth
a. The bony labyrinth:
 it is a series of cavities in the petrous portion of the
temporal bone.
 It can be divided into 3 areas.
Semicircular canals
Vestibules
Cochlea.
The bony labyrinth (continued)
--The first two contain receptors for equilibrium and
the cochlea contains receptors for hearing.
--the bony labyrinth is lined with periostium and
contains a fluid called perilymph which surounds the
membranous labyrinth.
b. The membranous labyrinth:
--it is a series of sacs and tubes and have the same
general form as the bony labyrinth.
--it is lined with epithelium.
--it contains a fluid called endolymph—similar to
intracellular fluid.
c. The vestibule constitute the oval central portion of
bony labyrinth. The membranous labyrinth in the
vestibule consists of 2 sacs calleed Utricle &
Saccule.
d. Semicircular canals:
--projecting upwards & posteriorly from vestibule.
--3 Canals: arranged at right angle to the other two.
--anterior and posterior canals are oriented vertically ,
lateral canal oriented horizontally.
--one end of each canal is enlarged called ampulla.
--The portion of the membranous labyrinth lie inside
the semicircular canals called semicircular ducts.
e. Cochlea:
i. Structural organization
--Anterior to the vestibule is the cochlea.
--consists of a bony spiral canal that makes about three
turns around a central bony core called modiolus.
--Cross section through the cochlea shows that it is
divided into three channels by partitions.
 Scala vestibule, ends at the oval window.
 Scala media(cochlear duct)
 Scala tympani, ends at the round window.
--scala vestibule and scala tympani contain perilymph
and are connected through a hole at the apex of the
cochlea called Helicotrema.
--Vestibular membrane separates scala vestibule from
scala media and basilar membrane separates scala
media from scala tympani.
ii.
Organ of Corti:
Resting on the basilar membrane is the spiral
organ called Organ of Corti or organ of hearing.
iii. Tectorial membrane:
A flexible delicate gelatinous membrane
projecting over and in contact with the hair cells
of the organ of Corti.
B. Sound waves:
--results from alternate compression and
decompression of air molecule
--human being can hear sound with a frequency of
1000—4000 Hertz(cycles / sec)
--the frequency of sound vibration is called Pitch
--sound wave characters are:
frequency.
Pitch.
Amplitude/intensity.
--more the frequency, more is the pitch.
--more the amplitude, louder the sound.
--intensity of sound is measured by decibel(dB)
C. Physiology of Hearing:
1. Events are as follows:
i. Sound waves in external ear.
ii. Waves strike tympanic membrane, causing it
to vibrate.
iii. Vibration is conducted to ossicles—down to
stapes.
iv. Stapes moves back and forth, pushing the
membrane of oval window(in & out).
v. Movement of the oval window sets up fluid
pressure waves in perilymph of the scala
vestibule.
vi. Pressure waves from scala vestibule are
transmitted to the scala tympani and finally to
the round window, causing it to buldge
outward into the middle ear.
vii. These pressure waves then push the vestibular
membrane & creates vibration of endolymph in
the scala media(cochlear duct).
viii. This then move the basilar membrane—which
move the hair cells of the organ of Corti
against the tectorial membrane.
ix. The bending of the hairs generate receptor
potential that lead to the production of nerve
impulses in the cochlear nerve fiber.
2. Appreciation of sounds of different frequencies:
a. High frequency (high pitched sound) cause the
basilar membrane to vibrate near the base of the
cochlea.
b. Low frequency (low pitched sound) cause the
basilar membrane to vibrate near the apex of the
cochlea.
3. Initiation of nerve impulse:
Hair cells converts a mechanical force (stimulus) into
an electrical signal (receptor potential): hair cells
release neurotransmitter which initiate nerve impulse.
D. Auditory pathway:
i.
ii.
Nerve impulse from the cochlear branch of the
vestibulocochlear nerve pass to the cochlear
nuclei in the medulla.
From here most impulses cross to the opposite
side and then travel to the mid-brain, to thalamus
& finally to the primary auditory area of the
temporal lobe of cerebral cortex.
E. Disorders:
i.
Cataract: loss of transparency of the lens. It can
lead to blindness.
ii. Glaucoma: is a condition where intraocular
pressure is high due to a build up of aqueous
humor , which destroy neurons in retina. This is
the second most cause of blindness.
iii. Deafness:
--It can be conductive: where there is impairment
of sound wave transmission in the external ear &
middle ear to reach cochlea.
--It can be nerve or sensorineural deafness: due to
impairment of the cochlear or cochlear branch of
the vestibulocochlear nerve.
iv. Meniere’s disease:/syndrome:
--malfunction of the inner ear leading to deafness
& loss of equilibrium.
v. Otitis media: acute infection of the middle ear.
Children are more susceptible.
Sense of Hearing
Topics:
1. Anatomy of the ear.
2. The bony labyrinth
3. Cochlea.
4. Organ of Corti.
5. Sound waves.
6. Physiology of hearing.
7. Appreciation of sounds.
8. Auditory pathway.
9. Disorders of vision and hearing