The Special Senses
Human Anatomy
Chapter 16
I. The chemical senses: taste and smell
Four of the five senses are localized in the
face and head area. The receptors of
touch are spread over the surface of the
body.
A. Taste (gestation)- Taste
is perceived by
chemoreceptors.
1. taste budsTaste buds are found in
the tongue, some in the
posterior region of
palate, inner surface of
cheeks, on the pharynx
and epiglottis. Taste
buds form papillae and
large vallate papillae.
Each taste bud has
supporting, gustatory,
and basal cells. The
taste pore allows
contact with the saliva
containing dissolve
chemicals that stimulate
taste. Replacement of
cells occurs every 7-10
days. If an entire taste is
bud is destroyed it is
replaced at the rate the
entire epithelial layer is
replaced (may take a
month)
2. taste sensations
and gustatory
pathway- there
are four basic
tastes: sweet,
sour, salty, bitter.
The facial nerve,
glossopharyngeal
nerve, and vagus
nerve carry taste
sensations. The
parietal lobe has
gustatory areas.
• B. Smell (olfaction)- Smell is perceived by chemoreceptors found in the
olfactory epithelium; they are olfactory receptor cells. This surface is
covered with a layer of mucus that dissolves odor molecules. These cells are
connected to filament of the olfactory nerve. Smell is sent to the limbic
system (associate smell with emotions) and to areas of the cortex for
identification and analysis.
• C. Disorders of
the chemical
senses-Anosmiaresults when one
can’t smell due to
damage to the
olfactory nerve or
zinc deficiency.
Uncinate fitsdetect imaginary
smells caused by
damage to
olfactory nerve or
epileptic aura.
II. The eye and vision
40% of the cerebral cortex is dedicated to processing images captured
by the eye. The eye is protected by a cushion of fat found within the
optical orbit.
• A. accessory structures of the eye- These are structures that protect the
eye.
•
1. eyebrows- shade from sunlight, keep sweat away from the eyes.
2. eyelids- used for blinking, and have glands that produce “eye grit”.
The eyelashes at the margin trap particles and produce reflexive
blinking. Tarsal glands release oil that forms a film and slows water
evaporation.
•
3. conjuctiva- palpebral conjuctive is a transparent mucous
membrane that covers the inner sufaces of eyelids. Ocular
conjuctiva covers the white of the eye except the area of the
cornea. If the conjuctiva become inflamed due to bacteria or virus, it is
known as pink eye.
•
4. lacrimal apparatus- consists of lacrimal gland and lacrimal
duct, it
secrets a fluid to keep eye moist. Blinking the eye spreads the fluid
on the surface. Lacrimal fluid kills bacteria.
•
5. extrinsic eye muscles- Six muscles that move the eye- superior,
inferior, lateral, and medial rectus; also superior and inferior oblique.
B. Anatomy of the eyeball- This organ is designed to protect the
photoreceptors, capture and process and image. The eye has an anterior
and posterior pole, it is protected by a three layer wall.
• 1. Fibrous tunic- most external, dense connective tissue, makes up the
sclera and cornea. The cornea is transparent and allows light to enter the
eye. It is transparent because it is avascular but contains pain receptors. It
can easily be damaged but has good regeneration ability.
•
2. Vascular tunic- composed of the choroids, the ciliary body, and iris.
– a. The choroid is dark in color because it contains melanin to absorb
light, highly vascularized.
– b. The ciliary body in an anterior continuation of the choroids, it
encircles the lens an helps to adjust it.
– c. The iris is the colored part of the eye, it’s color comes from the
amount of melanin in the iris body. The pupil is the opening of the iris.
The iris has smooth muscles that control the size of the pupil. The
papillary light reflex protects the eye from light damage.
•
3. Sensory tunic is also called the retina. It contains melanocytes and a
sheet of nervous tissue made up of photoreceptors.
– a. The photoreceptors consist of photoreceptor cells : rods= dark, grays
and cones= bright, colors. There are blue, red and green light cones.
They do not regenerate themselves.
– b. Rods and cones contact a bipolar neuron that synapses with
ganglion cells. The axons of the ganglion cells enter the optic nerve and
carry the stimulus to the occipital lobe.
– c. The retina has a “yellow spot” called the macula lutea at the posterior
pole. At its center is a pit called the fovea centralis. It is an area that
contains only cones thus providing visual acuity.
– d. The optic disc (blind spot) lies medial to fovea centralis. This is the
site where the retina gathers to from the optic nerve.
•
4. Internal chambers and fluids- Fluids in these chambers nourish the eye
tissues and maintain appropriate pressure.
– a. The posterior chamber is filled with vitreous humor that regulates eye
pressure, maintains retina against the eye wall, and allows the image to
travel across the eye to the retina. This is formed as an embryo and
lasts the entire life time.
– b. The anterior chamber in filled with aqueous humor, it is more liquid
than vitreous humor, periodically regenerated, and drains into scleral
venous sinus.
– 5. Lens- a transparent disc that changes shape to adjust to focus light. The
lens sends images that are upside down and reverse from left to right. The
cerebral cortex reverses the image. With age the lens thickens and becomes
less flexible reducing its ability to focus light.
• C. Eye as an optical device- eyes are design to see best at far distances,
when looking at objects close up, the pupil constricts. The cornea is
powerful at bending light so it converges on the retina at a single focal
point. The lens is powerful at focusing objects.
• D. Visual pathways= please study the figure on page 484. Image travels
from retina to optic nerve to optic chiasma to optic tract. This tract travels
to midbrain some tract branch out to control eye reflexes, other branch to
the hypothalamus. From the hypothalamus to the thalamus to cerbral
cortex (visual cortex in occipital lobe). Images are sent at different angles
from both eyes, the brain process that information to create just one
image.
• E. Disorders of the eye- this book covers many eye disorders, including
focusing disorders on pg 482-483). Please read these for your own
knowledge.
III. The Ear: Hearing and equilibrium
The ear is divided into three regions: outer, middle, and inner ear. The first two
are involved in hearing, the third is involved in equilibrium.
• A. Outer ear- Consists of the auricle (pinna) and the auditory canal
(acoustic meatus).
• 1. Auricles consist of elastic cartilage and skin. Its function is to gather
sound waves and funnel the to the auditory canal. The way the sound
bounces off the auricles allows the brain to detect the direction of sound.
• 2. External auditory canals run from the auricle to the eardrum. The skin
contains modified sebaceous glands called ceruminous glands that produce
cerumen (ear wax). Sound travels through the canal until it meets the
eardrum, this causes it to vibrate, thus the bones in the middle ear vibrate.
•
B. Middle ear is also known as the tympanic cavity that lies within the
temporal bone. It has four openings: the epitympanic recess that leads to t
he mastoid process, the oval window connecting to the inner ear, the round
window leads to the temporal bone and the pharyngotympanic tube
(auditory tube) leads to the pharynx.
•
C. The inner ear is also called the labyrinth that lies within the temporal
bone. The membranous labyrinth are sacs and ducts within the bony
labyrinth. Both types of labyrinths are filled with fluid.
The three ossicles are smallest bones in the body. The connect to each other in
a linear fashion, the malleus (hammer) attaches to the eardrum and to the
incus (anvil) which attaches to the stapes (stirrup). The stapes attaches to the
oval window and send vibrations into the inner ear. There are two smooth
muscles that connect to these ossicles and reduce vibration in very noisy
environments.
*read about middle ear infections for your own knowledge-its interesting.
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1. Vestibule- part of the bony labyrinth, lies medial to middle ear connecting to the
oval window. It contains sensory epithelium called macula that detects the
position of the head, changes in linear acceleration. The macula have receptors
called hair cells that synapse with the vestibular nerve. Macula can run in
horizontal and vertical positions to detected tiling and elevation or depression of
the head.
2. Semicircular canals-part of the bony labyrinth, lie posterior of vestibule. There
at three canals and each lies along a different plane. The crista ampullaris
detects rotational acceleration via the hair cells.
3. Cochlea-part of the bony labyrinth, forms a spiraling chamber, it contains
receptors for hearing arranged in rows of hair receptors that detect the vibrations
traveling through the fluid in the chamber (endolymph). These hearing receptors
are called the spiral organ of Corti. As the hair cells detect vibration they send a
signal to the cochlear nerve that joins the vestibular nerve, the impulse travels to
the brain.
D. Equilibrium and auditory pathways
Equilibrium information travels to the brain through the vestibular nerve to the
brainstem and then to the cerebellum or to the reflex centers.
Auditory information travels from cochlear nerve to brain stem to the thalamus and
then to auditory cortex or from brainstem to auditory reflex center,
E. Disorders of equilibrium and hearing- Once again there are many disorders
discussed in this chapter see page 495.
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IV. The special senses throughout life.
1- Smell and taste are sharp in newborns and children have more taste
buds than adults. Taste and smell declines around 40.
2- Before birth photoreceptors are fully formed, after birth visual experience
fine tunes the the neurological connections in the visual pathway. The
following also occurs:
a- birth to 3 months- see only gray tones, uncoordinated eye movements,
images are blurry, can’t follow objects.
b. 3-6- images can be focus on fovea centralis, can follow moving object
c. by 6 month- depth perception is present and color vision is well
developed.
3. As we age the lens loses its clarity, scattering light and the pupil does not
adjust as efficiently. Thus by age 70 is visual acuity is dramatically lowered.
4. Newborns have reflexive responses to sound. Ability to hear fine sounds
develops after birth. By 3rd/4th months sounds can be localized. By 12
months they know all the sounds in their language.
5. It is uncommon to have congenital defects in the eye but more common
for the ear.
6. We are born with 20,000 hair cells in each ear and they are progressively
destroyed, by the time you reach puberty you have lost ability to hear high
pitch sounds. Deterioration continues and its rate depends on the degree of
noise you are exposed throughout life.
• THE END