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Special Senses
Chapter 15
Olfactory:
Small patch of olfactory epithelium located
on the superior nasal concha that contains
three types of tissue:
- Olfactory receptors
- Supporting cells (sustentacular)
- Basal cells
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1- Olfactory receptors: bipolar neurons
Dendrites have olfactory hairs (cilium)
hanging from the dendrites
2- Supporting cells: mucous membrane lining the
nose containing columnar epithelium
3- Basal cells continually produce new olfactory
receptors (neurons). They regenerate every
month. Exception to the rule that mature
neurons are not replaced.
Olfactory bulb and tract.
Taste and smell centers lie very close together in
the brain (primary olfactory cortex)
Physiology of olfaction.
Odors are made up of multiple odorant molecules
and each odorant molecule will activate several
olfactory receptors. This system can allow us to
recognize and remember some 10,000 odors.
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Odor molecule dissolves on the mucous
membrane. The molecules will bind to Gproteins located on the plasma membrane of the
receptor cells. This will activate the enzyme
cAMP (adenylate cylase or cyclic AMP) and cause
the opening of sodium channels, depolarization
and nerve impulse.
From the olfactory tracts impulse will
travel to two main destinations.
1- thalamus to the pyriform (pear shaped)
area
- uncus
- entorhinal area
- limen insula (junction between the
cortex of the frontal and insula lobes)
- frontal lobe above the orbits where
smell is interpreted and identified
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2- Subcortical pathway to the amygdala,
hypothalamus and other regions of the
limbic system. Here is where smells are
associated with danger and other
emotional responses. i.e. smoke, natural
gas, skunk. Sympathetic responses are
initiated.
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Olfactory terms:
Anosmia: loss of smell
Hyposmia: decr. smell
Hyperosmia: incr. smell
Dysosmia: distorted smell
Gustation:
Gustatory receptor cells are located in the
taste buds
- papillae (elevations scattered on tongue,
roof of mouth and walls of pharynx)
Primary taste sensations: sour, bitter, salty
and sweet.
Discovered in Japan is umami. Sensed in
response to a salt (monosodium
glutamate) found in the amino acid
glutamate.
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Sour, salty and sweet are carried via CN
VII (facial nerve) and bitter carried via CN
IX (glossopharyngeal).
All taste sensations are relayed to the
ventral posterior (vp) medial nuclei of the
thalamus from the solitary nucleus
(gustatory nucleus) then on to the primary
gustatory cortical area of the inferior postcentral gyri of the parietal lobes.
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Taste is 80% smell
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Gustatory terms:
Ageusia: loss of taste
Hypogeusia: decr. taste
Hypergeusia: incr. taste
Dysgeusia: distorted taste
Hearing:
External, middle and inner ear
External ear:
Outer auricle or pinna will funnel sound towards
the ear drum. Remember external auditory
meatus (EAM) through the temporal bone
Sound waves are transmitted through the EAM
until they reach the tympanic membrane (ear
drum)
- semitransparent membrane covered by skin
on the outside and mucous membranes on the
inside.
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Middle ear: Petrous portion of temporal bone
The ear drum magnifies the vibrations of sound
waves mechanically. One of the main purposes
of the ear is to transform these air pressure
waves to an electrical nerve signal that the brain
can understand
The ossicles are
- hammer (malleus) attaches to drum
- short and long handle (processes)
-manubrium
- anvil (incus)
- stirrup (stapes). The stirrup attaches to the
oval window of the inner ear or cochlea
Otitis media
Tympanostomy tubes
Inner ear: labyrinth
Vibration of the stapes on the oval window
moves fluid (perilymph) within the inner
ear. Vibrations will stimulate the hearing
receptors.
Pressure on the oval window is 22x greater
than the wave pressure exerted on the ear
drum.
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Pressure waves are transmitted from the scala
vestibuli to the scala tympani and then into the
round window
Pressure waves deform walls of scala vestibuli
and scala tympani, collectively causing the
vestibular membrane to move (pressure in
endolymph increases & decreases)
Pressure fluctuations in endolymph move the
basilar membrane
Hairs of the organ of Corti, connected to the
basilar membrane, move against the tectorial
membrane and cause the generation of nerve
impulses
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Decibels (dB) measures sound intensity.
0 dB is least perceived by the human ear
10 dB is 10x louder than 0 dB
20 dB is 100X as loud as 0 dB etc..
Whisper: 40 dB
Normal speech: 60-70 dB
Rock concert: 120 dB
Jet plane: 140 dB (nociception)
Prolonged periods of 90 dB will cause
permanent hearing loss
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Hearing nerve fibers are located in the
cochlear branch of the vestibulocochlear
nerve (VIII).
Auditory pathway for hearing:
- cochlear branch enters b/w the medulla
oblongata and the pons to end in the
cochlear nuclei both dorsal and ventral.
- fibers cross and ascend into thalamus
(medial geniculate body) to terminate in
the primary auditory cortex of the
temporal lobe
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Equilibrium: feeling of the position of the head
at rest and movement
Static (rest): organ is located in the vestibule
between the semicircular canals and the
cochlea. Specifically, the utricle and saccule.
Helps keep head still and balanced.
Dynamic (motion): organ located in the
ampullae of the semicircular canals.
- anterior
- posterior
- lateral
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The semicircular canals contain fluid and
are responsible for detecting changes in
motion. The three semicircular canals (all
at different angles), are responsible for
detecting motion on a different plane.
Within the canals are nerve hairs which
sense changes in the movement of the
perilymph fluid and depolarize, thus
sending a message to the brain. Ex:
spinning in a circle- (remember the sit and
spin toy)
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Vestibular portion of CN VIII enters b/w
the medulla oblongata and the pons and
ends in the vestibular nuclear complex.
Works closely with the cerebellum to
signal appropriate skeletal muscles in
response to changes in equilibrium.
Vision:
Eyeball has three layers:
Outer: Fibrous tunic
Middle: Vascular tunic
Inner: Nervous tunic
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Fibrous tunic:
Anterior portion contains the clear avascular
cornea which helps focus light rays. Very pain
sensitive
It is continuous with the white sclera that
surrounds the eyeball and is the attachment site
for the extrinsic ocular muscles.
Dura mater is continuous with the sclera at the
site of the optic nerve that exits the back of the
eyeball.
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Vascular tunic (uveal layer) includes the iris,
ciliary body and choroid coat.
Choroid coat contains blood vessels that nourish
the eye and is loosely joined to the sclera.
Ciliary body located in the anterior part of the
eye has ciliary muscles that are attached to
suspensory ligaments that hold the lens
(transparent structure located behind the iris) in
place. When the muscles contract these
ligaments are able to change the shape of the
lens (ligaments relax causing lens to thicken)
aiding in focusing close objects.
(accommodation- remember parasympathetic
division: CN III)
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iris:
- colored smooth muscles portion of
eye
- thin diaphragm allowing light to
enter.
- lies b/w the cornea and the lens
- separates the eyeball in an anterior
(b/w the iris and cornea) and posterior
chamber (b/w the iris and vitreous humor)
ant.: aqueous humor
post.: vitreous humor
- Pupil
- hole in the iris
- pupil size is controlled by smooth
muscles in the iris called circular and radial
sets.
- circular set acts as sphincter to constrict
pupil in the presence of bright light.
(parasympathetic)
- radial set increases diameter of pupil
causes it to dilate. (sympathetic.)
Nervous tunic:
The retina is a thin, delicate structure that
contains specialized photoreceptor cells
and is continuous with the optic nerve.
Five types of neuron located in the retina
- Receptor cells (rods and cones)
- Bipolar neurons
- Ganglion cells
- Horizontal cells
- Amacrine cells
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In the center of the retina is yellowish spot
called the macula lutea. A depression in the
center called the fovea centralis is the
region where the sharpest vision is
produced.
Medial to the fovea is the optic disc. The
optic disc is where the optic nerve exits
the eyeball. No receptors are present for
vision so it is also called the blind spot.
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Refraction of light rays (bending of light)
by the anterior and posterior surfaces of
the cornea as well as refraction by the
lens cause light rays to come into exact
focus on the retina .
Images focused on the retina are inverted
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Photoreceptors and visual pigments
- Rods and cones
- stimulated when light reaches them
- Rods are more sensitive to light than cones
and provide vision in dim light.
- Cones provide sharp images and color vision.
Fovea centralis contains high concentration of
cones but no rods.
- Light sensitive pigment in rods is called
rhodopsin (visual purple). Rhodopsin, in the
presence of bright light, will break down into
opsin and retinal (made from Vit. A).
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With rhodopsin broken down, cones are
activated and we see in color.
In dim light, rhodopsin is made faster than it is
broken down and cones remain unstimulated,
therefore we see in shades of gray in dim light.
Light sensitive pigment in cones is called
iodopsin.
Three sets of cones within retina
1- erythrolabe- red light waves
2- chlorlabe- green light waves
3- cyanolabe – blue light waves
Extrinsic eye muscles
1- Superior rectus: turns eye upward and towards
the midline (III)
2- Inferior rectus: turns eye downward and
towards the midline (III)
3- Medial rectus: turns eye towards midline (III)
4- Lateral rectus: turns eye away from midline (VI)
5- Superior oblique: turns eye downward and
away from midline (IV)
6- Inferior oblique: turns eye upward and away
from midline. (III)
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Visual pathways:
Axons of the ganglion cells in the retina
leave the eye by way of the optic nerve
Some fibers of the optic nerve will cross
just anterior to the pituitary gland at a
junction called the optic chiasma
- fibers coming from the medial (nasal)
retina will cross while fibers coming from
lateral (temporal) retina will not.
Posterior to the optic chiasma is the optic tracts
- right optic tract is composed of
- right temporal and left nasal
- left optic tract is composed of
- left temporal and right nasal
Some fibers branch off to the superior colliculus
(visual reflex)
- Optic tracts synapse in the posterior portion of
thalamus at the lateral geniculate body.
- from the geniculate body the pathway travels
through optic radiations into the visual cortex of
the occipital lobe.
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