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Chapter 17
Olfaction- the sense of smell
▪Paired olfactory organs are located in the nasal cavity on either side of the nasal septum
▪Made up of two layers; olfactory epithelium and the lamina propria
▫Olfactory epithelium▫Lamina propria-Olfactory glands▪Olfactory receptors are highly modified neurons
▫Olfactory reception occurs on the surfaces of the olfactory cilia as dissolved chemicals
interact with receptors.
▪Receptors are G proteins, odorant binding to a receptor triggers production of cAMP which
opens Na+ channels causing a localized depolarization (Fig. 17.2)
▪Considerable convergence occurs along the olfactory pathway
▫Intervening synapses can prevent the sensation from reaching the olfactory cortex of
the cerebral hemispheres.
▪Olfactory is the only type of sensory information that reaches the cerebral cortex without first
synapsing in the thalamus.
▫Extensive limbic system and hypothalamus connections account for the profound
relationship between emotions and smells
▪Olfactory receptors decline in number with age, reducing one’s sensitivity to detection of odors.
Gustation- The sense of taste
▪Taste receptors are distributed over the superior surface of the tongue and adjacent portions of
the pharynx and larynx.
▪Taste receptors and specialized epithelial cells form sensory structures called taste buds.
▪Adults have about 10,000 taste buds. Taste buds are found within lingual papillae.
▪There are three types of lingual papillae: filiform, fungiform, foliate, and vallate.
▫Filiform
▫Fungiform
▫Foliate
▫Vallate
▪Taste buds contain basal cells that appear to be stem cells.
▫Basal cells divide and eventually produce gustatory cells
▫Gustatory cells have slender microvilli extending into the surrounding fluid
▪A typical gustatory cell survives for only about 10 days before it is replaced.
▪Taste buds are monitored by the facial, glossopharyngeal, and vagus nerves.
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▪The level of stimulation of the olfactory receptors play an overwhelming role in taste perception.
▪Gustatory discrimination; primary tastes
▫Salty, sour, sweet, bitter: no differences in the structure of taste buds, taste buds in all
locations of the tongue provide all four primary taste sensations.
▫Umami- pleasant savory taste, receptors are located in the vallate papillae
▫Water- water receptors link to the hypothalamus and affects several systems that deal
with water balance and regulation of blood volume.
▪Salt and sour receptors are activated by leak channels.
▪Sweet, bitter, and umami receptors are G proteins called gustducins.
▪More sensitive to acidic and bitter tastes.
▪Begin loosing taste buds around age 50.
Vision
▪Accessory structures of the eye include; eyelids, superficial epithelium of the eye, and the
lacrimal apparatus.
▫Eyelids (palpebrae)- continuous with the skin
-palpebrae fissure
-medial and lateral canthus
-tarsal glands
-lacrimal caruncle
-conjunctiva
▫Lacrimal Apparatus- produces, distributes,
and removes tears.
-Lacrimal gland
-Lacrimal lake
-Lacrimal puncta
-Lacrimal canaliculi
-Lacrimal sac
-Nasolacrimal duct
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▪The Eye
▫Eyeball is hollow, divided into two cavities; posterior cavity and the anterior cavity.
-Posterior cavity
-Anterior cavity
▫Contains three distinct layers; fibrous outer tunic, intermediate vascular tunic, inner
neural tunic.
-Fibrous Tunic-Vascular Tunic-Iris
-Ciliary Body
-Choroid
-Neural Tunic-Pigmented part
-Neural part
▫Organization of the retina
-Rods
-Cones
-Rods and cones are not evenly distributed across the retina.
-Macula lutea
-Fovea
-Rods and cones synapse with bipolar cells, which in turn synapse with ganglion
cells, adjacent to the posterior cavity.
-Horizontal cells and amacrine cells can facilitate or inhibit communication
between photoreceptors and ganglion cells.
▫Optic Disc- A circular region just medial to the fovea, the origin of the optic nerve. No
photoreceptors, results in a blind spot.
▫Chambers of the Eye- Eye is divided into two main cavities, anterior and posterior.
-Anterior cavity is made up of the anterior chamber—cornea to iris, and the
posterior chamber—iris to lens and ciliary body. Contains aqueous humor
-Posterior cavity (vitreous chamber) is the area behind the lens, contains the
vitreous body.
▫The Lens- lies posterior to the cornea, held in place by the suspensory ligaments.
Functions to focus the visual image onto the photoreceptors.
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▫Refraction- light “bending” when traveling from one medium to another with a different
density. Light entering the eye refracts when it enters the cornea (air-cornea) and
through the lens (aqueous humor-lens).
-Focal point
-Closer objects need a longer focal distance, lens must change shape.
(accommodation)
▫Accommodation
▫Image reversal
▫Visual Acuity- clarity of vision
-20/20 vision relates to a person seeing details at a distance of 20 feet as clearly
as a “normal” person would.
-Visual acuity below 20/200 with corrective lenses is legally blind.
▪Visual Physiology
▫Rods provide the CNS with the information about the presence or absence of photons
▫Cones provide information about the wavelength of arriving photons.
▫Anatomy of photoreceptors
-Outer segment
-Inner segment
▫Visual pigments- absorbs photons, first step in photoreception.
-Rhodopsin- (visual purple)- protein complex found in rods, contains:
-Opsin
-Retinal
▫Photoreception
1. Opsin is activated by a photon, retinal changes from 11-cis to 11-trans
2. Opsin activates transducin which activates phosphodiesterase
3. Phosphosdiesterase (PDE) breaks down cGMP. cGMP levels decline, Na+
channels close
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4. Neurotransmitter rate of release declines
▫Recovery- Retinal must be enzymatically converted back to the 11-cis form. Entire
rhodopsin molecule is broken down and reassembled.
▫Color vision- stimulation in various combinations of three types of cones: blue, green,
and red.
▫Light and Dark Adaptation- can be in either a dark-adapted state; highly sensitive, or a
light-adapted state; rate of bleaching and reformation is balanced.
▪The Visual Pathway- begins at the photoreceptors and ends at the visual cortex of the cerebral
hemispheres.
▫In the visual pathway, there are two synapses (photoreceptors to bipolar cells, and
bipolar cells to ganglion cells)
▫These extra synapses cause synaptic delay, but allow for processing and integration of
visual information before it leaves the retina.
▫There is considerable convergence from the photoreceptors to the end of the pathway
▫M cells (magnocells)- ganglion cells that monitor rods.
▫P cell (parvo cells)- ganglion cells that monitor cones
▫Central Processing of Visual Information
-Axons from the entire population of ganglion cells converge on the optic disc,
penetrate the wall of the eye, and proceed toward the diencephalon as the optic
nerve (II).
-After the optic chiasm, visual information travels to the lateral geniculate nuclei,
then to the visual cortex of the cerebral hemispheres.
-Depth perception is the interpretation of the 3-D relationships among objects in
view, it is obtained by comparing the relative positions of objects within the
images received by the left and right eyes.
Circadian rhythm- daily pattern of visceral activity that is tied to the day-night
cycle, established by the pineal gland of the epithalamus.
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Equilibrium and Hearing
▪Anatomy of the Ear
▫External Ear- spans the auricle to the ear drum
-Auricle
-External acoustic canal
-Tympanic membrane (eardrum)
-Ceruminous glands are found along the external acoustic canal, produce cerum,
helps protect the eardrum.
▫Middle Ear- spans from the ear drum to the vestibule
-contains the auditory ossicles (malleus, incus, and stapes)
-two muscles are connected to the ossicles to help prevent damage to the ear
drum
-communicates to the nasopharynx through the auditory tube
▫Inner Ear- spans from the vestibule, deep the inner part of the temporal bone.
-senses of equilibrium and hearing are provided by the receptors in the inner ear.
-the membranous labyrinth is found within the bony labyrinth.
-endolymph flows within the membranous labyrinth, perilymph is found between
the bony labyrinth and the membranous labyrinth.
▫Vestibule- located within the bony labyrinth, contains two membranous sacs; saccule
and utricle.
-receptors in the saccule and utricle provide sensations of gravity and linear
acceleration.
▫Semicircular Canals- located within the bony labyrinth, contains the semicircular ducts,
which provide the sensations of head movement and rotation.
▫The vestibule and semicircular canals make up the vestibular complex
▫Cochlea- spiral shaped bony chamber that contains the cochlear duct.
-Near the base of the cochlea is two small openings, the oval window and the
round window.
▪Equilibrium
▫Semicircular Ducts- sensory receptors in the semicircular ducts respond to rotational
movement of the head.
-Ampulla
-Crista
-Cupula
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-movement in any of the three directions (horizontal, nodding, tilting from side to
side) causes the movement of the endolymph which in turn moves the cupula,
affecting the hair cells.
▫Hair Cells- sensory receptors of the inner ear.
-Stereocilia
-Kinocilium
-When the stereocilia or kinocilium are moved, by external pressures (sound
waves, gravity, or acceleration) they change the amount of neurotransmitter
released.
▫Utricle and Saccule- provide equilibrium sensations, whether the body is moving or not.
-endolymphatic duct and sac
-Maculae
-Statoconia
-Otolith
-Gravity or acceleration cause the statoconia to move, which causes the
gelatinous material to move, which affects the hair cells.
▫Vestibular ganglia cells collect the sensory information from the hair cells, transfer it to
the vestibular branch of the vestibulocochlear nerve (VIII).
▫Vestibulocochlear nerve (VIII) innervates neurons within the pair of vestibular nuclei at
the boundary between the pons and the medulla oblongata.
▪Hearing- receptors in the cochlear duct provide our sense of hearing.
▫The cochlear duct- lies between two chambers filled with perilymph, the tympanic duct
and the vestibular duct.
-the tympanic duct and the vestibular duct are connected at the top of the
cochlear spiral, producing a continuous perilymphatic chamber.
-both ducts are surrounded by the bony labyrinth except at the oval window and
the round window.
▫The Organ of Corti- hair cells of the cochlear duct are located here.
-hair cells lie between a basilar membrane and a tectorial membrane, they
contain no kinocilia, just stereocilia.
-the basal membrane becomes distorted with an incoming sound wave through
the perilymph, causes the hair cells to be affected.
▫The Hearing Process- Can be divided into six basic steps:
1. Sound waves arrive at the tympanic membrane.
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2. Movement of the tympanic membrane causes displacement of the auditory
ossicles.
3. Movement of the stapes at the oval window establishes pressure waves in
the perilymph of the vestibular duct.
4. The pressure waves distort the basilar membrane on their way to the round
window of the tympanic duct.
5. Vibration of the basilar membrane causes vibration of the hair cells against
the tectorial membrane.
6. Information about the region and intensity of stimulation is relayed to the
CNS over the cochlear branch of the vestibulocochlear nerve (VIII).
▫Auditory Pathways
-Stimulation of hair cells activate sensory neurons whose cell bodies are in the
adjacent spiral ganglion
-afferent fibers of those neurons form the cochlear branch of the
vestibulocochlear nerve (VIII).
-From there the information travels to the cochlear nucleus of the medulla
oblongata, then it crosses to the inferior colliculus of the mesencephalon, then to
the medial geniculate nucleus of the thalamus, then finally to the auditory cortex
of the temporal lobe.
▫Auditory Sensitivity
-the range for the softest audible sound to the loudest tolerable blast is a
trillionfold increase in power
-young children have the greatest hearing range.
- as we age, the ear drum gets less flexible, articulations stiffen, the round
window may begin to ossify, and minor injuries can accumulate, resulting in
older individuals showing signs of hearing loss.
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