Transcript Special Senses

Chapter 17
The Special Senses
Special Senses
•
•
•
•
Olfaction
Taste
Visual system
Hearing and balance
Olfactory System
• Nose- olfactory epithelium-receptors, supporting cells, and basal cells
• Olfactory Receptors- bipolar neuron, exposed knob shaped dendrite
and axon that projects through the cribiform plate synapsing in the
olfactory bulb.
• Supporting cells- columnar epithelium-mucous membrane in the
lining of the nose.
• Basal cells- stem cells underneath the supporting cells that divide to
produce new olfactory receptor cells.
• Olfactory nerve (I)- olfactory receptor axons bundle and form the
olfactory nerve which terminate in the olfactory bulb. Axons of olfactory
bulb form the olfactory tract that projects to the primary olfactory area
(inf. and med. temporal lobe), frontal lobe, limbic system, and
hypothalamus.
Olfactory Physiology
• Olfactory receptors respond to odors and produce a generator potential that
triggers an action potential in olfactory nerves that terminate in the olfactory
bulb.
• Odorant--binds to a G-protein linked receptor--biochemical cascade--opening
Na+ channels--depolarizing generator potential--generation of action potential.
•Adaptation- Occurs rapidly in olfactory receptors. There is a component of
olfactory perception that adapts more slowly and is likely in the brain.
Olfactory Epithelium and Receptors
Olfaction
• Sense of smell
– Olfactory neurons in this
epithelium
• Bipolar neurons
– Olfactory hairs
• Cilia which lies in
mucous
• Odors
– Odorants bind to
chemoreceptor molecules
– Depolarize and initiate
action potentials in neurons
– Low threshold for odor
detection
Figure 16.8
Neuronal Pathways of Olfaction
• Sense is detected by
taste buds
Taste
• Papillae
– Vallate- 12 form an inverted
V at back of tongue-contain
100-300 taste buds.
– Fungiform- mushroom
shaped all over tongue each
has 5 taste buds.
– Foliate- lateral tongue,
degenerate in childhood
– Filiform- pointed structures
with no taste buds, help
tongue to move food in
mouth (friction).
• Histology
– Support cells
– Gustatory cells
• Hairs
• Function
– Receptors on hairs
detect dissolved
substances
– Adaptation to taste is
rapid but variable based
on taste type. Taste
aversion-type of
adaptation
• Taste types
–
–
–
–
–
Sour
Salty
Bitter
Sweet
Umami (meaty, MSG)
Taste Receptors
The sense of taste is mediated by taste receptor cells which are bundled in
clusters called taste buds. Taste receptor cells sample oral concentrations of a
large number of small molecules and report a sensation of taste to centers in
the brainstem. In most animals, including humans, taste buds are most
prevalent on small pegs of epithelium on the tongue called papillae.
When taste cells are stimulated by binding of chemicals to their receptors,
they depolarize and this depolarization is transmitted to the taste nerve fibers
resulting in an action potential that is ultimately transmitted to the brain. One
interesting aspect of this nerve transmission is that it rapidly adapts - after the
initial stimulus, a strong discharge is seen in the taste nerve fibers but within a
few seconds, that response diminishes to a steady-state level of much lower
amplitude.
Gustatory (taste) Receptors
Papillae and Taste Buds
Actions of Major Tastants
Neuronal Pathways for Taste
Parietal lobe
Front 2/3 of tongue
Rear 1/3 of tongue
Throat and epipglottis
Physiology of Gustation
1. Tastant dissolved in saliva
2. binds to gustatory hairs generating a receptor potential that
3. stimulates neurotransmitter secretion
4. Neurotransmitters bind to receptors on the first order sensory
neurons that are postsynaptic to the gustatory receptor cells.
• Receptor potential arises differently for different tastes.
1. Salty taste-Na+ enters the gustatory receptor cells
through channels, causes depolarization, opens Ca2+ channels
and triggers neurotransmitter secretion.
2. Sour taste-H+ ions entering H+ channels influencing
other channels resulting in depolarization and neurotransmitter
secretion.
3. Sweet, bitter, and umami tastes- tastants bind to Gprotein linked receptors that result in depolarization of the receptor
cells and ultimately neurotransmitter secretion.
Visual System
• Eye
• Accessory structures
– Eyebrows, eyelids, eyelashes, tear glands
– Protect eyes from sunlight and damaging particles
• Optic nerve (II)
– Tracts
– Pathways
• Eyes respond to light and initiate afferent action
potentials
Accessory Structures of Eye
• Eyebrows
– Prevent running perspiration
into eyes
– Shade
• Eyelids or palpebrae
– Consist of 5 tissue layers
– Protect and lubricate
• Conjunctiva
– Covers inner eyelid and
anterior part of eye
• Lacrimal apparatus
• Extrinsic eye muscles
Lacrimal Apparatus
1.
2.
Lacrimal Gland: Produces
tears to moisten, lubricate,
wash. Innervated by
parasympathetic fibers of
facial (VII) nerve.
Production=1ml/day
Lacrimal Canaliculi
–
3.
4.
5.
Collects excess tears
Punctum-small openings
Lacrimal Sac
Nasolacrimal duct
–
Opens into nasal cavity
Flow of tears: Lacrimal gland--excretory lacrimal ducts--sup. or inf. lacrimal
canal--lacrimal sac--nasolacrimal duct--nasal cavity.
Extrinsic Eye Muscles
Anatomy of the Eyeball
Anterior and Posterior chambers of the
Eye
Eyeball Anatomy
Cornea- tranparent coat curved to help focus light onto retina.
Sclera-the white of the eye made of collagen fibers and fibroblasts
Uveachoroid-lines the sclera
ciliary body-appears dark brown because of melanocytes
Iris-colored portion of the eyeball, contains melanocytes: brown-high
melanin, green-medium melanin, blue-low melanin.
Retina-beginning of the visual pathway contains photoreceptor cells.
Responses of the pupil to light
Retina
Exit of
optic nerve
The Retina
Four kinds of light-sensitive receptors are found in the retina:
･rods
･three kinds of cones, each "tuned" to absorb light from a portion of the spectrum of
visible light
･cones that absorb long-wavelength light (red)
･cones that absorb middle-wavelength light (green)
･cones that absorb short-wavelength light (blue)
Each type of receptor has its own special pigment for absorbing light. Each consists of:･
a transmembrane protein called opsin coupled to･the prosthetic group retinal. Retinal is
a derivative of vitamin A (which explains why night blindness is one sign of vitamin A
deficiency) and is used by all four types of receptors.
The retina also contains a complex array of interneurons:･bipolar cells and ganglion
cells that together form a path from the rods and cones to the brain･a complex array of
other interneurons that form synapses with the bipolar and ganglion cells and modify
their activity.Ganglion cells are always active. Even in the dark they generate trains of
action potentials and conduct them back to the brain along the optic nerve. Vision is
based on the modulation of these nerve impulses. There is not the direct relationship
between visual stimulus and an action potential that is found in the senses of hearing,
taste, and smell. In fact, action potentials are not even generated in the rods and cones.
Photoreceptors
Two type of photoreceptors:
1. Rods- function only in dim light and are blind to
color.
2. Cones- operate in bright light and are
responsible for high acuity vision, as well as
color.
Rods and cones form an uneven mosaic within the
retina, with rods generally outnumbering cones
more than 10 to 1 except in the retina's center,
or fovea. The cones are highly concentrated in
the fovea. Even though the fovea is essential
for fine vision, it is less sensitive to light than
the surrounding retina. Thus, if we wish to
detect a faint star at night, we must gaze
slightly to the side of the star in order to project
its image onto the more sensitive rods, as the
star casts insufficient light to trigger a cone into
action.
Photopigment
Rods-rhodopsin
Cones- 3 types
Photopigments:
1. Opsin-glycoprotein
2. Retinal-light absorbing
derivative of vitamin A
(formed from carotene”eat carrots for good
vision”)
Rod photoreceptors
The lens
The lens is located just behind the iris. It is held in position by zonules extending from
an encircling ring of muscle. When this ciliary muscle is･relaxed, its diameter
increases, the zonules are put under tension, and the lens is flattened;･contracted, its
diameter is reduced, the zonules relax, and the lens becomes more spherical. These
changes enable the eye to adjust its focus between far objects and near objects.
Farsightedness. If the eyeball is too short or the lens too flat or inflexible, the light rays
entering the eye particularly those from nearby objects will not be brought to a focus by
the time they strike the retina. Eyeglasses with convex lenses can correct the problem.
Farsightedness is called hypermetropia.
Nearsightedness. If the eyeball is too long or the lens too spherical, the image of distant
objects is brought to a focus in front of the retina and is out of focus again before the light
strikes the retina. Nearby objects can be seen more easily. Eyeglasses with concave
lenses correct this problem by diverging the light rays before they enter the eye.
Nearsightedness is called myopia.
Cataracts One or both lenses often become cloudy as one ages. When a cataract
seriously interferes with seeing, the cloudy lens is easily removed and a plastic one
substituted. The entire process can be done in a few minutes as an outpatient under local
anesthesia.
The iris and lens divide the eye into two main chambers･the front chamber is filled with a
watery liquid, the aqueous humor･the rear chamber is filled with a jellylike material, the
vitreous body.
Refraction
Accommodation
Increased curvature of
the lens for near vision
Visual Pathway
Visual Pathways
Figure 16.10
Hearing-ear anatomy
Properties of Sound
Pitch-perception of different sound
frequencies
Frequency-cycles per second of sound waves
(20-20,000 Hz
Loudness-sound wave intensity
Intensity-sound waves have greater
amplitude resulting in stimulation of more
fibers and louder perception of sound.
External Ear
External Ear The external ear includes the auricle (pinna) and external auditory
canal. The auricle is composed of elastic fibrocartilage covered by perichondrium and
skin. The skin over the lateral aspect of the ear is tightly adherent to the perichondrium
whereas on the medial surface, it is more loosely attached. The auricle is attached to the
tympanic portion of the temporal bone on the lateral aspect of the skull by extension of
the auricular cartilage into the cartilaginous external canal, by three ligaments (anterior,
superior, posterior), by six poorly developed muscles and by its skin and subcutaneous
tissue.
The auricle receives sensory innervation from branches of cranial nerves V
(auriculotemporal nerve), VII (auricular branch), X (auricular branch) and by the greater
auricular nerve from the cervical plexus. The external auditory canal extends from the
conchal cartilage of the auricle to the tympanic membrane. It is approximately 25 mm
long in the adult. It courses slightly anteriorly and inferiorly in the adult. The lateral one
half of the canal is cartilagenous, has thicker skin with subcutaneous tissue and
ceruminous glands. The medial one half is osseous with only epidermis lying on the
periosteum of the bony external canal.
Middle Ear
Middle Ear The middle ear is composed of the tympanic membrane, the tympanic
cavity, the ossicles and the eustachian tube. The tympanic membrane forms the lateral
wall of the middle ear. It is oval in shape, approximately 8 mm wide and 10mm high. The
tympanic membrane is about 0.1 mm thick and lies at an angle of 40 degrees in the
saggital plane with the lower aspect displaced medially. It is not flat, rather it is concave
medially. The umbro marks the middle of the tympanic membrane and corresponds to the
attachment of the tip of the malleus to the tympanic membrane. Superiorly, the short
process of the malleus extends laterally and forms a prominence on the tympanic
membrane. From this prominence extends the anterior and posterior malleolar folds.
Superior to the folds, lies the pars flaccida (or Shrapnell's membrane), below is the pars
tensa. The pars tensa inserts into a bony groove in the tympanic bone termed the
tympanic sulcus. The tympanic membrane is composed of three layers, an outer layer
of epidermis continuous with the epidermis of the exxternal auditory canal, a middle layer
of fibrous tissue (lamina propia) and a medial layer of mucosa. Sensory nerves to the
tympanic membrane include the auricular branch of cranial nerve X and the
auriculotemporal branch of the mandibular nerve. The tympanic cavity is a cleft or space
within the temporal bone located between the tympanic membrane laterally and the inner
ear medially. Posteriorly it commumnicates with the mastoid air cells anterior-inferiorly
with the eustachian tube orifice. Within the cavity lies the middle ear ossicles, the
chorda tympanic and a segment of the facial nerve (cranial nerve VII).The middle ear
contains three bones or ossicles which transmit sound vibrations to the inner ear. They
are from lateral to medial, the malleus, the incus and the stapes. The malleus is firmly
attached to the tympanic membrane and the stapes sits within the oval window of the
cochlea. Between them lies the incus.
Inner ear
Inner Ear
The inner ear consists of two main parts, the cochlea (end organ for
hearing) and the vestibule and semicircular canals (end organ for balance). The inner
ear can be thought of as a series of tunnels or canals within the temporal bone. Within
these canals are a series of membranous sacs (termed labyrinths) which house the
sensory epithelium. The membranous labyrinth is filled with a fluid termed endolymph; it
is surrounded within the bony labyrinth by a second fluid termed perilymph.The cochlea
can be thought of as a canal that spirals around itself similar to a snail. The bony canal of
the cochlea is divided into an upper chamber, the scala vestibuli and a lower chamber,
the scala tympani by the membranous (otic) labyrinth also known as the cochlear duct.
The scala vestibuli and scala tympani contain perilymph. The scala media contains
endolymph. Endolymph is similar in ionic content to intracellular fluid (high K, low Na) and
perilymph resembles extracellular fluid (low K, high Na). The cochlear duct contains
several types of specialized cells responsible for auditory perception. The floor of the
scala media is formed by the basilar membrane,the roof by Reissner's membrane.
Situated on the basilar membrane is a single row of inner hair cells medially and three
rows of outer hair cells laterally. The cells have specialized stereocilia on their apical
surfaces. Attached to the medial aspect of the scala media is a fibrous structure called
the tectorial membrane. It lies above the inner and outer hair cells coming in contact with
their stereocilia. Synapsing with the base of the hair cells are dendrites from the auditory
nerve. The auditory nerve leaves the cochlear and temporal bone via the internal auditory
canal and travels to the brainstem.
Middle ear-auditory ossicles
Internal ear
Figure 16.15
Figure 16.16
Figure 16.17
Figure 16.18
Figure 16.18a
Figure 16.18b
Hearing
Summary
Sound wave-tympanic membrane-middle ear ossiclesoval window-fluids within cochlea-distortions in
membranes (related to pitch and intensity)-bending of
sensory hair cells in the organ of Corti-electrical signalcochlear branch of the vestibulocochlear nerve (VIII).
Hair cells transduce mechanical signals into electrical
signals
*Basilar membrane tuned for pitch
*left-right timing and sound location
Figure 16.19
Figure 16.19a
Figure 16.19b
Figure 16.19c
Figure 16.20
Figure 16.20a
Figure 16.20b
Figure 16.20c
Figure 16.21
Maculae-structure and receptor
localization
Semicircular ducts
Eye development
Ear Development
Chapter 17
END