45.1.
Chemical Senses
A. Receptors, present in sense organs, monitor changes in the
external and internal environment.
1. Each type of receptor has a low threshold for a particular stimulus (e.g.,
light, changes in temperature).
2. Receptors do not interpret
stimuli; they are transducers that receive stimuli and generate
nerve impulses.
3. The brain, not sensory receptors,
interprets the stimulus by where it arrives in the brain.
4. Specific regions of the brain process
the information into a sensation.
B. Chemoreceptors are responsible for taste and smell by being
sensitive to chemicals in food, liquids, and air.
1. Chemoreception is found universally in animals; it is thought to be the most
primitive sense.
2. Chemoreceptors are present all
over a planarian but concentrated in auricles at side of the head.
3. Crustacea have chemoreceptors on
antennae and appendages.
4. Insects, such as houseflies,
taste with their feet.
5. In amphibians, chemoreceptors are
located in both mouth and all over skin.
6. In mammals, chemoreceptors for
taste are in the mouth, and chemoreceptors for smell are in the nose.
C. Sense of Taste
1. Taste buds are located primarily on the tongue.
2. Many lie along walls of papillae,
small elevations on surface of the tongue.
3. Isolated ones are present on
surfaces of hard palate, pharynx, and epiglottis.
4. Taste buds are embedded in tongue
epithelium and open at taste pores.
5. Taste buds have supporting cells
and elongated taste cells that end in microvilli.
6. Microvilli bear receptor
proteins for certain chemicals.
a. Molecules
bind to receptor proteins and impulses are generated in associated sensory
nerves
b. Nerve
impulses go to the brain corticular areas which interpret them as tastes.
7. Humans have four types of taste
buds.
a. Taste
buds for each are concentrated in particular regions.
1) Sweet receptors are most plentiful near tip of the tongue.
2) Sour receptors occur primarily along margins of the tongue.
3) Salty receptors are most common on tip and upper front portion.
4) Bitter receptors are located near back of the tongue.
b. The brain
appears to take an overall weighted average of taste messages as the perceived
taste.
D. Sense of Smell
1. Sense of smell depends on olfactory cells
located high in roof of the nasal cavity.
2. Olfactory cells are modified
neurons.
3. Each cell has a tuft of five
olfactory cilia that bear receptor proteins for various odor molecules.
a. There are
around 1,000 different odor receptors; many olfactory cells carry the same
type.
b. Odor
activates a characteristic combination of cells; this information is pooled in
olfactory bulb.
c.
Interneurons communicate this information via the olfactory tract to areas of
cerebral cortex.
4. Olfactory bulbs are directly
connected with limbic system; smells associate with emotions and memory.
5. Taste and smell supplement each
other: "smelling" food also involves the taste receptors; losing taste
when you
have a cold is usually due to loss of smell.
45.2. Sense
of Vision
A. Animals lacking photoreceptors depend on senses of hearing and smell
rather than sight.
B. Photoreceptors vary in complexity.
1. In its simplest form, a photoreceptor indicates only presence
of light and its intensity.
2. "Eyespots" of planaria
allow flatworms to determine direction of light.
3. Image-forming eyes
occur in four invertebrate groups: cnidaria, annelids, mollusks, and
arthropods.
4. Arthropods have compound
eyes composed of many independent visual units (ommatidia),
each
possessing
all elements needed for light reception. (Fig. 47.3)
a. Cornea
and crystalline cone of each visual unit focus rays toward photoreceptors.
b.
Photoreceptors generate nerve impulses, which pass to brain by way of optic
nerve fibers.
c. Image
resulting from all stimulated visual units is crude; small size of compound
eyes limits number
of visual units.
d. Insects
have color vision but utilize a shorter range of electromagnetic spectrum.
5. Some fishes, reptiles, and most
birds are believed to have color vision, but among mammals, only
humans and
other primates have color vision; this is adaptive for day activity.
6. Vertebrates and certain mollusks
(e.g., the squid and the octopus) have a camera-type eye.
a. Mollusks
and vertebrates are not closely related; this is convergent evolution.
b. A single
lens focuses an image of the visual field on closely packed photoreceptors.
c. In
vertebrates a lens changes shape to aid in focusing; in mollusks a lens move
back and forth.
d. Human eye
is considerably more complex than a camera.
C. The Human Eye
1. Human eye is an elongated sphere 2.5 cm in diameter with
three layers. (Fig. 47.6)
2. Sclera is outer,
white fibrous layer that covers most of eye; it protects and supports eyeball.
3. Cornea is a
transparent part of sclera at front of the eye that is window of the eye.
4. Middle, thin, dark-brown layer is
choroid containing many blood vessels and pigments absorbing
stray light rays.
5. To front of eye, choroid thickens
and forms ring-shaped ciliary body and finally becomes the iris
that
regulates
size of the opening called a pupil.
6. Lens divides eye
cavity into two portions: aqueous humor fills anterior cavity and
vitreous humor fills posterior.
7. Retina
a. Inner
layer retina contains photoreceptors: rod
cells and cone cells.
b. Fovea
centralis is a small area of retina that contain only cones; this area
produces acute
color vision in daylight.
c. Cone
cells are barely sensitive at low intensity at night; at this time, the rods
are still active.
D. Focusing the Eye
1. Light rays entering eye are focused on the retina.
2. Focusing involves light passing
through the cornea, lens and humors.
3. Because of refraction, image on
retina is rotated 180º from actual but seems to be righted in the brain.
4. Shape of lens is controlled by
ciliary muscle.
5. Lens is flat (ciliary muscle
relaxed) when viewing distant objects.
6. Lens is naturally elastic and
becomes rounder for viewing near objects because light rays must bend to
a greater
degree.
7. These changes are called visual
accommodation.
8. Aging lens loses ability to
accommodate for near objects; we may need reading glasses by middle age.
9. Lens is also subject to cataracts,
or becoming opaque; surgery is the only current treatment.
a. A surgeon
opens the eye near rim of the cornea.
b. Enzyme
zonulysin digests away ligaments holding lens in place.
c. A
cryoprobe freezes lens for easy removal.
d. An
intraocular lens attached to iris is implanted to avoid need for thick glasses
or contact lenses.
10. Persons who can see close up but
not far away are nearsighted.
a. They
often have an elongated eyeball that focuses distant images in front of retina.
b. They can
wear corrective concave lenses to refocus the image on the retina.
c. Radial
keratotomy is a new treatment that surgically cuts and flattens cornea.
11. Persons who can see far away but
not up close are farsighted.
a. They
often have a shortened eyeball that focuses near images behind retina.
b. They can
wear corrective convex lenses to refocus the image on retina.
12. When cornea is uneven, image is
fuzzy; this is astigmatism corrected by an unevenly ground lens
to
compensate
for unevenness.
E. Photoreceptors of the Eye
1. Vison begins when light has been focused on photoreceptors in
retina.
2. Rods and cones have outer segment
joined to inner by stalk.
3. Outer segment contains stacks of
membranous disks (lamellae) with many molecules of rhodopsin.
4. Rhodopsin molecules contain a
protein opsin and pigment molecule retinal derived
from Vitamin A.
5. When rod absorbs light, rhodopsin
splits into opsin and retinal, leading to cascade of reactions and closure
of ion
channels in rod cell plasma membrane.
6. This stops release of inhibitory
molecules from rod's synaptic vesicles and starts signals that result in
impulses to
brain.
7. Rods are stimulated by low light
and provide night vision; rods detect motion but not color or detail.
8. Cones located primarily in fovea
centralis are activated by bright light and detect detail and color.
9. Three kinds of cones contain
either blue, green, or red pigment.
10. Each pigment is composed of
retinal and opsin, but structure of opsin varies among the three.
11. Combinations of cones are
stimulated by intermediate colors; combined nerve impulses are interpreted
in brain as
one of 17,000 hues.
F. Integration of Visual Signals in the Retina
1. Retina has three layers of
neurons.
a. Rods and
cones are nearest the choroid.
b. Bipolar
cells form the middle layer.
c. Ganglion
cells, whose fibers become the optic nerve, form the innermost layer.
2. Rods and cones synapse with bipolar cells which pass the
impulse to ganglionic cells.
3.
There are more rods and cones than nerve fibers leaving ganglionic cells.
4. Up to 100 rods synapse with a
ganglion cells; this results in indistinct vision.
5. Each cone synapses with one
ganglionic cell; this accounts for detailed images.
6. As signals pass from bipolar to
ganglion cells, integration occurs.
a. If all
rod cells in receptive field are stimulated, ganglion cell is weakly stimulated
or neutral.
b. If only
center is lit, it is stimulated; if edge is lit, it is inhibited.
c. Therefore
considerable processing occurs in retina before impulse is sent to brain.
7. Blind spot is an
area where optic nerve passes through retina; it lacks rods and cones.
45.3. Sense
of Hearing and Balance
A. Anatomy of the Ear
1. Human ear has three divisions: an outer, middle, and inner
ear. 2. Outer ear
consists of pinna (external flap) and the auditory canal.
a. Auditory
canal opening is lined by fine hairs that filter air.
b. Modified
sweat glands in auditory canal secrete earwax to guard ear against foreign
matter.
3. Middle ear begins
at tympanic membrane and ends at a bony wall with
membrane-covered openings
(oval
window and round window).
a. It
contains ossicles: malleus (hammer), incus (anvil), and stapes
(stirrup).
b. Malleus
adheres to tympanum; stapes touches oval window.
c. Auditory
(eustachian) tube extends from middle ear to pharynx to equalize inside
and outside air.
4. Inner ear has three
regions: semicircular canals, vestibule, and cochlea.
5. Cochlea resembles a
snail shell because it spirals.
B. Process of Hearing
1. Process of hearing begins when sound waves enter auditory canal, causing
ossicles to vibrate.
2. Sound is amplified 20 times by
size difference between tympanic membrane and oval window.
3. Stapes strikes membrane of oval
window, passing pressure waves to fluid in cochlea.
4. Vestibular canal connects with
tympanic canal, which leads to oval window membrane.
5. Three canals are located
within cochlea: vestibular canal, cochlear canal, and tympanic canal.
6. Along the basilar membrane are
hair cells whose stereocilia are embedded in tectorial membrane.
7. Hair cells of the spiral organ (organ
of Corti) synapse with nerve fibers of cochlear (auditory) nerve.
8. When stapes strikes membrane of
oval window, pressure waves move from vestibular canal to tympanic
canal and
across basilar membrane, and the round window bulges.
9. Basilar membrane vibrates up and
down bending stereocilia of hair cells embedded in tectorial membrane.
10. Nerve impulses in cochlear nerve
travel to brain stem.
11. In auditory areas of cerebral
cortex, this is interpreted as sound.
12. Spiral organ is narrow at its
base and widens as at tip; each part is sensitive to different pitches.
13. Nerve fibers from each region
(high pitch base or low pitch tip) lead to slightly different regions
of brain
producing sensation of pitch.
14. Sound volume causes more
vibration; increased stimulation is interpreted as louder sound intensity.
15. Tone is interpretation of brain
based on distribution of hair cells stimulated.
C. Sense of Balance
1. Sense of balance is divided into:
a. dynamic
equilibrium (angular or rotational movement of the head), and
b. static
equilibrium (vertical or horizontal movement).
2. Dynamic equilibrium
utilizes the semicircular canals.
a. Semicircular
canals are oriented at right angles to one another in three different
planes.
b. Enlarged
base of each semicircular canal is called an ampulla.
c. Fluid
flowing over and displacing a cupula causes stereocilia of the hair cells
to bend; the pattern
of impulses carried by the vestibular nerve to the brain changes.
d.
Continuous movement of fluid in semicircular canals causes vertigo motion
sickness.
e. By
spinning and stopping, we see a room still spin; this indicates vision is
involved in balance.
3. Static equilibrium
utilizes the utricle and saccule.
a. A vestibule
or space between semicircular canals and cochlea contains utricle
and saccule.
b. Utricle
and saccule are small membranous sacs, each of which contains hair
cells.
c. Hair cell
stereocilia are embedded within a gelatinous material called the otolithic
membrane.
d. Calcium
carbonate granules (otoliths) rest on this membrane.
e. Utricle
is sensitive to horizontal movements; the saccule responds best to up-down
movements.
f. When a
body is still, otoliths in utricle and saccule rest on otolithic membrane above
hair cells.
g. As head
bends or body moves, otoliths are displaced and otolithic membrane sags,
bending the
larger stereocilia (kinocillium) of hair cells beneath; this tells brain the
direction of movement.
D. Other Sensory Receptors
1. Lateral line system of fish and amphibians detects water currents and
pressure waves.
2. Primitive fishes have the system
on surface; advanced fishes enclose it in a canal on the side.
3. Lateral line receptor is a
collection of hair cells with cilia embedded in a mass of gelatinous material
(cupula).
4. Static equilibrium organs called
statocysts are found in cnidaria, mollusks, and crustacea.