47.1. Sensing Chemicals (p. 840)
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 sensory receptors that are responsible for taste and smell by being sensitive to certain chemical substances in food, liquids, and air.
1. Chemoreception is found universally in animals and is therefore thought to be the most primitive sense.
2. Chemoreceptors are present all over a planarian, but are concentrated in the auricles at the side of the head.
3. Crustacea have chemoreceptors on the antennae and appendages.
4. Insects, such as houseflies, taste with their feet.
5. In amphibians, chemoreceptors are located in both the mouth and all over the skin.
6. In mammals, chemoreceptors for taste are in the mouth, and chemoreceptors for smell are in the nose.
C. Tasting With Taste Buds
1. Taste buds are located primarily on the tongue. (Fig. 47.1) [transp. 261]
2. Many lie along the walls of the papillae, small elevations on the surface of the tongue.
3. Isolated ones are present on the surface of the hard palate, the pharynx, and the epiglottis.
4. Taste buds have supporting cells and a number of elongated taste cells that end in microvilli.
5. The microvilli bear plasma membrane receptors for certain chemicals.
a. Taste receptors do not generate nerve impulses but make extensive contacts with ends of sensory nerve fibers that do generate impulses.
b. Binding of a molecule to a plasma membrane receptor is believed to cause changes in permeability of the receptor-cell membrane.
c. Electrochemical changes in these cells cause the associated sensory nerve fibers to generate nerve impulses that go to the parietal lobe of the cerebrum.
6. Evidence indicates humans have four types of taste buds, each type stimulated by chemicals that result in a bitter, a sour, a salty, or a sweet sensation.
a. Taste buds for each are concentrated in particular regions. (Fig. 47.1a)
1) Sweet receptors are most plentiful near the tip of the tongue.
2) Sour receptors occur primarily along the margins of the tongue.
3) Salty receptors are most common on the tip and upper front portion.
4) Bitter receptors are located near the back of the tongue.
b. Associated sensory fibers have graded, rather than all-or-nothing, sensitivities to the four tastes.
c. The brain appears to take an overall weighted average of taste messages as the perceived taste.
d. Some nerve impulses go directly to the cerebrum and also to the brain stem; this may have survival value as when babies continue to nurse even in the absence of conscious control.
D. Smelling With the Nose (Fig. 47.2a) [transp. 262]
1. Our sense of smell depends on olfactory cells located high in the roof of the nasal cavity.
2. Olfactory cells are modified neurons.
3. Each cell has a tuft of about five olfactory cilia that bear receptor proteins for various chemicals.
a. When molecules bind to receptors, nerve impulses pass to olfactory bulb located at front of brain.
b. Some processing occurs here before olfactory information is sent primarily to the temporal lobe of the cerebrum, which produces the sensation of smell.
c. There are around 1,000 different odor receptors; many olfactory cells carry the same type.
d. An odor activates a characteristic combination of cells; this information is pooled in the olfactory bulb.
e. The brain then determines the precise pattern of the types of receptors activated.
4. Olfactory receptors, like touch and temperature receptors, adapt to outside stimuli; after time, the presence of a chemical no longer causes the olfactory cells to generate nerve impulses.
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.
47.2. Sensing Light (p. 842)
A. Animals that lack photoreceptors depend largely on the sense of hearing and smell rather than sight.
B. Photoreceptors vary in complexity.
1. In its simplest form, a photoreceptor indicates only the presence of light and its intensity.
2. The "eyespots" of planaria also allow the flatworms to determine the direction of light.
3. Image-forming eyes are found among four invertebrate groups: cnidaria, annelids, mollusks, and arthropods.
4. Arthropods have compound eyes composed of many independent visual units (ommatidia), each possessing all the elements needed for light reception. (Fig. 47.4)
a. The cornea and crystalline cone of each visual unit focus an image on the light sensitive membranes of a small number of photoreceptors within the unit.
b. The photoreceptors generate nerve impulses, which pass to the brain by way of optic nerve fibers.
c. Image resulting from all stimulated visual units is crude because the small size of compound eyes limits the number of visual units.
d. The advantage of a compound eye is its excellent motion detection.
e. Insects have color vision but utilize a shorter range of electromagnetic spectrum than do humans. (Fig. 47.5)
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 appears adaptive for activity during the day.
6. Vertebrates and certain mollusks (e.g., the squid and the octopus) have a camera type of eye.
a. Since mollusks and vertebrates are not closely related this is an example of convergent evolution.
b. A single lens focuses an image of the visual field on the photoreceptors, which are closely packed together.
c. In vertebrates the lens changes shape to aid in focusing, but in mollusks the lens move back and forth.
d. The human eye is considerably more complex than a camera, however.
C. Seeing With the Eye (Table 47.1)
1. The human eye is an elongated sphere about 2.5 cm in diameter with three layers. (Fig. 47.6) [transp. 263]
2. The sclera is the outer, white fibrous layer that covers most of the eye; it protects and supports the eyeball.
3. The cornea is a transparent part of the sclera at the front of the eye that is the window of the eye.
4. The middle, thin, dark-brown layer is the choroid; it contains many blood vessels and pigments that absorb stray light rays.
5. Toward the front of the eye, the choroid thickens and forms the ring-shaped ciliary body and finally becomes the iris that regulates the size of the opening called the pupil.
6. The lens divides the cavity of the eye into two portions: the aqueous humor fills the anterior cavity and the vitreous humor fills the posterior of the eye.
D. How the Retina Is Structured
1. Inner layer of an eye, the retina contains photoreceptors for sight: rod cells and cone cells. (Fig. 47.7) [transp. 264]
2. Nerve impulses initiated by rods and cones are passed to bipolar cells, which pass them to ganglionic cells.
3. Fibers of bipolar cells pass in front of the retina which forms the optic nerve, which conducts impulses to the brain.
4. There are more rods and cones than nerve fibers leaving ganglionic cells; this indicates that there is a considerable amount of mixing of messages and certain amount of integration before nerve impulses leave the eye to go to the brain.
5. The blind spot is an area where the optic nerve passes through the retina; it lacks rods and cones.
6. The fovea centralis is a small area of retina that contain only cones; this area produces acute color vision in daylight.
7. Cone cells are barely sensitive at low intensity at night; at this time, the rods are still active.
8. Blindness has many causes; glaucoma occurs late in life as pressure of aqueous humor on arteries kills nerve fibers.
E. Focusing Uses the Lens
1. Light rays entering eye are refracted as they pass through cornea, lens, and humors and are brought to focus on the retina.
2. A lens is relatively flat when viewing distant objects but becomes rounder for viewing near objects because light rays must bend to a greater degree; these changes are called accommodation. (Fig. 47.8) [transp. 265]
3. Because of refraction, the image on the retina is rotated 180º from the actual, but seems to be righted in the brain.
F. Helping the Eye
1. With aging, a lens loses ability to accommodate for near objects; many people need reading glasses by middle age.
2. The lens is also subject to cataracts, or becoming opaque; surgery is the only current treatment.
a. A surgeon opens the eye near the rim of the cornea.
b. The enzyme zonulysin digests away the ligaments holding the lens in place.
c. A cryoprobe freezes the lens for easy removal.
d. An intraocular lens attached to the iris is implanted to avoid need for thick glasses or contact lenses.
3. 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 the 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 the cornea.
4. Persons who can see far away but not up close are farsighted.
a. They often have a shortened eyeball that focuses near images behind the retina.
b. They can wear corrective convex lenses to refocus the image on the retina.
5. When the cornea is uneven, the image is fuzzy; this is astigmatism and it is corrected by an unevenly ground lens to compensate for the unevenness.
G. Seeing Uses Chemistry
1. Rods are stimulated by low light levels and provide night vision; rods also detect motion but not color or detail.
2. The outer segment of rods contains stacks of membranous disks (lamellae) with many molecules of rhodopsin.
3. Rhodopsin is a molecule containing a protein opsin and pigment molecule retinal derived from Vitamin A.
4. When light strikes retinal, it changes shape and rhodopsin is activated.
5. Activation of rhodopsin leads to reduction of cyclic guanosine monophosphate (cGMP), which triggers changes in membrane permeability leading to a change in frequency of impulses in bipolar and ganglionic cell layers that send messages to the brain. (Fig. 47.9)
6. Rod cells are sensitive to dim light; one molecule of rhodopsin acts on many cGMP molecules, amplifying original stimulus. [micro. slide 97]
7. Cones located primarily in the fovea are activated by bright light and detect detail and color.
a. There are three kinds of cones, containing blue, green, or red pigment.
b. Each pigment is also composed of retinal and opsin, but the structure of opsin varies among the three.
c. Combinations of cones are stimulated by intermediate colors; combined nerve impulses are interpreted in brain.
47.3. Sensing Mechanical Stimuli (p. 848)
A. Mechanoreceptors
1. Mechanoreceptors are sensitive to mechanical stimuli (e.g., changes in pressure, sound waves, and gravity).
2. Human skin contains various mechanoreceptors (e.g., touch receptors and pressure receptors).
a. Pacinian corpuscles are pressure receptors shaped like an onion with concentric layers of connective tissue wrapped around the end (dendrite) of a sensory neuron.
b. Pain receptors are unmyelinated (naked) ends (dendrites of) sensory nerve fibers; some are sensitive to mechanical stimuli and others to temperature or chemicals.
3. Vertebrate mechanoreceptors are composed of hair cells such as those found in lateral line of fishes. (Fig. 47.10)
a. This forms the lateral line system of fish and amphibians that detects water currents and pressure waves.
b. Primitive fishes have the system on the surface; advanced fishes have it enclosed in a canal on the side.
c. Lateral line receptor is a collection of hair cells with cilia embedded in a mass of gelatinous material (cupula).
d. The otic vesicles of fishes are derived from a portion of the lateral line system.
B. Structure of the Human Ear
1. The human ear has three main parts: an outer, middle, and inner ear. (Fig. 47.11)
2. Evolution of the inner ear of humans can be traced back to the lateral line system of fishes.
3. The inner ear of humans contains both equilibrium and sound receptors.
4. The outer ear consists of the pinna (external flap) and the auditory canal. (Fig. 47.11) [transp. 266]
a. The auditory canal is lined by fine hairs, which filter air.
b. Modified sweat glands in the auditory canal secrete earwax, which helps guard the ear against foreign matter.
5. The middle ear begins at the tympanic membrane and ends at a bony wall that has small membrane-covered openings (the oval window and the round window).
a. It contains the ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup).
b. The auditory (eustachian) tube extends from middle ear to pharynx and permits air to equalize between inside and outside.
c. The middle ear is continuous with mastoid air spaces in temporal bone of skull; a middle ear infection is serious.
6. The inner ear has three spaces that form a maze, the bony labyrinth.
a. The spaces are the semicircular canals, the vestibule, and the cochlea. (Fig 47.11) [transp. 266]
b. The membranous ducts and sacs are filled with a clear fluid endolymph.
c. Semicircular canals are three fluid-filled canals oriented at right angles to one another in three different planes.
d. The enlarged base of each semicircular canal is called an ampulla. (Fig. 47.12) [transp. 267]
e. A vestibule or space between the semicircular canals and the cochlea contains the utricle and the saccule.
1) The utricle and saccule are small membranous sacs, each of which contains hair cells whose stereocilia are embedded within a gelatinous material called the otolithic membrane.
2) Calcium carbonate granules (otoliths) rest on this membrane.
f. The cochlea resembles the shell of a snail because it spirals. (Fig. 47.12) [transp. 267]
1) Three canals located within the cochlea are vestibular canal, the cochlear canal, and the tympanic canal.
2) Along the basilar membrane are hair cells whose stereocilia are embedded in tectorial membrane.
3) Hair cells known as the spiral organ (organ of Corti) synapse with the nerve fibers of the cochlear (auditory) nerve to the brain, where they are interpreted as sound.
C. Hair Cells Keep Us Balanced
1. Sense of balance is divided into dynamic equilibrium (angular or rotational movement of the head) and static equilibrium (vertical or horizontal movement).
2. With three semicircular canals, each ampulla responds to head rotation in a different plane of space.
3. 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. (Fig. 47.12a)
4. Continuous movement of fluid in semicircular canals causes one form of motion sickness.
5. The utricle is especially sensitive to horizontal movements; the saccule responds best to up-down movements.
6. When a body is still, otoliths in the utricle and saccule rest on otolithic membrane above hair cells. (Fig. 47.12b)
7. When the head bends or the body moves, the otoliths are displaced and the otolithic membrane sags, bending the stereocilia of the hair cells beneath; this tells the brain the direction of movement.
8. Similar organs called statocysts are found in cnidaria, mollusks, and crustacea; these organs are static equilibrium organs.
D. Hair Cells Account for Hearing
1. Process of hearing begins when sound waves enter the auditory canal, causing ossicles to vibrate.
2. Sound is amplified because of size difference between tympanic membrane and oval window.
3. The stapes strikes the membrane of the oval window, passing the pressure waves to the fluid in the cochlea.
4. The vestibular canal connects with the tympanic canal, which leads to the round window membrane.
5. When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal and across the basilar membrane, and the round window bulges.
6. As basilar membrane vibrates up and down, stereocilia of hair cells embedded in tectorial membrane bend.
7. Nerve impulses in cochlear nerve travel to brain stem; in temporal lobe of cerebrum, they are interpreted as sound.
8. The spiral organ is narrow at its base and widens as it approaches the tip of the canal; each part is sensitive to different wave frequencies.
9. The nerve fibers from each region, such as the high pitch base or low pitch tip, lead to slightly different regions of the brain producing the sensation of pitch.
10. Sound volume causes more vibration; the increased stimulation is interpreted as louder sound intensity.