Human Physiology From Cells to Systems 8th Edition
Lauralee Sherwood 9781111577438 1111577439
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The Peripheral Nervous System: Afferent Division; Special Senses
CHAPTER OUTLINE
RECEPTOR PHYSIOLOGY
The peripheral nervous system (PNS) consists of nerve fibers that carry information between the CNS and other parts of the body. A stimulus is a change detectable by the body. Modalities of input refer to the different forms of energy such as heat, light, sounds, pressure, and chemical changes. When stimulated, receptors produce graded potentials called receptor potentials in the receptor; this conversion of stimulus energy into receptor potentials is called sensory transduction. Receptor potentials trigger action potentials in afferent fibers.
Receptors have differential sensitivities to various stimuli. Each receptor will respond to one type of stimulus and can be categorized according to this responsiveness.
Photoreceptors respond to visible wavelengths of light.
Mechanoreceptors are sensitive to mechanical energy such as stretch and pressure.
Thermoreceptors are sensitive to heat and/or cold.
Osmoreceptors detect changes in solute concentration in the ECF.
Chemoreceptors are sensitive to specific chemicals.
Nociceptors (pain receptors) are sensitive to tissue damage such as cutting, tearing, or burning.
The CNS can process inputs from several different types of simultaneously activated primary receptors to produce an integrated sensation such as wetness (touch, pressure, and thermal receptor inputs).
Stimuli of the same intensity do not always produce the same magnitude of receptor potentials. Some receptors diminish the extent of their depolarization despite sustained stimulus strength. This is known as receptor adaptation.
Receptors may adapt slowly or rapidly. Tonic receptors do not adapt or adapt very slowly. Phasic receptors rapidly adapt and may completely stop responding (off response).
Each somatosensory pathway is “labeled” according to modality and location. Pathways conveying conscious somatic sensations are the somatosensory pathways. These pathways consist of discrete chains of neurons (labeled lines). The neuron with the peripheral receptor is known as the first-order sensory neuron. This will synapse on the second-order sensory neuron in either the spinal cord or medulla. This cell will synapse in the thalamus on the third-order sensory neuron, and so on until it reaches the final destination.
Peripheral Nervous System: Afferent Division; Special Senses
Acuity is influenced by receptive field size and lateral inhibition. Each somesthetic sensory neuron responds to stimuli only in specific regions of skin surface surrounding the receptor, the receptive field. Smaller receptive fields with higher receptor density lead to greater acuity or discriminative ability. Acuity is also influenced by lateral inhibition, where information from the receptive fields surrounding the area of most intense stimulation is inhibited within the CNS. This facilitates the ability to localize from where the stimulus is emanating.
PAIN
Pain is considered a protective mechanism meant to bring about awareness of tissue damage that is occurring or about to occur. Pain receptors (nociceptors) do not adapt to sustained stimulus strength. Experience with pain helps us avoid potentially harmful events in the future. Stimulation of nociceptors elicits the perception of pain plus motivational and emotional responses. Pain perception can be modified by past or present experiences. Fear of re-experiencing painful stimuli may heighten the perceived pain, while other situations may reduce the perception of pain such as during a competitive event.
Nociceptors are categorized by the energy to which they react. Mechanical nociceptors respond to cutting, crushing, or pinching. Thermal nociceptors respond to temperature extremes, especially heat. Polymodal nociceptors respond equally to damaging inputs including irritating chemicals released from injured tissue.
Pain signals are transmitted from the receptors to the CNS via either fast pain pathways ( A- fibers; 30 m/sec.) or slow pain pathways (C fibers; 12 m/sec or slower). Afferent pain fibers utilize two neurotransmitters. Substance P is unique to pain fibers and activates ascending pathways that transmit nociceptive signals to higher levels for further processing. Glutamate, a major excitatory neurotransmitter, is released from primary afferent pain terminals.
The brain has a built-in analgesic system. This system suppresses transmission in the pain pathways as they enter the spinal cord. Three brainstem regions are part of this system. The periaqueductal gray matter stimulates inhibitory neurons to release enkephalin (an endogenous opiate peptide). There are specific analgesia-inducing nuclei in the medulla and the reticular formation.
EYE: VISION
The eyes capture patterns of illumination onto the sensory retina. This coded information is transmitted through several steps of processing before final perception of the image by the CNS. Protective mechanisms help prevent eye injuries. Specific protective structures include the eyelids, tear-producing lacrimal glands, and eyelashes. The eye is a fluid-filled sphere enclosed by three specialized tissue layers. The amount of light entering the eye is controlled by the iris.
The eye refracts the entering light to focus the image on the retina. The cornea and flexible lens provide the refractive mechanism. The shape of the lens is altered by the ciliary muscle. This change in shape alters the refractive qualities of the lens (accommodation) and allows one to focus on objects at varying distances from the eye. The greater the curvature is of the lens, the greater the strength of the lens to “bend” light rays and focus on closer objects. Light must pass through several retinal layers before reaching the photoreceptors. The retina is an extension of the CNS, not a separate peripheral organ. The photoreceptors, rods and cones, are located in the outermost layer of the neural retina. The light-sensitive ends of these photoreceptors face away from the incoming light (face the pigmented layer of the retina). The middle layer of the neural retina contains bipolar cells and interneurons. The inner layer is composed of ganglion cells whose axons join to form the optic nerve. Light must pass through the layers of ganglion and bipolar cells to reach the photoreceptors in all areas of the retina except the fovea, where the inner two layers are pulled aside to allow light to strike the photoreceptors (only cones are found in the fovea), directly providing the most distinct vision.
Phototransduction by retinal cells converts light stimuli into neural signals.
Rods provide indistinct gray vision in low-light situations, whereas cones provide sharp color vision during periods of high light intensity There are 120 million rods and 6 million cones per retina. Rods are more numerous in the periphery of the retina and are highly sensitive, but have low acuity. Cones are concentrated in or near the fovea. They have low sensitivity to light, but have high acuity. There are four different photopigments, one specific to the rods and one in each of the three different types of cones red, green, and blue cones. The photopigments are composed of retinal and four different opsins. As each opsin binds retinal differently, the four photopigments absorb different wavelengths of light. Each photopigment absorbs maximally at a particular wavelength (strong stimulation), but also absorbs a range of wavelengths, with those wavelengths farther from the peak wavelength producing weaker stimulation. Color vision depends on the ratios of stimulation of the three cone types.
As light is absorbed by the photoreceptor, retinal is changed from the 11-cis form to the all-trans form. This sets off a cascade of events leading the breakdown of the photopigment. This ultimately hyperpolarizes the photoreceptor, closes Ca++ channels, and reduces the amount of neurotransmitter released from the photoreceptor. This decrease in neurotransmitter release depolarizes on-center bipolar cells and subsequently the on-center ganglion cells leading to propagation of impulses along the optic nerve. However, off-center bipolar cells and the subsequent off-center ganglion cells are hyperpolarized. This bipolar and ganglion cell processing allows for a distinct visual image to be formed. The sensitivity of the eyes can vary markedly through dark and light adaptation. The sensitivity depends on the amount of light-responsive photopigments present in the rods and cones. Exposure to strong light depletes the photopigments, thus reduces the sensitivity. This is why when one comes into a dark room from a brightly lit area, there is a temporary blindness. As the photopigments are regenerated, vision in the low-light areas increases. This is called dark adaptation. Visual information is modified and separated before reaching the visual cortex. The optic nerves carry information from lateral and medial areas of the retinas they serve. As the two optic nerves meet at the optic chiasm, fibers from the lateral regions project to the ipsilateral side of the brain, while fibers from the medial regions project to the contralateral side of the brain for processing The thalamus and visual cortices elaborate the visual message. Visual input goes to other areas of the brain not involved in vision perception. Example of non-sight related activities include: (1) contribution to cortical alertness and attention e.g., getting drowsy in a dimly lit room; (2) control of pupil size (constriction in bright light, dilation in dim light); and (3) control of eye movements. Furthermore, some ganglion cells make melanopsin, a light-sensitive pigment involved in helping set the body’s biological clock. Some sensory input may be detected by multiple sensory-processing areas in the brain.
EAR: HEARING AND EQUILIBRIUM
Sound waves (traveling vibrations of air) consist of alternate regions of compression (area of high pressure) and rarefaction (area of low pressure) of air molecules.
The external ear plays a role in sound localization. The external ear consists of the pinna (ear), the external auditory meatus (ear canal), and the tympanic membrane (eardrum). The pinna collects sound waves and directs them to the ear canal where the sound waves are channeled to the eardrum. Humans cannot move their pinna, but because the pinna partially shields sound arising from behind, a person can distinguish sounds coming from directly in front or behind. Sound localization for sounds coming from left or right depends on the time delay of the sound reaching the ear farther from the sound compared with the ear closer to the sound.
The tympanic membrane vibrates in unison with sound waves in the external ear. The middle-ear bones act as a bridge between the eardrum and the cochlea. The malleus is attached to the tympanic membrane. As the eardrum vibrates it causes the malleus to vibrate. The vibrating malleus is attached to the incus, which functions to amplify the movements of the malleus. The incus transmits the amplified vibrations to the stapes, which conveys the vibrations to the oval window at the entrance to the fluid-filled cochlea. The cochlea contains the organ of Corti, the sense organ for hearing. As the oval window vibrates the sound energy is transferred to the fluid. Some of the movement of this fluid is to compensate for pressure changes in the cochlea, while some of the energy is transferred to the hair cells in the organ of Corti for transduction to neural signals.
Hair cells in the organ of Corti transduce fluid movements into neural signals.
Pitch discrimination depends on the region of the basilar membrane that vibrates.
Loudness discrimination depends on the amplitude of basilar-membrane vibration.
The auditory cortex is mapped according to tone.
Deafness is caused by defects either in conduction or neural processing of sound waves.
The inner ear has a non-hearing component called the vestibular apparatus, important for the sense of equilibrium and for coordinating head movements with eye and postural movements. The semicircular canals detect rotational or angular acceleration and deceleration of the head. The otolith organs provide information concerning the position of the head relative to gravity.
CHEMICAL SENSES: TASTE AND SMELL
About 10,000 taste receptors (chemoreceptors) are located in the taste buds present in the oral cavity and throat, although most are on the upper surface of the tongue.
Discrimination of the thousands of taste sensations is coded by patterns of activity in various taste bud receptors. The primary tastes are salty, sour, sweet, bitter, and umami (savory). It is proposed that a separate sensor for fat may be present as well, as scientists have located sensors in the mouth that respond to long-chain fatty acids. Taste
discrimination depends on perceiving the subtle differences in the stimulation patterns of the various taste receptors. Taste is also influenced by other, non-taste receptors, especially odor receptors; temperature and texture of food can affect the perception of taste as can the emotional state of the individual.
The olfactory receptors in the nose are specialized endings of renewable afferent neurons. Various parts of an odor are detected by different olfactory receptors and sorted into “smell files.” Humans have five million olfactory receptors, of which there are 1000 different types. The odors are dissected into different components based on the ratio of response of the various types of olfactory receptors. Afferent fibers arising from the olfactory receptor endings synapse in the olfactory bulb for processing. Odor discrimination is coded by patterns of activity in the olfactory bulb glomeruli.
The olfactory system adapts quickly and odorants are rapidly cleared.
The vomeronasal organ detects pheromones.
LIST OF KEY TERMS
Peripheral receptors
- chemoreceptors
- osmoreceptors
- mechanoreceptors
- nociceptors
- photoreception
- thermoreceptors
Processing stimuli
- receptive field
- receptor potential
- acuity
- lateral inhibition - generator potential
- intensity discrimination
- receptor adaptation
- phasic receptor
- tonic receptor
Touch/tactile
- hair receptor
- Pacinian corpuscle
- Meissner’s corpuscle
- Merkel’s disk
- Ruffini endings
- visceral afferent
- perception
Pain
- A delta fibers
- fast pain
- C fibers
- slow pain
- analgesic system
- periaqueductal grey
- substance P
- morphine
- endogenous opiate peptide
- phantom pain
Vision
- accommodation
- cornea
- cataract
- lens
- sclera
- choroid
- vitreous humor
- glaucoma
- retina - blind spot/optic disc
- fovea
- cones
- rods
- phototransduction
- rhodopsin
- myopia
Hearing and equilibrium
- basilar membrane
- cochlea
- cochlear implants
- kinocilium
- motion sickness
- organ of Corti
LECTURE HINTS AND SUGGESTIONS
- ossicles
- incus
- malleus
- stapes
- endolymph
- otoliths
- oval window
- scala media
- scala vestibule
- saccule
- semicircular canal
- stereocilia
- tone
- tympanic membrane
- utricle
Taste and smell
- gustation
- tastant
- umami
- taste bud
- taste pore
- olfactory receptor
- olfactory bulb
- pheromones
- basal cell
- glomeruli
- mitral cell
- odorant
1. Obtain preserved specimens of bovine or sheep eyes and dissect them for the demonstration of their important structures.
2. The shark’s vestibular apparatus and the middle ear bones of mammals are excellent demonstration materials.
3. Students can localize receptors in their skin for touch, pressure, pain, and temperature. See any general physiology lab manual for techniques and supplies. For example, calipers can be used for two-point discrimination tests on several regions of the skin.
4. Use models of the eye and ear to illustrate their structures. During your presentation, describe the functions of these structures as you locate them. Challenge the students to do the same with the models during their lab class. The models can be obtained from Carolina Biological Supply Company, Burlington, NC.
5. To demonstrate the interaction of the senses of taste and smell, students can detect the difference between the tastes of two foods (e.g., an apple and an onion) while pinching shut their nostrils.
6. Demonstrate the test for color blindness. See any general physiology lab manual for techniques and supplies. Booklets with various charts are available to diagnose the different kinds of color blindness.
7. Students can examine their own eyes in a mirror and with an ophthalmoscope. Use any general physiology lab manual for techniques and supplies.
8. To demonstrate sensory adaptation, aerosol some perfume into the air and measure the time required for students to lose the ability to detect its presence.
9. Use tuning forks to illustrate the relationship between the frequency of sound waves and their pitch when students hear the waves in the classroom. The tuning forks can be obtained from Wards Biology, Rochester, NY.
10. Demonstrate dark and light adaptation by turning the lights off and then on while discussing the physiology of these events.
11. Demonstrate the sense of equilibrium by spinning a student and asking them to walk, or by having a student walk while shaking their head in a downward position.
12. Be sure to remind students of the learning resources available on the textbook website and of the literaturesearching capability of InfoTrac®.
AUDIOVISUAL AIDS
Videos/Films
The following films may be suitable for presentation in your class.
http://cambridge.films.com
Human Senses, six-part series, 30 min each. This VHS, DVD, or streaming video includes dramatizations explaining the biological roots of smell, vision, taste, touch, hearing, and balance.
The Anatomy of Pain, 49 min.
An examination of the neurological aspects of pain and some methods of treating pain symptoms.
The Anatomy of Pleasure, 49 min.
An examination of the neurological aspects of pleasure. Examining a wide range of ways to follow ones bliss including sex, artistic creation, the consumption of favorite foods, and high-risk behaviors like gambling and rock-climbing the program explains the crucial role played by endorphins and other body chemicals in these activities.
Secret Senses, 54 min. A Discovery Channel production. This program examines the senses of animals such as gorillas, hippos, and bats.
http://ffh.films.com
The Senses of Smell and Taste, 28 min. This program describes the functions and interrelatedness of the senses of smell and taste.
Smell and Taste, 30 min. A Dartmouth-Hitchcock Medical Center production. A patient is followed through the Taste and Smell Clinic at the University of Connecticut.
Sound of Silence, 60 min.
This program looks at the extreme sensitivity of the rods in the human eye; examines the limits of sensitivity of the human ear; looks at some examples of extraordinary sensual sensitivity in animals; and analyzes how the human senses analyze and discriminate between signals over wide operating ranges and then transmit their messages along the rather low-quality cables constituted by the nerve fibers connecting them to the brain.
Sixth Sense... and the Rest, 26 min.
This program looks at some of the sensory experiences of animals that humans can glimpse only through physical instruments of detection and measurement: rattlesnakes use infrared detector organs to sense the position of their warm prey; many insects can sense the plane of polarization of light; bats use ultrasound echo-location to guide their flight; many fishes have electric field organs; and bacteria and birds have magnetic sense.
The Senses: Skin Deep, 26 min.
This VHS, DVD or streaming video explores the human cutaneous sensory receptors. Eyes and Ears, 26 min.
This VHS, DVD or streaming video examines the senses of vision and hearing.
The Senses, 20 min.
The program demonstrates how the senses of sight and balance operate as well as how they interact with each other.
http://www.shoppbs.org/
Mystery of the Senses, five-part series, 280 min. A NOVA program. Dramatizations describing hearing, smell, taste, touch, and vision.
Software
Anatomy Projects, PTH, three-disk set neuroanatomy CD, including the ANS.
Dynamics of the Human Ear, EI, presents vocabulary and diagrams of ear.
Dynamics of the Human Eye, EI, details the part of the eye, emphasizing retinal receptors.
Dynamics of the Human Senses of Touch, Taste, and Smell, EI, covers the structures relating to touch, taste, and smell.
Nervous System, CBS, covers the nature of the nervous system and chemical transmitters.
Neurology of Eye Movements, RAM, an interactive CD.
Senses, CBS, covers the eye, ear, nose, tongue, and skin.
Senses, VES, CD-ROM covering all senses.
Relevant Educational Websites
http://www.leffingwell.com/olfaction.htm
This site includes an overview of olfaction; includes hyperlinks to more in-depth discussions.
http://www.brainnet.org/
Free registration for neuroscience information.
http://www.cf.ac.uk/biosi/staff/jacob/sensmnu.html
Interactive tutorials dealing with olfaction and taste.
http://www.rybak-et-al.net/iod.html
Report: “Experimental Study and Computational Modeling of Orientation Selectivity in the Visual Cortex.”
http://www.hhmi.org/senses/
Report: “Seeing, Hearing, and Smelling the World”
http://www.sinetwork.org/
Homepage for the Sensory Processing Disorder Foundation. Hyperlinks to various activities provided by this organization.
http://www.visionscience.com/
A resource for research in human and animal vision science. Downloadable literature is available free.
http://thalamus.wustl.edu/course/basvis.html
Basic review of visual pathways and visual processing.
http://www.atlasophthalmology.com/
Approximately 3000 pictures from the Atlas of Ophthalmology.
http://www.innerbody.com/anim/ear.html
Animations and text for the physiology of the ear.
American Academy of Audiology
http://www.audiology.org
American Academy of Ophthalmology
http://www.aao.org
American Optometric Association
243 North Lindbergh Blvd., 1st floor St. Louis, MO 63141
http://www.aoanet.org/
American Academy of Otolaryngology
One Prince Street Alexandria, VA 22314-3357
http://www.entnet.org/
Association for Research in Otolaryngology
19 Mantua Rd
Mt. Royal, NJ 08061
http://aro.org
International Hearing Society
http://www.ihsinfo.org
International Society for Eye Research
P.O. Box 193940
San Francisco, CA 94119
http://www.iser.org/
National Eye Institute
2020 Vision Place Bethesda, MD 20892-3655
http://www.nei.nih.gov
National Institute on Deafness and Other Communication Disorders
National Institutes of Health
31 Center Drive, MSC 2320
Bethesda, MD USA 20892-2320
http://www.nidcd.nih.gov/
Nerve Research Foundation
http://www.nrf.med.usyd.edu.au/
Sensory Integration International PO Box 5339
Torrance, CA 90510-5339
http://www.sensoryint.com/
Vestibular Disorders Association PO Box 13305
Portland, OR 97213-0305
http://www.vestibular.org/
Vision Sciences Society
http://www.vision-sciences.org/
Answers to End of Chapter Essays
1. Photoreceptors are responsive to visible wavelengths of light. Mechanoreceptors are sensitive to mechanical energy including stretch. Thermoreceptors are sensitive to heat and cold. Osmoreceptors are sensitive to changes in the concentration of solutes in the ECF. Chemoreceptors are sensitive to specific chemicals and include taste and smell receptors. Nociceptors, or pain receptors, are sensitive to tissue damage. There are three categories of nociceptors: Mechanical nociceptors respond to cutting, crushing, or pinching; thermal nociceptors respond to temperature extremes, especially heat; and polymodal nociceptors respond equally to all damaging stimuli, including irritating chemicals released from injured tissues.
2. Tonic receptors adapt slowly, or not at all, to a sustained stimulus. These receptors provide continuous information about a stimulus. Phasic receptors adapt rapidly to a sustained stimulus and often exhibit an off response when the stimulus is removed. This type of receptor provides information about changes in the intensity of signaling rather than information about the status quo.
3. Neurons with small receptive fields have a higher acuity (sensitivity) than those with large receptive fields. This occurs because small receptive fields have receptors densely packed into a small area and can monitor smaller areas of skin. This allows fine discrimination between separate receptive fields. Lateral inhibition increases acuity by ensuring that the receptor at the site of the most intense stimulus is activated to the greatest extent. The most intensely activated receptor activates inhibitory interneurons that inhibit receptive field tracts that lie laterally, thereby enhancing its activity.
4. Fast pain occurs on stimulation of mechanical and thermal nociceptors; is carried by small, myelinated A delta fibers at rates of up to 30 m/sec.; and produces sharp pain sensations that are easily localized. Slow pain occurs on stimulation of polymodal nociceptors; is carried by small, unmyelinated C fibers at rates of up to 12 m/sec.; and produces dull, aching pain sensations that are not easily localized.
5. This system includes three brainstem regions: the periaqueductal gray matter, medulla, and reticular formation. This system, when activated, leads to the release of endogenous opiates from inhibitory interneurons in the dorsal horn of the spinal cord onto the substance-P-releasing neurons, resulting in the suppression of the release of substance P, thereby blocking afferent pain transmission.
6. Phototransduction is the process of converting light energy into bioelectrical signals. Most receptors depolarize when stimulated, however, photoreceptors hyperpolarize when stimulated with light. In the presence of light, 11-cis retinal (the light-absorbing component of the photopigment rhodopsin) changes to all-trans retinal form Retinal no longer fits into the protein opsin, thus activating the G protein transducin Activated transducin, in turn, activates the intracellular enzyme phosphodiesterase (PDE). PDE decreases the concentration of cGMP, which causes Na+ channels in the outer segment to close. This produces hyperpolarization of the photoreceptor, leading to a light intensity dependent (graded) decrease in the release of the neurotransmitter glutamate from the photoreceptor. Further processing occurs in the bipolar and ganglion cells. The decrease in glutamate release depolarizes on-center bipolar and ganglion cells, leading to action potentials in the optic nerve arising from these cells. In contrast, offcenter bipolar and ganglion cells are hyperpolarized when neurotransmitter release is decreased, so no action potentials occur in the off-center ganglion cells. This processing allows greater discrimination of relative brightness of areas within the visual field.
7. Rods 120 million/retina, most numerous in the periphery of retina, high sensitivity, low acuity, and function for low-light vision by detecting shades of grey. Cones 6 million/retina, concentrated in the fovea, low sensitivity, high acuity, and process color vision.
8. Sound waves are traveling vibrations of air that consist of regions of high pressure alternating with regions of low pressure. Pitch is determined by the frequency of vibrations. Intensity is determined by the amplitude of the sound wave. Timbre, or quality of sound, is determined by overtones, which are additional frequencies that are superimposed on the fundamental wave.
9. Pinna collects sound waves and passes them down the ear canal at the end of which is the tympanic membrane. The tympanic membrane, or ear drum, vibrates when sound waves strike it. The movement of the tympanic membrane produces movement of the ossicles, which are the middle ear bones: the malleus, the incus, and the stapes. The ossicles not only pass along the sound wave characteristics, but also amplify the auditory signal. The last of these bones, the stapes, is attached to the oval window, which is part of the cochlea of the inner ear. As the stapes moves it pushes on the oval window membrane, which in turn causes the fluid in the inner ear to move. As the fluid in the inner ear moves, it moves the basilar membrane that is located inside the cochlea. The transduction of sound waves to action potentials occurs as auditory hair cells (mechanoreceptors) on top of the basilar membrane move as the membrane moves. This movement of hair cells causes cilia on their surface to bend. This bending opens ionic channels in the cilia, causing depolarizing or hyperpolarizing graded potential currents, depending on the direction of the movement. If depolarization takes place, neurotransmitter is released from the hair cells, which in turn causes action potential production in the afferent cells synapsing on the hair cells.
10. Semicircular canals detect rotation or angular acceleration or deceleration. The utricle and saccule detect changes in linear motion and the position of the head relative to gravity due to interactions of the receptor hair cells and movements of the otolith-embedded gelatinous mass contained within these organs.
11. Taste receptor cells are found lining the taste pores associated with taste buds. These cells are modified epithelial cells containing microvilli to increase functional surface area. These cells are activated by the five primary tastes: salty, sour, sweet, bitter, and umami (meaty or savory taste). In general, the binding of chemicals associated with each taste (salt is Na+; sour is H+; sweet is glucose; bitter is over 100 different chemicals; umami is amino acids, especially glutamate) produces depolarizing potentials in each cell. There may also be receptors for fat in the mouth. Olfactory receptor neurons are located in the nasal epithelium lining the ceiling of the nasal cavity. These cells are true neurons and send axons into the brain to relay smell information. The dendrites of these cells contain cilia that lie on the surface of the nasal lining in a layer of mucus. Odorant molecules in the nasal passages dissolve into the mucus layer and interact with receptors on the cilia. This interaction produces EPSPs in the olfactory neurons, and action potentials are passed into the brain.
12. Color vision discrimination involves the ratios of stimulation of the three cone types (blue, green, and red cones); pitch discrimination depends on the region of the basilar membrane that vibrates maximally in response to different frequencies of incoming sound waves, with high frequency sounds vibrating
maximally at the narrow oval-window end of the membrane and low frequency sounds vibrating maximally at its wide helicotrema end; taste discrimination depends on subtle differences in the stimulation patterns of all taste buds in response to varying combinations of the five primary tastes (salty, sour, sweet, bitter, and umami); and odor discrimination is based on different patterns of glomeruli in the olfactory bulb, activated as a result of variable stimulation of 1000 different types of olfactory receptors in response to the various odor components of each scent.