Chemical Senses

Receptors sensitive to specific chemicals are chemore-ceptors. Some of these respond to chemical changes in the internal environment, two examples being the oxygen and hydrogen-ion receptors in certain large blood vessels (Chapter 15). Others respond to external chemical changes, and in this category are the receptors for taste and smell, which affect a person's appetite, saliva flow, gastric secretions, and avoidance of harmful substances.

Taste

The specialized sense organs for taste are the 10,000 or so taste buds that are found primarily on the tongue. The receptor cells are arranged in the taste buds like the segments of an orange (Figure 9-43). A long narrow process on the upper surface of each receptor cell extends into a small pore at the surface of the taste bud, where the process is bathed by the fluids of the mouth.

Many chemicals can generate the sensation of taste, but taste sensations (modalities) are traditionally divided into four basic groups: sweet, sour, salty, and bitter, each group having a distinct transductional system. For example, salt taste begins with sodium entry into the cell through plasma-membrane ion channels, which depolarizes the plasma membrane; depolarization causes neurotransmitter release from the receptor cell at synapses with afferent nerve fibers. In contrast, molecules such as carbohydrates interact with plasma-

membrane receptors that regulate second-messenger cascades. It is likely that different mechanisms occur in separate cell types.

Although more than one afferent fiber synapses with each receptor cell, the taste system is organized into independent coded pathways into the central nervous system. Single receptor cells, however, respond in varying degrees to substances that fall into more than one taste category and awareness of the specific taste of a substance depends also upon the pattern of firing in a group of neurons. For example, sensations of pain ("hot" spices), texture, and temperature contribute to taste.

The pathways for taste in the central nervous system project to the parietal cortex, near the "mouth" region of the somatosensory cortex (see Figure 9-6).

Smell

Eighty percent of the flavor of food is actually contributed by the sense of smell, or olfaction, as is attested by the common experience that food lacks taste when one has a head cold. The odor of a substance is directly related to its chemical structure. Since we are able to recognize and identify hundreds of different odors with a great deal of accuracy, neural circuits that deal with olfaction must encode information about different chemical structures, store (learn) the different code patterns that represent the different structures, and at a later time recognize a particular neural code to identify the odor.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

Olfactory mucosa

Olfactory cells

Nose

Upper lip

Olfactory bulb

Olfactory bulb

Olfactory mucosa

Olfactory cells

Nose

Upper lip

Olfactory Cell

Olfactory receptor cell

Supporting cell

Inner chamber of nose

Hard palate

FIGURE 9-44

(a) Location and (b) enlargement of a portion of the mucosa showing the structure of the olfactory receptor cells. In addition to these cells, the olfactory epithelium contains stem cells, which give rise to new receptors, and supporting cells. %

Inner chamber of nose

Hard palate

Olfactory receptor cell

Supporting cell

FIGURE 9-44

(a) Location and (b) enlargement of a portion of the mucosa showing the structure of the olfactory receptor cells. In addition to these cells, the olfactory epithelium contains stem cells, which give rise to new receptors, and supporting cells. %

The olfactory receptor cells, the first cells in the pathways that give rise to the sense of smell, lie in a small patch of membrane, the olfactory epithelium, in the upper part of the nasal cavity (Figure 9-44a). These cells are specialized afferent neurons that have a single enlarged dendrite that extends to the surface of the epithelium. Several long nonmotile cilia extend out from the tip of the dendrite and lie along the surface of the olfactory epithelium (Figure 9-44b) where they are bathed in mucus. The cilia contain the receptor proteins (binding sites) for olfactory stimuli. The axons of the neurons form the olfactory nerve, which is cranial nerve I.

For an odorous substance (that is, an odorant) to be detected, molecules of the substance must first diffuse into the air and pass into the nose to the region of the olfactory epithelium. Once there, they dissolve in the mucus that covers the epithelium and then bind to specific odorant receptors on the cilia. Proteins in the mucus may interact with the odorant molecules, transport them to the receptors, and facilitate their binding to the receptors.

Although there are many thousands of olfactory receptor cells, each contains one, or at most a few, of the 1000 or so different plasma-membrane odorant receptor types, each of which responds only to a specific chemically related group of odorant molecules. Each odorant has characteristic chemical groups that distinguish it from other odorants, and each of these groups activates a different plasma-membrane odorant receptor type. Thus, the identity of a particular odorant is determined by the activation of a precise combination of plasma-membrane receptors, each of which is contained in a distinct group of olfactory receptor cells.

The axons of the olfactory receptor cells synapse in the brain structures known as olfactory bulbs, which lie on the undersurface of the frontal lobes. Axons from olfactory receptor cells sharing a common receptor specificity synapse together on certain olfactory-bulb neurons, thereby maintaining the specificity of the original stimulus. In other words, specific odorant receptor cells activate only certain olfactory-bulb neurons, thereby allowing the brain to determine which receptors have been stimulated. The codes used to transmit olfactory information probably use both spatial (which neurons are firing?) and temporal (what is the timing of the action-potential responses?) components.

Information is passed from the olfactory bulbs to olfactory cortex, which is in the limbic system (see Figure 8-42), a part of the brain intimately associated with emotional, food-getting, and sexual behavior. There, different odors elicit different patterns of electrical activity in a variety of cortical areas, allowing humans to discriminate between some 10,000 different odorants even though they have only 1000 different olfactory receptor types.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Olfactory discrimination varies with attentiveness: hunger—sensitivity is greater in hungry subjects; gender—women in general have keener olfactory sensitivities than men; smoking—decreased sensitivity has been repeatedly associated with smoking; age—the ability to identify odors decreases with age, and a large percentage of elderly persons cannot detect odors at all; and state of the olfactory mucosa—as we have mentioned, the sense of smell decreases when the mucosa is congested, as in a head cold.

SECTION B SUMMARY

Somatic Sensation

I. Sensory function of the skin and underlying tissues is served by a variety of receptors sensitive to one (or a few) stimulus types.

II. Information about somatic sensation enters both specific and nonspecific ascending pathways. The specific pathways cross to the opposite side of the brain.

III. The somatic sensations include touch-pressure, the senses of posture and movement, temperature, and pain.

a. Rapidly adapting mechanoreceptors of the skin give rise to sensations such as vibration, touch, and movement, whereas slowly adapting ones give rise to the sensation of pressure.

b. Skin receptors having small receptive fields are involved in fine spatial discrimination, whereas receptors having larger receptive fields signal less spatially precise touch-pressure sensations.

c. A major receptor type responsible for the senses of posture and kinesthesia is the muscle-spindle stretch receptor.

d. Cold receptors are sensitive to decreasing temperature; warmth receptors signal information about increasing temperature.

e. Tissue damage and immune cells release chemical agents that stimulate specific receptors that give rise to the sensation of pain.

f. Stimulation-produced analgesia, transcutaneous nerve stimulation (TENS), and acupuncture control pain by blocking transmission in the pain pathways.

Vision

I. Light is defined by its wavelength or frequency.

II. The light that falls on the retina is focused by the cornea and lens.

a. Lens shape is changed to permit viewing near or distant objects (accommodation) so that they are focused on the retina.

b. Stiffening of the lens with aging interferes with accommodation. Cataracts decrease the amount of light transmitted through the lens.

c. An eyeball too long or too short relative to the focusing power of the lens causes nearsighted or farsighted vision, respectively.

III. The photopigments of the rods and cones are made up of a protein component (opsin) and a chromophore (retinal).

a. The rods and each of the three cone types have different opsins, which make each of the four receptor types sensitive to different wavelengths of light.

b. When light falls upon the chromophore, the photic energy causes the chromophore to change shape, which triggers a cascade of events leading to hyperpolarization of the photoreceptors and decreased neurotransmitter release from them. When exposed to darkness, the rods and cones are depolarized and therefore release more neurotransmitter.

IV. The rods and cones synapse on bipolar cells, which synapse on ganglion cells.

a. Ganglion-cell axons form the optic nerves, which lead into the brain.

b. The optic-nerve fibers from half of each retina cross to the opposite side of the brain in the optic chiasm. The fibers from the optic nerves terminate in the lateral geniculate nuclei of the thalamus, which send fibers to the visual cortex.

c. Visual information is also relayed to areas of the brain dealing with biological rhythms.

V. Coding in the visual system occurs along parallel pathways, in which different aspects of visual information, such as color, form, movement, and depth, are kept separate from each other.

VI. The colors we perceive are related to the wavelength of light. Different wavelengths excite one of the three cone photopigments most strongly.

a. Certain ganglion cells are excited by input from one type of cone cell and inhibited by input from a different cone type.

b. Our sensation of color depends on the output of the various opponent-color cells and the processing of this output by brain areas involved in color vision.

VII. Six skeletal muscles control eye movement to scan the visual field for objects of interest, keep the fixation point focused on the fovea centralis despite movements of the object or the head, prevent adaptation of the photoreceptors, and move the eyes during accommodation.

Hearing

I. Sound energy is transmitted by movements of pressure waves.

a. Sound wave frequency determines pitch.

b. Sound wave amplitude determines loudness.

II. The sequence of sound transmission is as follows:

a. Sound waves enter the external auditory canal and press against the tympanic membrane, causing it to vibrate.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

b. The vibrating membrane causes movement of the three small middle-ear bones; the stapes vibrates against the oval-window membrane.

c. Movements of the oval-window membrane set up pressure waves in the fluid-filled scala vestibuli, which cause vibrations in the cochlear duct wall, setting up pressure waves in the fluid there.

d. These pressure waves cause vibrations in the basilar membrane, which is located on one side of the cochlear duct.

e. As this membrane vibrates, the hair cells of the organ of Corti move in relation to the tectorial membrane.

f. Movement of the hair cells' stereocilia stimulates the hair cells to release neurotransmitter, which activates receptors on the peripheral ends of the afferent nerve fibers.

III. Each part of the basilar membrane vibrates maximally in response to one particular sound frequency.

Vestibular System

I. A vestibular apparatus lies in the temporal bone on each side of the head and consists of three semicircular ducts, a utricle, and a saccule.

II. The semicircular ducts detect angular acceleration during rotation of the head, which causes bending of the stereocilia on their hair cells.

III. Otoliths in the gelatinous substance of the utricle and saccule move in response to changes in linear acceleration and the position of the head relative to gravity, and stimulate the stereocilia on the hair cells.

Chemical Senses

I. The receptors for taste lie in taste buds throughout the mouth, principally on the tongue. Different types of taste receptors operate by different mechanisms.

II. Olfactory receptors, which are part of the afferent olfactory neurons, lie in the upper nasal cavity.

a. Odorant molecules, once dissolved in the mucus that bathes the olfactory receptors, bind to specific receptors (protein binding sites). Each olfactory receptor cell has one of the 1000 different receptor types.

b. Olfactory pathways go to the limbic system.

SECTION B KEY TERMS

somatic sensation

zonular fiber

kinesthesia

iris

nociceptor

pupil

wavelength

rod

frequency

cone

visible spectrum

photopigment

retina

rhodopsin

lens

opsin

cornea

chromophore

fovea centralis

retinal

accommodation

bipolar cell

ciliary muscle

ganglion cell

suprachiasmatic nucleus

organ of Corti

opponent color cell

hair cell

saccade

stereocilia

external auditory canal

tectorial membrane

tympanic membrane

vestibular apparatus

middle ear cavity

semicircular duct

auditory tube

utricle

inner ear

saccule

cochlea

labyrinth

malleus

semicircular canal

incus

cupula

stapes

ampulla

oval window

chemoreceptor

cochlear duct

taste bud

scala vestibuli

olfaction

scala tympani

olfactory epithelium

basilar membrane

odorant

SECTION B REVIEW QUESTIONS

Describe the similarities between pain and the other somatic sensations. Describe the differences. List the structures through which light must pass before it reaches the photopigment in the rods and cones.

Describe the events that take place during accommodation for far vision.

What changes take place in neurotransmitter release from the rods or cones when they are exposed to light?

Beginning with the ganglion cells of the retina, describe the visual pathway.

List the sequence of events that occur between entry of a sound wave into the external auditory canal and the firing of action potentials in the cochlear nerve.

Describe the anatomical relationship between the cochlea and the cochlear duct.

What is the relationship between head movement and cupula movement in a semicircular canal?

What causes the release of neurotransmitter from the utricle and saccule receptor cells?

In what ways are the sensory systems for taste and olfaction similar? In what ways are they different?

CHAPTER 9 CLINICAL TERMS

phantom limb hyperalgesia referred pain analgesia stimulation-produced analgesia transcutaneous electric nerve stimulation (TENS) acupuncture presbyopia cataract nearsighted myopic farsighted hyperopic astigmatism glaucoma color blindness hearing aid cochlear implant nystagmus vertigo motion sickness Meniere's disease

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

CHAPTER 9 THOUGHT QUESTIONS

(Answers are given in Appendix A.)

1. Describe several mechanisms by which pain could theoretically be controlled medically or surgically.

2. At what two sites would central nervous system injuries interfere with the perception of heat applied to the right side of the body? At what single site would a central nervous system injury interfere with the perception of heat applied to either side of the body?

3. What would vision be like after a drug has destroyed all the cones in the retina?

4. Damage to what parts of the cerebral cortex could explain the following behaviors? (a) A person walks into a chair placed in her path. (b) The person does not walk into the chair but does not know what the chair can be used for.

Vander et al.: Human I II. Biological Control I 10. Principles of Hormonal I I © The McGraw-Hill

Physiology: The Systems Control Systems Companies, 2001

Mechanism of Body

Vander et al.: Human I II. Biological Control I 10. Principles of Hormonal I I © The McGraw-Hill

Physiology: The Systems Control Systems Companies, 2001

Mechanism of Body

Hormone Visual Mcgraw Hill
Acupuncture For Cynics

Acupuncture For Cynics

Have You Always Been Curious About Acupuncture, But Were Never Quite Sure Where To Stick The Needles? If you associate acupuncture with needles, pain and weird alternative medicine then you are horribly misinformed about the benefits of the world's oldest form of medicinal treatment.

Get My Free Ebook


Responses

  • marcel
    Where are olfactory cells located?
    7 years ago
  • Kathrin
    How is structure related to function taste buds and taste olfactory cells and smell rods and cones?
    7 years ago
  • tobias
    What is the function of the olfactory receptor cells in the nose?
    7 years ago
  • asfaha petros
    What sense depends on stimulation of olfactory receptor cells and cilia?
    6 years ago

Post a comment