Peripheral Nervous System: Afferent Division

• SENSORY PHYSIOLOGY

Chapter GoalsAfter studying this chapter, students should be able to . . .

1. explain how sensory receptors are categorized, give examples of functional categories and explain how tonic and phasic receptors differ.

2. explain the law of specific nerve energies.

3. describe the characteristics of the generator potential.

4. give examples of different types of cutaneous receptors and describe the neural pathways for the cutaneous senses.

5. explain the concepts of receptive fields and lateral inhibition.

6. describe the distribution of taste receptors on the tongue and explain how salty, sour, sweet and bitter tastes are produced.

7. describe the structure and function of the olfactory receptors, and explain how odor discrimination might be accomplished.

8. describe the structure of the vestibular apparatus and explain how it provides information about acceleration of the body in different directions.

9. describe the functions of the outer and middle ear.

Chapter Goals10. describe the structure of the cochlea and explain how movements of the

stapes against the oval window result in vibrations of the basilar membrane.

11. explain how mechanical energy is converted into nerve impulses by the organ of Corti and how pitch perception is accomplished.

12. describe the structure of the eye, and how images are brought to a focus on the retina.

13. explain how visual accommodation is achieved and describe the defects associated with myopia, hyperopia, and astigmatism.

14. describe the architecture of the retina, and trace the pathways of light and nerve activity through the retina.

15. describe the function of rhodopsin in the rods and explain dark adaptation is achieved.

16. explain how light affects the electrical activity of rods and their synaptic input to bipolar cells.

17. explain the trichromatic theory of color vision.

18. compare rods and cones with respect to their locations, synaptic connections, and functions.

SENSORY PHYSIOLOGY

• Sensory Receptors

• Pain

• Vision

• Hearing

• Chemical Senses

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Receptor Physiology

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Vision

• Structure of Eye (camera analogy)

• Vision Problems

• Light Receptors

• Chemistry

• Histology

• Fovea

• Accommodation

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Accommodation

• Pupil accommodation reflex - the smaller the aperture of the pupil, the greater the depth of field. Near objects cause reflex constriction of the pupil.

• Changing shape of lens

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Vision Problems

• Emmetropia - normal vision

• Hypermetropia - far-sightedness (eyeball abnormally flattened)

• Myopia - near sightedness (abnormally elongated eyeball)

• Astigmatism - irregular refractive surface (corrected with irregularly ground lens.

• Presbyopia - vision of old age (loss of lens elasticity)(corrected with bifocals)

• Cataracts - opaque lens (corrected with lens replacement)

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Light Receptors

• Rods - sensitive to any visible light

• Cones - respond to one of three different wavelengths– i. Dominator cones - all wavelengths– ii. Modulator cones - respond to red, green, or

blue (ratio determines color) - Brain can discriminate between 107 different colors. Color blindness = one type of cone missing.

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Fovea

• Only modulator cones - point where image of focus falls; therefore, if one does not focus directly on an object in dim light, it will be seen more clearly than if focused directly.

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Analysis of Firing Frequencies

Impulses/sec: Red: Green:Blue Ratio Interpretation

50 10 0 5 A shade of red

500 100 0 5 Same, but

brighter

10 50 0 0.2 A shade of green

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Hearing

• Anatomy

• Determination of Sound Quality

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Anatomy

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Chemical Senses

• Taste

• Smell

Taste

• Taste buds: groups of taste cells that have chemical receptors in their membranes. When dissolved molecule binds to receptor, an action potential is generated which the brain interprets as one of 4 primary sensations of taste. (10,000 at birth & slowly decrease)

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Taste (cont’d)

• Sour - H+

• Sweet - organic compounds (Saccharine is 500 times sweeter than sucrose; fructose 2X)

• Salty - Salts (NaCl strongest)

• Bitter - alkaloids (Most find to be objectionable; therefore, protection against poisoning?)

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Olfaction (Smell)

• Olfactory epithelium - (contains receptors) - cells have small projections which increase surface area.– Molecules must be volatile then liquefied.

– Olfactory epithelium high in nasal cavity; hence, sniffing.

• Most adaptable sense - we don't need to be able to constantly detect an odor. We need to detect changes in odors. If we were inundated by a harmless, but strong odor, we might miss the odor of a faint, lethal substance

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Chapter SummaryCharacteristics of Sensory ReceptorsI. Sensory receptors may be categorized on the basis of their structure, the

stimulus energy they transduce, or the nature of their response.

A. Receptors may be dendritic nerve endings, specialized neurons, or specialized epithelial cells associated with sensory nerve endings.

B. Receptors may be chemoreceptors, photoreceptors, thermoreceptors, mechanoreceptors, or nociceptors.

1. Proprioceptors include receptors in the muscles, tendons, and joints.

2. The senses of sight, hearing, taste, olfaction, and equilibrium are grouped as special senses.

C. Tonic receptors continue to fire as long as the stimulus is maintained. They monitor the presence and intensity of a stimulus.

1. Phasic receptors respond to stimulus changes; they do not respond to a sustained stimulus.

2. Phasic receptors partly account for sensory adaptation to sustained stimuli.

Chapter SummaryII. According to the law of specific nerve energies, each sensory receptor

responds with lowest threshold to only one modality of sensation.

A. That stimulus modality is called the adequate stimulus.

B. Stimulation of the sensory nerve from a receptor by any means is interpreted in the brain as the adequate stimulus modality of that receptor.

III. Generator potentials are graded changes (usually depolarizations) in the membrane potential of the dendritic endings of sensory neurons.

A. The magnitude of the potential change of the generator potential is directly proportional to the strength of the stimulus applied to the receptor.

B. After the generator potential reaches a threshold value, increases in the magnitude of the depolarization result in increased frequency of action potential production in the sensory neuron.

Chapter SummaryCutaneous Sensations

I. Somatesthetic information, from cutaneous receptors and proprioceptors, is carried by third-order neurons to the postcentral gyrus of the cerebrum.

A. Proprioception and pressure sensation ascend on the ipsilateral side of the spinal cord, synapse in the medulla and cross to the contralateral side, and then ascend in the medial lemniscus to the thalamus; neurons in the thalamus in turn project to the postcentral gyrus.

B. Sensory neurons from other cutaneous receptors synapse and cross to the contralateral side in the spinal cord and ascend in the lateral and ventral spinothalamic tracts to the thalamus; neurons in the thalamus then project to the postcentral gyrus.

II. The receptive field of a cutaneous sensory neuron is the area of skin that, when stimulated, produces responses in the neuron.

A. The receptive fields are smaller where the skin has a greater density of cutaneous receptors.

B. The two-point threshold test reveals that the fingertips and the tip of the tongue have a greater density of touch receptors, and thus a greater sensory acuity, than other areas of the body.

Chapter SummaryCutaneous Sensations

III. Lateral inhibition acts to sharpen a sensation by inhibiting the activity of sensory neurons coming from areas of the skin around the area that is most greatly stimulated.

Chapter SummaryThe Eyes and Vision

I. Light enters the cornea of the eye, passes through the pupil (the opening of the iris) and then through the lens, from which point it is projected to the retina in the back of the eye.

A. Light rays are bent, or refracted, by the cornea and lens.

B. Because of refraction, the image on the retina is upside down and right to left.

C. The right half of the visual field is projected to the left half of the retina in each eye, and vice versa.

II. Accommodation is the ability to maintain a focus on the retina when the distance between the object and the eyes is changed.

A. Accommodation is produced by changes in the shape and refractive power of the lens.

B. When the muscles of the ciliary body are relaxed, the suspensory ligament is tight and the lens is pulled to its least convex form.

1. This gives the lens a low refractive power for distance vision.

2. As an object is brought closer than 20 feet from the eyes, the ciliary body contracts, the suspensory ligament becomes less tight, and the lens becomes more convex and more powerful.

Chapter SummaryIII. Visual acuity refers to the sharpness of the image. It depends in part on the

ability of the lens to bring the image to a focus on the retina.

A. People with myopia have an eyeball that is too long, so that the image is brought to a focus in front of the retina; this is corrected by a concave lens.

B. People with hyperopia have an eyeball that is too short, so that the image is brought to a focus behind the retina; this is corrected by a convex lens.

C. Astigmatism is the condition in which asymmetry of the cornea and/or lens causes uneven refraction of light around 360 degrees of a circle, resulting in an image that is not sharply focused on the retina.

Chapter SummaryThe RetinaI. The retina contains rods and cones, photoreceptor neurons that synapse with

bipolar cells.

A. When light strikes the rods, it causes the photodissociation of rhodopsin into retinene and opsin.

1. This bleaching occurs maximally with a light wavelength of 500 nm.

2. Photodissociation is caused by the conversion of the 11-cis to the all-trans form of retinene, that cannot bond to opsin.

B. In the dark, more rhodopsin can be produced, and increased rhodopsin in the rods, makes the eyes more sensitive to light. This increased concentration of rhodopsin is partly responsible for dark adaptation.

C. The rods provide black-and-white vision under conditions of low light intensity. At higher light intensity, the rods are bleached out and the cones provide color vision.

Chapter SummaryII. In the dark, a constant movement of Na+ into the rods produces

what is known as a "dark current.’’

A. When light causes the dissociation of rhodopsin, the Na+ channels become blocked and the rods become hyperpolarized in comparison to their membrane potential in the dark.

B. When the rods are hyperpolarized, they release less neurotransmitter at their synapses with bipolar cells.

C. Neurotransmitters from rods cause depolarization of bipolar cells in some cases, and hyperpolarization of bipolar cells in other cases; thus, when the rods are in light and release less neurotransmitter these effects are inverted.

III. According to the trichromatic theory of color vision, there are three systems of cones, each of which responds to one of three colors: red, blue, or green.

A. Each type of cone contains retinene attached to a different type of protein.

B. The names for the cones signify the region of the spectrum in which the cones absorb light maximally.

Chapter SummaryVestibular Apparatus and EquilibriumI. The structures for equilibrium and hearing are located in the inner ear within

the membranous labyrinth.

A. The structure involved in equilibrium, known as the vestibular apparatus, consists of the otolith organs (utricle and saccule) and the semicircular canals.

B. The utricle and saccule provide information about linear acceleration, whereas the semicircular canals provide information about angular acceleration.

C. The sensory receptors for equilibrium are hair cells that support numerous stereocilia and one kinocilium.

1. When the stereocilia are bent in the direction of the kinocilium, the cell membrane becomes depolarized.

2. When the stereocilia are bent in the opposite direction, the membrane becomes hyperpolarized.

Chapter SummaryII. The stereocilia of the hair cells in the utricle and saccule project into the endolymph of

the membranous labyrinth and are embedded in a gelatinous otolithic membrane.

A. When a person is upright, the stereocilia of the utricle are oriented vertically; those of the saccule are oriented horizontally.

B. Linear acceleration produces a shearing force between the hairs and the otolithic membrane, thus bending the stereocilia and electrically stimulating the sensory endings.

III. The three semicircular canals are oriented at nearly right angles to each other, like the faces of a cube.

A. The hair cells are embedded within a gelatinous membrane called the cupula, which projects into the endolymph.

B. Movement along one of the planes of a semicircular canal causes the endolymph to bend the cupula and stimulate the hair cells.

C. Stimulation of the hair cells in the vestibular apparatus activates sensory neurons of the vestibulocochlear nerve (VIII), which projects to the cerebellum and to the vestibular nuclei of the medulla oblongata.

Chapter SummaryThe Ears and Hearing

I. The outer ear funnels sounds waves of a given frequency (measured in hertz) and intensity (measured in decibels) to the tympanic membrane, causing it to vibrate.

II. Vibrations of the tympanic membrane cause movement of the middle-ear ossicles, malleus, incus, and stapes, which in turn produces vibrations of the oval window of the cochlea.

III. Vibrations of the oval window set up a traveling wave of perilymph in the scala vestibuli.

A. This wave can pass around the helicotrema to the scala tympani, or it can reach the scala tympani by passing though the scala media (cochlear duct).

B. The scala media is filled with endolymph.

1. The membrane of the cochlear duct that faces the scala vestibuli is called the vestibular membrane.

2. The membrane that faces the scala tympani is called the basilar membrane.

Chapter SummaryIV. The sensory structure of the cochlea is called the spiral organ or organ of

Corti.

A. The organ of Corti rests on the basilar membrane and contains sensory hair cells.

1. The stereocilia of the hair cells project upward into an overhanging tectorial membrane.

2. The hair cells are innervated by the vestibulocochlear (VIII) nerve.

B. Sounds of high frequency cause maximum displacement of the basilar membrane closer to its base near the stapes; sounds of lower frequency produce maximum displacement of the basilar membrane closer to its apex near the helicotrema.

1. Displacement of the basilar membrane causes the hairs to bend against the tectorial membrane and stimulate the production of nerve impulses.

2. Pitch discrimination is thus dependent on the region of the basilar membrane that vibrates maximally to sounds of different frequencies.

3. Pitch discrimination is enhanced by lateral inhibition.

Chapter SummaryIV. The fovea centralis contains only cones; more peripheral parts of the retina

contain both cones and rods.

A. Each cone in the fovea synapses with one bipolar cell, which in turn synapses with one ganglion cell.

1. The ganglion cell that receives input from the fovea thus has a visual field equal to only that part of the retina which activated its cone.

2. As a result of this 1:1 ratio of cones to bipolar cells, visual acuity is high in the fovea but sensitivity to low light levels is less than in other regions of the retina.

B. In regions of the retina where rods predominate, large number of rods provide input to each ganglion cell (there is great convergence). As a result, visual acuity is impaired, but sensitivity to low light levels is improved.

Chapter SummaryV. The right half of the visual field is projected to the left half of the retina of

each eye.

A. The left half of the retina sends fibers to the left lateral geniculate body of the thalamus.

B. The left half of the right retina also sends fibers to the left lateral geniculate body. This is because these fibers decussate in the optic chiasma.

C. The left lateral geniculate body thus receives input from the left half of the retina of both eyes, corresponding to the right half of the visual field; the right lateral geniculate receives information about the left half of the visual field.

Chapter SummaryTaste and SmellI. The sense of taste is mediated by taste buds.

A. A particular taste bud is most sensitive to one of four taste modalities: sweet, sour, bitter, and salty.

B. Taste buds are located in characteristic regions of the tongue according to the modality to which they are most sensitive.

C. Salty and sour taste are produced by movements of sodium and hydrogen ions, respectively, through membrane channels; sweet and bitter tastes are produced by binding of molecules to protein receptors that are coupled to G-proteins.

II. The olfactory receptors are neurons that synapse within the olfactory bulb of the brain.

A. Odorant molecules bind to membrane protein receptors. There may be as many as a 1,000 different receptor proteins responsible for the ability to detect as many as 10,000 different odors.

B. Binding of an odorant molecule to its receptor causes the dissociation of large numbers of G-protein subunits. The effect is thereby amplified, which may contribute to the extreme sensitivity of the sense of smell.