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Chapter 10
The Nervous System:
Sensory systems
1.
General principles of sensory physiology
感覺生理學的一般原則
2.
The somatosensory system
體感覺系統
3.
Vision
視覺
I. General Principles of sensory
physiology
 The afferent division 傳入的分支of the peripheral nervous system 末梢神經
系統 transmits information 傳遞訊息 from the periphery 末梢 to the central
nervous system 中樞神經系統
 The information is detected by sensory receptors 感覺接受器 that
respond to specific types of stimuli  eg. visceral receptors 臟器接受器
detect stimuli that arise within the body
 The sensory systems that enable us to perceive the external environment
include the somatosensory system 體感覺系統 and the special sensory
systems 特殊感覺系統
 The somatosensory system is necessary for perception of sensations
associated with receptor in the skin (somesthetic sensations 體感覺) and
for proprioception 本體感覺, the perception for the position of the limbs and
the body
 The special senses are necessary for senses of vision 視覺, hearing 聽覺,
balance and equilibrium 平衡感覺, taste 味覺, and smell 嗅覺
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Receptor physiology
 Sensory receptors are specialized neuronal structures that detect a
specific form of energy in either the internal or external environment
 the energy form of a stimulus is called its modality
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
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Sensory transduction
 The function of sensory receptors is transduction 傳遞作用—that
is, the conversion of one form of energy into another
 In sensory transduction 感覺傳遞, receptors covert the energy of
a sensory stimulus into changes in membrane potential called
receptor potentials 接受器電位 or generator potentials
 Receptor potentials are graded potentials 漸進電位 caused by
the opening or closing of ion channels
 The greater the strength of the stimulus, the greater the change in
membrane potential  receptor potentials are triggered by
sensory stimuli
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Sensory transduction
Figure 10.2 Structure and function of
sensory receptors. (a) A sensory
receptor that is a specialized ending of
an afferent neuron. The stimulus acts on
the sensory receptor by opening or
closing ion channels, thus producing a
receptor potential. (b) A sensory
receptor that is a separate cell from the
afferent neuron. The stimulus changes
the membrane potential of the receptor
cell, which opens (or closes) a calcium
channel, and cytosolic calcium
concentration increases (or decreases).
Changes in calcium concentration trigger
(or inhibit) the release of a chemical
transmitter by exocytosis. The transmitter
communicates to the afferent neuron by
binding to receptors on the afferent
ending and inducing a graded potential.
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Receptor adaptation
 Receptor adaptation 接受器適應性
is a decrease over time in the
magnitude of the receptor potential
in the presence of a constant
stimulus
 Slowly adapting or tonic receptors
show little adaptation and therefore
can function in signaling the intensity
of a prolonged stimulus
 Examples of slowing adapting
receptors are muscle stretch
receptor 肌肉牽張接受器, and
proprioceptors 本體接受器, which
detect the position of the body in
three-dimensional space
Figure 10.3 Responses of slowly
adapting receptors and rapidly
adapting receptors. (a) Slowing
adapting receptors respond with a
change in receptor potential that
persists for the duration of the
stimulus.
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Receptor adaptation
 Rapidly adapting or phase receptors adapt quickly, and thus function
best in detecting changes in stimulus intensity
 Examples of rapidly adapting receptors are olfactory receptor
嗅覺接受器, which detect odors
Figure 10.3 Responses of slowly
adapting receptors and rapidly
adapting receptors. (b) Rapidly
adapting receptors respond with a
change in receptor potential at the
onset of a stimulus, but then adapt.
The “off response” is a second, smaller
response that occurs upon termination
of a stimulus.
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Sensory pathways
 The specific neural pathways 專一的神經路徑 that transmit information
pertaining to a particular modality are referred to as labeled lines, and
each sensory modality follows its own labeled line 標線 or pathway 路徑
 The pathways for different modalities terminate in different sensory area
感覺區 of the cerebral cortex 大腦皮質
前庭皮質
Figure 10.4 Sensory area of the
cerebral cortex. Note that the
vestibular cortex 前庭皮質 actually lies
on the underside of the somatosensory
cortex 體感覺皮質 in the parietal lobe
頂葉, and the olfactory cortex 嗅覺皮質
is on the inferior surface of the temporal
lobe 顳葉.
體感覺皮質
味覺皮質
視覺皮質
嗅覺皮質
聽覺皮質
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Sensory pathways
 A sensory unit 感覺單位 is a single afferent neuron and all the
receptors associated with it  all the receptors are of the same type
 The area over which an adequate stimulus can produce a response
(which can be either excitatory or inhibitory) in the afferent neuron is
called the receptive field 接受區 of that neuron
 The afferent neuron that transmits information from the periphery to
the CNS is called the first-order neuron 一級神經  may diverge 發散
within the CNS and communicate with several interneurons
 Some of these interneurons transmit the information to the thalamus
丘腦, the major relay nucleus for sensory input  such interneurons are
examples of second-order neurons 二級神經
 In the thalamus, these second-order neurons from synapses with thirdorder neurons 三級神經 that transmit information to the cerebral
cortex 大腦皮質, where sensory perception 感覺知覺 occurs
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Figure 10.5 Sensory units and receptive fields. (a) A sensory unit in
which the receptors are specialized endings of the afferent neuron. (b) A
sensory unit in which the receptors are separate cells, each of which
communicates to a single afferent neuron. (c)The receptive field for the
sensory unit depicted in part (a).
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Figure 10.6 Generalized pathway for sensory systems.
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Sensory coding
 Stimulus type 刺激型態 is codes 編碼 by
the receptor 接受器 and pathway
activated 路徑活化 when stimulus is
applied  for example, light waves 光波
activate photoreceptors 光接受器, which
communicate via a specific pathway
專一路徑 to the visual cortex 視覺皮質
 Stimulus intensity 刺激強度 is coded by
the frequency of action potentials 動作
電位的頻率(frequency coding) and the
number of receptors activated 活化的
接受器數目 (population coding)
Figure 10.7 Coding of stimulus intensity.
Changes in membrane potential along the axon
(top) and at the receptor (middle) are plotted for
two different stimulus intensities (bottom). Detection
of a stronger stimulus by the receptor leads to
more frequent action potentials along the axon.
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Figure 10.8 Coding of stimulus intensity by recruitment (population coding).
(a) Receptor recruitment within a single sensory unit. (b) Receptor recruitment of
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additional sensory units.
 In population coding, a stronger stimulus activates, or recruits, a greater number
of receptors  these receptors may be associated with a single afferent neuron, in
which case the receptor potentials that are generated at the individual receptors
sum and produce a greater frequency of action potentials in that neuron
 A stimulus may also recruit receptors associated with different afferent neurons, in
which case more afferent neurons transmit signals to the CNS concerning the
presence of the stimulus
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 The basis for coding the locations of
most stimuli is receptive fields 接受區
 the precision 準確性 with which the
location of a stimulus is perceived is
called acuity 敏度
 In sensations associated with the skin,
acuity depends on the size and
number of receptive field, the amount
of overlap between the receptive
fields, and a phenomenon called lateral
inhibition 外側抑制
 Localization of a stimulus is better in
areas served by neurons with small
receptive fields
 Localization is improved by the
overlapping of the receptive field of
different afferent neurons
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Figure 10.9 Overlapping receptive
fields of two afferent neurons.
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 In lateral inhibition 外側抑制, stimulus that
strongly excites receptors in a given location
inhibits activity in the afferent pathways of
other nearby receptors
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Figure 10.10 Lateral inhibition. Lateral
inhibition enhances contrast between sites of
strong and weak stimulation. In this example,
the stimulus is applied in the center of the
receptive field for afferent neuron Y1, and in
the periphery of the receptive fields for
afferent neurons X1 and Z1. Lateral inhibition
occurs when collaterals of one afferent
neuron (here, Y1) activate interneurons that
inhibit communication between neighboring
afferent neurons and their second-order
neurons. The graphs show the frequencies
of action potentials in the afferent and
second-order neurons.
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
P261
 One measure of tactile acuity is two pint
discrimination, the ability of a person to
perceive two fine points pressed against the
skin as two distinct point
 The minimum distance that must exist between
two points for them to be perceived as separate
is termed the two-point discrimination threshold
 points that are closer together are perceived
as a single point
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Figure 10.11 Two-point discrimination. The ability
to discriminate between two separate points
depends on the activation of separate receptive
fields. The smaller the receptive field, the greater
the ability for two point discrimination, and the
greater the tactile acuity.
Copyright © 2008 Pearson Education, Inc.,
publishing as Benjamin Cummings.
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 Tactile acuity varies over
different regions of the body
 The skin in some areas of the
body is innervated by afferent
neurons with few branches
and, therefore, small
receptive fields, whereas
other areas are innervated by
afferent neurons with
extensive branching and
large receptive fields
 In addition, greater overlap
occurs for the smaller
receptive fields, which also
contributes to greater tactile
acuity
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 Localization in some sensory
systems is unrelated to
coding by receptive fields
 Localization in olfaction and in
hearing is based on the arrival
of stimuli at the two nostrils or
at the two ears at slightly
different times
 The brain uses the difference
in the time of arrival of action
potentials at the olfactory or
auditory cortex to determine
where the stimulus originated
Figure 10.12 Localization in hearing and
olfaction. Localization of a sound or an odor
depends on the difference between the time
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the stimulus reaches the left and right ears or
nostrils, respectively.
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II. The somatosensory system
Somatosensory receptors
 The somatosensory system 體感覺系統 is involved with body
sensations such as pressure, temperature, pain, and body position
 The somatosensory system responds to a variety of stimuli arising in
many areas of the body, and thus it utilizes many receptor types 
such as mechanoreceptors, thermoreceptors, nociceptors
 Most somatosensory receptors in the skin are specialized
structures at nerve endings, which are easily identified under the
light microscope
 A few somatosensory receptor types lack identifiable specialized
structures and are therefore called free nerve endings
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Mechanoreceptors in the skin
表皮層
真皮層
皮下組織
Figure 10.13 Sensory receptors in the skin.
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Thermoreceptors in the skin
 Thermoreceptors respond to the
temperature of the receptor endings
themselves and the surrounding tissue,
not the temperature of the surrounding air
 warm receptors and cold receptors
 Warm receptors  respond to 30~45 0C
 Cold receptors  respond to 35~20 0C
Figure 10.14 Thermoreceptor
responses. (a) The frequency of action
potentials in afferents associated with
warm or cold receptors when temperature
is held at a set level. (b) The response of
afferents of cold receptors (upper panel)
and warm receptors (middle panel) to a
decrease in temperature (lower panel)
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Nociceptors in the skin
 Nociceptors 傷痛接受器 are the sensory receptors responsible for the
transduction of noxious stimuli that we perceive a pain  are free nerve
endings that respond to tissue-damaging (or potentially damaging) stimuli
 There are three types of nociceptors:
 Mechanical nociceptors, which respond to intense mechanical stimuli,
such as stubbing your toe
 Thermal nociceptors, which respond to intense heat (> 440C), such as
touching a hot stove
 Polymodal nociceptors, which respond to a variety of stimuli, including
intense mechanical stimuli, intense heat, intense cold, and chemicals
released from damage tissue
 Chemicals that released from damaged tissue and are capable of activating
polymodal nociceptors include histamine 組織胺, bradykinin 遲緩痛素,
and prostaglandins 前列腺素
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Somatosensory pathways
 The perception 知覺 of somatic sensations 體感覺 from all parts of the body
begins in the primary somatosensory cortex 主要的體感覺皮質
 Two main pathways 路徑 transmit information from peripheral
somatosensory receptors 末梢體感覺接受器 to the CNS 中樞:
 dosal column-medial lemniscal pathway
 spinothalamic tract
 These pathways transmit different types of sensory information to the
thalamus 丘腦, and then to the primary somatosensory cortex
 In both cases the pathways enter the spinal cord 脊髓 on one side and
cross to the other side before reaching the thalamus  somatosensory
information from the right side of the body is perceived in the left
somatosensory cortex, and vice versa
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主要的體
感覺皮質
三級
神經
丘腦
二級
神經
延腦
脊髓
一級
神經
傷痛接受器
本體接受器
機械性接受器
感溫接受器
Figure 10.15 The two somatosensory pathway. (a) Dorsal column-medial
lemniscal pathway, which transmits information from mechanoreceptors and
proprioceptors to the CNS. (b) Spinothalamic tract, which transmits information
from thermoreceptors and nociceptors to the CNS.
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Pain perception
 Pain 痛, one of the somesthetic sensations 體感覺, is important because it
teaches us to avoid subsequent encounters with potentially damaging stimuli
 it is also important clinically, because it indicates that tissue damage may
have occurred
 The activation of nociceptors leads not only to the perception of pain but also
to a variety of other body responses :
 autonomic responses 自律神經系統的反應, such as  BP, HR, blood
epinephrine, blood glucose, dilation of the pupils, or sweating
 emotional responses 情緒反應, such as fear or anxiety
 a reflex 反射 withdrawal from the stimulus
 Each of two types of pain, fast pain and slow pain, is perceived differently
and is transmitted by a different class of afferent neurons
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Pain perception
 Fast pain is perceived as sharp pricking sensation that can be easily
localized  it is transmitted by Ad (A delta) fibers, thin, lightly myelinated
axons有髓鞘的軸突 with a conduction velocity of approximately 12-30 m/sec
 Slow pain is perceived as a poorly localized, dull aching sensations  it is
transmitted by C fibers, thin, unmyelinated axons 沒有髓鞘的軸突 with a
conduction velocity of approximately 0.2-1.3 m/sec
 The primary afferents, whether Ad or C fibers, from synapses with secondorder neurons in the dorsal horn of the spinal cord
 Communication between these first- and second-order neurons involves
different neurotransmitters, one of which is substance P
 Substance P is released from primary afferent neurons and binds to
receptors on second-order neurons  they ascend to the thalamus via the
spinothalamic tract, the pathway involved in the perception and
discrimination of pain
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Visceral pain
 The viscera 臟器 are subject to tissue damage,
and nociceptors in the organs detect this
damage
 Generally, activation of nociceptors in the
viscera produces pain that is called referred
pain 轉移痛 (because it has been “referred” to
the body surface)
 Referred pain occurs because the secondorder neurons that receive input from visceral
afferents also receive input from somatic
afferents
Figure 10.16 Mechanisms and sites of referred
pain. (a) Referred pain occurs when visceral and
somesthetic afferents converge on the same
second-order neurons in the spinal cord. (b) Pain
in specific visceral organs is generally referred to the
areas of the body surface indicated in this map.
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Modulation
of pain
signals
Figure 10.17 Gate-control theory of pain. (a) In unmodulated pain transmission,
collaterals of the nociceptor afferents (C-fibers) inhibit inhibitory interneurons, allowing
transmission of pain signals to second-order neurons in the dorsal horn of the spinal
cord and then to the thalamus. (b) In the modulation of pain transmission, collaterals of
large-diameter afferents (Ab fibers) branching from touch and pressure receptors excite
the inhibitory interneuron, thereby decreasing the transmission of pain signals.
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Figure 10.18 Endogenous analgesia
systems. When a painful stimulus activates
nociceptors, information is transmitted to
the CNS via the spinothalamic tract. In the
presence of stress, endogenous analgesia
system can block pain transmission at
the level of the synapse between the
nociceptive afferent neuron and the secondorder neuron, as follows: The periaqueductal
gray matter in the midbrain communicates to
the lateral reticular formation and to the
nucleus raphe magnus of the medulla.
These regions have neurons that descend to
the dorsal horn of the spinal cord and
activate inhibitory interneurons that release
the neurotransmitter enkephalin, which then
blocks communication between the
nociceptive afferent and the second-order
neuron via two mechanisms: presynaptic
inhibition of substance P releases from
the nociceptive afferent, and production of
IPSPs on the second-order neuron
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II. Vision
Anatomy of the eye
肌肉
Copyright © 2008 Pearson Education,
Inc., publishing as Benjamin Cummings.
眼角膜
虹膜
中央小窩
瞳孔
視神經盤
水狀液
晶狀體
懸韌帶
血管
睫狀體
透明液
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視神經
鞏膜
脈絡膜 Figure 10.19 Anatomy of the eye.
視網膜
The major structures of the eye as
viewed in a horizontal section.
Anatomy of the eye
 鞏膜(sclera)：眼球最外層的堅韌薄膜(眼白)
 眼角膜(cornea)：覆蓋在眼睛前方(黑眼球)的透明構造；是形成眼睛
光學系統的一部份，幫助物體影像聚焦在視網膜上
 脈絡膜(choroid)：眼睛色素層，接近視網膜
 虹膜(iris)：環繞眼睛瞳孔周圍的環狀構造
 瞳孔(pupil)：眼睛虹膜中的開口，讓光線由此通過而到達視網膜
 視網膜(retina)：位於眼球底部的薄層神經組織，包含視覺接受器
 晶狀體(lens)：眼睛視覺系統中可調整的部位，幫助將觀測物成像在
視網膜
 中央小窩(fovea centralis)：影像聚焦在視網膜上的特殊區域，
為視網膜上可引起最清晰視覺之處
資料來源：Vander’s Human Physiology
Anatomy of the eye
 視神經盤(optic disc)：視神經離開視網膜的地方，該部位沒有光接受器，
也沒有感光細胞，只有很多的神經纖維，又稱為盲點
 水狀液(aqueous humor)： 位於眼球內，用來維持眼球的形狀，為視神
經提供養分。每日都會製造新鮮的水狀液，並會不斷排出。當排出的管
道閉塞，而水狀液又不斷的製造出來，導致無法排出，眼內的壓力會持
續增加，慢慢視神經便被壓壞，最後可能導致失明。
 透明液(vitreous humor)：又稱為玻璃體液，是晶狀體後方與視網膜之
間的 一種膠狀物質
 睫狀肌(ciliary muscle)：在適應期間，可移動並改變晶狀體的形狀
 懸韌帶(zonular fiber)：連接睫狀肌與晶狀體的構造
 調適(accommodation)： 藉由改變晶狀體的形狀，調整眼睛以看清各種
距離的物體
資料來源：Vander’s Human Physiology
The nature and
behavior of light waves
 Light is a form of energy  light exists
as electromagnetic waves 電磁波
 Visible light 可見光 includes those
electromagnetic waves having
wavelengths 波長 between about 350
nm and 750 nm  different colors
correspond to different wavelengths
within this range
Figure 10.21 The electromagnetic
spectrum. The numbers indicate
wavelength in nanometers (1 nm =
1x10-9 meter). The band of visible
light is highlighted.
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The nature and behavior of light waves
 Both the cornea and the lens have convex surfaces that function to converge
the light wave entering the eye onto the retina, a process that is necessary if
visual images are to be in focus
Figure 10.23 Refraction of light waves passing through curved surfaces.
(a) Concave surfaces cause divergence of light waves. (b) Convex surfaces cause
convergence of light waves to a focal point. The distance from the long axis of a convex
lens to the focal point is called the focal length
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The nature and behavior of light waves
 The ability of the lens to adjust its refractive power for viewing near objects is a
process called accommodation
Figure 10.24 Refraction of light waves in the eye. A given point in the visual
field comes to focus on a single point in the retina. Refraction of light waves as
they pass through the convex cornea and lens of the eye causes the image to
be inverted and reversed on the retina.
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Accommodation
 Accommodation 調視作用 is under control of the parasympathetic nervous
system 副交感神經系統, which triggers contraction of the ciliary muscle 睫狀肌
for near vision  in the absence of parasympathetic activity, the ciliary muscle
relaxes
Figure 10.25 Focusing light from distant and near sources. (a) Light waves
reflected from a distant object approach the lens parallel to one another. A
relatively flat (weak) lens is sufficient to converge the light waves on the retina.
(b) Light waves reflected from a near object diverge as they approach the lens.
A rounder (strong) lens is needed to converge the light waves on the retina.
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睫狀肌鬆弛，懸韌帶
拉緊，晶狀體被拉成
扁平，從遠處來的光
線剛好可以幾乎平行
的聚焦在中央小窩
看遠物
副交感神經活化，使
睫狀肌收縮，懸韌帶
鬆弛，晶狀體放鬆成
球狀，近處的光線可
以聚焦而看清近物，
此作用是為調適作用
Figure 10.26 Mechanism of accommodation. (a) Vision of distant objects. In the
absence of parasympathetic stimulation, the ciliary muscle relaxes, putting tension on
the zonular fibers. The zonular fibers pull on the lens, flattening the lens. (b)
Accommodation for near vision. Under parasympathetic stimulation, the ciliary muscle
contracts, reducing the tension on the zonular fibers and enabling the elastic lens to
become rounder.
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Clinical defects in vision
Figure 10.27 Normal, near-sighted, and far-sighted vision. (a) In
emmetropia 正常 視力, or normal vision, distant objects are focused on the
retina without accommodation, and near objects are focused with
accommodation.
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Clinical defects in vision
Figure 10.27 Normal, near-sighted, and far-sighted vision. (b) In myopia
近視, or near-sightedness, the lens (or cornea) is too strong for the length
of the eyeball. Near objects are focused without accommodation, and distant
objects come into focus in front of the retina even without accommodation.
Myopia can be corrected by using a concave lens to produce divergence of
light waves before they enter the eye.
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Clinical defects in vision
Figure 10.27 Normal, near-sighted, and far-sighted vision. (c) In hyperopia
遠視, or far-sightedness, the lens (or cornea) is too waek for the length of
the eyeball. Distant objects are focused with accommodation, and near objects
come into focus behind the retina, even with accommodation. Hyperopia can
be corrected by using a convex lens to produce convergence of light waves
that supplements the convergence produced in the eye.
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Clinical defects in vision
 白內障(cataract)：由於老化產生的晶狀體顏色變化，使得晶狀體 呈現
不透明的現象，是最常見的 眼部疾病之ㄧ
 老花眼(presbyopia)：老化造成 晶狀體硬度增加，使得觀看近物的
視覺調適日益困難。這是老化過程中的的正常現象
 近視(nearsighted, myopic)：若眼球的長度過長，遠物的影像會落在
視網膜前方的位置，而看不清遠方的物體
 遠視(farsighted, hyperopic)： 若眼球的長度過短，遠物的影像可以
落在視網膜上，而近物的影像則會聚焦在視網膜後方，觀看近物時會
有模糊的現象
 散光(astigmatism)：晶狀體或眼角膜的表面不光滑而造成的視力缺陷
 青光眼(glaucoma)：有時是因水狀液的形成比排除快，水狀液增加
造成眼內壓增加，壓迫視網膜，是造成永久性失明的主因之ㄧ
資料來源：Vander’s Human Physiology
Regulating the amount of light
entering the eye
Figure 10.28 Regulation of the amount of light entering the eye. (a) The
iris, which consists of two layers of smooth muscle—an inner circular and an
outer radial layer—controls pupil size. The size of the pupil determines the
amount of light that enters the eye. (b) Pupillary constriction, which is caused
by parasympathetic stimulation of the circular muscle layer of the iris. (c )
Pupillary dilation, which is caused by sympathetic stimulation of the radial
muscle layer of the iris.
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The retina
Figure 10.29 Anatomy of the retina. Located on the inner surface of the eye,
the retina consists of three layers of neural tissue composed of the various
types of cells depicted. Note that light must pass through the inner and middle
layers of the retina before striking the photoreceptors in the outer layer. Deep
on the retinal pigment epithelium, which absorbs light.
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The retina
Figure 10.30 Distribution of rods
and cones in the retina. The
abundance of cones is greatest at
the fovea and declines rapidly with
distance from it. Rods are absent
from the fovea but very abundant
near it; they slowly decrease in
abundance with distance from the
fovea.
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Phototransduction
Figure 10.31 Morphology of
the photoreceptors. Rods and
cones have the same basic
structural components: The outer
segment consists of disks that
contain the photopigment; the
inner segment contains the
nucleus and most of the
organelles. The synaptic terminal
contains the synaptic vesicles,
which store a chemical
transmitter used for
communication.
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Phototransduction
Figure 10.32 Components of rods. The photopigment, rhodopsin, is located
in the membrane of the stacked located in the rod’s outer segment. Rhodopsin
is coupled to a G protein called transducin, which activates the enzyme
photodiesterase, which in turn catalyzes the breakdown of cGMP.
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Figure 10.33 Phototransduction of light. (a) In the dark, photoreceptors release their
chemical transmitter. (b) When light is present, it is absorbed by the photopigment,
initiating a sequence of events that decrease release of the transmitter.
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Phototransduction
Figure 10.34 Absorbance spectra for the different photoreceptors. Rods
can absorb light over the widest range of wavelengths. The absorbance
spectra of the three types of cones overlap.
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Neural pathways for vision
Figure 10.36 Neural pathways for vision.
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