Chapter 10 The Nervous System: Sensory systems

Transcription

1 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

2 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 P

3 P254 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.

4 P 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

5 P255 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.

6 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. P

7 P256 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.

8 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 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. 嗅覺皮質 前庭皮質 聽覺皮質 體感覺皮質味覺皮質視覺皮質 P256

9 P 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

10 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). P257

11 Figure 10.6 Generalized pathway for sensory systems. P258

12 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. P

13 Figure 10.8 Coding of stimulus intensity by recruitment (population coding). (a) Receptor recruitment within a single sensory unit. (b) Receptor recruitment of additional sensory units. P260 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 P258

14 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 P Figure 10.9 Overlapping receptive fields of two afferent neurons. P260

15 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 P259 Figure 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

16 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 P Figure 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. P261

17 P260 P263 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

18 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 P260 Figure Localization in hearing and olfaction. Localization of a sound or an odor depends on the difference between the time the stimulus reaches the left and right ears or nostrils, respectively. P261

19 P 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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21 Mechanoreceptors in the skin 表皮層 真皮層 皮下組織 Figure Sensory receptors in the skin. P262

22 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 0 C Cold receptors respond to 35~20 0 C Figure 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) P

23 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 (> 44 0 C), 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 P263

24 P 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

25 Figure 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. P265 三級神經 主要的體感覺皮質 丘腦 二級神經 延腦 脊髓 一級神經 本體接受器 機械性接受器 傷痛接受器 感溫接受器

26 P266 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

27 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 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 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 P266

28 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 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. P267

29 Modulation of pain signals Figure 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. P268

30 Figure 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 P269

31 眼角膜 虹膜 瞳孔 II. Vision Anatomy of the eye 肌肉 Copyright 2008 Pearson Education, Inc., publishing as Benjamin Cummings. 中央小窩 水狀液 視神經盤 晶狀體懸韌帶睫狀體 P 透明液 血管 視神經鞏膜脈絡膜 Figure Anatomy of the eye. 視網膜 The major structures of the eye as viewed in a horizontal section.

32 資料來源 :Vander s Human Physiology 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

33 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

34 P 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 The electromagnetic spectrum. The numbers indicate wavelength in nanometers (1 nm = 1x10-9 meter). The band of visible light is highlighted.

35 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 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 P

36 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 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. P273

37 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 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. P273

38 Figure 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. P274 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

39 P275 Clinical defects in vision Figure 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.

40 Clinical defects in vision Figure 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. P275

41 Clinical defects in vision Figure 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. P275

42 資料來源 :Vander s Human Physiology 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

43 Regulating the amount of light entering the eye Figure 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. P276

44 The retina Figure 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. P277

45 P277 The retina Figure 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.

46 P278 Phototransduction Figure 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.

47 Phototransduction Figure 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. P278

48 Figure 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. P279

49 Phototransduction Figure 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. P279

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51 Neural pathways for vision Figure Neural pathways for vision. P281