第九章 感觉器官的功能

【目的要求】了解各主要感觉器官的结构与分类，感觉通路中的信息编码和处理，本体感觉、触-压觉和温度觉，眼的折光异常，视觉信息的处理及机制，听神经动作电位，嗅觉和味觉。熟悉感受器的一般生理特性，视锥细胞的色觉和色觉学说，视觉和听觉形成基本过程。掌握感觉器官和感受器的概念，眼的调节，视网膜的两种感光换能系统（概念、功能特点），视紫红质的光化学反应，暗适应和明适应、视野、视敏度，基底膜的震动和柯蒂氏器的换能作用，行波学说，耳蜗微音器电位，内耳前庭器官的适宜刺激和生理功能。

【教学内容】

1.        感受器和感觉器官的概念，感受器的一般生理特性（感受器的适宜刺激、换能作用、编码作用和适应现象）。感觉通路中的信息编码和处理。

2.        躯体感觉（本体感觉、触-压觉和温度觉），内脏感觉。

3.        视觉产生基本过程，眼的折光系统与简化眼，视调节，近点与远点的概念，瞳孔近反射和瞳孔对光反射，眼的折光能力异常，房水和眼内压。视网膜的结构和感光换能功能（两种感光换能系统的概念、功能特点），视杆细胞外段的超微结构，视紫红质的光化学特性及其代谢，视杆细胞光-电转换感受器电位的产生机理。视锥细胞和颜色视觉，色觉学说。视网膜的信息处理。暗适应和明适应，视野，视敏度，视后像和融合现象，双眼视觉和立体视觉。

4.        人耳的听阈和听域，听觉产生基本过程，外耳的传音功能，中耳的传音增压效应，咽鼓管功能，声音的气传导和骨传导。耳蜗的结构，基底膜的震动和柯蒂氏器的换能作用，行波学说。耳蜗的生物电现象，耳蜗微音器电位，听神经动作电位。

5.        内耳前庭器官的组成、适宜刺激和生理功能。前庭反应。

6.        嗅觉和味觉。

【计划学时】3学时。

Chapter IX Sense System

  Sensory systems process information that may lead to a sensation or to perception of an event within the body or in the outside world.

Basic Characteristics of Sensory Coding

  A single afferent neuron with all its receptor endings is a sensory unit. The area of the body that, when stimulated, causes activity in a sensory unit or other neuron in the afferent pathway is called the receptive field for that neuron. The type of stimulus perceived is determined by the type of receptor activated, the specific pathways activated, and the part of the brain in which these pathways terminate. Stimulus intensity is coded by the rate of firing of individual sensory units and by the number of sensory units activated. Location of the stimulus depends on the size of the receptive field covered by a single sensory unit and on the overlap of nearby receptive fields. Information coming into the nervous system is subject to control by ascending pathways and by descending pathways. 

Vision

   Light is described by its wavelength or frequency. The light that falls on the retina must be focused by the cornea and lens. Lens shape is changed in response to viewing near or distant objects so that both are focused on the retina. Presbyopia interferes with accommodation. Cataract prevents the passage of light to the retina. An eyeball too long or too short relative to the focusing power of the lens causes nearsighted or farsighted vision, respectively. The photopigments of the rods and cones are made up of a protein component (opsin) and a chromophore. The rods and each of the three cone types have different opsins that make each of the four-receptor types sensitive to a different wavelength of light. When light falls upon the chromophore, the photic energy causes the chromophore to change shape, which triggers a cascade of events involving the second-messenger cyclic GMP and Ca++. When exposed to darkness, the rods and cones are depolarized and release their neurotransmitter. When exposed to light, they become hyperpolarized, and transmitter release is reduced. The rods and cones synapse on bipolar cells, which synapse on ganglion cells. Ganglion cell axons form the optic nerves, which lead into the brain. Half the optic nerve fibers cross to the opposite side of the brain in the optic chiasm. Fibers from the optic nerves terminate in the lateral geniculate nuclei of the thalamus, which send fibers to visual cortex.. Visual information is also relayed to areas of the brain dealing with biological rhythms. 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. The colors we perceive are related to the wavelength of light. Different wavelengths excite one of the three cone photopigments most strongly. Certain ganglion cells are excited by input from one type of cone cell and inhibited by input from a different cone type.  The sensation of color depends on the output of the various cone opponent-color cells and the processing of this output by brain areas involved in color vision. 

Hearing

  Sound energy is transmitted by movements of pressure waves. The sound wave frequency determines pitch. The sound wave amplitude determines loudness. The sound transmission sequence is as follows: Sound waves enter the ear canal and press against the tympanic membrane, causing it to vibrate. The vibrating membrane causes movement of the three small middle-ear bones, and the stapes presses against the oval-window membrane. Movements of this membrane set up pressure waves in the fluid-filled scala vestibuli, which cause movements in the cochlear duct wall, setting up pressure waves in the fluid there. These pressure waves cause vibrations in the basilar membrane, located on one side of the cochlear duct. As this membrane vibrates, the hair cells of the organ of Corti move in relation to the tectorial membrane. Movement of the hair cells' stereocilia stimulates them to release neurotransmitter that activates receptors on the peripheral ends of the afferent nerve fibers. Each part of the basilar membrane vibrates maximally in response to one particular sound frequency.

Vestibular System

   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. The semicircular ducts detect angular acceleration during rotation of the head, which cause bending of the stereo-cilia on the hair cells. Otoliths in the gelatinous substance over the utricle and saccule hair cells 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

   The receptors for taste lie in taste buds throughout the mouth, principally on the tongue, and can respond to many different substances. Olfactory receptors, which are part of the afferent olfactory neurons, lie in the mucosa in the upper nasal cavity. Odorant molecules once dissolved in the mucus that bathes the olfactory receptors are transported to the receptors by odorant binding proteins. Olfactory pathways go to the limbic system.

内容提要

（一）眼的折光成像和感光换能

1．视觉产生的基本过程是：外界物体的反射光或发出的光入眼→折光系统→视网膜上成像→视锥或视杆细胞感光换能→视神经上的动作电位以不同的组合形式上传→视交叉→视束→外膝体→视反射→枕叶视皮质→产生视觉及视效应。

2. 眼有折光成像和感光换能作用，人眼的折光系统让进入眼内的光线会聚成像，在视网膜上形成清晰的物象。平行光进入眼内，不需要眼的调节就能汇聚形成清晰的像，视近物需要眼调节才能形成清晰的物象。视调节的过程包括晶状体变凸，增大折光能力，使进入眼内的光线提前汇聚，焦点落在视网膜上；瞳孔缩小，减少球面相差和色像差；双眼会聚，使物象落在两眼视网膜的对应点上以避免复视，从而产生单一的清晰视觉。

3．视网膜中存在视锥和视杆两种感光换能系统。视锥系统由视锥细胞及与之相连的双极细胞和视神经节细胞构成；视锥细胞对光敏感度低，分布在视网膜中央，能分辨颜色，视锥系统司明视觉和色觉；视杆系统由视杆细胞及与之相连的双极细胞和视神经节细胞构成，视杆细胞光敏感度高，分布在视网膜周边，视杆系统司暗视觉。 

4．视杆细胞感光换能是以视紫红质的光化学反应为基础。视紫红质由视蛋白和11-顺视黄醛在暗处结合而成，光照使视紫红质分解为为全反型视黄醛和视蛋白，并导致视杆细胞外段产生超极化的感受器电位。

5. 视锥细胞分别含三种视锥色素，不同波长的光照射视网膜时，会使三种视锥细胞以不同的比例兴奋，这样的信息传到中枢，就会产生不同颜色的感觉。

（二）听觉的形成和耳蜗基底膜的振动——行波理论

1．听觉产生的主要过程是：声波→耳廓的集音和外耳道的传音→鼓膜和听骨链的传音增压→前庭窗膜振动→前庭阶和鼓阶外淋巴位移→蜗管内淋巴位移→基膜上的毛细胞受到刺激→声波的机械能转换为听神经上的动作电位及其序列，上传至大脑皮质听觉中枢，产生听觉及其他功能活动的变化。

2．耳蜗可将传入的机械振动转变为听神经上的动作电位。

耳蜗感音的机制是声波传入耳蜗→基底膜振动和毛细胞兴奋→感受器电位→传至毛细胞顶部→神经递质释放→听神经上的动作电位→传入中枢形成听觉。耳蜗微音器电位是多个毛细胞产生的感受器电位的复合型电位变化。

耳蜗微音器电位的特点——潜伏期短，无不应期，可总和，可诱发动作电位，耐缺氧和麻醉。

3．音调的感觉取决于基底膜产生振动的部位。不同频率的声波在基底膜上传播的距离不一样，引起基底膜上最大振幅的位置也不同，所以不同频率的振动波被不同的基膜部位感受。

低频——行波距离远，靠顶部，高频——行波距离短，靠底部

（三）前庭器官的适宜刺激及前庭反射的作用

前庭器官感受头部的空间位置和人体自身运动状态，同时可引起前庭反射（各种姿势调节反射、植物神经性反应以及眼震颤）。其中姿势调节反射维持身体原有的姿势和在运动中保持平衡。

半规管的适宜刺激是旋转变速运动，椭圆囊和球囊是直线变速运动。