…… (p. 423) Visual Function of the Eye Vision is the most important source of information people obtain from the outside world, and at least 70% of total outside information comes from vision. The eye is (p. 424) the peripheral receptor organ that causes vision, and Figure 31-11 shows the horizontal section of the human right eyeball. The appropriate stimulus for human speech is electromagnetic waves with wavelengths of 380 to 760 nm, i.e., visible light. The light emitted from external objects is imaged on the retina by the refractive system of the eye, and then the light-sensitive transducer system of the eye transforms the visual information contained in the retinal image into bioelectrical signals and carries out preliminary processing of these signals in the retina, and the visual information initially processed by the retina is then transmitted to the center, which will be further analyzed and processed at all levels of the center, especially the cerebral cortex, before vision is finally formed. (p. 428) Second, the phototransduction system of the human eye has the function of forming and initially processing visual information The principle that external objects are imaged on the retina through the refractive system of the eye can be classified as a physical study, which is not essentially different from the imaging of objects on a camera negative; however, the final formation of sensation in the subjective consciousness through the visual system belongs to the scope of physiological and psychological studies. Although vision is ultimately formed in the visual center, visual information is first formed in the retina where it is initially processed. The basic function of the retina is to perceive external light stimuli and to convert this form of stimulus energy into electrical signals on nerve fibers. (i) The retina has a complex functional structure The retina (…… contains two types of photoreceptor cells, the optic rod cells and the cone cells, and four other types of neurons, namely, bipolar cells, ganglion cells, horizontal cells, and anaglyph cells. …… (p. 29) 2. Photoreceptor cells and their characteristics Two types of photoreceptor cells exist in any mammalian retina: the optic rod cells and the optic cone cells, which are morphologically divided into 3 parts: the outer segment, the inner segment, and the synaptic part. …… 3. Connections of retinal cells Both retinal rod cells and cone cells form chemical synapses with bipolar cells by their synaptic sections, and bipolar cells are also connected to ganglion cells by chemical synapses. This longitudinal connection of cells in the retina is an important basis for visual information transmission. Axons from ganglion cells converge on the retinal surface in bundles and cross the retina and posterior wall of the eye approximately 3 mm lateral to the nasal aspect of the central recess to (p. 430) form the optic nerve. The site where the ganglion cell axons cross the retina is called the optic nerve papilla. …… In the retina, in addition to the vertical connections between cells described above, there are also horizontal connections. Horizontal cells located in the outer retina act as liaisons between photoreceptor cells, while anaplastic cells located in the inner retina act as liaisons between ganglion cells with different lengths and forms of protrusions. In addition, the permeability of gap junctions between the synaptic parts of photoreceptor cells, between horizontal cells and between longitudinal cells, and even between neurons, is variable, and thus changes in extracellular potentials can affect photoreceptor activity through gap junctions. (b) There are two different photoreceptor systems in the retina, the optic rod and the optic cone 1. i.e., the optic rod system and the optic cone system. The optic rod system is more sensitive to light and can perceive low light stimuli in a dark environment and cause vision, but has no color vision and has a low ability to discriminate the fine structure of the viewed object, so it is also called the late photoreceptor or dark vision system. The visual cone system is less sensitive to light and can only be activated in bright light, but it can discriminate colors when seeing objects and has a higher ability to discriminate the fine structure of the object being seen, so it is also called the diurnal light perception or bright vision system. …… (1) Different distribution of different photoreceptor cells in the retina: optic rod cells are mainly distributed in the peripheral area of the retina, and their number is highest at 10°-20° outside the central concavity, and decreases as you move to the peripheral area of the retina; optic cone cells are highly concentrated in the central concavity of the retina, and only optic cone cells are distributed here, and decrease significantly toward the peripheral area of the retina (Figure 1). The cone cells were highly concentrated in the central retinal recess, where only cone cells were distributed, and decreased significantly toward the peripheral area (Figure 31-20). Consistent with this, in the bright, the human eye has good color discrimination and the ability to discriminate the fine structure of the object being viewed, and the ability to discriminate is already stronger in the central recess; in the dark, the human eye cannot discriminate color and can only discriminate the general outline and brightness difference of the object being viewed, and the sensitivity to light is higher in the peripheral area of the retina. For example, when one is observing the starry sky and casts a very faint star, one will often find that the star will disappear because the star image on the retina is falling in the central concave; however, when one moves one’s eyes to the side, it can appear again because the star image on the retina falls in the peripheral area outside the central concave. (2) Different cellular contacts in the rod and cone systems: In the human eye, there are 1.2*10 rod cells and 6*10 cone cells in the retina, while there are only 1.2*10 optic nerve fibers in the optic nerve. Overall, the convergence of photoreceptor cells through bipolar cells to ganglion cells is 105:1. Up to 250 optic rod cells are seen in the peripheral region of the retina converging on a single ganglion cell via a few bipolar cells; whereas in the central recess it is common for an optic cone cell to associate with only one bipolar cell and then with only one ganglion cell. It can be seen that there is a higher degree of convergence in the cellular connections of the optic pole system, whereas the degree of convergence is much lower in the optic cone system. Obviously, the lower the degree of convergence in the sensory pathway, such as in the optic cone system, the higher the sensory discrimination ability, while the higher the degree of convergence, such as in the optic rod system, the lower the sensory discrimination ability. (3) Different habits of different species of animals: some animals that are active only during daytime, such as chickens, pigeons, squirrels, etc., their photoreceptor cells are mainly cone cells, while other animals that are active at night, such as owls, etc., their photoreceptors in the retina are mainly rod cells. (4) Different photoreceptor cells contain different optic pigments: as mentioned above, there is only one type of optic pigment in the rod cells, i.e., violet red, while the cone cells contain three types of optic pigments with different spectral absorption properties, which is consistent with the fact that the rod system has no color vision function while the cone system has color vision function. 2, dark adaptation and light adaptation after a long time in the bright suddenly into the dark, initially can not see any object, after a certain period of time (p. 432), only gradually can see the objects in the dark, this phenomenon is called dark adaptation. On the contrary, after a long time in the dark suddenly enter the light, initially feel a dazzling light, but also can not see the object, wait a few moments to restore vision, this phenomenon is called light adaptation. Dark adaptation is the process of gradually increasing the sensitivity of the human eye to light in the dark. Generally, within the first 5-8 min after entering a dark place, there is a significant drop in the threshold of light perception by the human eye, and then a more significant drop again; about 25-30 min after entering a dark place, the threshold drops to the lowest point and stabilizes at this level. The first drop in visual threshold mentioned above is mainly associated with an increase in the synthesis of cone pigments. Bright adaptation is much faster, usually within a few seconds. The mechanism is that the optic rod cells accumulate a large amount of retinal pigment in the dark, which is rapidly broken down when entering the light, thus producing a dazzling light perception. Only after the rapid breakdown of the more optic pigments can the relatively light-insensitive retinal pigments restore vision in the light.