Stimuli, Receptors, and Responses (TE 16) PDF

TE 16 Stimuli, receptors and responses e-aristo.hk/r/ bioccfc16.e It is estimated that about 80% of the information we rec...

TE 16 Stimuli, receptors and responses e-aristo.hk/r/ bioccfc16.e It is estimated that about 80% of the information we receive from the environment comes from our eyes. Links to prior knowledge Chapter preview In Chapter 1, we learned that all organisms can detect and respond to 16.1 Irritability stimuli. In this chapter, we will discuss 16.2 Human eyes as the sense organs how humans detect two stimuli—light for detecting light and sound. This draws on what we have learned in Junior Science about 16.3 Human ears as the sense organs sight and hearing as two of our most for detecting sound important senses. Then, we will discuss 16.4 Phototropic responses in plants plant responses to light.  TE 16 Stimuli, receptors and responses Orthokeratology It may sound like a dream to correct your short sight while you are asleep. Now, with a treatment called orthokeratology, this dream can come true. The treatment involves wearing specially designed, gas permeable contact lenses during sleep. Overnight, the lenses gently reshape the cornea of your eye. When you wake up in the morning and remove the lenses, you will be able to see  Orthokeratology corrects short sight while clearly for the day without the help of glasses. you are asleep. cornea lens with no lens with lens fitted after overnight lens removed  How orthokeratology works The effect of orthokeratology is temporary. The cornea will gradually return to its original shape if you stop wearing lenses consecutively for a week. Think about … 1. What is the function of the cornea? 2. What are the causes of short sight? 3. How do glasses correct short sight? (Refer to p.A1 for answers.) Answer orthokeratology 角膜塑形 16- 2  TE 16 Stimuli, receptors and responses Learning objective 16.1 Irritability Understand the roles of sense organs and receptors in detecting Irritability refers to the ability of organisms to detect stimuli changes in the environment (singular: stimulus) and produce appropriate responses accordingly. Irritability is important to survival as it enables organisms to escape from danger, and look for food and mates. a b The lynx sees the rabbit and runs to catch it. The The Venus’ flytrap detects the movement of the fly and rabbit detects the danger and runs away. closes its leaves quickly to catch it. Figure 16.1 Both animals and plants show irritability. Stimuli are changes in our environment and can be internal or external. They are detected by sensory cells, called receptors. Different types of receptors detect different stimuli. Some receptors are grouped with other tissues to form a sense organ. The table below shows four types of human receptors, the stimuli they detect and the sense organs containing the receptors. Type of receptor Stimulus detected Sense organ(s) Photoreceptor Light Eye Mechanoreceptor Sound (a pressure wave) Ear Touch (pressure) Skin Chemoreceptor Chemicals in the air Nose Chemicals in food Tongue Thermoreceptor Changes in temperature Skin Table 16.1 Four main types of receptors in humans irritability 感應性 sensory cell 感覺細胞 photoreceptor 光感受器 thermoreceptor 温度感受器 stimulus 刺激 receptor 感受器 mechanoreceptor 機械感受器 16- 3 response 反應 sense organ 感覺器官 chemoreceptor 化學感受器  16 Stimuli, receptors and responses When receptors detect stimuli, they convert the stimuli into nerve impulses. The nerve impulses travel along a nerve to the control centre, usually the brain, where they are interpreted as a sensation. The brain may also send nerve impulses to effectors, such as muscles or glands, which produce responses (Figure 16.2).  Receptors in the boy's ears  The receptors send nerve  The brain sends nerve impulses detect the hooting of the impulses to the brain. The brain to neck muscles (effectors) to approaching taxi (stimulus). interprets the nerve impulses turn the boy's head (response) and produces the sensation of to look at the source of the hearing. sound. Figure 16.2 An example of irritability in daily life In the above example, the receptors and effectors are coordinated by the nervous system. The brain serves as the control centre. This Link it type of coordination is called nervous coordination. You will learn more about We have another coordinating system—the endocrine system. The coordination in humans in Chapter 17. coordination brought about by the endocrine system is called hormonal coordination. detected produce by coordinating systems stimuli receptors nervous system effectors responses endocrine system Figure 16.3 Coordination between receptors and effectors Key point Irritability is the ability of organisms to detect stimuli and produce appropriate responses. nerve impulse 神經脈衝 nervous system 神經系統 hormonal coordination 激素協調 16- 4 sensation 感覺 nervous coordination 神經協調 effector 效應器 endocrine system 內分泌系統  TE 16 Stimuli, receptors and responses 16.2 Human eyes as the sense Learning objective organs for detecting light Relate the structure of major parts of the eye to vision Our eyes are the sense organs for detecting light. The brain Explain the causes of eye defects receives nerve impulses from the eyes and interpret them to Describe how short sight and produce vision. long sight are corrected with glasses Be aware of the surgical A. Structure of the human eye methods for eyesight correction 1. Structures surrounding the eye Flipped classroom The eye is a spherical organ. It is situated in an orbit, which is a How the eye bony socket in the skull (Figure 16.4). The orbit encloses and works e-aristo.hk/r/ protects all but the front of the eye. The eyeball is attached to the bioccflip1601.e wall of the orbit by three pairs of eye muscles. These muscles contract and relax to allow the eyeball to rotate. The front of the eye is protected by several structures (Figure 16.5): Eyebrows prevent sweat from entering the eye. Eyelashes trap dirt before it can enter the eye. The upper and lower eyelids can close to protect the eye from foreign objects and strong light. Tear glands, located above the eyeball, secrete tears. When we blink, the eyelids spread tears over the eye surface, keeping it moist and clean. Tears contain lysozyme, an enzyme which can kill bacteria. Key: tear gland eye muscles flow of tears upper eyelid eyebrow upper eyelid tear gland eyelash optic nerve lower eyelid lower eyelid duct draining tears orbit eyeball to the nasal cavity Figure 16.4 Side view of the left eye Figure 16.5 Front view of the left eye orbit 眼窩 eyelid 眼瞼 eyebrow 眼眉 tear gland 淚腺 16- 5 eyelash 眼睫毛 lysozyme 溶菌酶  TE 16 Stimuli, receptors and responses 2. Internal structures of the eye 3D model The wall of the eyeball is composed of three layers: Human eye e-aristo.hk/r/ the outer layer consists of the sclera and the cornea. biocc3dm1601.e the middle layer consists of the choroid, ciliary body and the iris. the inner layer consists of the retina. Internally, the eye is divided into two chambers (the anterior chamber and the posterior chamber) by the lens. lens eye muscle anterior chamber (filled sclera with aqueous humour) choroid cornea retina pupil yellow spot iris suspensory ligament blind spot blood vessels ciliary muscle ciliary body optic nerve posterior chamber (filled with vitreous humour) Figure 16.6 Section through the eye AR Extras: Do you know... Sclera Conjunctiva The sclera, also known as the white of the eye, is the white, opaque, fibrous coat of the eye. It is very tough so that it gives shape to the A thin, transparent layer of tissue, called the conjunctiva, eyeball and protects the structures inside. It is also the surface covers the front part of the where the eye muscles attach to the eyeball. sclera. The conjunctiva contains many blood vessels. Cornea In cases of conjunctivitis (inflammation of the The cornea is located in the front part the eye and is continuous conjunctiva), these blood with the sclera. The cornea is transparent so that light can enter the vessels become dilated and filled with blood, and the eye eye. Its curved surface refracts (bends) light. appears red. sclera 鞏膜 ciliary body 睫狀體 lens 晶體 opaque 不透光的 16- 6 cornea 角膜 iris 虹膜 conjunctiva 結膜 choroid 脈絡膜 retina 視網膜 conjunctivitis 結膜炎  TE 16 Stimuli, receptors and responses Choroid The choroid is the middle layer of the eye. It contains many blood vessels. Blood supplies the cells of the retina with nutrients and oxygen, and removes wastes. The choroid is dark in colour because it contains a black pigment to absorb light. This prevents reflection of light in the eye and contributes to sharp vision. Ciliary body, lens and suspensory ligaments The ciliary body is a forward continuation of the choroid. It is a ring of muscular tissue that surrounds the edge of the lens and is connected to the lens by suspensory ligaments. The lens is a transparent, elastic and biconvex structure, consisting of living cells that have lost their nuclei. It helps refract and focus light onto the retina. The ciliary muscles in the ciliary body contract or relax to change the tension of suspensory ligaments, hence the shape of the lens, to focus light from objects at different distances. Iris and pupil The iris is connected to the ciliary body, lying in front of the lens. The iris contains the pigment which gives the eye its colour (Figure 16.7). It consists of circular muscles and radial muscles, and has a central hole called the pupil. These muscles change the size of the pupil, thus controlling the amount of light that enters the eye. iris pupil Figure 16.7 The colour of the iris varies among individuals. suspensory ligament 懸韌帶 radial muscle 放射肌 ciliary muscle 睫狀肌 pupil 瞳孔 16- 7 circular muscle 環肌  TE 16 Stimuli, receptors and responses Retina The retina is the inner layer of the eye. It contains photoreceptors (also called light-sensitive cells), which generate nerve impulses when stimulated by light. There are two types of photoreceptors: rod cells and cone cells. The central region of the retina contains cone cells only, and is called the yellow spot. The photoreceptors on the retina are connected to neurones. The nerve fibres from these neurones are bundled to form the optic nerve, which transmits nerve impulses to the brain. The region where the optic nerve leaves the eye contains no photoreceptors, and is called the blind spot (Figure 16.8). retina choroid sclera light yellow blind spot spot x100  hotomicrograph showing the wall of P  he yellow spot and the blind spot viewed through T the eyeball an ophthalmoscope layers of neurones nerve fibres to layer of photoreceptors optic nerve cone cell rod cell choroid sclera blind spot to brain blood vessels Key: transmission of optic nerve Figure 16.8 Structure of the retina nerve impulses light-sensitive cell 感光細胞 yellow spot 黃點 16- 8 rod cell 視桿細胞 optic nerve 視神經 cone cell 視錐細胞 blind spot 盲點  TE 16 Stimuli, receptors and responses Anterior and posterior chambers The anterior chamber is the space between the cornea and the lens. It is filled with the aqueous humour. The aqueous humour is a clear, watery solution secreted by the ciliary body. It supplies nutrients and oxygen to the cornea and the lens, which have no blood vessels. The posterior chamber is the space between the lens and the retina. It is filled with a clear jelly-like fluid called the vitreous humour, which is also secreted by the ciliary body. The aqueous humour and the vitreous humour help keep the shape of the eyeball, and refract and focus light onto the retina. Practical 16.1 Examination of a human eye model Examine a human eye model. Identify the various structures of the eye. Test yourself How is the model different from a real eye? What are the model's limitations? (Refer to p.A1 for answers.) Answer  A human eye model Practical 16.2 Dissection and examination of an ox eye In this practical, you will dissect an ox eye. An ox eye and the human eye have Video a similar overall structure. By examining the anatomy of an ox eye, you can Practical 16.2 gain a better understanding of the structure and function of different parts of e-aristo.hk/r/ bioccpv1602.e the eye.  An ox eye cont'd aqueous humour 水狀液 vitreous humour 玻璃狀液 16- 9  16 Stimuli, receptors and responses Procedure 1. Place an ox eye on a dissecting board. Examine the external structure Caution of the eye. Locate and identify the eye muscles, the eyelashes, the Wear a mask and disposable eyelids (if still present), the sclera, the cornea and the optic nerve. gloves. 2. Remove the eyelids, the fat and the eye muscles around the eyeball Scissors and scalpels are sharp. with scissors. Be careful not to cut away the optic nerve. Handle them with care. 3. Make a small cut at the centre of the cornea with a scalpel. Then cut through the cornea twice diagonally with scissors. Note the liquid that flows out when the cornea is cut open. This is the aqueous humour. 4. Spread out the four flaps of the cornea. Examine the iris and the pupil. 5. Cut through the iris and into the sclera to about half-way around the wall of the eye. Locate and identify the lens, the suspensory ligaments, the ciliary body and the vitreous humour. 6. Pick up the lens carefully with forceps and avoid making any scratches on the lens. Put it on a piece of newspaper with text printed on it. Observe what the text looks like through the lens. Squeeze the lens gently with your fingers and feel the texture of the lens. 7. Empty out the jelly-like vitreous humour from the eyeball. Examine the retina and locate the blind spot. 8. Lift up the retina with forceps and examine the choroid underneath. Caution After the practical, dispose of the ox eye, the mask and the gloves as directed. collect the apparatus used for central sterilization by the technician. wash your hands thoroughly with soap and water. scalpel cornea pupil cornea pupil iris iris sclera optic nerve blind spot lens suspensory ligament vitreous humour ciliary body iris  Steps for dissecting an ox eye 16- 10  TE 16 Stimuli, receptors and responses B. How we see Remember this When you look at an object, light rays from the object enter your The image formed on the retina is eyes through the cornea. The light rays are refracted by the cornea, a real, vertically and laterally inverted image that is smaller than the aqueous humour, the lens and the vitreous humour, and focused the object. on the retina to form an inverted image (Figure 16.9). vitreous lens humour light rays focused light ray from onto the retina cornea an object inverted image aqueous optic nerve humour Figure 16.9 Formation of an image on the retina When the photoreceptors on the retina are stimulated by light falling on them, they generate and send nerve impulses along the optic nerve to the visual centre of the brain. The brain interprets the nerve impulses and produces vision. Therefore, we see an upright image of the object. Test yourself Despite the presence of the blind What will happen if the image is formed on the blind spot? Since the spot, you do not consciously blind spot has no photoreceptors to detect light, no nerve impulses experience a ‘hole’ in your vision. are generated to send to the brain. Thus, the image formed on the Why? (Refer to p.A1 for answers.) Answer blind spot is not visible. Mini lab Activity 16.1 Finding the blind spot There is a blind spot in each eye. Try this activity to find the blind spot in your right eye. Hold this book at arm’s length. Close your left eye. Focus on the cross with your right eye and slowly move the book towards yourself. When the image of the dot falls on the blind spot, you will not see it. 16- 11  16 Stimuli, receptors and responses Clear your concepts No image is formed on the blind spot. An image is formed wherever incoming light rays meet on the retina, so it can be formed on the blind spot. However, any image formed on the blind spot is not visible as there are no photoreceptors to detect light. Extras: Do you know… Why we need two eyes Humans have two forward-facing eyes. Since the two eyes are some distance apart, they view the same object from slightly different angles and the image formed on the retina of each eye is therefore different. The brain combines the two images from both eyes into one three-dimensional image. This gives us a perception of depth and enables us to judge distances more accurately. binocular field of vision left field right field of vision of vision left eye right eye optic nerve three-dimensional image ‘seen’ by the brain  Humans have binocular vision, i.e. the two eyes have overlapping visual fields. Key point 1. Light rays that enter the eye are refracted by the cornea, the aqueous humour, the lens and the vitreous humour and focused on the retina. 2. Photoreceptors detect light falling on them and send nerve impulses along the optic nerve to the visual centre of the brain. 3. The brain interprets the nerve impulses and produces vision. binocular vision 雙眼視覺 16- 12  All answers TE 16 Stimuli, receptors and responses Checkpoint Directions: Questions 1 and 2 refer to the 1. Refraction of light occurs at surface(s) diagram below, which shows a section of the A. 1. B. 1 and 3. human eye. C. 2 and 3. D. 1, 2 and 3. 4 2. Which of the following is/are the function(s) of 1 structure 4? (1) to supply nutrients to the eyeball 2 (2) to generate nerve impulses 3 (3) to reduce reflection of light within the eye A. (1) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) C. Rod cells and cone cells The two types of photoreceptors, rod cells and cone cells, are named from their characteristic shapes (Figure 16.10). rod cell cone cell x2000 Figure 16.10 SEM showing rod cells and cone cells Rod cells contain the photosensitive pigment visual purple, which is very sensitive to light. This explains why rod cells are stimulated by low light intensities and can work well in dim light. Because rod cells cannot detect colours, they are responsible for black and white vision. Rod cells are evenly distributed across the retina, except that they are absent from the yellow spot and the blind spot. Cone cells are stimulated by higher light intensities and are used mainly in bright light for colour vision. Cone cells are less numerous than rod cells. They are most concentrated at the yellow spot, but few are found in the periphery of the retina. This is why objects appear most clearly when their images fall on the yellow spot. visual purple 視紫 colour vision 色覺 16- 13  TE 16 Stimuli, receptors and responses There are three types of cone cells—red, green and blue, which are Remember this most sensitive to red, green and blue light, respectively (Figure Each of the three types of cone 16.11). The colours we see depend on the relative stimulation of the cells contains a different photosensitive pigment, which three types of cone cells (Table 16.2). For example, light reflected absorbs light of different by a red apple stimulates mainly red cone cells and we see it as red; wavelengths, corresponding light reflected by a ripe banana stimulates both red and green cone roughly to red, green and blue light. cells, and we see it as yellow. When all three types of cone cells are stimulated equally, we see white. Red cone Green cone Blue cone Colour we Blue Green Red cells cells cells see Sensitivity (percentage of light absorbed) cones cones cones 100 ✔ red ✔ green 75 ✔ blue 50 ✔ ✔ yellow ✔ ✔ cyan 25 ✔ ✔ magenta 0 ✔ ✔ ✔ white 380 450 500 550 600 650 700 750 black Wavelength of light (nm) Key: ✔ represents stimulation of cone cells Figure 16.11 Sensitivities of the three types of cone Table 16.2 Different colours are seen as a result cells to light of different wavelengths of stimulation of different types of cone cells. Extras: Do you know… What an afterimage is Stare at the image on the right for 30 seconds, and then look at a piece of white paper. What can you see? You should be able to see a green four-leaf clover. This effect is called 'afterimage'. It occurs as the red cone cells become fatigued after we stare at the red image for a while, and only the green and blue cone cells are stimulated when we look at the piece of white paper. Key point 1. Rod cells can function in dim light and are responsible for black and white vision. 2. Cone cells function well in bright light and are responsible for colour vision. afterimage 後像 16- 14  TE 16 Stimuli, receptors and responses Worked example 16.1 Figure 1 shows a horizontal section of the left eye. Figure 2 shows the densities of the two types of photoreceptors (rod cells and cone cells) in the retina. X Number of photoreceptors per mm2 side of the centre of head the head 160,000 rod cells 120,000 80˚ 80˚ 80,000 60˚ 60˚ cone cells 40,000 40˚ Y 40˚ 20˚ 20˚ 0 80° 60° 40° 20° 0° 20° 40° 60° 80° X Y Figure 1 Figure 2 Use the information from Figures 1 and 2 to explain the following: (a) A person cannot see an object if its image falls on area X. (2 marks) (b) It is easier to see a faint star if you look slightly to the side rather than looking directly at it. (4 marks) Solution Reminder (a) Since there are no photoreceptors in area X,............................(1) No marks can be scored by no nerve impulses are generated to stimulate the visual centre of simply stating that ‘X is the blind the brain...................................................................................(1) spot’ without reference to the distribution of photoreceptors. Therefore, the image falling on area X cannot be seen. (b) When looking slightly to the side of the star, its image falls on the periphery of the retina which has a higher density of rod cells than the central area........................................................................(1) Rod cells are stimulated by dim light to generate nerve impulses, Reminder so the star can be seen.............................................................(1) Rod cells are much more When looking directly at the star, its image falls on Y where there sensitive to light than cone cells, are only cone cells, no rod cells................................................(1) and can respond to lower light intensities. The light from the faint star is too weak to stimulate cone cells.........................................................................................(1) Try Exam practice Q3 (p.46) 16- 15  TE 16 Stimuli, receptors and responses D. Control of the amount of light entering the eye The pupil is the hole in the centre of the iris. The circular muscles and radial muscles of the iris contract or relax to change the size of the pupil. They thus regulate the amount of light entering the eye. This is important because it is difficult for us to see when light is insufficient, but too much light can damage the photoreceptors on the retina. 1. In dim light In dim light, the radial muscles of the iris contract and the circular muscles relax. The pupil becomes larger in size (dilates) to allow more light to enter the eye (Figures 16.12 and 16.13). This increases stimulation of the photoreceptors to allow us to see in dim light. Side view Front view radial muscles of the iris contract iris pupil dilates circular muscles of the iris relax Figure 16.12 The radial muscles of the iris contract to Figure 16.13 Pupil dilated dilate the pupil. 2. In bright light In bright light, the circular muscles of the iris contract and the radial muscles relax. The pupil becomes smaller in size (constricts) to reduce the amount of light entering the eye (Figures 16.14 and 16.15). This prevents damage to the photoreceptors on the retina. Side view Front view radial muscles of the iris relax iris pupil constricts circular muscles of the iris contract Figure 16.14 The circular muscles of the iris contract to Figure 16.15 Pupil constricted constrict the pupil. 16- 16 All answers 16 Stimuli, receptors and responses Clear your concepts When bright light shines into the eye, the pupil contracts. The pupil is a hole and cannot contract. It can only be constricted or dilated by the iris muscles. Key point 1. The circular muscles and the radial muscles of the iris can change the size of the pupil, thereby controlling the amount of light entering the eye. 2. In dim light, the radial muscles of the iris contract and the circular muscles relax. The pupil dilates to increase the amount of light entering the eye. 3. In bright light, the circular muscles of the iris contract and the radial muscles relax. The pupil constricts to reduce the amount of light entering the eye. Checkpoint The graph below shows the changes in the diameter of the pupil of a person’s eye during a period of 30 seconds. pupil diameter X 0 10 20 30 time (s) Which of the following combinations correctly states the change in light intensity and the type of iris muscles that begins to contract at X? Light intensity Iris muscles contracting A. increases circular muscles B. increases radial muscles C decreases circular muscles D. decreases radial muscles 16- 17  TE 16 Stimuli, receptors and responses Animation E. Eye accommodation Eye accommodation e-aristo.hk/r/ In order for a person to see an object clearly, light rays from the bioccani1601.e object must be focused on the retina. Light rays from a nearby object are more divergent (spread out) and require more refraction. On the contrary, light rays from a distant object are nearly parallel and require less refraction. The lens of the eye is elastic. Its shape (curvature) can be changed to adjust the amount of refraction of light, so that light rays from near and distant objects can be focused on the retina. This is called eye accommodation. 1. Focusing on a near object When focusing on a near object, the circular ciliary muscles around the lens contract. Thus, the tension in the suspensory ligaments is reduced (i.e. they become slack). The lens becomes thicker (more convex) due to its elasticity and refracts light more (Figure 16.16).  Circular ciliary muscles contract.  Tension in suspensory ligaments decreases. light rays from a light rays focused  Tension in suspensory   Lens becomes near object on the retina ligaments decreases. thicker.  Circular ciliary muscles contract.  L ens becomes thicker. Side view Front view of the lens Figure 16.16 Eye accommodation: focusing on a near object 2. Focusing on a distant object When focusing on a distant object, the circular ciliary muscles relax. Thus, the tension in the suspensory ligaments is increased (i.e. they become tighter). The lens is pulled into a thinner (less convex) shape and refracts light less (Figure 16.17 on the next page). eye accommodation 視覺調節 divergent 發散 16- 18 curvature 曲率  All answers 16 Stimuli, receptors and responses  Circular ciliary muscles relax.  Tension in suspensory ligaments increases. light rays from a light rays focused  Tension in suspensory   Lens becomes distant object ligaments increases. thinner. on the retina  Circular ciliary muscles relax.  Lens becomes thinner. Side view Front view of the lens Figure 16.17 Eye accommodation: focusing on a distant object Key point 1. Eye accommodation refers to changing the shape of the lens to focus on objects at different distances. 2. When focusing on a near object, the circular ciliary muscles contract; the tension in the suspensory ligaments decreases; the lens becomes thicker (more convex) and light is refracted more. 3. When focusing on a distant object, the circular ciliary muscles relax; the tension in the suspensory ligaments increases; the lens becomes thinner (less convex) and light is refracted less. Checkpoint Which of the following combinations correctly shows the conditions of different parts of the eyes when a person is looking at an object moving towards him? Lens Suspensory ligament A. becoming thinner slackening B. becoming thinner tightening C becoming thicker slackening D. becoming thicker tightening HKDSEE Biology 2015 Paper 1 Section A Q27 16- 19  TE 16 Stimuli, receptors and responses Simulation F. Eye defects and their correction Short sight and long sight e-aristo.hk/r/ bioccsim1601.e 1. Short sight A person with short sight can see near objects more clearly than distant objects (Figure 16.18). The causes may be that the eyeball is too long, or that the lens is too thick, or a combination of both. Light rays from distant objects are focused in front of the retina, resulting in a blurred image on the retina. Short sight can be corrected by wearing glasses containing concave (diverging) lenses, which are thinner at the centre and thicker at the Figure 16.18 Distant objects edges. Light rays from distant objects are diverged before entering seen by a person with short the eyes. This allows the image to be brought back into focus on the sight retina (Figure 16.19). Remember this People with severe short sight a Focusing on a near object have a higher risk for retinal detachment. This happens as the light rays from eyeball continues to lengthen, a near object light rays focused on pulling the retina away from the the retina choroid. b Focusing on a distant object light rays from a distant object light rays focused in front of the retina c Correction by wearing a concave lens light rays from a distant object diverged by a concave lens light rays focused on the retina concave lens Figure 16.19 Short sight and its correction short sight 近視 retinal detachment 視網膜脫落 16- 20  TE 16 Stimuli, receptors and responses 2. Long sight A person with long sight can see distant objects more clearly than near objects (Figure 16.20). The causes may be that the eyeball is too short, or that the lens is too thin, or a combination of both. Light rays from near objects are focused behind the retina, resulting in a blurred image on the retina. Figure 16.20 A near object Long sight can be corrected by wearing glasses containing convex seen by a person with long (converging) lenses, which are thicker at the centre and thinner at sight the edges. Light rays from near objects are converged before entering the eyes, so when the cornea and the lens further refract the light rays, a sharp image is formed on the retina (Figure 16.21). a Focusing on a distant object light rays from a distant object light rays focused on the retina b Focusing on a near object light rays from a near object light rays focused behind the retina c Correction by wearing a convex lens light rays from a near object converged by a convex lens light rays focused on the retina convex lens Figure 16.21 Long sight and its correction Sharpen your skills Note the following when drawing ray diagrams: Use solid straight lines to present the light rays that enter the eyes and reach the retina. Light rays behind the retina should be drawn with dotted lines until they intersect. Light rays from a distant object should be parallel and the object should not be drawn. Light rays from a near object should be diverging from a single point. Arrowheads should be added to the light rays to indicate the direction in which the light is travelling. long sight 遠視 converge 會聚 16- 21  16 Stimuli, receptors and responses 3. Colour blindness Poor functioning or a deficiency of one or more types of cone cells can lead to colour blindness, an inability to distinguish some or all colours. Total colour blindness is rare in humans. Red-green colour blindness is the commonest type. The person cannot distinguish red and green (Figure 16.22) because of poor functioning or a deficiency of red or green cone cells. normal colour vision red-green colour blindness Figure 16.22 The view of people with normal colour vision and with red- green colour blindness Colour blindness is an inherited eye defect, although in rare cases it can be caused by damage to the retina or the brain. There is temporarily no cure for colour blindness but colour-blind people generally live a normal life. Mild forms of colour blindness are often not even recognized by the person concerned. Figure 16.23 shows some test cards for detecting colour blindness. a b c d Figure 16.23 Test cards for detecting colour blindness AR colour blindness 色盲 16- 22 red-green colour blindness 紅綠色盲  TE 16 Stimuli, receptors and responses STSE connections Aids for colour-blind people People with red-green colour blindness may wear glasses with special filter lenses to help them see colours more accurately. There are also mobile phone applications that help colour- blind people see colours using the camera of their smartphones.  Mobile phone application to help colour-blind people Taking it further Other common eye problems Astigmatism Cataract Astigmatism is a refraction error of the eye. Incoming A cataract is a dense, cloudy area that forms inside the light rays do not fall into a single point in the retina, lens of the eye, interfering with vision. It can lead to causing blurred vision. blindness. Cause: The cornea or lens having an irregular shape Cause: Aging, injuries or genetic factors Treatment: Wearing lenses with different curvatures in Treatment: Surgical replacement of the lens different regions or undergoing surgery (e.g. LASIK) Glaucoma Macular degeneration This is a group of eye diseases that causes damage to This is an eye disease that affects the region of the the optic nerve. It causes a loss of peripheral vision at retina around the yellow spot, causing the loss of vision first and can lead to blindness. in the centre of the visual field. Cause: High eye pressure (due to improper drainage of Cause: Loss of photoreceptors in the yellow spot of the the aqueous humour) retina Treatment: Using eye drops or surgery to reduce the Treatment: Medications or surgery to slow the eye pressure development of the disease astigmatism 散光 glaucoma 青光眼 LASIK 角膜激光矯視手術 macular degeneration 視網膜黃斑變性 16- 23 cataract 白內障  TE All answers 16 Stimuli, receptors and responses Video STSE connections LASIK e-aristo.hk/r/ Laser surgery for correcting vision bioccuv1601.e Short sight, long sight and astigmatism are problems of focusing that can be corrected with a type of surgery called LASIK. It makes use of a laser beam to change the curvature of the cornea to correct the focus of light rays on the retina. flap laser beam   reate a flap on the C   eshape the cornea R   eplace the flap to R cornea. with a laser beam. its original position. Conventionally, the corneal flap was cut manually with a blade. With modern advanced technology, the corneal flap can now be created with a laser beam which offers greater precision. A recent method, called SMILE, is performed without uncovering the corneal flap, so that the healing time is shorter than conventional LASIK. Key point 1. In short sight, the eyeball is too long or the lens is too thick, resulting in the image of distant objects being focused in front of the retina. It can be corrected by wearing concave lenses. 2. In long sight, the eyeball is too short or the lens is too thin, resulting in the image of near objects being focused behind the retina. It can be corrected by wearing convex lenses. 3. In colour blindness, one or more of the three types of cone cells function poorly or are deficient. There is no cure for colour blindness. Checkpoint A person suffers from colour blindness and is unable to focus on distant objects. Which of the following can explain his conditions? A. deficiency of visual purple; lens too thick B. deficiency of visual purple; eyeball too short C. deficiency of cone cells; lens too thin D. deficiency of cone cells; eyeball too long SMILE 小切口透鏡切除術 16- 24  TE 16 Stimuli, receptors and responses 16.3 Human ears as the sense Learning objective organs for detecting sound Relate the structure of major parts of the ear to hearing Our ears are the sense organs for detecting sound. The brain receives nerve impulses from the ears and produces the sensation of hearing. 3D model A. Structure of the human ear Human ear e-aristo.hk/r/ The human ear can be divided into three parts: the outer ear, the biocc3dm1602.e middle ear and the inner ear (Figure 16.24). auditory canal ear bones semicircular auditory canals nerve pinna to brain cochlea oval window round window eardrum Eustachian tube to pharynx outer ear middle ear inner ear Figure 16.24 Structure of the human ear AR Practical 16.3 Examination of a human ear model Examine a human ear model. Identify the various structures of the ear.  Human ear model outer ear 外耳 middle ear 中耳 16- 25 inner ear 內耳  TE 16 Stimuli, receptors and responses 1. Outer ear The outer ear consists of the pinna and the auditory canal. The pinna is the part of the ear that we see on each side of our head. It is a flap of elastic cartilage covered with skin. It has the shape of a funnel, which facilitates the collection of sound waves into the auditory canal. The auditory canal is a passageway for sound from the pinna to the eardrum. The eardrum is a thin, elastic membrane at the end of the auditory canal. It separates the outer ear from the middle ear. When sound waves reach the eardrum, they cause the eardrum to vibrate. The eardrum thus converts sound waves to mechanical vibrations. 2. Middle ear The middle ear is an air-filled cavity behind the eardrum. It contains three ear bones (Figure 16.25). The ear bones amplify the vibrations from the eardrum and transmit them to the oval window, which is a flexible membrane separating the middle ear from the inner ear. The oval window transmits the vibrations to the cochlea in the inner ear. Situated below the oval window is another flexible membrane, called the round window. It bulges out into the middle ear when the oval x3 window presses in. This allows the pressure built up in the cochlea Figure 16.25 Ear bones to be released into the middle ear. The Eustachian tube connects the middle ear to the pharynx. It helps maintain equal pressure on either side of the eardrum by allowing air to enter or leave the middle ear. Extras: Do you know... Why your ears pop Normally the air pressure inside the middle ear and the air pressure outside are essentially the same. When outside air pressure changes suddenly—for example, during the ascent or descent of an aeroplane or a high-speed lift—air must move through the Eustachian tube to equalize the pressure in the middle ear with the outside pressure. Otherwise, the eardrum will bulge and cannot vibrate freely. This affects hearing and causes ear pain. Swallowing or yawning opens the Eustachian  Ear pain can be felt in tube to allow air to enter or leave the middle ear. You will feel a ‘pop’ as the eardrum an ascending or a returns to its normal position. descending aeroplane. pinna 耳殼 ear bone 聽小骨 Eustachian tube 耳咽管 16- 26 auditory canal 聽道 oval window 卵圓窗 eardrum 耳膜 round window 圓窗  TE 16 Stimuli, receptors and responses 3. Inner ear The inner ear consists of fluid-filled tubes that form the cochlea and the semicircular canals. The cochlea is for hearing, whereas the semicircular canals detect head movements and help us have a sense of balance. The cochlea is a tube coiled into a spiral shape, similar to a snail shell. The lumen of the cochlea is divided lengthwise by membranes into three liquid-filled canals (Figure 16.26). The liquid in the upper and lower canals is called perilymph. The upper canal is in contact with the oval window. The lower canal is in contact with the round window. The liquid in the central canal is called endolymph. The central canal contains sensory hair cells that are connected to the auditory nerve. The movements of the endolymph cause the hairs of the sensory hair cells to bend. This stimulates the sensory hair cells to generate nerve impulses. The nerve impulses are sent along the auditory nerve to the auditory centre of the brain for interpretation. semicircular canals upper canal filled with perilymph ear bone in central canal filled the middle ear with endolymph round cochlea window lower canal filled with perilymph auditory nerve membrane hair sensory hair cell to auditory nerve Figure 16.26 Structure of the cochlea cochlea 耳蝸 endolymph 內淋巴 semicircular canal 半規管 sensory hair cell 感覺毛細胞 16- 27 perilymph 外淋巴 auditory nerve 聽神經  TE 16 Stimuli, receptors and responses Animation B. How we hear How the ear works e-aristo.hk/r/ The diagram below shows how we hear. bioccani1602.e  The pinna collects sound  The ear bones amplify and waves into the auditory transmit the vibrations from  Nerve impulses travel along the eardrum to the oval the auditory nerve to the canal. window. auditory centre of the brain to produce the sensation of hearing.  The sound waves cause the eardrum to vibrate.  The oval window vibrates and causes wave movements of the perilymph in the upper canal of the cochlea.  The wave movements are  Sensory hair cells are Key: transmitted to the endolymph stimulated to generate transmission of vibrations/ in the central canal. and send nerve impulses. wave movements transmission of nerve impulses Figure 16.27 How we hear STSE connections Hearing aids and cochlear implants Problems in the outer or middle ear, such as infections or a punctured ear drum, can cause conductive hearing loss. If the condition is serious and cannot be cured, a hearing aid may be needed. It works by increasing the amplitude of sound waves so that the person is more likely to detect them. Sometimes, hearing loss is due to damage to the sensory hair sound processor cells in the cochlea. A cochlear implant may help people with this type of hearing loss. implant How a cochlear implant works:   he sound processor captures sounds from the T surroundings, and sends signals to the implant under the skin.   he implant generates electrical impulses and sends T them to the electrodes in the cochlea. electrodes  E lectrical impulses from the electrodes stimulate the auditory nerve to send nerve impulses to the brain.  A cochlear implant conductive hearing loss 傳導性弱聽 electrode 電極 16- 28 hearing aid 助聽器 cochlear implant 人工耳蝸 All answers 16 Stimuli, receptors and responses Taking it further Detecting movements of the head The three semicircular canals in the inner ear are arranged in three planes at right angles to each other. They contain endolymph and sensory hair cells for detecting head movements. As the head moves, sensory hair cells are stimulated by the movements of the endolymph and send nerve impulses to the brain for interpretation. The brain is kept informed of head movements and coordinates skeletal muscles to maintain body balance. Posterior canal senses the head tilting towards the right or left shoulder Anterior canal senses the head nodding Horizontal canal senses the head shaking  Positions and orientations of the semicircular canals Key point 1. The pinna of the outer ear collects and directs sound waves along the auditory canal to the eardrum, which converts sound waves to mechanical vibrations. 2. In the middle ear, the ear bones amplify and transmit the vibrations from the eardrum to the oval window. 3. Vibrations of the oval window cause wave movements of the liquids in the cochlea. 4. Movements of the endolymph cause the hairs of the sensory hair cells in the cochlea to bend; sensory hair cells are stimulated to send nerve impulses along the auditory nerve to the brain. 5. Nerve impulses are interpreted by the brain to produce the sensation of hearing. Checkpoint Which of the following combinations correctly matches the structure of the human ear and its function? Structure Function A. ear flap protecting the ear B. ear drum amplifying sound waves C ear bones transmitting vibrations D. round window setting the endolymph in motion HKDSEE Biology 2013 Paper 1 Section A Q29 16- 29  TE 16 Stimuli, receptors and responses 16.4 Phototropic responses in Learning objective plants Recognize the significance of phototropism Like all living things, plants show irritability. They can detect stimuli Understand the mechanism of phototropic responses in shoots such as light and water, and respond to them. and roots However, most plants do not respond to stimuli as quickly as Flipped classroom animals. This is because animals mainly respond by muscle Phototropism contractions while plants mainly respond by growing certain parts e-aristo.hk/r/ of their body (e.g. shoots and roots) towards or away from the stimuli. bioccflip1602.e Growth responses in plants are called tropism or tropic responses. A tropic response is described as positive when the plant part grows towards the stimulus and negative when it grows away from the stimulus. A. Responses of shoots and roots to light Light is an important stimulus to plants. The directional growth movement of a plant part in response to unilateral light (i.e. light coming from one direction only) is called phototropism. Practical 16.4 Investigation of the phototropic responses of shoots and roots Procedure Video Practical 16.4 1. Set up the apparatus as shown below. e-aristo.hk/r/ bioccpv1604.e light-proof box shoot light light cardboard root light light culture solution stand rotating clinostat Set-up A Set-up B 2. Observe and record how the seedlings have grown in the two set-ups after two days. cont'd tropism 向性 16- 30 phototropism 向光性  TE 16 Stimuli, receptors and responses Results and discussion In set-up A, the seedlings are subjected to unilateral light. The shoots grow towards the light source, and the roots grow away from it. In set-up B, the effect of unilateral light is cancelled out by the rotation of the clinostat, and so the seedlings are equally illuminated on all sides. The shoots grow vertically upwards, and the roots grow vertically downwards. The results of Practical 16.4 show that shoots and roots respond differently to light. Shoots are positively phototropic. They respond to unilateral light by growing towards it. This brings the leaves to face the light source so that they can obtain the maximum amount of light for photosynthesis (Figure 16.28). Roots are negatively phototropic. They respond to unilateral light by growing away from it. This ensures that the roots grow downwards Figure 16.28 Plant shoots show positive phototropism. into the soil for anchorage. Taking it further Types of tropism Tropisms are classified according to the type of stimulus. In addition to phototropism, where light is the stimulus, two other common types of tropisms are geotropism and hydrotropism. Geotropism Geotropism is the growth movement of plants in response to gravity. Shoots grow away from the direction of gravity (negative geotropism), and roots grow towards the direction of gravity (positive geotropism).