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Chapter 1: Stimuli and Responses 1.2 Stimuli and Responses in Humans Humans face constant changes in surroundings. These changes are called stimuli. Examples of stimuli include light, sound and chemical substances. Humans use their sensory organs to detect stimuli. Humans have five sensory organs: eyes, ears, nose, skin and tongue as shown in Photograph 1.6. Which sensory organ is the largest? Eyes (sense of sight) Ear (sense of hearing) Nose (sense of smell) Tongue (sense of taste) Skin (sense of touch) Photograph 1.6 Human sensory organs Eye Study Figure 1.7. Can you identify the parts of the eye? Let us learn more about the parts of the eye by referring to Figure 1.8 on page 12. Sclera Iris Pupil Figure 1.7 Front view of the eye 1.2.1 11 Ciliary muscle Eye lens Sclera Choroid 12 Muscle that changes Transparent and Strong layer that maintains Black layer that prevents the thickness of the elastic convex the shape of the eye reﬂection of light in the eye and eye lens through lens which and protects it. supplies oxygen and nutrients Suspensory contractions focuses light to the eye. ligaments and relaxations. onto the retina. Strong ﬁbres which hold the Retina eye lens in its Layer containing position. photoreceptors which detects light and produces nerve impulses. Cornea Transparent layer which refracts and Yellow spot focuses light onto Part of the retina which is the retina. most sensitive to light as it has many photoreceptors. Iris The coloured part of the eye which controls Optic nerves the size of the pupil. Nerve ﬁbres which carry nerve impulses from the Pupil retina to the brain to be Opening in the interpreted. centre of the iris which controls the Aqueous humour Vitreous humour Blind spot quantity of light entering the eye. Transparent ﬂuid which Conjunctiva Transparent jelly-like substance Part of the retina which is not maintains the shape of Transparent membrane which maintains the shape of sensitive to light as there are no the eyeball and focuses which protects the front the eyeball and focuses light photoreceptors and an exit point for light into the eye. part of the sclera. onto the retina. all optic nerve ﬁbres. 1.2.1 Figure 1.8 Parts of the human eye and their functions Chapter 1: Stimuli and Responses What is the Colour of the Object Seen? The retina has two types of photoreceptors: rod cells and cone cells as shown in Figure 1.9. Rod cells are sensitive to different light intensities including faint light but are not sensitive to the colours of light. Cone cells are sensitive to the colours of light under bright conditions. There are three different types of cone cells, where each is sensitive to red light, green light and blue light. Rod cell Light Cone cell Vitreous humour Sclera Retina Choroid Figure 1.9 Photoreceptors – rod and cone cells Ear What are the parts of the ear and their functions? Study Figure 1.10 and Table 1.1 on page 14. Outer ear Middle ear Inner ear Ossicles Semicircular canals Auditory nerve Cochlea Earlobe Ear canal Eardrum Oval Eustachian tube window Figure 1.10 Parts of the human ear 1.2.1 13 Table 1.1 Functions of the parts of the human ear Part of ear Structure of ear Functions Earlobe Collects and directs sound waves into the ear canal Outer ear Ear canal Directs sound waves to the eardrum Eardrum (thin Vibrates according to the frequency of the sound waves membrane) received and transfers the vibrations to the ossicles Ossicles (made up of Amplify sound vibrations and transfer them to the three small bones) oval window Middle ear Oval window Collects and transfers sound vibrations from the ossicles to the cochlea Eustachian tube Balances the air pressure on both sides of the eardrum Cochlea (contains fluid) Detects and converts sound vibrations into nerve impulses Semicircular canals Detect the position of the head and help to balance the Inner ear (contain fluid) body Auditory nerve Sends nerve impulses from the cochlea to the brain to be interpreted Nose What are the parts of the nose? Study Figure 1.11. Nerve Nerves to the brain Nasal cavity rs ente Air Sensory cells for smell (smell receptors) Nose Sensory cells for smell Mucus Tongue Nostrils Figure 1.11 Parts of the human nose 14 1.2.1 Chapter 1: Stimuli and Responses Structure of the Nose The nose is the sensory organ of smell. Smells are chemical BRAIN substances present in the air. About 10 million sensory TEASER cells for smell are located at the roof of the nasal cavity Why is a person suffering from flu as shown in Figure 1.11. normally unable to detect smells? Function of Sensory Cells for Smell Sensory cells for smell are tiny and covered with a layer of mucus. Chemical substances in the air will dissolve in this layer of mucus and stimulate the cells to produce nerve impulses. The nerve impulses are then sent to the brain to be interpreted to determine the type of smell. Tongue What are the parts of the tongue? Study Figure 1.12. Pore Supporting cell Taste receptor Nerves to the brain Papillae on the tongue Taste bud on the papillae Tongue Figure 1.12 Parts of the human tongue Structure of the Tongue The tongue is the sensory organ of taste. Observe the SCIENCE INFO surface of your tongue using a mirror. There are tiny Umami is classified as a basic nodules known as papillae on the surface of the tongue. taste because there are taste The surface of a papillae is covered by hundreds of taste receptors that can only detect buds. Each taste bud contains 10 to 50 taste receptors. umami taste. This is the same as These taste receptors can detect five types of basic tastes other basic tastes such as sweet, which are sweet, salty, sour, bitter and umami. salty, sour and bitter. Umami taste is related to delicious tastes such Function of Taste Buds as the taste of meat in soups or the taste of fermented foods such When food is chewed, part or all the chemical substances as cheese and mushrooms or in the food dissolve in the saliva. These dissolved chemical monosodium glutamate (MSG). substances will diffuse into the taste buds through their pores and stimulate the taste receptors in them to produce nerve impulses. These nerve impulses are then sent to the brain to be interpreted as sweet, salty, sour, bitter, umami tastes or a combination of the basic tastes. 1.2.1 15 Skin What are parts of the skin? There are five types of receptors found in the skin. What are their functions? Study Figure 1.13. Pain receptor Cold receptor Detects stimuli that Detects cold stimuli causes pain Hair Oil gland Epidermis Heat receptor Detects heat stimuli Dermis Touch receptor Detects touch stimuli Fat layer Nerve Pressure receptor Detects pressure exerted Figure 1.13 Parts of the human skin The skin is the largest sensory organ in the human BRAIN body. The human skin is made up of a thin outer layer TEASER known as epidermis and an inner layer known as dermis. How does the skin function as a The skin has five types of receptors at different positions sense of ‘sight’ for the blind? to detect different stimuli as shown in Figure 1.13. State the five types of stimuli which can be detected by the receptors in the skin. When the receptor in the skin is stimulated, nerve impulses are produced and sent through the nervous system to the brain to be interpreted and to produce an appropriate response. 16 1.2.1 Chapter 1: Stimuli and Responses Mechanism of Hearing How do we hear? Study Figure 1.14. Ossicles Brain Auditory nerve Cochlea Earlobe Ear Ear canal Eardrum Oval window Direction of sound sound sound vibrations Source Ossicle Earlobe Ear canal Eardrum of sound bones Earlobe e receives rec Ear canal channels thee The vibrations ns are and gathers sound waves to the ampliﬁed by the vibrations sound waves. eardrum causing it to ossicles and then vibrate. sent to the cochlea through e ova the oval window. nerve nerve impulses impulses vibrations Sound is Auditory Oval Brain Cochlea interpreted nerve window The nerve rve im impulses are then sent through the auditory en se audi Nerve cells in n the cochlea convert the nerve to the brain to be interpreted. sound vibrations to nerve impulses. Figure 1.14 Mechanism of hearing in humans Activity 1.4 To study the mechanism of hearing using a model CPS, ICS Instructions Innovation- 1. Work in groups. based activity 2. Each group is required to present the mechanism of hearing using a model prepared by the teacher. 3. Construct a flow chart that shows the direction of sound in the mechanism of hearing. 1.2.2 17 Mechanism of Sight How do we see? Study Figure 1.15. Ciliary muscle Vitreous humour Light rays Retina from an object Cornea Brain Aqueous humour Eye lens Optic nerves Light rays from an object enter the eye through the cornea, aqueous humour, eye The light rays stimulate The brain interprets the lens and vitreous humour before reaching photoreceptors to nerve impulses. The the retina. These parts of the eye focus the produce nerve impulses smaller inverted image light rays from the object onto the retina. that are sent to the on the retina will then The object appears smaller and inverted. brain. appear upright. Figure 1.15 Mechanism of sight in humans Activity 1.5 To study the mechanism of sight using a model CPS, ICS Instructions Innovation- 1. Work in groups. based activity 2. Each group is required to present the mechanism of sight using a model prepared by the teacher. 3. Construct a flow chart that shows the direction of light in the mechanism of sight. Photograph Ph t h 11.77 A human h eye model dl 18 1.2.2 Chapter 1: Stimuli and Responses Sensitivity of the Skin on Different Parts of the Body towards Stimuli Photograph 1.8 shows a few examples of daily activities of humans which make use of the sensitivity of skin on different parts of the body towards different stimuli. Photograph 1.8 Sensitivity of the skin on different parts of the body Why are the daily activities shown in Photograph 1.8 carried out on different parts of the body? Let us investigate this in Activity 1.6. Activity 1.6 Inquiry-based activity To investigate the sensitivity of the skin on different parts of the body towards the touch stimulus Material Cellophane tape Apparatus Ruler (30 cm), toothpick and handkerchief (or blindfold) Instructions 1. Work in pairs. 2. Set up the apparatus as shown in Figure 1.16. Toothpick 2 Ruler Cellophane tape Toothpick 3 Toothpick 1 Figure 1.16 Using the cellophane tape, attach: Toothpick 1 on the 0 mark of the ruler. Toothpick 2 opposite the first toothpick on the ruler. Toothpick 3 on the 0.5 cm mark of the ruler. 1.2.3 19 Different Areas of the Tongue are More Sensitive to Specific Taste KEY: Areas of the tongue are sensitive to all five tastes. Bitter However, different areas of the tongue have different sensitivities towards specific taste. For Sour example, the area in front of the tongue is more Sweet sensitive to sweet taste whereas the sides are more sensitive to sour and salty tastes. The area at the Salty back of the tongue is more sensitive to bitter taste. The area at the centre of the tongue, however is Umami more sensitive to umami taste. Study Figure 1.19. Figure 1.19 Different areas of the tongue BRAIN are more sensitive to specific taste TEASER Nowadays, there is a toothbrush equipped with a tongue cleaner. Does the use of the tongue cleaner reduce the sensitivity of the tongue? Combination of the Sense of Taste and the Sense of Smell Look at Photograph 1.10. Can the child enjoy the fried chicken? Does the sense of smell play a role when a person tastes food? Let us investigate this matter in Photograph 1.10 Food eaten Activity 1.8. without smelling Activity 1.8 Inquiry-based activity To investigate the relationship between sense of taste and sense of smell Materials Cordial drinks of different flavours (grape, orange, mango, strawberry) and distilled water Apparatus Handkerchief (or blindfold) and cups Instructions 1. Work in pairs. Your teacher will provide each pair of students with cordial drinks of different flavours such as grape, orange, mango and strawberry in different cups. 2. Cover the eyes of your partner with a handkerchief and ask him to pinch the nose as shown in Photograph 1.11. 3. Give your partner a cup of distilled water and ask him to gargle. 22 1.2.3

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