CHAPTER 1 STIMULI AND RESPONSES The Human Nervous System 1. The human nervous system is divided into: a) The central nervous system (control centre) ; the brain controls all activities of the body & the spinal cord controls the involuntary actions like knee jerks. b) The peripheral nervous system transmits impulses from the sensory organs through the central nervous system to the muscles or glands 2. Voluntary actions are actions we aware of and involuntary actions are actions which carry on automatically. Human Nervous System The Brain Voluntary and Involuntary action Involuntary action Reflex arc when hand is touch hot object Eye 1. The eye is the sensory organ of sight that responds to the light. 2. Sight mechanism: a) Light rays from the object entering the eye through the cornea, aqueous humour, pupil, eyepiece and vitreous humour and focusing on the retina. b) The photoreceptors that are trigged and nerve impulses that are formed are sent through the optic nerve to the brain. 3. The rod cell in the retina is sensitive to light of different intensity, such as at night. It is not sensitive to colour and only a black and white image I produced. 4. The cone cell in the retina is sensitive to light of high intensity to detect colour. Choroid Sclera Retina Suspensory ligament Cornea Yellow spot Blind spot Pupil Aqueous humour Eye Lens Optic nerve Iris Ciliary muscles Vitreous humour Structure of Human eye Eyelid Pupil Sclera Iris Front view of eye Side view of eye The functions of the different parts of the eye PART Sclera Choroid STRUCTURE FUNCTION -White, tough and - Protect the eye fibrous coat - Gives the eye fixed shape - Dark colour - Absorbs light to - Has many blood prevent reflection of light inside the capillaries eye PART Retina Yellow spot STRUCTURE - Light-sensitive layer of the eye - Contains photoreceptors that detect light Shallow yellowish depression in the retina directly opposite the centre of the lens FUNCTION - Place on which images are formed - The area that is the most sensitive to light - The area where images are normally focused PART Blind spot STRUCTURE - Located immediately over the optic nerve - Has no photoreceptors Cornea - Transparent layer at the front of the eye - Curved shape FUNCTION - The spot in the eye where the optic nerve enters the eye - Not sensitive to light - Allow light to enter the eye - Directs light towards the lens PART STRUCTURE Conjunctiv - Thin transparent a membrane covering the exposed part of the eye Iris - Part of the choroid layer which can be seen from the front of the eye as a discshaped structure - Consists of muscles - Coloured, for example, brown or blue FUNCTION - Protects the cornea - Controls the size of the pupil thus controlling the amount of light entering the eye PART Pupil STRUCTURE - A small opening in the centre of the iris Ciliary - Consists of the muscles ciliary muscle - Part of the choroid layer FUNCTION - Enables light to enter the eye - Controls amount of light entering the eye - Contracts and relaxes to change the thickness of the lens PART Eye Lens STRUCTURE - A transparent, elastic and biconvex disc FUNCTION - Focuses light that enters the eye onto the retina to form an image - Focuses light from near and distant objects by changing its thickness PART Vitreous humour STRUCTURE - Transparent jelly-like substance - Fills the space behind the lens Suspensory - Consists of fibres ligaments attaching the lens to the ciliary body FUNCTION - Helps maintain the shape of the eyeball - Helps focus images on the retina - Holds the lens in position PART STRUCTURE Aqueous - Watery, humour transparent fluid - Fills the space between cornea and lens FUNCTION - Helps focus images on the retina - Helps maintain the shape of the eyeball - Enables oxygen and nutrients from the choroid to diffuse to the lens, cornea and conjunctiva PART Optic nerve STRUCTURE FUNCTION - Nerve - Carries impulses from connecting the the retina to the brain photoreceptors in the retina to the brain How Human See Light reflected from an object enters the eye through the pupil Lens bends the light rays and focuses them onto the retina Inverted image formed on the retina Brain interprets the image as upright Impulses formed and sent through the optic nerve to the brain Photoreceptors stimulated The Human Sight Mechanism Aqueous humour Cornea Eye lens Retina (Photoreceptor) Brain Pupil Vitreous humour Optic nerve Ear 1. The ear is the sensory organ for hearing. 2. Responds to sound stimuli. The Human Ear Outer - Pinna - ear canal (filled with air) - eardrum Middle - ossicles - oval window - Eustachian tube. (filled with air) Inner -Cochlea - semicircular canals - auditory nerve (contain fluid) Hearing Mechanism 1. The ear pinna receives and sends sound waves through the auditory canal to the eardrum. 2. The vibration from the ear drum are amplified by the ossicles and then sent to the cochlea through the oval window. 3. The cochlea converts sound vibration to nerve impulses and are transmitted through the auditory nerve to the brain. PART STRUCTURE Outer ear - Made of (a) Pinna cartilage and skin (b) Ear canal FUNCTION - Collects and directs sound waves into the ear canal -A narrow passage - Directs sound - Walls near the towards the ear outside of the ear drum covered with fine hairs -Leads to the ear drum PART Middle ear (a) Ear drum (b) Ossicles STRUCTURE FUNCTION - A thin membrane - Vibrates when sound waves reach it - Three small bones called the hammerbone, anvilbone and stirrupbone. - Amplifies and transmits vibrations of the ear drum to the membrane covering the oval window PART STRUCTURE (c) Oval window - A small opening covered by a membrane (d) Eustachian tube - A narrow tube - Connects the middle ear with the throat FUNCTION - Transmits vibrations from middle ear to inner ear - Balances air pressure on both sides of ear drum so that the ear drum cam vibrate freely PART STRUCTURE Inner ear - Coiled structure (a) - Filled with fluid Cochlea - Inner wall contain receptors sensitive to vibrations (b) - Nerve fibres that Auditory connect nerve receptors in the cochlea to the brain FUNCTION - Changes sound vibrations to nerve impulses - Transmits impulse from receptors in the cochlea to the brain PART (c) Semicircular canals STRUCTURE - Three semicircular canals positioned at right angles to each other - Contains sensory cells and fluid FUNCTION - Involved in maintaining the body balance - Not involved in hearing The Hearing Mechanism Impulses interpreted by the brain as sound The pinna catches sound waves and directs the sound waves to the ear Nerve impulses carried by the auditory nerve to the brain The ear drum vibrates when the sound waves strike it Vibrations amplified and transferred by the ossicles to the membrane of the oval window The membrane of the oval window vibrates Figure 4. How we hear Vibrations of the membrane of the oval window causes the fluid in the cochlea to move in waves and stimulate receptors that produce nerve impulses Nose 1. The nose is the sensory organ for smell. 2. Cells sensitive to smell are found in the epithelium located high in the nasal cavity. 3. Chemicals in the air dissolve in the mucus layers that coat the sensory cell of smell (olfactory cell) and then stimulate it to produce nerve impulses. Nose Structure Chemicals in the air Air enters nasal cavity through nostrils Chemicals dissolve in the mucus layer Brain interprets the messages as a specific smell Receptors (olfactory nerve) send messages to the brain Receptors stimulated by the chemicals The detection of Smell Tongue 1. The tongue is the sensory organ for taste. 2. Chemicals in food dissolve in saliva and are absorbed into the taste buds through the pores and stimulate the taste receptors in them to produce nerve impulses. 3. There are five tastes: sweet, salty, sour , bitter and umami 4. Tiny bumps found on the tongue called taste buds contain the receptors that detect different tastes. 5. Different areas of the tongue detect different tastes. 6. The saliva in the mouth has two functions: ◦ Dissolve substances so that they can be detected by the taste receptors. ◦ Make chewed food easier to swallow. Chemicals in food released by chewing Chemicals dissolved by saliva Taste receptors stimulated by chemicals in saliva Brain interprets the messages as a specific taste Taste receptors send messages to the brain Detected of Taste Skin 1. The skin has five receptors that are sensitive to heat, cold, pressure, touch and pain stimuli respectively. 2. The thinner the epidermis or the more receptors found on the skin, the more sensitive is that part of the skin. The Structure of Human Skin Three layers: Epidermis Dermis Hypodermis (fat layer) Epidermis - Outer layer of the skin. divided into three layers. - Outermost layer is made up of dead cells. - Tough and water-resistant . - It also protects the sensitive cells under it - Prevents the entry of germs into the body. Dermis - Inner layer of skin. - Blood vessels, glands and receptors are found -The glands ~sweat glands and sebaceous glands. - Receptors~ touch receptors, pain receptors, heat receptors, cold receptors and pressure receptors. Hypodermis (fat layer) - is a layer directly below the dermis and serves to connect the skin to the fibrous tissue of the bones and muscles. 1. Cold receptors sensitive to cold. 3. Pain receptors - nearest to the epidermis. - nerves endings. 2. Heat receptors sensitive to heat. Receptors in the Skin 4. Touch receptors - sensitive to slight pressure -found more abundantly in certain parts (fingertips). 5. Pressure receptors - sensitive to pressure. - located the furthest from the epidermis. - stimulated when any object presses hard against the skin. Sensitivity of the Skin at Different Parts of the Body 1. The skin on different parts of the body has different sensitivity. 2. The skin is more sensitive ~ fingertips, neck and cheek. 3. These areas have more touch receptors or a thinner epidermis. 4. Some areas of the body are less sensitive than others. 5. Example: the skin on the back, arms and legs have fewer nerves endings. 6. The elbows, knees and soles of the feet are not very sensitive because they have thicker epidermis. Limitation of the Sense of Sight 1. The limitation of the sense of sight is the ability limit the eye to see the object. 2. The optical illusion occurs when our brain cannot interpret accurately what is actually seen by the eye. 3. The blind spot does not have any photoreceptor and we cannot see an object if its image is formed on it. Optical Illusion Eye Defects In normal vision, light is focused accurately on the retina to form an image on the retina. This will produce a clear and sharp image on the retina. Defects of vision: (a) short sightedness. (b) long sightedness. (c) astigmatism. A short-sighted person 1. Can see nearby objects clearly but distant objects appear blur due to the image of the object which falls in front of the retina. 2. The eyeballs are too long and the eye lenses are too thick. This is because the ciliary muscles are too weak to make the eye lens thinner. 3. Can be corrected by wearing concave lenses. A long-sighted person 1. Can see distant objects clearly but nearby objects appear blur because the image of a near object falls behind the retina. 2. The eyeballs are too short and the eye lenses are too thin. This is because the ciliary muscles are too weak to make the eye lens thicker. 3. The defect can be corrected by wearing convex lenses. Vision defect causes Short sightedness Long sightedness Eyeball is too long or lens is too thick Symptom Can see near objects clearly but distant objects appear blur Eyeball is too short or lens is too thin Near objects appear blur but can see distant objects clearly Short sightedness Light focus in front of retina Long sightedness Light focus behind the retina Vision Short sightedness defect Method of Use a concave correction lens to diverge (spread out) light rays just before they enter the eyes so that image if formed further inside and exactly on the retina. Long sightedness Use a convex lens to converge light rays (make them come closer together) just before they enter the eyes so that image is formed at a shorter distance and exactly on the retina. Astigmatism 1. Caused by irregular curvature of the cornea or the lens. 2. The light rays from an object is split and focused at different points in the eye. 3. Some light may be focused on the retina, some light will be focused in front or behind the retina. 4. Causes blurred vision for both near and distant objects 5. Can be corrected with cylindrical lenses Limitation of Sight and Hearing Device to overcome the limitation of sight Limitation of Sight Limitation of sight Cannot see through objects that are not transparent Devices X-ray machine, ultrasound scanning device Cannot see objects that Binoculars, telescope are very far away Cannot see objects that Magnifying glass, are very tiny microscope Cannot see around corners Periscope Ultrasound machine Telescope Microscope X-ray machine Limitation of Sight and Hearing Device to overcome the limitation of hearing Stethoscope Hearing aid Microphone Loud speaker The Response in Plants 1. The response of plants to stimuli is called tropism. 2. The tropic movements such as phototropism are important to help the plants to obtain the basic needs like light, water and minerals. Stimuli and Responses in Plants Responses of Plants to Stimuli Plants respond by either positive tropism or negative tropism. Positive tropism- the growth of the part of the plant towards the direction of the stimulus. Negative tropism- the growth of the part of the plant away from the direction of the stimulus. Nastic movement – not dependent on the direction of the stimulus. Phototropism Phototropism- the growth of a plant to light. The shoots of plants show positive phototropism- they grow towards the light. The roots of plants show negative phototropism- they grow away from the light. Geotropism ◦ Response to gravity. ◦ Root show positive geotropism since they grow in the direction of gravity. ◦ Shoots show negative geotropism since they grow in the opposite direction to the pull of gravity Hydrotropism The growth of a plant part in response to water. Roots grow towards water- positive hydrotropism. Positive hydrotropism of roots is stronger than the positive geotropism of roots. When the direction of water source and the direction of gravity is not same, roots will grow towards the water. Thigmotropism ◦ Response towards touch or contact with a solid object. ◦ The stems and tendrils of climbing plants grow and twine around a support when they come into contact with the support. ◦ Stems and tendrils- positive thigmotropism. Use tendril use twinning stem Nastic Movement The response of a plant part to a stimulus in which the movement of the plant part is independent of the direction of the stimulus. Nastic movement takes place more quickly than a tropism. One type- seismonasty ~ Mimosa pudica and Venus flytrap. Nastic Movement Leaves of Mimosa pudica close immediately when they are touched. The leaves will open again after a while. Thus, these leaves show seismonastic response. Venus flytrap- closes and trap insects that land on it. Photonastiy - nastic movement involved in growth. Some flowers open in bright light and close in dim light or vice versa. Visions in Humans and Animals 1. Stereoscopic vision is the vision whereby both the eye located in front of the head. 2. The production of three-dimensional images enables humans and predators to estimate distance accurately. 3. Monocular vision is the vision whereby both of the eye are located at the side of the head. 4. The production of two-dimensional images makes the preys difficult to estimate distance accurately. 5. However, the field of monocular vision is wider. Monocular vision Properties of stereoscopic vision: a smaller field of vision compared to monocular vision the visual fields of both eyes overlap in the middle. it allows distance to be estimated more accurately it is found in predators such as tigers, cats, dogs, and owls. Properties of monocular vision: - a very wide field of vision compared to stereoscopic vision. - there is little or no overlap between the visual fields of both eyes. - distance cannot be estimated accurately. - it is found in prey such as mice, rabbits, fish and most birds. Both stereoscopic and monocular vision are important to the survival of animals. Predators hunt other animals – need to estimate the distance correctly to capture their prey. Prey- need to have wide field of vision in order to detect predators which may try to approach from the side or back.
