• UNIT 8 GENERAL PRINCIPLES OF RECEPTION AND RESPONSE IN ANIMALS

    UNIT 8: GENERAL PRINCIPLES OF RECEPTION AND

    RESPONSE IN ANIMALS.

    Key Unit Competence
    xplain the general principles of reception and response in animals.
    Learning Objectives
    By the end of this unit, I should be able to:
    – Explain the necessity of responding to internal and external changes in the
        environment.
    – Describe the main types of sensory receptors.
    – Discuss the main functions of a sensory system.
    – Explain the significance of sensory adaptation.
    – Describe the structure of the human eye.
    – Describe the structure of the retina.
    – Explain how rods transduce light energy into nerve impulses.
    – Explain how retinal convergence improves sensitivity.
    – Explain how the cones achieve visual acuity.
    – Explain how cone cells produce colour vision.
    – Discuss the significance of binocular vision.
    – Describe the structure of the human ear and the functions of its main parts.
    – Describe the process of hearing and balance.
    – Locate the taste buds on the tongue and sensory cells in the skin.
    – Observe the structure of the skin, retina, cochlea and vestibular apparatus from
        prepared slides or micrographs and relate them to their functions.
    – Interpret graphs on sensory adaptation in response to a constant stimulus.
    – Relate the number of retinal cells to sensitivity and visual acuity
    – Recognise the role of sense organs in the perception of different stimuli.
    – Appreciate the role of sensory adaptation in protecting the sense organs from

        overload with unnecessary or irrelevant information.

    Introductory activity
    This scenario is involving bat and moth, snail and a cultivating human. Imagine
    the situation in which a moth is flying in the darkness. At the same time there
    is a bat flying in the same zone. There is also another situation in which a snail
    is moving on the land as usual nearby its crawling area, there is a human who
    is cultivating in the land where the above snail is moving. The two scenarios

    are illustrated below

    1. What do you think would happen to a moth during the darkness when
         it is in area where the bat is living?

    2. What would be the reaction of the snail to the human digging?

    Animals realize different activities including searching for food, select a mate, and
    escape from predators. They also have the ability to feel changes in environmental
    factors and keep their internal environment within tolerable limits. These and
    other activities depend on the animal’s ability to gather information about what
    is happening inside and outside the body. The survival of animals depends upon
    the ability to respond in an appropriate way to environmental changes through the
    ability of detecting stimuli. Some other animals have become highly specialized to
    detect a particular form of energy by the use of specialized receptor cells which
    are able to perceive whichever form of energy and elaborate adequate response
    respond to nervous impulse.

    8.1 Types of sensory receptors and stimuli

    Activity 8.1
    Use the school library and search additional information on the internet, read
    the information related to different types of sensory receptors, while taking
    short notes on each type of sensory receptors. What are the main sensory

    receptors?

    The physical and chemical conditions in an animal’s internal and external
    environments are continually changing. A change that can be detected is called

    a stimulus. To some extent, all animal cells are sensitive to stimuli, and some cells

    called receptors have become especially sensitive to particular stimuli. There are
    a huge number of environmental variables that an animal could sense. However,
    each species has evolved receptors only to environmental variables that have an
    appreciable effect on its chances of survival. For example, humans can sense all the

    colors of the rainbow but can sense neither infrared nor ultraviolet light.

    Classification of receptors
    Receptors are commonly classified according to the type of stimulus energy they
    detect. The main types are:
    – Mechanoreceptors which detect changes in mechanical energy, such as
        movements, pressures, tensions, gravity, and sound waves.
    – Chemoreceptors which detect chemical stimuli, for example, through taste and
        smell.
    – Thermoreceptors which detect temperature changes.
    – Electroreceptors which detect electrical fields.

    – Photoreceptors which detect light and other forms of electromagnetic radiation.

    Receptors can also be classified according to their structure. Simple receptors, known
    as primary receptors, consist of a single neurone, one end of which is sensitive to
    a particular type of stimulus. A primary receptor gathers sensory information and
    transmits it to another neurone or an effector. For example, Pacinian corpuscles
    are mechanoreceptors located in the skin, tendons, joints and muscles. Their ends
    consist of concentric rings of connective tissue. Application of pressure against the
    connective tissue deforms stretch-mediated sodium ion channels in the cell surface

    membrane, causing an influx of sodium ions which leads to a generator potential.

         Figure 8.1: Primary receptor and secondary receptor (CNS: Central Nervous System)

    A secondary receptor is more complex. It consists of a modified epithelial cell which
    is sensitive to a particular type of stimulus. The cell senses changes and passes this
    information on to a neurone which transmits it as nervous impulse. Sense organs are
    complex stimulus – gathering structures consisting of grouped sensory receptors.
    In many sense organs, several receptors make synaptic connections with a single
    receptor neuron.
    A third classification of receptors is based on the source of stimulation and includes
    exteroceptors responding to stimuli outside the body, interceptors responding to
    stimuli inside the body, and proprioceptors respond to changes of joint angle and

    amount of tension in muscles.

    Application 8.1
    1. Describe the main types of sensory receptors
    2. Distinguish between a primary receptor and a secondary receptor
    3. Which type of receptor detects changes in the internal environment of the
          body?
    4. Which one of the five categories of sensory receptors is primarily dedicated

          to external stimuli?

    8.2 Components of the sensory system: transduction, trans-
    mission and processing

    Activity 8.2
    Use the school library and search additional information on the internet, read
    the information related to the sensory system while taking a short summary
    on sensory system, make a table showing the component and the functions
    of the sensory system. What do you think about those components and

    functions?

    8.2.1 Sensory systems
    Receptors are the first component of a sensory system, which has three main
    functions:
    Transduction: Receptor cells gather sensory information and then convert it
        into a form of information that can be used by the animal (nerve impulses)
    – Transmission: Sensory neurones transmit nerve impulses from the receptors
        to the central nervous system
    – Processing: the central nervous system processes the information so that
    appropriate responses can be made to environmental changes.
    A receptor converts the energy from the stimulus into an electrical potential that
    is proportional to the stimulus intensity. This graded electrical potential is known
    as the receptor potential or generator potential. If the stimulus is sufficiently high
    (above a critical threshold level) the graded potential is high enough to fire an action

    potential. If the stimulus is beneath the threshold, no action potential takes place.

    8.2.2 Sensory adaptation
    Receptors are adapted to detect potentially harmful or beneficial changes in the
    environment. When given an unchanging stimulus, most receptors stop responding
    so that the sensory system does not become overloaded with unnecessary or
    irrelevant information. Loss of responsive is brought about by a process called sensory
    adaptation. An unchanging stimulus results in a decline in the generator potentials
    produced by sensory receptors. Consequently, the nerve impulses transmitted in
    sensory neurones become less frequent and may eventually stop. The mechanism
    of sensory adaptation involves changes in the membranes of receptor cells and
    explains why, for example, a person becomes insensitive to the touch of clothing on

    skin. Even a hair shirt becomes tolerable after wearing it for a long period of time.

    8.2.2. Transferring information
    After gathering and transducing the stimuli, the sensory system transmits
    information about the stimulus to the central nervous system and effectors. The
    frequency of nerve impulses propagated along a sensory neurone usually gives
    information about stimulus strength. The transfer of information is rarely direct. In
    mammals, much of the sensory information goes to sensory projection areas in the

    brain where information processing takes place.

    Application 8.2
    1. Distinguish between an action potential and a generator potential
    2. Explain the significance of sensory adaptation
    3. Distinguish between transduction, transmission and perception
    4. If you stimulated a sensory neuron electrically, how would that

        stimulation be perceived?

    8.3 Structure and functioning of the eye

    Activity 8.3
    Dissection of a mammalian eye
    Materials needed:
    Diagram of a dissected eye, scissors (optional), wax paper, plastic garbage bag,
    a cutting board or other surface, on which you can cut, a sheet of newspaper,
    soap, water, and paper towels for cleaning up, one cow’s eye for every six

    learners, and one single-edged razor blade or scalpel for every team

    Procedure
    – Examine the outside of the eye and see how many parts you can identify.
    – Cut away the fat and the muscle.
    – Use scalpel to make an incision in the cornea.
    – Cut until the clear liquid in the cornea is released.
    – Use the scalpel to make an incision through the sclera in the middle of the
        eye.
    – Cut around the middle of the eye until you get two halves.
    – Remove the front part and place it on the board.
    – Cut the front part with scalpel or razor
    – During cutting of the front part, listen and explain what happens.
    – Pull out the iris between the cornea and the lens.
    – Observe in the centre of the iris after pulling out the iris.
    – Remove the lens and mention its texture.
    – Hold the lens in front of you and observe. What do you observe?
    – Empty the vitreous humor out of the eyeball.
    – Remove the retina and mention whether the spot is attached to the back of
         the eye.
    – Find the optic nerve and pinch the nerve with your fingers or with a pair of

         scissors. What do you see there?

    Questions
    1. Draw and label the internal structure of the mammalian eye.

    2. Write in your own words the functions of each part of a mammalian eye

    The eye is a complex light – sensitive organ that enables us to distinguish minute
    variations of shape, color, brightness, and distance. The function of eye is to
    transduce light (visible frequencies of electromagnetic radiation) into patterns of
    nerve impulses. These are transmitted to the brain, where the actual process of

    seeing is performed.

                                                                   Figure 8.2: External structure of human eye


                                                          Figure 8.3: Internal structure of human eye

    8.3.1. Functions of parts of eye
    – The lens: Refracts light and focuses it on retina. Made up of elastic material that
        adjusts when the eye focuses on far or near object.
    – The ciliary body: Made up of muscle fibres which contract or relax to change
        the shape or curvature of the lens. It produces aqueous humour.
    – The suspensory ligament: The suspensory ligament is a tissue that attaches
         the edge of the lens to the ciliary body.
    – The iris: It is coloured part of the eye, it has radial and circular muscles which
        control the size of the pupil; it has melanin pigment that absorbs strong light to
        prevent blurred vision.
    – Pupil: It is a hole at the centre of the iris through which light pass into the eye.
    – Aqueous humour: Has fluids to maintain the shape of eye ball and to refract
       light rays. It contains oxygen and nutrient for cornea and lens. It is a transparent
       and allow light to pass through
    – Vitreous humour: It is the space behind the lens and it is filled with fluids, a
       transparent, jelly-like substance. Vitreous humour keeps the eyeball firm and
       helps to refract light onto the retina.
    – Cornea: Is transparent part of the eye and allows the passage of light. It refracts
       light ray. It is made up of tough tissues to strength the eye.
    – Choroid: The choroid is the middle layer of the eyeball that lies between the
       sclera and retina. It has two functions, one being able to prevent internal
       reflection of light as it is pigmented black. Secondly, it contains blood vessels
       that bring oxygen and nutrients to the eyeball and remove metabolic waste
       product.
    – Retina: The retina is the innermost layer of the eyeball. It is the light sensitive
       layer on which images are formed. It contains light sensitive cells called
       photoreceptors. Photoreceptors consist of rods and cones. Cones enable us to
       see colours in bright light while rods enable us to see in black and dim light. The
       photoreceptors are connected to the nerve endings from the optic nerve.
    – Blind spot: The blind spot is the region where the optic nerve leaves the eye. It
       does not contain any rods or cones. Therefore, it is not sensitive to light.
    – Optic nerve: It is a nerve that transmits nerve impulses to the brain for
       interpretation when the photoreceptors in the retina are stimulated.
    – Fovea or yellow spot: It is a small yellow depression in the retina. It is situated
       directly behind the lens. This is where images are normally focused. The fovea
       contains the greatest concentration of cones, but has no rods. The fovea enables
       a person to have detailed colour vision in bright light.
    – Conjunctiva: Thin and transparent to allow light to pass through.
    – Sclera: It is a tough, white outer covering of the eyeball, which is continuous
       with the cornea. It protects the eyeball from mechanical damage.
    – The eye brows: Prevent sweat and dust from entering the eye.
    – The eye lashes: Prevent dust particles from entering the eye.
    – The tears glands: Secrete tears that wash away dust particles in the eye and

        keep the eye moist.

    8.3.2. Accommodation of the eye
    The ability of the eye to see far and near objects on the retina is possible because the
    eye is able to adjust the size of the lens and its power to bend light. Adjustment of
    the size of the lens is done by the ciliary muscles inside the ciliary body which exert
    a force on the suspensory ligament and then onto the lens. Changes that occur in

    the eye during accommodation include:

                                                Figure 8.4: Illustration of seeing a near object

    When the eye focuses on a near object, several changes occur:
    – The ciliary muscles contract, relaxing their pull on the suspensory ligaments.
    – The suspensory ligaments slacken, also relaxing their pull on the lens.
    – The lens, being elastic, becomes thicker and more convex, decreasing its focal
        length.
    – Light rays from the near object are sharply focused on the retina.
    – Photoreceptors are stimulated.
    – The nerve impulses produced are transmitted by the optic nerve to the brain.
       The brain interprets the impulses and the person sees the near object.
    b. Focusing on a distant object: When a person is looking at a distant object,
       the light rays reflecting off the object are almost parallel to each other
       when they reach the eye. These ‘parallel’ light rays are then refracted

       through the cornea and the aqueous humour into the pupil


                                                Figure 8.5: Illustration of seeing a distant object

    When the eye focuses on a distant object, several changes occur.
    – The ciliary muscles relax, pulling on the suspensory ligaments.
    – The suspensory ligaments then become taut, pulling the edge of the lens.
    – The lens become thinner and less convex, the focal length is increased. The
        focal length is the distance between the middle of the lens and the point of
        focus on the retina.
    – Light rays from the distant objects are sharply focused on the retina and
        photoreceptors are stimulated.
    – The nerve impulses produced are transmitted by the optic nerve to the brain.
       The brain interprets the impulses and the person sees the distant object

    Table 8.1. Summary of changes that occur in the eye during accommodation



    8.3.4. Some changes that occur in eye when you see in bright and dim

    light
    In bright light
    – Circular iris muscle contracts.
    – The radial iris muscles relax.
    – The iris elongates in wards each other.
    – The pupil is reduced (narrowed).

    – Small amount of light rays enters the eye.

             Figure 8.6: Illustration of changes that occur in eye when you see in bright light

    Table 8.2: Illustration of changes that occur in eye during bright and dim light



    8.3.5. The retina of the eye
    The retina possesses the photoreceptor cells. These are of two types, cones and rods.
    Both converts light energy into the electrical energy or nerve impulses. Both rods
    and cones are embedded in the pigment epithelial cells of the choroid layer. In cats
    and some other nocturnal mammals. They have reflecting layer called the tapetum

    which reflects light back into the eye and so allow the rod cells to absorb it.

                                                                     Figure 8.8: Structure of the retina


                                                         Figure 8.9: Structure of photosensitive cells

    8.3.6. Adaptations of photosensitive cells.

    – They have numerous mitochondria to provide energy in form of ATP.
    – They have photosensitive pigment i.e. rhodopsin in rods and iodopsin in cones
         to absorb light rays.
    – They have lamellae (vesicles) to increase the surface area for holding the
        pigment molecules.
    – Many rods cells share a single bipolar neurone such that a single stimulation
        builds up a big generator potential.
    8.3.7. Changes which occur on rod cells when light strikes the retina
        Each rod cell has in its outer segment up to 1000 vesicles, each containing a

        photosensitive pigment called rhodopsin. Rhodopsin is made up of the protein

    opsin and retinal, a derivative of vitamin A. Light causes retinal to change shape
    from its normal cis-isomeric form to trans-isomeric form. As result, retinal and opsin
    break apart. This process is called bleaching. This triggers a series of events which
    alters the permeability of rod’s cell surface membrane.
    If light stimulation exceeds the threshold level, an action poetical is set up in a
    bipolar neurone, and then passes along a neurone in the optic nerve. The pattern of
    nerve impulses transmitted along different neurones is interpreted in the brain as
    patterns of light and dark. Before the rod cell can be activated again, the opsin and
    retinal must first be resynthesized into rhodopsin.
    This re-synthesis is carried out by the mitochondria found in the inner segment
    of rod cell, which provide ATP for the process. Re-synthesis takes longer time than
    splitting of rhodopsin but is more rapid in lower light intensity. Rhodopsin of rods
    spits into opsin protein and retinal (derivative of vitamin A). About 3 minutes are
    required to reform again. That is why our eyes need some minutes to adapt to dark

    when we come from bright light.

    The splitting of iodopsins of cone cells also produces an action potential (impulse)
    but they quickly re-form. There are three types of iodopsins and each responds to
    the wavelength of a particular colour: red – green – blue.
    The impulses are then transmitted along the optic nerve to the visual area of the
    brain. There, the image is interpreted. Note that the image that is cast on the retina
    is virtual I to mean not real, small, inverted upside down and laterally, and reversed

    for example from right to left.

    8.3.8. Changes which occur on cones when light strikes the retina
    When light of high intensity strikes the cones, the iodopsin pigment decomposes
    into iodide ions and opsin, this process is called bleaching. On the contrary, when
    enough iodopsin is decomposed, the membrane develops an action potential
    when it reaches threshold level. An impulse is fired via bipolar neurone to the optic
    nerve to the brain for interpretation. A comparison between cone and rod cells is

    summarized in the table 8.3.

    Table 8.3: Differences between rods and cones


    8.3.9. The process of vision
    When light enters the eye, it is refracted by the curved surface of the cornea, the
    lens, the aqueous and vitreous humour. The refraction of light causes the image to
    be formed upside down on fovea centralis. When cones and rods are stimulated by
    light, they send impulses through the optic nerves to the brain where the correct
    impression of the object is formed

    Colour vision in organism is explained by the trichromatic theory which states that,
    there are three forms of iodospin each responding to light of different wave length
    that is each responds on one of the three primary colours which are, blue, green
    and red. When these colours are mixed in appropriate intensities they can give rise
    to any other colour for example equal stimulation of red and green cones gives
    yellow perception. Alternative theory of colour vision known as the retinex theory,
    suggests that the brain cortex as well as retina is involved in colour perception. This
    would explain why we usually perceive a particular object as being the same colour

    under different types of illumination.

    a. Stereoscopic vision: combining two images
    Having two eyes (binocular vision) is better than having one because it gives a larger
    field of vision, a defect in one eye does not result in blindness. In animals with two
    forward facing eyes, it provides the potential for stereoscopic vision which depends
    on each eye being able to look at the same object from slightly different perspective.
    The visual centre in the brain combines the two views to make a three dimensional
    image. Stereoscopic vision provides information about the sizes and shapes of object
    and enables distance to be judged accurately. However, because the eyes have to
    be relatively close together for stereoscopic vision, the field of vision is relatively
    small. Mammalian predators tend to have well developed stereoscopic vision, while
    herbivores tend to have eyes wide apart, sacrificing stereoscopic vision for a wide

    field of view

    b. Nocturnal animals
    Nocturnal animals have a lot of rods in their retinas, but no cones. The levels of light
    at night are very low, so even if the animals have lot of cones, they would not be able
    to see in colour because the level of light is too low to stimulate the cone cells. At
    night, animals need to be able to detect shape and movement and the very sensitive

    rod cells are ideal of this because they are stimulated by very low levels of light.

    Application 8.3
    1. What is meant by the term adaptation of the eye?
    2. Describe the adjustments which occur in the eye in bright and dim
        light.
    3. If you perceive an object floating across your field of view, how can you
        determine whether the image represents a real object or a disturbance
       in your eye or a neural circuit of your brain?
    4. Distinguish between visual acuity, adaptation and photoreception of
       the eye
    5. Describe the shape of the lens when the eye is focused on a near object?
    6. Study the section of the human eye and then complete the table, by

        filling in the letter and the name of the correct part

    7. Which type of photoreceptors occur in the fovea

    8.4 Structure and functioning of the ear
    Activity 8.4
    Use textbooks and other additional sources (e.g. internet), read the information
    related to the human ear and make notes about it.
    1. Draw and label a diagram of human ear

    2. Give the functions of each part of the ear

    The human ear is a complex sensory organ that enables us to hear sounds, detect
    body movements, and maintain balance. The ear has three main parts: an air-filled

    (outer ear), an air-filled middle ear, and a fluid- filled inner ear

                                                   Figure8.10: Illustration of external and internal structures of human ear

    Each part of the ear has specifc feature and function as it is indicated in the table 8.4.

    Table 8.4: The functions of the parts the ear


    8.4.1. Sound perception in the ear (Hearing)
    The most function of the ear is hearing. The hearing process include the following
    processes:
    – Sound waves are collected by the pinna and directed to the auditory canal,
        which then strike the ear drum (tympanic membrane)
    – The sound waves cause the tympanic membrane to vibrate and the vibrations
        are sent to the ossicles.
    – The ossicles amplify the vibration and amplified vibration are received by
       the oval window that setting up vibration in the perilymph of tympanic and
       vestibular canal.
    – Vibration in perilymph cause movement of Reissner’s membrane which in
        turn displaced relative to the tectorial membrane, the sensory hair cell located
        between the basilar membrane and tectorial become distorted.
    – This distortion set up an action potential, which is transmitted along the

        auditory nerve to the brain which interprets the impulses as sound.

                             Figure8.11: The diagram showing the process of hearing

    8.4.2. The cochlea and the organ of corti
    The cochlea is coiled around above and their internal region is crossed by two
    membranes, i.e. upper Reissner’s membrane and lower basilar membrane. In
    between there is a membrane which is short called tectorial membrane. From the
    basilar membrane are sensitive sensory hair cells whose hair tips are close to the
    tectorial membrane. These cells have fibres which take impulses to the brain along
    the auditory nerve for interpretation. The upper and lower chambers of the cochlea
    are filled with perilymph while the middle chamber is filled with endolymph. The
    basilar membrane, tectorial membrane, Reissner’s membrane and sensitive hair cells

    are collectively known as the organ of corti and are directly concerned with hearing.


                                              Figure 8.12: Structure of cochlea and organ of corti

    8.4.3. The vestibular apparatus and sense of balance
    Our sense of balance and information about position and movement come from
    the vestibular apparatus in the inner ear. The vestibular apparatus consists of the
    semicircular canals, containing organs called cristae sacs including the saccule and
    utricle. The utricle and saccule are receptors containing sense organs called maculae
    that give information on the position of head in space in relation to gravity (static
    equilibrium).
    These receptors consist of sensory hair cells which are embedded in fine granules
    of calcium carbonate called otoliths. According to the position of the head, the pull
    of gravity on the otolith will vary and otolith will be titled accordingly. The different
    distortions of the sensory cells that result from impulses discharge in the vestibular
    nerve fibres and this is interpreted by the brain, which sends impulses to the relevant
    organs which then restore the balance of the body


                           Figure 8.13: The diagram illustrating the macula

    8.4.4. The role of semicircular canals in the maintenance of balance
    Semicircular canals are responsible for maintaining the balance of the body during
    motion (dynamic equilibrium). These are fluid – filled canals, three in number and
    arranged in three mutually perpendicular planes: vertical canals detect movement
    in the upward direction, horizontal canals detect back ward and forward motion
    while lateral canals detect sideways movement of the head.

    A swelling, the ampulla in the canal contains the receptor. This consists of sensory
    hair cells supported by hairs embedded in a dome – shaped of a gelatinous structure
    called cupula. Movements of head in any of the planes causes the fluid in the relevant
    canal to move and therefore displacing the cupula. Due to inertia, the cupula is
    deflected in direction opposite to that of head. This put strain on the sensory cells
    and causes them to fire impulses in the different nerve fibres to the brain.
    The pattern of impulses sent to the brain varies depending on the canal stimulated.
    The brain interprets impulses and detects the speed and direction of movement
    of head. Then impulses from brain are sent to the relevant organs which then

    maintained the balance of the body.


                                                                      Figure 8.14: Diagram of semi-circular canals

                                             Figure 8.15: Internal structure of semicircular canal

    8.4.5 Ear as a balance organ
    The vestibular apparatus is concerned mainly with detecting changes in the head
    position and body posture. When the head moves quickly, the cupula, knob in the
    ampulla, moves in the opposite direction. Sensory hairs below the cupula detect the

    impulse that is brought by a vestibular nerve to the brain.





                                                            Figure 8.16: Illustration of the ear as a balance organ

    Likewise, as the head moves by changing its posture, some crystals of CaCO3 known

    as otoliths also move. The membrane of the otoliths also moves pulling on the

    sensitive hairs and making them bend. The sense cells are stimulated to varying
    degrees, causing an action potential to be sent to the cerebellum (hindbrain) that
    actually controls the muscles in maintenance of body balance. The cerebellum sends
    out impulses to the muscles of the body which contract or relax or maintain body

    balance.

    Application 8.4
    1. In which part of the ear are the organs of balance?
    2. What is the role of ossicles during transmission of sound waves?
    3. Which structure equalizes the pressure on either side of the eardrum?
    4. Distinguish between pitch and intensity of sound
    5. Suppose a series of pressure waves in your cochlea causes a vibration of
        the basilar membrane that moves gradually from the apex toward the
        base. How would your brain interpret this stimulus?
    6. If the stapes became fused to the other middle ear bones or to the oval

        window, how would this condition affect hearing? Explain

    8.5 Structure and functioning of the tongue
    Activity 8.5
    Use the school library and search additional information on the internet,
    read the information related to the tongue while taking a short summary on
    tongue, list all taste buds on the tongue and answer the following questions:
    1. Which taste buds are found at the tip of the tongue?

    2. Which taste buds are found on sides of the tongue?

    The tongue is the receptor organ for taste. Taste is due to chemicals taken into the

    mouth and for this reason the tongue is called chemoreceptor.

    The tongue is able to distinguish between four different kinds of taste including
    sweet, sour, salt and bitter which are also called primary taste. This is possible
    with the help of group of sensory cells found in taste buds located on the surface
    of the tongue in specific taste areas through four types of taste buds in which they
    are located in overlap as shown on the Figure 8.18, the detection of sour and bitter
    substances is important for they can be easily rejected if harmful. For a chemical
    to be tasted it must be dissolved in the moisture of the buccal cavity where it can

    stimulate the sensory cells grouped in taste buds.

    Different types of taste and their sites on the tongue
    In human, there are four kinds of taste including sweet, salty, sour and bitter.
    Different taste buds are sensitive to different chemicals: Those which are sensitive to
    sugary and salty fluids are usually found at the tip of the tongue while those at the
    sides of the tongue are sensitive to acidic substances and thus give the sensation of

    sourness while those at the back are responsible for the sensation of the bitterness.

                                                              Figure 8.18: Location of different papillae


                                                                              Figure 8.18: Location of different papillae

    Application 8.5
    1. Explain why some taste receptor cells and all olfactory receptor cells use
    G protein-coupled receptors, yet only olfactory receptor cells produce
       action potentials
    2. If you discovered a mutation in mice that disrupted the ability to taste
       sweet, bitter, but not sour or salty, what might you predict about the

       identity of the signalling pathway used by the sour receptor?

    8.6 Structure and functioning of the skin
    Activity 8.6
    Use the school library or the internet, make a research about the human skin
    and make a short summary on it with all the sensory cells in it
    1. Draw and label a diagram of human skin
    2. How many types of sensory cells found in human skin?

    3. Write in your own words the functions of each part of human skin.

    The human skin is the largest organ of the body. Being a vast organ, it has
    many functions including protection from microbes, regulation of the body
    temperature, and permits the sensations of touch
    , heat, and cold. This is possible
    thanks to the presence of different glands. The skin consists of three main layers: The
    epidermis, the outermost layer of skin that provides a waterproof barrier and creates
    our skin tone, the dermis, beneath the epidermis that contains tough connective
    tissue, hair follicles, and sweat glands and the deeper subcutaneous tissue called
    hypodermis that is made of fat and connective tissue.

    The epidermis consists of three regions:

    – The Cornfield layer also known as keratinized layer. This is the thin outermost
        layer made up of dead cells. It is resistant to bacterial infections and damage,
        and reduces water loss from the body. It is very thick on the soles of the feet and
        the palm and is also modified as nails.
    – The Granular layer that contains living cells which give way to the cornfield
         layer.
    – Malpighian layer that is the continuous layer of living cells and they
        continuously divide to produce new cells. This layer has melanin pigment
        granules that determine the skin colour and act as screen against ultraviolet
        light.
    The dermis consists of the thick connective tissue. It consists of blood capillaries,
    receptors (sensory organs), lymphatic, sweat glands, sebaceous glands and hair
    follicles with different functions:
    – Capillaries supply food and oxygen, remove excretory waste products and
    help in temperature regulation.
    – Sweat glands are coiled tubes consisting of secretory cells with duct that
    passes sweat to the skin surface.
    – Hair follicles are deep pit (hole) of cells which divide and build the hair inside
       the follicle. They are richly supplied with sensory nerve endings which are
       stimulated by the hair movements.
    – Sebaceous gland opens into the hair and secretes oil which makes the hair
       waterproof.
    – Sensory nerve endings include sensory receptors for temperature, touch,
       pressure and pain.
    Subcutaneous layer attaches dermis to underlying structures, composed of adipose
    and connective tissue. It serves as shock absorbers for vital organs, it stores energy. It

    varies in thickness according to age, sex, general health of individual.

                                                                                  Figure 8.19: Human skin structure


                                              Figure 8.20: Figure the location of human skin receptors

    A comparative study of sense organs
    Sense organs have different biological functions beneficial to the living organisms.
    A brief summary is given in the table 8.5.

    Table 8.5: The functions of sense organs

    Application 8.6
    1. Describe how the skin contributes to the regulation of body temperature,
        storage of blood, protection, sensation, excretion and absorption, and
         synthesis of vitamin D.
    2. Why do eating food containing hot peppers sometimes cause you to sweat?
    3. If you stimulated a sensory neuron electrically, how would that stimulation

         be perceived?

    End of unit assessment 8
    A. Multiple choice questions: choose the best answer
    1. Human receptors are classified into:
    a. sensory and motor receptors
    b. Photoreceptors, mechanoreceptors, chemoreceptors,
        thermoreceptors
    c. Pacinian, Meissner, and Ruffini receptors
    d. Central, peripheral and sympathetic receptors
    e. Mechanical, electrical and gravitational
    2. The eye contains:
    a. Mechanoreceptors
    b. Photoreceptors
    c. Chemoreceptors
    d. Proprioceptors
    3. The small bones located in the middle ear, collectively as ossicles,
        include:
    a. Tympanum, oval and round windows.
    b. Pinna, vestibule and Eustachian.
    c. Malleus, incus, and stapes.

    d. Ossicles I, II and III.

    B. Answer by True or False
    4. Pain receptors are a type of mechanoreceptor.
    5. Receptors for a particular sensation, such as touch, are spread evenly
          throughout the skin surface.

    6. The image formed on the retina is inverted.

    C. Essay questions
    7. Describe what would happen to rhodopsin when it absorbs light
    8. According to the trichromatic theory of colour vision, discuss which
        colours of light are the three different types of cone sensitive to.
    9. The diagram represents enlarged section of part of the retina and

        choroid of a human eye.

    a. Draw an arrow on a sketch of the diagram to show the direction in
         which light passes through the retina
    b. Suggest a function of the black pigment which occurs in the choroid
         layer of the eye
    c. Use information in the diagram to explain how a person is able to:
         i. see light of low intensity
         ii. see in great detail in bright light
    10. Describe the significance of three semi-circular canals being in different

         planes?

    UNIT 7 EXCRETION AND OSMOREGULATIONUNIT 9 NERVOUS COORDINATION