UNIT 8 GENERAL PRINCIPLES OF RECEPTION AND RESPONSE IN ANIMALS
UNIT 8: GENERAL PRINCIPLES OF RECEPTION ANDRESPONSE 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 fromoverload 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 scenariosare 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 sensoryreceptors?
The physical and chemical conditions in an animal’s internal and external
environments are continually changing. A change that can be detected is calleda 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 thecolors 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 surfacemembrane, 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 andamount 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 dedicatedto 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 andfunctions?
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 actionpotential. 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 onskin. 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 thebrain 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 thatstimulation 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 sixlearners, 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 ofscissors. 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 ofseeing 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 andkeep 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 inthe 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 refractedthrough 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 tapetumwhich 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 aphotosensitive 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 darkwhen 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 reversedfor 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 issummarized 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 colourunder 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 widefield 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 sensitiverod 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, byfilling 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 ear2. 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 theauditory 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 cellsare 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 thenmaintained 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 theimpulse 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 knownas 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 bodybalance.
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 ovalwindow, 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 themouth 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 canstimulate 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 ofsourness 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 theidentity 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. Itvaries 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 stimulationbe 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 andchoroid 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 differentplanes?