O Level Notes : Biology - Animal Receptor Organs

Sense organs are the organs through which we sense or detect changes in the external environment. Each sense organ has special sensory cells, which receive the stimuli and transmit the impulses produced through the concerned nerve to the brain or the spinal cord.


Sense organs are the organs through which we sense or detect changes in the external environment. Each sense organ has special sensory cells, which receive the stimuli and transmit the impulses produced through the concerned nerve to the brain or the spinal cord. The brain sorts out the impulses, interprets them and transmits message for the required response. In human there are typically five sense receptors, eyes for seeing, ears for hearing, nose for smelling, tongue for taste and skin for sensing touch, pain, heat, etc.


The eye is nearly spherical in shape, bulging a little in front, and is able to rotate freely in the bony socket. It is a hollow ball containing several structures inside The wall of the eyeball is made up of three layers: the sclera, choroid and retina.

_ Sclera is the outermost tough white layer. In front it is continued as the transparent cornea.

_ Choroid is the middle layer. It is composed of connective tissue having a dense network of blood vessels. Its inner surface is dark brown or black. This prevents reflection, which would otherwise interfere with the clarity of the image.

_ Retina is the innermost sensitive layer. It contains two kinds of sensory cells–the rods (sensitive to dim light) and cones (sensitive to bright light and colours).

_ Yellow spot lying at the visual axis is the place of best vision in the normal

eye. It contains maximum number of sensory cells and particularly the cone.

The rest of the retina has fewer cones and more rods.

_ Blind spot is the point where the nerve fibres (axons) from all the sensitive cells of the retina converge to form the optic nerve which connects the eye to the brain. There are no sensory cells at the blind spot and any image formed here is not perceived.

  • The parts of the eye:

Internally the eye is divided into two main chambers separated by the lens.

_ Aqueous chamber is the front part containing a watery fluid (aqueous humour) and vitreous chamber is the back part containing a thick jelly like glassy substance (vitreous humour, vitro : glass). The aqueous humour keeps the lens moist and protects it from physical shocks. The vitreous humour helps in maintaining the shape of the eyeball and protects the retina.

_ The lens is biconvex in shape and semi-solid. It is composed of soft gelatinous tissue. It is held in position by suspensory ligament, which attaches it to the muscular ciliary body. The shape of the lens is influenced by the amount of tension in the suspensory ligament.

_ Iris is a sort of circular curtain in front of the lens. It is black, brown or blue. The colour of the eye is the colour of its iris. It contains two kinds of muscles : circular muscles for narrowing the pupil, and radiating muscles for dilating it. The size of the pupil is adjusted involuntarily to control the amount of light entering the eye. Can you think of the situations when the pupil gets narrower and when it becomes wider?

  • How Do We See:

_ Transmission of light : Reflected light rays from the object enter the eyes

through the transparent structures of the eye i.e. conjucativa, cornea, aqueous humour, lens and vitreous humour.

_ Formation of image. The curvature of the cornea bends the rays to some extent and the lens bends them further to form an image on the retina.

_ Nature of image. The image is inverted and real.

_ Production of nerve impulse and its transmission. The light energy of the

image produces chemical changes in the sensory cells (rods and cones). These changes produce nerve impulses, which travel through the optic nerve and reach the brain.

_ Perception. The brain interprets the image in many ways; e.g. it sees the object vertical although the actual image formed is inverted.

_ Accomodation (focusing). Focusing the image on retina is called accommodation. Changing the curvature of the elastic lens brings about the accommodation.

  1. For distant vision : The lens is more flattened or thinner; this is the normal condition of the lens, which is kept stretched by the suspensory ligaments.
  2. For near vision : The ciliary muscles which are circular, contract and tend to reduce the circumference of the eyeball there. This releases the tension on the suspensory ligament and the lens becomes thicker (more rounded) on account of its own elasticity.

(A normal eye is constantly accommodating while walking, playing or just looking around).

_ Binocular vision. In all primates including humans, both eyes are placed

forward. Each eye views at a slightly different angle. The images from the two eyes are perceived overlapped inside the brain giving the impression of depth.



  • 3-dimensional/stereoscopic vision:


Three Common defects of the eye


  1. Near sightedness (Myopia). Nearby objects are clearly seen but not the distant ones by those suffering from myopia because the image of the object is formed in front of the retina. This can be corrected by using concave lens (worn in frames (spectacles) or as contact lenses).
  2. Long sightedness (Hypermetropia). Distant objects are clearly seen but not the nearby because the image of the object is formed behind the retina. This can be corrected by convex lens (worn in frames as spectacles or as contact lenses).
  3. Cataract (opacity of the lens). The lens usually loses its transparency and turns opaque with age. Such a lens can be surgically removed and either replaced by an intra-ocular lens or by simply using suitable glasses.


The ear serves two sensory functions: hearing and maintenance of body balance.

The ear has three main parts – external ear, middle ear, and internal ear

  • The external ear consists of the following :


– an outwardly projecting ear to be called pinna supported by cartilage. It directs the sound waves inwards.

– The auditory canal through which the sound waves travel up to the ear drum (tympanic membrane)


  • The middle ear consists of the following:

_ An air-filled tympanic cavity

_ The tympanum or ear drum

_ Three tiny bones-malleus (hammer) connected to the ear drum, incus (anvil) in between and stapes (stirrup) forming a contact with the oval window of the internal ear.

_ Eustachian tube connects the tympanic cavity with pharynx. It equalizes the pressure on both sides of the eardrum or tympanum :

  • The internal ear contains two main parts:

(a) Cochlea – It is a long coiled structure which looks like the coils of the shell of a snail. It has two and a half turns. The inner winding cavity of the cochlea is divided into three parallel tubes of canals separated by membranes. The canals are filled with a fluid called endolymph. The middle canal possesses sensory cells (organ of corti) for hearing.

(b) Vestibule – is concerned with physical balance of the body. It consists of three semicircular canals arranged at right angles to each other and a part joining the cochlea and differentiated into a utriculus and a sacculus. One end of each semicircular canal is widened to form an ampulla, which contains sensory cells, and the nerve fibres from them continue into auditory nerve.

Mechanism of hearing

– The sound waves enter the auditory canal and cause the eardrum to vibrate

– The vibrations of the eardrum are transferred to malleus, to incus, and then to stapes. Stapes transfers the vibrations through oval window into the cochlea.

– These vibrations move the fluid in the cochlea. The organ of corti catches the movement of the fluid and transfers it to the auditory nerve that carries the impulses to the brain.

Perception of body balance

Static balance due to gravity – Any bending or change in the body posture causes the fluid inside the semicircular canals to move. The semi circular canals are arranged in different planes. The sensory hairs in the ampulla of the canal pick up these movements and the impulses are transmitted through the auditory nerve.

Balance during motion – Utriculus and sacculus perceive dynamic equilibrium (while the body is in motion). Fine particles of calcium carbonate present in the endolymph press on the sensory hairs whenever the body is in some motion. The impulses are carried through the auditory nerve.


The tongue perceives the taste and the nose perceives the smell. The perception depend upon the nature of chemical substance coming in contact with the sensory cells. For taste there is a direct contact of the substance with the sensory cells located in the taste buds on the tongue. For smell, the molecules of the chemical are carried inward by the air inhaled and they stimulate the sensory epithelium of the nose.

There are a variety of nerve endings in the skin. Some of these are concerned with touch (gentle pressure), some with deep pressure and others with cold, heat and pain.

The sense of hunger is due to receptors in the stomach wall. The sense of thirst is due to stimulation