Sense Organs

Types of receptors

(1) Exteroreceptors : Receive stimulation directly from external environment. These may be of following type

(i) Photoreceptor : Sensitive to light (Eye)                   

(ii) Thigmoreceptor : Sensitive to touch.

(iii) Tectoreceptor : Sensitive to touch.

(iv) Tangoreceptor : Sensitive to touch pressure.

(v) Phonoreceptor : Sensitive to sound (Ear).

(vi) Olfactoreceptor : Sensitive for smell (Nose).

(vii) Gustoreceptor : Sensitive to taste (Tongue).

(viii) Thermoreceptor : Sensitive to temperature.

(ix) Calo receptor : Sensitive to heat.

(x) Frigido receptor : Sensitive to cold.

(xi) Galvano receptor : Sensitive to electric current.

(xii) Rheoreceptor : Sensitive to water or air current.

(xiii) Geo receptor : Sensitive to gravity.

(xiv) Telero receptor : Sensitive to distance. (Receptors of vision, hearing and smell receive stimuli from a distance hence called teleroreceptor).

(2) Proprioceptors : Proprioceptors are located in skeletal muscles, joints, tendons etc. It is from these receptors that we know the position of our arm or leg without having to look at it.

(3) Intero receptor : These are present in internal organ. Ex. receptor for hunger, thirst, pain and balancing.

Few important receptor.       

(1) Important tangoreceptor

(i) Merckel&aposs corpuscles : Found in epidermis (stratum malphighi) of skin.

(ii) Merckel&aposs disk : Found in epidermis (stratum malphighi) of skin.

(iii) Meissner&aposs corpuscles : Present in skin around the base of hair and feather. These are sensitive for touch and pressure both.

(iv) Genital corpuscles : These are sensitive cells with nerve endings in skin around the genital organ.

(v) Grandey&aposs corpuscles : Found in birds at the base of the beak. These are kidney shaped in structure.

(vi) Herbert corpuscles : Found in buccal cavity of birds.

(vii) Capsulated corpuscles : These are sensitive cells encapsulatedਊnd found in skin.

(viii) Paccinian corpuscles : Found in deep layer of dermis and sensitive to touch, pain and pressure.

(x) Golgi corpuscles and Mazzoni corpuscles : These are sensitive to touch and found in subcutaneous region.

(2) Important Olfactoreceptor

Jacobson&aposs organ (Vomero-nasal organ) : It is concerned with smell. These were 1^st appear in amphibians and well developed in snake, lizzard and sphenodon that is reptile. These are less developed in birds and mammal. Structure is blind sac like and lined by olfactory epithelium (Shneiderian membrane). Jacobsons organ is not found in rabbit. In man it is vestigeal organ. In human foetus Jacobson organ present. Jacobson organ also found in marsupiales, Rodents, and Insectivora.

(3) Important Thermoreceptor

(i) Krause end bulb : These are sensitive for temperature and pain and freidgo in nature. Found in lips, tongue, conjunctiva of eyes and corium of fingure.

(ii) Organ of Ruffeni : Sensitive to temperature and mainly related with heat.

(iii) Ampulla of Lorenzini : Found in snout region of cartilagenous fishes these are helpful in detecting the temperature of water.

Eye

Human have binocular vision. The eye can discriminate colour, appraise length, width and depth visually and form true inverted image.

Structure of eye

The eyes are two in number and lodged in orbits (bony socket) of skull. The eye is a hollow, spherical organ, about 2.5 cm in diameter and about 6 to 8 gram in weight. It has two parts –

(1) Protective devices : Eye has four protective devices.

(i) Eye brows : The outwardly directed hair of the eyebrows carry the sweat and rain drops trickling down the forehead to the sides to prevent their falling into the eyes.

(ii) Eye lids (Palpebrae) : In man two eyelids are present, upper is movable. They are regularly closed at short intervals to clean the cornea. This is called blinking. In frog out of two upper eyelid is immovable and lower eyelid is movable. Nictitating membrane is present in frog which protect eye in water. Movement of nictitating membrane takes place by retractor bulbi. It becomes folded by levator bulbi.

A nonfunctional vestigeal nictitating membrane, called plica semilunaris, occurs in human eyes. It remains permanently retracted at the inner angle of each eye.

(iii) Eye lashes : The eyelids bear at the free edge a row of stiff hair, the eye lashes. These check the entry of dust particles, tiny insects and rain drops into the eyes.

(iv) Eye glands

(a) Meibomian gland : The eye-lids bear at the free edge a row of meibomian gland that is modified sebaceous gland. (Act as a lumbricant).

(b) Lacrimal gland or Tear gland : It lies in the upper outer part of the orbit and secretes a slightly saline, watery fluid that contains a bacteriolytic enzyme named lysozyme. This secretion moistens the surface of the eyeball. The excess of this secretion passes through nasolacrimal duct. It is modified sweat gland.

(c) Harderian gland : Some aquatic mammals (whale) possess harderian gland which lubricate nictitating membrane. It is also found in frog and birds.

(d) Glands of zeis (zis) : These are modified sebaceous gland, found at base of hair follicle of eye lashes, pour lubricating fluid in hair follicle. Infection of these glands is sty.

(e) Glands of Moll : It is modified sweat gland and open into the follicles of eyelashes.

In human meibomian, lachrymal, Moll&aposs glands, and zeis glands are present.

(v) Connective tissue : A layer of fatty connective tissue surrounds the eyeball. It serves as a soft shockproof pad.

(2) Eye ball : Eye ball is made up of 3 coats or tunic.

(i) Sclerotic layer (Fibrous tunica) : Outer most and opaque, fibrous and non-vascular layer easily seen as white of the eye. It is a coat of dense connective tissue made up of collagen fibers and fibroblasts. Sclera covers entire eye ball except cornea, gives shape to eye ball. Sclera in frog is cartilaginous.

(a) Cornea : In the centre, sclerotic layer it merges with the transparent round window called cornea.

(b) Conjunctiva : The cornea and exposed part of sclera are covered externally by a thin, transparent membrane the conjunctiva.

(ii) Choroid layer (Vascular tunica) : Also known as uvea middle. it is vascular layer which supplies nutrients to the eye. It is distinguished into three parts choroid, ciliary body and iris.

(b) Choroid :  It is highly vascular posterior portion of vascular tunic. The choroid occurs in the main part of eye ball adhered to the sclerotic. (The pigment is reddish in rabbit and black, brown or bluish in man).

(b) Ciliary body : Ciliary body is vascular and pigmented like choroid, made up of ciliary processes and ciliary muscles (only circular type). The ciliary body is hidden by iris. The ciliary body helps in accommodation by altering the focus of eye from object or the shape of lens near or far vision.

(c) Iris : Beyond the ciliary body, the vascular tunic sharply turns inwards, forming a circular, shelf-like diaphragm called iris. The colour of the iris is responsible for colour of eye e.g., brown, black, blue or green. In albinos, iris is deficient of pigments.

Lens : Lens is colourless, transparent and fibrous crystaline structure made up of protein (a and b crystalline protein) and enclosed in lens membrane. It is ectodermal in origin. Lens is lodged in eye ball by suspansory ligament of ciliary body. Suspansory ligaments are known as "Zonula of Zinn". In man lens is biconvex while in frog it is elliptical (subspherical).

Lens divide the eye ball in 2 chamber outer aqueous chamber (partially divided into a large anterior and a smaller posterior chamber) filled with aqueous humor (watery) formed by ciliary body and inner vitreous chamber filled with vitreous jelly (or Wharton&aposs jelly) containing 99% water, some salt a little mucoprotein (vitrein) and hyaluronic acid.

(iii) Retina (Neurosensory tunica) : It is innermost, thin and transparent, purplish red due to the present of the eye pigment rhodopsin (in rods) or visual purple which was extracted by Kuhne (1876) and named &aposSchpurpur&apos (Visual purple). Made up of 4 distinct layer –

(a) Cuboidal pigmented epithelium (towards choroid).

(b) Layer of rods and cones.

(c) Layer of bipolar neurons.

(d) Layer of ganglia (Towards vitreous chamber innermost).

Area centralis of retina : A little part of retina that lies upon the optical axis is called area centralis. Here, the retina is very thin and contains only cone cells filled with a yellow pigment. Hence, this part is called yellow spot or maculla lutea. In man (Rabbits) and other mammals, but not in frogs, this area has a small shallow dispression called fovea centralis. The latter is the most sensitive part of an eye, i.e. the area of most acute vision. It is also claimed that the cone cells in fovea centralis are placed somewhat obliquely. So that these can form magnified images of object.

Blind spot (Optic disc) : At this point, the optic nerve turns towards the outer side, pierces through the whole thickness of the wall of eyeball, forming an optic foramen and runs to the brain. Obviously, the region of optic foramen has no retina. It therefore, does not take part in image formation and is called blind spot.

Ora seratta : The function retina terminates anteriorly along an irregular border, the ora seratta.

Working of eye

(i) Mechanism of light perception : The human eye has two functional parts – Dioptric or Focussing part and Receptor part.

(ii) Focussing part : It consists of conjunctiva, cornea, aqueous humour lens and vitreous humour. These part are transparent and act as lenses. They refract the light rays passing through the eye to bring them to a focus on the retina. Maximum refraction is caused by the cornea, which places the image approximately on the retina. The lens effects fine adjustment and brings the image into a sharp focus.

(iii) Receptor part : It comprises the retina. The image formed on the retina is inverted and smaller. It converts the energy of specific wave lengths of light into action potential in nerve fibre.

(a) Pathway of sensory impulses from eye to brain : The nerve impulses generated in the retina of the eye in response to light follow a definitive path and terminates in visual cortex in each optic lobe which act as primary visual center.

Biochemistry of eye

The receptor cells of eye are called photoreceptor or visual cells. They are of two types – Rod cells and Cone

(1) Rod cells : The rod cells contain a purplish pigment called visual purple or rhodopsin. They function in dim light and at night. They produce poorly defined images. Bright light splits rhodopsin into a lipoprotein scotopsin and a carotenoid pigment retinal (retinine) a process called bleaching. The spiliting of rhodopsin depolarizes the rod cell and it releases a neurotransmitter, passing the nerve impulse via bipolar neuron and ganglion cells to the optic nerve. In the dark, rhodopsin is resynthesized from scotopsin and retinal. This process is called &aposdark adaptation&apos. It makes the rods functional. It takes sometime for rhodopsin to be reformed. This is why on entering a dark room at daytime or on coming out of a well lighted room at night we feel blind for a while, when we go from darkness into bright light, we feel difficulty in seeing properly for a moment till rhodopsin is bleached and cones become functional.

(2) Cone cells : Cones contain iodopsin which is visual violet and made up of photopsin + retinal. The 3 types of cones are erythrolobe (775 nm sensitive tored), cyanolabe (430 nm sensitive to blue) and chlorolabe (sensitive to green 535 nm). However, if all the cone, types are simultaneously stimulated by equal amounts of coloured light than sensation for white light is perceived.

Diurnal animals are adapted to see during day light (Photopic vision) and can perceive colour. In dark, colours are not perceived. Such animals have more cones in their eye than rods.

Accommodation and types of vision

(1) Accommodation : Light passes through many refractive surfaces before it is focussed on the retina forming an inverted and true image. The main sites of refraction are cornea $\to$ aqueous humor – iris – lens (position can be altered by ciliary body : accommodation) – posterior chamber (= vitreous humor) $\to$ retina ( in fovea). The refractive index of the eye varies from 59 diopter (when the lens is at rest) to about 71 diopter (when lens is bulging in maximum accomodation). The accommodation reflex occurs when the eye changes its focus from a far away object to nearer one. The change in strength of the lens provides the physiological basis of accommodation. Radial and circular muscle fibres of ciliary muscles play an important role in this as they contract reflexively (parasympathetic control) and increase lens strength. The pupil constricts. This facilitates increase in sharpness of image. Ageing causes loss of accomodation.

(2) Types of vision

(i) Binocular vision : Man has binocular vision in which both the eyes are focussed on the same object but from slightly different angles. The visual fields of both eyes overlap and the foveae of both are focused on the same object. This provides depth to the images, i.e., gives stereoscopic or 3D effect and enables man to judge distances correctly.

(ii) Vision in other animals : Primates and predatory animals, such as owl and cat, have binocular vision. In some animals, such as rabbit, birds, each eye is focussed on a separate object. This is termed monocular vision.

(iii) Colour vision : It is the ability of some animals to detect colours in an object. Humans, apes, monkeys, and most fishes, amphibians, reptiles and birds have strong colour vision. The insects and crayfish also have colour vision. In vertebrates, colour vision results from the activity of cone cells. Most domestic and nocturnal mammals and sharks lack colour vision. They probably see objects in shades of grey (monochrome vision).

(iv) Nocturnal and Diurnal vision : Man has both day vision and night vision as he has both rods and cones in considerable numbers in the retina. Most birds have only day vision as their retina contains mainly cones. Owls have much better night vision than day vision for they possess a large number of rods and few cones in their retina.

Range of vision : The visible range of spectrum varies in animals. Bees, ants, spiders and goldfish can see ultraviolet light, which is invisible to man.

Eye movement

In eye orbit eyeball remain attached with 6 extrinsic muscles.

Out of six, first four are rectus and last two are oblique muscles.

(1) Anterior rectus or Internal ractus              

(2) Posterior rectus or External ractus            

(3) Inferior rectus

(4) Superior rectus

(5) Inferior oblique muscle

(6) Superior oblique muscle

Eye defects

(1) Myopia

(i) Also known as near sightness.

(ii) Short sightness.

(iii) Near object is clear. Far object is not clear.

(iv) Eyeball become longer.

(v) Image is formed before retina. Can be removed by concave lens.

(2) Hypermetropia

(i) Also known as hypermetropia or long sightness.

(ii) Far sightness.

(iii) Far object is clear, near object is not clear.

(iv) Eye ball become short.

(v) Image is formed behind the retina.

(vi) Can be removed by convex lens or lens convient.

(3) Astigmatism

(i) Curvature of cornea become irregular and image is not clearly form.

(ii) Can be removed by cylindrical lens.

(4) Cataract

(i) It is due to defective protein metabolism.

(ii) During this lens or cornea sometime both become opaque.

(iii) Operation is needed.

(5) Gloucoma

(i) It is due to increase in intraocular pressure in aqueous chamber.

(ii) Operation is needed at early stage due to blockage of schlemm’s canal.

(6) Trachoma 

(i) It is increased in redness of eye and more secretion of watery fluid.

(ii) It is due to infection of bacteria, chlamidia trachamastis.

(iii) Due to this follicles may form in conjunctiva.

(7) Xerothalmia

(i) It is due to deficiency of vitamin A. (A₂)

(ii) During this conjunctiva or cornea becomes keratinized.

(iii) It may lead to blindness.

(8) Strabimus

(i) In this type eyeball remain in some what in bended position.

(ii) It is due to long extra ocular muscles during development of eye.

(iii) Operation is needed at early stage.

(iv) Also associated with squint.

(9) Presbiopia

(i) During this power of accommodation of lens decreases due to age factor and defected metabolism.

(ii) Also known as age sightness.

(iii) Can be removed by bifocal lens.

(10) Photofobia : No clear image in bright light.

(11) Emmetropia : Normal vision.

Phonoreceptor and Mechanism of hearing or auditory sensations and equilibrium

Also known as stato-acuostic organ. It is the receptor for balancing and hearing which is sensitive for gravity and sound waves. It is also sensitive in orientation of body. It is also known as mechano receptor because of it change mechanical energy of sound waves in to action potential.

Structure of Ear

Ear of mammal is divided in to 3 parts –

(1) External ear : It is made up of pinna and auditory meatus. Pinna is found in only mammals. Its upper rounded part is helix and lower is ear lobe. It is made up of adipose connective tissue and elastic cartilage and has ear muscles which are vestigeal in case of human beings. Pinna collect the sound waves and drive towards auditory meatus.

Auditory meatus is 25 mm. long canal lined by simple columnar epithelia and made up of fibro elastic cartilage. It possesses ceruminus gland which secrete cerumin (ear wax). Cerumin trap the dust particles and microbes.

Tympanic membrane : It is also called ear drum and present at the junction of auditory meatus and tympanic cavity.

(2) Middle ear : The cavity of middle ear is known as tympanic cavity which is enclosed by tympanic bulla bone of skull and filled with air. Middle ear separated from external ear by ear drum and from internal ear by thin bony portion or partition with two openings known as oval and round window.

(i) Ear ossicle : A chain of three small, movable bones, the auditory or ear ossicles crosses the tympanic cavity. The outer ossicle is attached to the inner surface of the tympanic membrane

In man ear ossicles are known as H.A.S. stapes is the smallest bone of the body. In frog only stapes is present.

(ii) Joints 

(iii) Muscles

Tencer tympani : Limits movements and increases tension on eardrum to prevent damage to inner ear from loud sound.

(iv) Eustachian tube : It is made up of elastic cartilage and it connect middle ear to nosopharynx. It maintain equilibrium in and out side of the tympanic membrane. Blocking of eustachian tube impairs hearing due to imperfect vibrations of drum. Eustachian tube is normally closed, it opens during swallowing and yawning.

(v) Fenestrae : Between middle ear and internal ear a thin bony membrane is present which possess two apertures (Windows).

(a) Fenestra ovalis : It is upper window, connect middle ear to internal ear and guarded by membrane. End of stapes is fit on the upper window. It is towards vestibule so it is also known as F. vestibuli.

(b) Fenestra rotundus : It is ventral window, connect middle ear to internal ear and guarded by membrane. It is towards scala tympani so it is also known as F. Tympani (also known as F. cochleae).

(3) Internal ear (Membranous labyrinth) : Internal ear is also known as membranous labyrinth and enclosed by bony labyrinth. Bony labyrinth is formed by periotic bone or petrous. A cavity is present between membranous labyrinth and bony labyrinth known as perilymphatic space. It is filled with perilymph and endolymph is found in membranous labyrinth. The membranus labyrinth consists of 2 parts – Vestibule and Cochlea.

(i) Vestibule : The vestibule is a central sac like part. It further consists of 2 chambers large – Utriculus (Upper) and smaller – sacculus (lower).

(a) Semicircular canal : From utriculus 3 semicircular canals arise these are –

Anterior semicircular canal (Superior)

Posterior semicircular canal (Inferior)

Horizontal semicircular canal (External)

They are perpendicular each other.

Crus commune : A common part of anterior and posterior semicircular canal arise from dorsal region of utriculus is known as crus commune.

Ampulla : Terminal part of the each semicircular canal is enlarged to form an ampulla.

Crista : Each ampulla has a sensory spot called crista ampullaris or simply crista, for equilibrium.

(b) Sacculus : It is a lower chamber of vestibule. From the lower part of the sacculus arises a short tube, the ductus reuniens, that joins the cochlear duct.

Ductus endolymphaticus : It is filled with endolymph and arises from the junction of utriculus and sacculus.

Macula : are present in utriculus and sacculus. it is a group of sensory cells. In man (Rabbit) 2 maculas are present. (A crista resembles a macula in structure except that lies on an elevation, the acoustic ridge, its sensory cells have longer "hair", and its gelatinous mass is dome shaped, lacks otoliths and is called cupula.)

Otolith : Also known as otoconia made up of protein and calcium carbonateਊnd present in endolymph.

(ii) Cochlear duct and Cochlea : It is a spirally coiled tube (2 – 3 coiling) which is connected to sacculus by a short duct. It is divided into 3 chambers by 2 membranes.

(a) Scala vestibuli : Upper chamber filled with – perilymph - connect with middle ear by F. ovalis, or oral window.

(b) Scala media (Real cochlear duct) : Middle chamber filled with – endolymph.

(c) Scala tympani : Lower chamber filled with – perilymph connect with middle ear by F. Tympani or round window.

(d) Reissner&aposs membrane : Present at the roof of scala media, it saparate S.M. to S.V.

(e) Basilear membrane : Present at the base of S.M. It is thicker than Reissner&aposs membrane and it separates S.M. to S.T.

(f) Modiolus : A bony core around which bony spiral canal of cochlea make $2\frac{3}{4}$ turns or coils in man.

(g) Helicotrema : A aperture present in scala media which connect scala vestibuli to scala tympani is known as helicotrema.

(h) Tectorial membrane : Tectorial membrane is a leaf like gelatinous structure present at the dorsal side of organ of corti.

(i) Organ of corti : Discovered by Italian anatomist Alfanso-Corti. Also known as ridges of corti which are present in basiler membrane. Organ of Corti contains a variety of cells. They receives nutrients from endolymph. The cells of organ of Corti areਏollowing types –

Mechanism of sound perception

Vone Beskey won the Nobel prize for his work on ear. The mechanism found in ear involve two unrelated functions : Hearings and equilibrium.

(1) Hearing : The ear not only detects sound but also notes its direction, judges its loudness and determines its pitch (frequency) sound waves are collect by the pinna and directed inward through the external auditory meatus (frequency 430 cycle per second). Here they strike the tympanic membrane. The latter begins to vibrate at the same frequency as that of the sound waves. From the tympanic membrane, the vibration are transmitted across the tympanic cavity by the ear ossicles to the membrane of the fenestra ovalis. The force of vibrations is considerably increased in the middle ear by leverage of the ossicles and also by much smaller surface area of the membrane of fenestra ovalis than that of the tympanic membrane. (The frequency is 2400 cycle/sec). Increase in frequency is important because the sound wave are transmitted from air to a fluid medium. The membrane of fenestra ovalis transmits the vibrations to the perilymph of the scala vestibuli and hence via Reissner&aposs membrane to the endolymph in the scala media. From here the vibrations are transferred to the basilar membrane and the perilymph in the scala tympani.

Vibration of the endo lymph of the scala media cause the basilar membrane of this chamber vibrate. Vibrations of the basilar membrane make the "sensory hair" of receptor cells in the organ of corti move in the overlying gelatinous membrane (Tectorial membrane) and get distorted. This stimulation causes depolarisation of the receptor cells and initiation of nerve impulse in the fibres of the auditory nerve. The nerve impulse travels via relay centers e.g. spiral ganglion $\to$ cochlear nuclei $\to$ superior auditory nuclei $\to$ inferior colliculi $\to$ auditory cortex of cerebrum (The cerebral cortex interprets the impulses as sound). The various steps in the mechanism of hearing

Human ear can hear a frequency of 500 to 5000 hertz (Hz 1 Hz = 1 cycle/second). However, it can hear the complete range of frequencies from 20 – 20,000 Hz only with intense sound. Sound energy is measured in terms of units called decibels (dB). Sounds in our city homes average 40 – 50 dB, but street noise averages 70 – 80 dB. Sounds up to 80 dB are considered bearable by man, but higher sound intensity are hazardous, causing nervous stress, irritability, increased blood pressure etc. Non stop noise of 90 or more dB produces temporary deafness. 160 dB sound can cause total deafness by rupturing our ear drum. Sound becomes uncomfortable to normal ear at about 120 dB.

(2) Equilibrium : Sound become painful above 140 dB. Exposure to certain antibiotics, such as gentamycin some anticancer drugs, loud sound, loud music, or engine rear of jet planes, vacuum cleaners, damages hair cells of cochlea.  

(i) Static equilibrium and linear acceleration : Maculae detect changes in the head (or body) with respect to gravity (static equilibrium) and in the movement in one direction (linear acceleration). With a change in the position of the body, the otoliths, being heavier than the endolymph, press upon the sensory hairs of the maculae. This stimulates the sensory cells which initiates nerve impulse in the fibres of the auditory nerve. The macula of utricle responds to vertical movements of the head, and the macula of saccule responds to lateral (sideways) movement of the head.

On rapid forward movement, the otoliths, because of having greater inertia than the surrounding endolymph, lag behind and press back the sensory hair, stimulating the sensory cells to generate nerve impulses.

(ii) Dynamic equilibrium : Cristae detect turning or rotational movements of the head (angular acceleration). When the head is turned, the endolymph in the semicircular ducts, due to its inertia, does not move as fast as the head and the sensory cells of the crista, but continues to move after the head stops moving. Because of this difference in the rate of movement, the sensory hair of the cristae are swept through the endolymph and become bent over. This disturbance stimulates the sensory cells and sets up action potential in the fibres of the auditory nerve, which transmits it to the brain. Since the three semicircular ducts are arranged in three different planes, a movement of the head in any direction will stimulate the sensory cells of at least one crista.

Defects of ear

(1) Labyrinthine disease : Malfunction of inner ear.

(2) Meniere&aposs disease : Loss of hearing due to defect in cochlea.

(3) Otitis media : Acute infection of middle ear.

(4) Eustachitis : Inflammation of eustachian tube.

(5) Myringitis (Tymanitis) : Inflammation of eardrum.

(6) Otalgia : Earache (pain in ear)