NEET Biology Sense Organs Notes

NEET Biology Sense Organs

Environmental information received by a sense organ is first changed into the language of the nervous system and the process is called transduction.
Stimuli are perceived by receptors but interpretation of the stimuli is done by specific nervous centres in the brain. Receptors may be exteroreceptors, proprioreceptors or interoreceptors. Exteroreceptors are present in special sense organs like skin, nasal chamber, taste buds, eyes and ears.
The five kinds of sensory receptors are mechanoreceptors which respond to physical deformations of tissues; chemoreceptors which respond to certain molecules dissolved in solution; thermoreceptors which respond to heat; and painreceptors which respond to excess heat, pressure, or certain chemicals; and electromagnetic which respond to energy of different wavelengths.
Sensory receptors can also be categorized as exteroreceptors which receive information from the external environment; interoreceptors which receive information from the internal environment; and proprioreceptors which receive information about the body’s orientation in space.
Skin receptors may be naked free nerve endings or covered bulbs or corpuscles. These may be tango receptors, thermoreceptors, algesia receptors etc.

Sense Organs Classification Of Some Receptors By Stimuli

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NEET Biology Sense Of Smell

Olfactoreceptors are present in the olfactory epithelium and are chemoreceptors to specific odours. These show olfactory adaptation as are soon fatigued when exposed to specific odour for a long period.

Evolution Neet Notes

In the case of humans 7 groups of primary odours have been classified :

Camphoraceous (Scent of camphor)
Musky ( scent of musk )
Floral (flowers)
Pepperminty ( Scent of oil or peppermint)
Ethereal ( Scent of ether)
Pungent (Scent of spices)
Putrid ( Decaying meat)

NEET Biology Sense Taste Receptors

Gustatoreceptors are present in taste buds on the tongue. These are also chemoreceptors. Man can perceive four basic modalities of taste with the help of different taste buds located on different parts of the tongue.
Human beings recognise four basic modalities of taste, viz. sweet, sour, salty and bitter. Sweet, sour, salty and bitter tastes are principally perceived at the tip, along the lateral edges, on the upper side of the front half, and the back of the tongue respectively.
The taste of food is of vital importance in the process of digestion because taste stimulates reflexes causing secretions of saliva, gastric juice and pancreatic juice.

Rheo Receptors. They are responsible for perceiving water currents. They are of four types.

Lateral line sense organs
Scattered pit organs
Ampulla of Lorenzini
Vesicles of Sair
Watering of eyes due to gases or smoke is due to irritoreceptors.

Principles Of Inheritance And Variation Neet Notes

NEET Biology Sense The Eye

The eye captures light reflected from objects in the visual field and transforms it into electrical signals, which the brain interprets as visual perception.

Light is emitted and absorbed in distinct quanta of photons. It propagates as waves of electromagnetic radiation, with wavelengths perceptible to the human eye ranging from 380 to 760 nm, and is most sensitive at 500 nm (i.e., green color).
Free-living animals require photoreception to identify food, predators, and refuge. Photoreceptors have evolved for this purpose, undergoing progressive improvement over time.
Planarians (flatworms) possess a “eye cup” that functions as a photosensory organ to perceive light intensity and direction.
In annelids (polychaetes) and subsequent higher animals, the eye has evolved into a distinct organ.
In insects and crustaceans, the compound eye consists of individual units known as ommatidia. They are adapted for apposition (mosaic) vision in bright illumination and superposition imagery in low light (darkness).
It additionally produces colored visuals. Honeybees possess the ability to perceive ultraviolet energy.
Among all invertebrates, only cephalopods (molluscs) possess eyes with real lenses like to those of vertebrates.

NEET Biology Sense Human (Mammalian) Eye

A fluid-filled ball with about 21 mm (17.5 mm just born) metro-posterior diameter (optical or visual avis) lodged in the bony orbit of sphenoid bone with a cushion of fat.
It weighs about 7 gin.
About 1/6 of the eyeball (l/3rd in frog and 1/5 in rabbit) projects outside while the rest parts remain within the socket. 1 Iris anterior part is covered with transparent cornea.
Each eye is protected by upper and lower eyelids with eyelashes.
The eyes are somewhat ball-like and are lodged in the deep skull cavities (orbits). The clevis can rapidly close to prevent dust or other things from entering the eyes. The eyelashes also help in this function. On any irritation, due to a fallen particle due to gas or smoke or under certain emotions, the eyes are flooded with a watery secretion or tears.
This secretion comes in front of the (car glands, located near the outer border of each eye and it is poured through minute ducts on the inner surface of the upper eyelid. Movements of the upper eyelid spread the secretion over the eye to wash off any dirt.

Structure. The wall of each eye is made of the three concentric layers sclerotic, choroid and retina. In the front, the sclerotic layer forms a transparent, somewhat bulging circular comea.

The cornea can be stored and grafted on the other person. The cornea and the rest of the exposed sclerotic layer are covered by another very thin transparent membrane, the conjunctiva, which is the continuation of the inner surface of the eyelids.
The pigmented iris, formed by the middle or choroid layer encloses the pupil, visible from outside as a dark window. The iris consists of delicate muscles which regulate the size of the pupil and consequently control the amount of light passing to the retina ciliary body.
Just behind the pupil lies a biconvex lens which is held in position by ligaments and muscles. A transparent watery fluid (aqueous humour) fills the narrow space between the lens and the comea. The large chamber behind the lens is filled with a clear, gelatinous substance (vitreous humour).
The sensory layer called the retina, consists of two types of cells, rods and cones. The rods are sensitive to dim light and do not distinguish colours, whereas the cone cells are sensitive to bright light and can distinguish colours.
The cone cells are most numerous near the back of the eye, opposite the pupil. This part is somewhat depressed and is known as the yellow spot or fovea. Vision is the sharpest here. The sensory fibres from the retina bundle together and emerge from the back of the eyeball as the optic nerve.
At the exit of the optic nerve, there are no rods or cones. The part of the image falling at this place is not perceived, and so this place is called the blind spot.

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NEET Biology Sense Associated Glands

Lacrimal gland (tear gland).Consists of a chain of three small glands located at the outer angle of each eye. Tear flows into the lacrimal duct through the small pore, and lacrimal punctum to pass into the nasal chamber of its side.

Tear is a salty fluid, that contains amino acids, glucose and lysozyme.
A continuous layer of tear is always present upon the comea to keep it moist, and away from dust and other foreign particles. It also nourishes comea and acts as a medium for refraction of light; lysozyme in it is antibacterial.
In humans tear secretion begins at four months of birth.
Tearing is stimulated by injury, irritation, emotions etc.

The meibomian gland (tarsal gland) is a modified sebaceous gland located on the mar¬gin of eyelids. Its oily secretion makes a film over the tear to prevent it1 fall from the surface of comea.
The Zeis gland is also a modified sebaceous gland in the hair follicle of eyelashes to keep them smooth and waterproof by its oily secretion.
Glands of Moll are modified sweat glands on the inner surface of eyelids found only in humans, of uncertain function.
Harderian Gland. It is a special type of mucous-secreting gland found in aquatic, fossorial and bush dwellers. Mucous keeps the eye moist and protected from abrasion. In frogs it remains beneath the lower eyelid towards the inner corner of the eye, also found in all reptiles and aquatic mammals, rabbits, etc., but absent in man. Its secretion is known as crocodile tears.

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NEET Biology Sense Refraction Or Focusing

Tear → conjunctiva → cornea → aqueous humor → lens → vitreous humor refract light rays to converge on the retina.

The cornea exhibits the highest degree of refraction, followed by the lens.
The lens precisely directs the beam onto the retina to create a picture. Accommodation is achieved by altering the curvature (convexity) of the lens through the action of ciliary muscles, depending on the object’s distance.
When the ciliary muscle is relaxed, the eye remains at rest and focuses on distant things. To focus on nearby objects, the ciliary muscle contracts to enhance the lens’s convexity.
The frog possesses inadequate accommodation capabilities; it is hypermetropic in water and myopic on land. Nevertheless, a minor adjustment can be achieved by moving the lens anteriorly and posteriorly, akin to a camera.
The inverted virtual image is produced on the retina, and normal vision is referred to as emmetropic.
In humans, each eye possesses a vision field of around 170°. The significant overlap of visual fields aids in assessing the relative positions of objects, constituting depth awareness or stereoscopic vision.
The typical range of vision in the human eye extends from 30 centimeters to 6 meters.

Evolution Neet Notes

NEET Biology Sense Common Defects Of Eye

Myopia (Near-sightedness). When the image of a distant object is focussed in front of the retina due to lengthening of the eyeball or due to increased convexity of the lens. It can be corrected by using appropriate concave lenses.
Hypermetropia or Far-sightedness. It is due to just the opposite condition as in myopia. It can be corrected by wearing suitable convex lenses.
Astigmatism. It is due to an error in the shape of the lens and cornea, as they are of different curvatures. Cylindrical glasses are used to correct this defect.
Presbyopia (old age sight). It is the diminished ability to focus the eye on near objects due to the gradual loss of elasticity of the crystalline lens with age. It can be corrected by use of the convex lens.
Cataract. The lens becomes opaque due to different reasons. It can be corrected by removing the lens and replacement by a convex lens by the specialist.
Glaucoma. It is an increase in intraocular tension when the pressure within the eye is raised above the normal level (15-20 mm Hg.) If prolonged it brings about blindness.
Trachoma. It is caused due to infection by virus conjunctivitis. The cornea may be ulcerated and vision is lost.
Squint/ Strabismus/Diplopia – It is the formation of two images. Corrected surgically.

NEET Biology Muscular System Notes

NEET Biology Sense The Ear

The ear consists of three parts :

External
Middle
Internal ear.

External Ear. The external or the outermost projecting part of the ear is a skin-covered cartilaginous organ called pinna. It collects sound waves and admits them into the tubular auditory passage.

Some animals can move their external ears in the direction of sound, but such muscles are vestigial in man although some people do have the capacity to move their ears slightly. A waxy layer of cerumen glands in the auditory passage entangles the bacteria and any other minute organisms that may creep in.
Similarly, the outer openings have fine hair for protection purposes. Somewhat obliquely placed at the inner end of the auditory passage is a delicate blue-grey membrane called the ear drum (tympanum).

Middle Ear. The middle ear consists of a set of three tiny bones, which are lodged in a chamber. The first bone is the hammer-shaped malleus, attached to the inner surface of the car drum (tympanum), by a muscle called tensor tympani.

Next is the anvil-shaped incus and the third is the stirrup-shaped stapes, the inner end of which is attached to fenestra malls. An air passage of the castellan tube connects the floor of (the middle ear with the pharynx, to balance the air pressure on either side of the tympanum.
The tympanic cavity opens into the internal ear’s cavity through two apertures – fenestra ovalis and fenestra rotundas.

The car ossicles help in the amplification of sound waves and their transmission to the internal car. These are three in mammals as a characteristic feature :

The malleus (hammer) comes first connected to the tympanum, derived from the articular bone of the low er jaw.
The incus (anvil) is the second bone upon which the malleus hammers. It is derived from the quadrate bone of the upper jaw.
Stapes (stirrup) the last, fork-shaped bone is connected to the membrane of fenestra ovalis. It is derived from oromandibular, a homologous bone, columella auris is the only ear ossicle found in frogs.
The middle car is connected with the pharynx through a cartilaginous eustachian tube which gets filled with air and helps maintain equal pressure on both sides of the tympanum.
The internal car is very delicate. It has two main regions – the upper utriculus and the lower sacculus. Both are connected by a small and narrow saccule-utricular duct. A small duct, the endolymphatic duct, opens into it; the other end of which opens into a closed endolymphatic sac which is present on the posterior side of the temporal bone.
The upper portion i.e. utriculus is connected with three semi-circular canals, which are concerned with the sense of position of the body and not with hearing.
From the sacculus arises a long tubular part or cochlea which is coiled like-conch shell and is embedded in a bone of the skull. It carries a system of canals and spaces which are filled with lymph-like fluid and there are fine sensitive membranes running across it.
Branches from the auditory nerve enter these membranes. This part is connected with the sense of hearing. The cochlea is attached to the sacculus through ductus reuniens. Cochlea has about 2 coils. The cochlea can be divided into three parts the upper one is scala vestibuli, the middle scala media and the lower one is scala tympani.
The upper and lower parts are filled with perilymph whereas the middle one with endolymph. Both the scala vestibule and scala tympani are continuous with each other through a passage called helicotrema.
The dorsal wall of scala media is called Reissner’s membrane and the ventral one is the Basilar membrane. Several sensitive structures are found on the basilar membrane like an organ of Corti, cell of deiter, the cells of Hensen and the tectorial membrane.

NEET Biology Muscular System Notes

NEET Biology Sense Equilibrium

Any alteration in body posture causes the otoliths within the endolymph of the cristae and maculae to contact sensory hairs, generating an impulse that is transmitted to the brain, which then adjusts to maintain balance.
The cristae in the ampulla of semicircular canals sustain dynamic balance. It is activated when the body is in motion or rotation.
The maculae in the utriculus and sacculus ensure static balance or orientation of the body or head during a static position (sitting or standing).

NEET Biology Sense Hearing

The process of hearing and its path of sound waves is as follows:

Sound waves collected by pinna → external auditory meatus→ tympanum →tympanic cavity → ear ossicles (malleus→ incus → stapes) → membrane of fenestra ovalis → perilymph of scala vestibuli →helicotrema →scala tympani (vibrations cause movement of Reissner’s and basilar membranes result in the vibration of endolymph of scala media) → Tectorial membrane →Sensory hair cells of the organ of Corti → Impulse ’ generated → Auditory nerve→ Brain (perception and interpretation of sound vibrations) and sound Is perceived.

Evolution Neet Revision Notes

NEET Biology Sense Organs In Focus

Receptors are merely involved in receiving the stimuli and initiating the nerve impulses but cannot interpret the impulses.
Interpretation power of nerve impulses lies in the specific sensory areas of the brain,
Proprioceptors are maximally present in the sole of feet.
Exteroreceptois and proprioceptors are somatic receptors while interoceptors are visceral receptors, Classification of receptors was given by Sherrington. Reading of a page is an example of exteroception i.e., sensing of the external environment, or Sensing of internal condition and position is inerorecspection.
Perception is the conscious awareness and interpretation of sensation.
Each eye weighs roughly 7 gins.
Photoreceptor cells are present on the innermost layer of the eye. the retina.
The Iris of the eye contains radial bands and rings of circular smooth muscles.
The size of the pupil is larger in human females than in males. The Iris of the eye acts as a diaphragm of eye. Iris, being pigmented. gives colour to the eye which depends upon the amount of melanin present example, more melanin in brown eye, less melanin in blue eye but no melanin in albino eye.
Rods of the retina have rhodopsin (visual purple) while cones have iodopsin (visual violet), and Rods help in night vision. twilight and Black and white vision, while cones help in day vision and colour vision.
There are about 7 million cones in the human eye.
Zonula of Zinn is the other name for the suspensory ligament.
Monocular vision is found in animals like dogs, rabbits, pigeons, frogs, etc. In this, only one eye is focused on one object at a time.
Stereoscopic vision. Binocular vision is Found in most primates and owls among the birds. In this, both eyes can be focused on the same object at the same time.
In flatfishes, both eyes are on the right side of the body. Palpebrae. Eyelids. These act as shutters.
Meibomian glands are modified oil glands.
Glands lying at the edges of eyelids Gland of Zeis and Gland of Moll.
Sty Bacterial infection of the gland of Zeis. It is also called
hordeolum.
Eyelids are absent in cyclostomes, bony fishes and snakes.
Plica semilunaris. The third eyelid, also called the nictitating membrane, is vestigial in man.
The cornea is non-vascu, a part of the eye. So cornea is most easiest part to transplant as n does not stimulate the immune system.
Nutrition to the cornea is provided by alkaline lacrymal secretion. It is differentiated into Bulbar conjunctiva (outside cornea) and palpebral conjunctiva (inner to eyelids). It is a modified stratified epithelium.
Owls have a large number of rods and only a few cones in the retina of their eyes.
The retina of the fowl’s eye contains only cones.
Deer have the biggest eyes in proportion to body size.
The region of the environment from which each eye collects light is called the visual field.
The normal eye can accommodate light from objects from about 25 cm to infinity.
In the ear canal, there are about 4.000 specialized ceruminous glands.
The cochlea contains 16.000 and 24.000 hair cells and each hair cell has about 100 hair. They create waves in the lymph fluid of the cochlea and the wave causes the basilar membrane to ripple.
The ear acts as a statoacoustic organ that controls hearing and equilibrium.
The Pinna of the ear is called the auricle.
The pharyngeal opening of the eustachian tube opens during yawning, swallowing and during the ascent or descent of an aeroplane to equalise the pressure inside the tympanic cavity to that outside the eardrum for free vibrations of the eardrum.
Audiology. Study of hearing power.
Otology. Study of the structure, working and disorders of the ear.
Aquatic animals like seals, whales, etc. and monotremes like duck-billed platypi and spiny ant-eaters lack pinna.
Thermoreceptors are more in number on the face and hand of man.
The most sensitive chemical receptors are the taste and smell receptors.
Taste receptors in the tongue respond specifically to sweet, sour, salty and bitter molecules.
The nose contains mucus-coated olfactory receptors, over 2 million in number.
The mucus produced by Bowman’s gland absorbs odour-iferous substances that stimulate the reception of the cells.
The taste of chillies, black pepper and ‘hot’ sauces is not a true sensation. It is a sensation of burning pain produced by the stimulation of pain receptors of the tongue.
Many insects such as honey bees, flies, butterflies and moths possess chemoreceptors to taste sensation on their feet.
Some mammals such as rhesus monkeys, pigs, cats and dogs possess taste buds for tasting water. But man is deprived of them.

Evolution Neet Chapter Summary

NEET Biology Sense Organs Conclusion

In fishes, accommodation for near objects is brought about by elongating the eyeball but in other animals, it is brought about by increasing the lens curvature.
lives, capable of focussing the images of objects possessed by vertebrates and some higher invertebrates like prawns, crabs and insects.
Prawns, crabs and cockroaches possess compound eyes, each made of many elongated tube-like units called ommatidia. They are crowded over a spherical surface and produce a composite blurred image of the object.
All animals do not have their olfactory receptors located in the nose. For instance, moths and butterflies possess olfactory chemoreceptors on their antennae.
The cornea has no vascular or immune systems. Therefore a complete transplant is not rejected by the body and is highly successful.
Cornea from the eye of a dead person can be stored and transplanted to restore vision in another person. It is so because it is non-vascular and does not stimulate the immune system.
Owls have the keenest eyesight at night because their eyes have a large number of rods and a few cones in the retina.
Aquatic mammals like seals, whales, etc., and monotremes like duck-billed platypus and spiny ant eater lack pinna. Tympanum. It is the specialized hearing organ of insects.
Sea sickness. Tendency of nausea and vomiting due to unusual stimulation of semicircular ducts by the movement
ment of a ship in the sea.
The area of the body most sensitive to the sensation of cold is the chest Merkel’s disc is slowly adapting receptors for touch.
Organ of Ruffini arc the receptors for warmth.
Cynnolabs. Cone cells are sensitive to blue light radiations.
Erythrolabs. Cone cells are sensitive to red light radiations.
Jacobson’s organs. Additional smelling organs of reptiles like Sphenodon, lizards and snakes (best developed). These are also called vomeronasal organs.
Ampullae of Lorenzini. Thermoreceptors are found in the snout region of fishes.
A sense of direction is given by the nose.
Olfactory cells are modified bipolar neurons.
Grandy’s corpuscles. A special type of Merkel’s corpuscles present in the skin and tongue of birds
Otoliths/Otoconia. Calcium carbonate crystals
Harberf s corpuscles. A simple type of Pacinian corpuscles in birds
Pacinian corpuscles. Vater’s corpuscles and hence Vater Pacinian corpuscles
Vibration to which the human ear is most sensitive. 1000 cycles /sec
Harderian glands -Present at an angle of the eye, secrete lubricant for nictitating membrane.
Eyelids are absent in snakes.