Sense Organs:

• We smell things by our nose, taste by the tongue, hear by ear and see objects by eyes.

  • The nose contains mucus-coated receptors which are specialized for receiving the sense of smell and are called olfactory receptors.

    • These are made up of olfactory epithelium that consists of three kinds of cells.

      • The neurons of the olfactory epithelium extend from the outside environment directly into a pair of broad bean-sized organs, called the olfactory bulbs, which are extensions of the brain’s limbic system.

        • Both nose and tongue detect dissolved chemicals.

        • The chemical senses of gustation (taste) and olfactory (smell) are functionally similar and interrelated.

          • The tongue detects tastes through taste buds, containing gustatory receptors.

          • With each taste of food or sip of the drink, the brain integrates the differential input from the taste buds and a complex flavor is perceived.

Eye:

• Our paired eyes are located in sockets of the skull called orbits

Parts of an Eye:

• The adult human eyeball is nearly a spherical structure.

• The wall of the eyeball is composed of three layers.

  • The external layer is composed of dense connective tissue and is called the sclera.

  • The anterior portion of this layer is called the cornea.

    • The choroid layer is thin over the posterior two-thirds of the eyeball, but it becomes thick in the anterior part to form the ciliary body.

  • The middle layer, the choroid, contains many blood vessels and looks bluish in color.

• The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible colored portion of the eye.

  • The eyeball contains a transparent crystalline lens that is held in place by ligaments attached to the ciliary body.

• In front of the lens, the aperture surrounded by the iris is called the pupil.

• The diameter of the pupil is regulated by the muscle fibers of the iris.

  • The inner layer is the retina and it contains three layers of neural cells – from inside to outside –

    • Ganglion cells

    • Bipolar cells

    • Photoreceptor cells.

• There are two types of photoreceptor cells, namely, rods and cones.

  • These cells contain light-sensitive proteins called photopigments.

    • The daylight (photopic) vision and color vision are functions of cones and the twilight (scotopic) vision is the function of the rods.

      • The rods contain a purplish-red protein called rhodopsin or visual purple, which contains a derivative of Vitamin A.

    • In the human eye, there are three types of cones that possess their own characteristic photopigments that respond to red, green, and blue lights.

      • The sensations of different colors are produced by various combinations of these cones and their photopigments.

      • When these cones are stimulated equally, a sensation of white light is produced.

• The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eyeball.

  • Photoreceptor cells are not present in that region and hence it is called the blind spot.

    • At the posterior pole of the eye lateral to the blind spot, there is a yellowish pigmented spot called macula lutea with a central pit called the fovea.

      • The fovea is a thinned-out portion of the retina where only the cones are densely packed.

      • It is the point where the visual acuity (resolution) is the greatest.

    • The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humor.

    • The space between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called the vitreous humor.

Mechanism of Vision:

• The light rays in visible wavelength focussed on the retina through the cornea and lens generate potentials (impulses) in rods and cones.

• As mentioned earlier, the photosensitive compounds (photopigments) in the human eyes are composed of opsin (a protein) and retinal (an aldehyde of vitamin A).

  • Light induces dissociation of the retinal from opsin resulting in changes in the structure of the opsin.

    • This causes membrane permeability changes.

  • As a result, potential differences are generated in the photoreceptor cells.

  • This produces a signal that generates action potentials in the ganglion cells through the bipolar cells.

• These action potentials (impulses) are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analyzed and the image formed on the retina is recognized based on earlier memory and experience.

The Ear:

• The ears perform two sensory functions, hearing and maintenance of body balance.

• Anatomically, the ear can be divided into three major sections called

  • The outer ear:

    • The outer ear consists of the pinna and external auditory meatus (canal).

    • The pinna collects the vibrations in the air which produce sound.

    • The external auditory meatus leads inwards and extends up to the tympanic membrane (the ear drum).

    • There are very fine hairs and wax-secreting glands in the skin of the pinna and the meatus.

    • The tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside.

  • The middle ear:

    • The middle ear contains three ossicles called the malleus, incus, and stapes which are attached to one another in a chain-like fashion.

      • The malleus is attached to the tympanic membrane and the stapes are attached to the oval window of the cochlea.

    • The ear ossicles increase the efficiency of transmission of sound waves to the inner ear.

  • The inner ear:

    • A Eustachian tube connects the middle ear cavity with the pharynx.

      • The Eustachian tube helps in equalizing the pressures on either side of the eardrum.

    • The fluid-filled inner ear called labyrinth consists of two parts, the bony and the membranous labyrinths.

      • The bony labyrinth is a series of channels.

        • Inside these channels lies the membranous labyrinth, which is surrounded by a fluid called perilymph.

      • The membranous labyrinth is filled with a fluid called endolymph.

        • The coiled portion of the labyrinth is called the cochlea.

          • The membranes constituting cochlea, the reissner’s and basilar, divide the surrounding perilymph-filled bony labyrinth into an upper scala vestibuli and a lower scala tympani.

          • The space within the cochlea called scala media is filled with endolymph.

        • At the base of the cochlea, the scala vestibule ends at the oval window, while the scala tympani terminates at the round window which opens to the middle ear.

    • The organ of Corti is a structure located on the basilar membrane which contains hair cells that act as auditory receptors.

      • The hair cells are present in rows on the internal side of the organ of Corti.

      • The basal end of the hair cell is in close contact with the afferent nerve fibers.

        • A large number of processes called stereocilia are projected from the apical part of each hair cell.

      • Above the rows of the hair cells is a thin elastic membrane called a tectorial membrane.

    • The inner ear also contains a complex system called vestibular apparatus, located above the cochlea.

      • The vestibular apparatus is composed of three semi-circular canals and the otolith (the macula is the sensory part of the saccule and utricle).

      • Each semi-circular canal lies in a different plane at right angles to each other.

      • The membranous canals are suspended in the perilymph of the bony canals.

      • The base of the canals is swollen and is called the ampulla, which contains a projecting ridge called crista ampullaris which has hair cells.

      • The saccule and utricle contain a projecting ridge called macula.

      • The crista and macula are the specific receptors of the vestibular apparatus responsible for the maintenance of the balance of the body and posture.

Mechanism of Hearing:

• The external ear receives sound waves and directs them to the ear drum.

  • The eardrum vibrates in response to the sound waves and these vibrations are transmitted through the ear ossicles (malleus, incus, and stapes) to the oval window.

    • The vibrations are passed through the oval window onto the fluid of the cochlea, where they generate waves in the lymph.

    • The waves in the lymph induce a ripple in the basilar membrane.

      • These movements of the basilar membrane bend the hair cells, pressing them against the tectorial membrane.

    • As a result, nerve impulses are generated in the associated afferent neurons.

  • These impulses are transmitted by the afferent fibers via auditory nerves to the auditory cortex of the brain, where the impulses are analyzed and the sound is recognized.