SENSE ORGANS - EYE AND EAR (Structure & Function)

Here we will discuss about structure and function of sense organ (eye)

    EYE:

• Eye is the sense organ of sight in the body. 

• It is situated in the orbital cavity 

•  It is supplied by the optic nerve (2nd cranial nerve)

    Structure

    There are three layers of tissue in the walls of the eye:

   

    1. Outer fibrous layer:

• Sclera and
• Cornea

    2. Middle vascular layer:

• Uveal tract of the choroid,
• Ciliary body and
• Iris

      3. Inner nervous tissue layer:

• The retina

    Sclera:

• It is the white parts of the eye.
• It forms the outermost layer of the posterior and lateral aspects of the eyeball.
• It continuous anteriorly with the cornea.
• It consists of a firm fibrous membrane that maintains the shape of the eye and attached the extrinsic muscles of the eye.

    Cornea:

• It is clear transparent epithelial membrane.
• Light rays pass the cornea to reach the retina.
• Convex anteriorly and is involved in refracting (bending) light rays to focus them on the retina.

     Choroid:

• It is lining the posterior five-sixth of the inner surface of the sclera.
• It is very rich in blood vessels and is deep chocolate brown in color.
• Light enters the eye through the pupil, it also stimulates the sensory receptors in the retina and is then absorbed by the choroid.

    Ciliary body:

• It is interior continuation of the choroid.
• It is consisting of ciliary muscle (smooth muscle fibers) and secretory epithelial cells.
• The lens is attached to the ciliary body by radiating suspensory ligaments, like the spokes of a wheel
• Epithelial cells release aqueous fluid into the anterior segment of the eye.
• Ciliary body is supplied by parasympathetic branches of the oculomotor nerve.

    Iris:

• It is visible coloured ring at the front of the eye
• It occurs behind the cornea. 
• It divides the anterior segment of the eye into anterior and posterior chambers.
• Composed of pigment cells.
• In the centre there is an aperture called the pupil.
• Two layers of smooth muscle fibre

        1.Circular
        2. Radiating

    Retina:

• It is the innermost lining of the eye.
• Extremely delicate structure of the eye.
• Well adapted for stimulation by light rays.
• Composed of several layers of nerve cell bodies and their axons, lying on a pigmented layer of epithelial cells.
• Light-sensitive layer consists of sensory receptor cells, rods and cones,
• These cells contain photosensitive pigments that convert light rays into nerve impulses.
• The retina lines about three-quarters of the eyeball and is thickest at the back
• Thins out anteriorly
• Near the centre of the posterior part is the macula lutea, or yellow spot.
• In the centre of the yellow spot is a little depression called the fovea centralis, consisting of only cones. 

    Accessory structures of eye: 

1. Eyelids:

• It close eyes during sleep.
• Protect from excessive light and foreign object.
• Spread lubricating secretions over eyeballs.

2. Eyelashes & Eyebrows:

• It protects from excessive light and foreign objects.
• Sebaceous glands of the eyelashes at base of the hair follicles, called sebaceous ciliary glands that release lubricating fluid.

3. Lacrimal apparatus: produces and drains lacrimal fluid or tears

4. Extrinsic eye muscles:

        

    Six muscles move each eye:

• The superior rectus,
• Inferior rectus,
• Lateral rectus,
• Medial rectus,
• Superior oblique, and
• Inferior oblique

Function of eye

Cornea: It admits and refracts (bends) light.
Sclera: It provides shape and protects inner parts of the eye.
Iris: It regulates amount of light that enters into the eyeball.
Ciliary body: It release the aqueous humor and alters the shape of lens for near or far vision.
Choroid: It supply blood and absorbs scattered light that enters into the eye.
Retina: It receives light and converts it into receptor potentials and nerve impulses. 
Lens: It refracts light which are visible through eye.
Anterior cavity: It has the aqueous humor that helps to maintain shape of eyeball and supplies oxygen and nutrients to lens and cornea of the eye.
Vitreous chamber: It has vitreous body that helps the eyeball and keeps the retina attached to the choroid.  

    EAR

• It is the organ of hearing.
• It is external auditory part, also known as vestibulocochlear organ.
• It maintains our sense of balance and hearing.

Parts of Ear: 

1. External ear: It collects sound waves and channels them inward of the ear.

2. Middle ear: It conveys sound vibrations to the oval window of the ear.

3. Internal ear: It houses the receptors for hearing and equilibrium.

 External ear:

It is having pinna and the external auditory canal, which receives sounds waves and transmits it into the middle ear via the eardrum, also known as tympanic membrane. The eardrum is circular and flexible. 

Function:

• Auricle (pinna): It collects sound waves. 
• External auditory canal (meatus): It directs the sound waves into the eardrum.
• Tympanic membrane (eardrum): The sound waves cause it to malleus to vibrate. 

Middle Ear:

It includes the ossicles, three minuscule bones called the malleus, incus and stapes (the smallest bone in the human body). These three bones form a bridge between the eardrum and the inner ear called oval window that covers the cochlea. A narrow tube called the Eustachian tube, connects the ear to the outer part of the nose. This makes the pressure balanced on either side of the eardrum.

Function:

• Auditory ossicles: It transmit and amplify vibrations from tympanic membrane (eardrum) to oval window.
• Auditory (eustachian) tube: It equalizes or balance air pressure on both sides of the tympanic membrane. 

Internal Ear (labyrinth):  

It formed by cochlea, a structure that has a spiral shape., which has several membranous sections filled with fluids called endolymph and when these liquids move, they make changes in the cochlea’s hair-like structures called stereocilia. Organ of Corti transforms the mechanical energy of the sound waves into nerve energy by making electric impulses that are sent to the brain through the auditory or vestibulocochlear nerve.

Function:

• Cochlea: It contains a series of fluids, channels, and membranes that transmit vibrations to the spiral organ, the organ of hearing; and hair cells in the spiral organ produce receptor potentials, which changes the nerve impulses in the cochlear branch of the vestibulocochlear (VIII) nerve.
• Vestibular apparatus: It includes semicircular ducts, utricle, and saccule, which generate nerve impulses that passes along the vestibular branch of the VIII nerve.
• Semicircular ducts: It contain cristae, site of hair cells for dynamic equilibrium (maintenance of body position, mainly the head, in response to rotational acceleration and deceleration).
• Utricle & Saccule: It contains macula, site of hair cells for static equilibrium (maintenance body position, mainly the head, relative to the force of gravity).

Physiology of hearing:  

• Pinna directs sound waves into the external auditory meatus of the ear. 
• The sound waves cause the tympanic membrane (eardrum) to vibrate. 

         Slowly for low- frequency sounds

         Rapidly for high-frequency sounds

         The membrane travels during these vibrations relates to loudness or decibels.

• Vibrations are transmitted from the tympanic membrane to the auditory ossicles.
• Stapes vibrates in a way that, back and forth in the oval window, that cause vibrating the oval window membrane.
• Vibration of oval window membrane causes fluid pressure waves in the perilymph of the scala vestibuli.
• Perilymph pressure waves are transmitted to the scala tympani and to the round win causing the secondary tympanic membrane to bulge outward. 
• The vibrations of the vestibular membrane cause vibrations of the endolymph within the cochlear duct.
• Endolymph pressure waves cause the basilar membrane to vibrate. 

       a. Vibrations of the basilar membrane cause the the hair cells of the Organ of Corti to vibrate.

        b. Hair cells vibrate upward, bending the stereocilia against the tectorial membrane.

        c. Bending the stereocilia produces a receptor potential that ultimately leads to an action potential on Cochlear nerve.

Physiology of equilibrium: 

• It occurs in two types: 

1. Static equilibrium: It is the maintenance of the position of the body (mainly the head) relative to the force of gravity.  Eg: tilting the head and linear acceleration or deceleration, such as when the body is being moved in an elevator.
2. Dynamic equilibrium: It is the maintenance of body position (mainly the head) in response to rotational acceleration or deceleration.

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