Sense Organ

Eye

Eyes are the Sense Organ of Vision. Eyes are the sensitive detectors high hence called photoreceptors. It is situated in deep protection bony cavities called Orbits or eye sockets of the skull. It is about 2.5cm in diameter & weight about 7gm.

Protection:

1. Eyebrows: present on supraorbital region.
2. Eyelashes:
3. Meibomian gland: These are some glands that opens at the base of follicle of eyelashes called Meibomian gland (modified sebaceous gland). That secrete oil & lubricates eyeball + reduces friction during movement.
4. Eyelids: Three eyelids are present in which 2 are functional eyelid & one is reduced nictitating membrane (vestigial).
5. Adipose tissue: around the eyeball.
6. Lacrymal (Tear) gland: is present on outer side of eye just inside the orbit. It is almond shape & secrete secretion in the form of tear (lysozyme) with the help of duct.
7. Glands of Zeis and Moll: Glands are present on the eyelid edges such as Gland of Zeis & Gland of Moll. Infection in the gland of Zeis by bacteria causes the disease called sty.

[Diagram: External Eye Protection]
Labels: Eyebrow, superior canal, lacrimal gland, lacrymal duct, eyelashes, inferior canal, open in nose, lacrymal sac, nictitating membrane, canthus.

Sensory Reception & Processing:

The sensory organ detect all types of changes in the environment & lead appropriate signal to CNS where all the inputs are processed & analysed. Signals are then sent to different centers of the brain. Sense organs are also called Receptors.

Sense Organs:

We smell thing by our nose, taste by tongue, hear by ear & see object by eye. The nose contains mucus-coated receptor specialized for receiving the sense of smell called olfactory receptors which opens into olfactory bulb which are the extensions of brain.

Both nose & tongue detect dissolved chemicals like the chemical sense of Gustation (taste) & Olfactory (smell) are functionally similar & interrelated. The tongue detect taste through taste bud containing gustatory receptors.

Structure of human Eye:

[Diagram: Internal Structure of Human Eye]
Labels: Canal of sclera, Aqueous humor, conjunctiva, cornea, pupil, inner part, suspensory ligament, ciliary part, vitrous humor (Whorston's jelly), Sclera, fovea (cones), choroid, Area of centralis, Blind spot, Optic nerves.

In human eye, the wall of eyeball consist of 3 layers:

1. Outermost layer: Consist of Sclera & cornea.
2. Middle layer: Consist of choroid, ciliary body & Iris.
3. Inner Most layer: Retina. It has photoreceptor cell called rods & cones.

(1) Outermost layers:

• Sclera: An opaque white outermost covering composed of dense connective tissue. It maintains the shape of eyeball.
• Cornea: Is a thin transparent front part of sclera which lacks blood vessels but rich in nerve ending. Cornea allows light to pass into the eye.
• Conjunctiva: Protects cornea & also secretes oil + mucus that moistens & lubricates the eye.

(ii) Middle vascular layers:

• Choroid: is present beneath the sclera & contains numerous blood vessels & nourish the retina.
• Ciliary body: It becomes thick in the anterior part to form the ciliary body which holds the lens in position.
• Iris: It is the visible coloured position of the eye ie pigmented circle attached to ciliary body in front of lens.
• Pupil: The diameter of pupil is regulated by muscles fibres of Iris.

(iii) Inner nervous layer:

The inner layer is the retina & it contains different layer of cells from inside to outside such as layer of ganglion cells, bipolar cell, photoreceptor & pigment cell.

• The photoreceptor cells are of two types ie rod & cone cells.
• Rods: contains photosensitive pigment called Rhodopsin.
• Cones: are of three types which posses characteristic photopigment (porphyropsin, iodopsin and cyanopsin) that respond to red, green and blue light.
• The daylight (photopic) vision & colour vision are function of cones & twilight (scotopic) vision is the function of rods.

Fluid Chambers:

• Aqueous chamber: The space between cornea & lens is called aqueous chamber & is filled with aqueous humor. It helps to maintain the shape of front part of eye & also provide nutrition to cornea & lens.
• Vitreous chamber: The space between lens & retina is called vitreous chamber which is filled with vitreous humour or Whortson's jelly.

Important Points on Retina:

• Optic nerve: contains fibres of sensory neuron & leaves the eyeball from back side called blind spot or optic disc that lacks rods & cones.
• Macula lutea: At the posterior pole of eye just opposite to lens there is a yellowish pigmented spot called macula lutea with a central pit called fovea centralis.
• Fovea centralis: is the out portion of the retina where only cones are densely packed and is the point where resolution is greatest (sharp image is formed).

Mechanism of Vision:

When the light rays from any object fall on cornea they enter the eye through the pupil. Iris control the amount of light passing through by controlling the size of pupil. The rays of light converges as they pass through aqueous humour, lens & vitreous humour & finally focus at retina forming an inverted image of the object.

The lens makes the fine adjustment to bring a shape of focus on retina. The ability of the lens by which the light ray from near & far away object are focused on retina is called accommodation.

The ciliary muscles relax & the lens becomes thinner to focus the ray of distant object (more than 6m). Similarly, ciliary muscles contract & lens becomes thicker to focus the ray from nearer objects. Thus normal eye is able to accommodate light from object 25cm to infinity.

The stimulus received by rod & cone result in dissociation of photosensitive pigment (rhodopsin into opsin & retinal (vit A)). The stimulated photoreceptor initiate the nerve impulse. Path taken by light ray in the eyeball:

Conjunctiva → Cornea → Aqueous humour → lens (through pupil) → Vitreous humour → Retina

The inverted image formed at the retina is picked up by the optic nerve which takes it to brain where it is reversed & nerve impulse is analysed & finally an upright image of the object is perceived by the individual.

Since, both our eyes are frontally placed there is a overlap between the visual field of each eye this is called binocular vision.

[Diagram: Structure of Retina]
Layers: Sclera, Choroid, Rod, Cone (first layer), Intermediate layer, Ganglia, fibre of optic nerve (Third or internal layer).

Detailed Layers of Retina:

Retina is the innermost layer to posterior wall of the eye. It contains cells which are sensitive to light and trigger nerve impulse that passes via optic nerves to the brain where image is formed. Retina consist of 3 layer:

1. Photoreceptor cells: It is mainly composed of sensory epithelial cells & is stimulated by light. They are of 2 types:
   • Rods: They are elongated, rod like and contains photosensitive pigment called rhodopsin. It help in conversion of light ray into nerve impulse in dim light or night & produce poor image. Vit A is required for the regeneration of rhodopsin. So the light bleaches the rod that produces nerve impulse which is transmitted to brain through optic nerve.
   • Cones: They are short & tapering at base forming cone like structure. It contains iodopsin as photosensitive pigment. It respond to daylight & help in colour vision.
2. Bipolar neuron: The neuron having one axon & one dendrite. So Bipolar neuron communicate below with rods & cones & above with ganglionic neuron through their process.
3. Ganglionic cells: It contains nuclei of ganglion cells of axon which become optic nerve for message.

Flowchart: Changes in retina when light falls on it:

Light falls at the receptor cells (rods & cones) in retina → Breaking up of light sensitive pigment present in rods & cones by specific wavelength of light → Stimulation of rod & cones & generation of nerve impulse → Nerve impulse transmitted to bipolar nerve cells → Nerve impulse transmitted to ganglion cells → Nerve impulse in the axon of ganglion cell which converge & forms the optic nerve → Nerve impulse transmitted to brain → Perception of light by the brain cell (In the visual area of cerebrum).

Comparison between Rods and Cones:

Eye defects & their corrective measures:

EAR

Ear are the Statocoustic Organs meant for both balancing & hearing. Ears are paired structure attached to the skull posterio-laterally in all vertebrates. And the ear has two receptors ie photoreceptors for hearing & statoreceptor for body equilibrium or balancing.

Structure:

The human ear consist of three main parts, the air filled external & middle ear + fluid filled inner ear.

[Diagram: Human Ear Structure]
Labels: Pinna, Auricle, Helix, lobule, External meatus or auditory canal, Temporal lobe, malleus, incus, stapes, Ampulla, vestibule, cochlea, Eustachian tube, Round window (fenestra rutunda).

a) External Ear:

It consist of pinna, auditory canal and tympanic membrane. Its function is to collect and direct the sound wave to the middle ear.

1. Pinna: It is an immovable cartilagenous str. It has ridges called helix and lower lobule. It leads into an auditory canal or meatus. It focus, collect & send sound wave into auditory canal.
2. Auditory Canal: It is also known as ear tube or external meatus & end at the eardrum. If canal has hair & secretion of ceruminous glands (ie ear wax). This wax traps insect, dust thus guard against infection and provide lubrication.
3. Tympanic membrane: It is also known as eardrum. It is delicate membranous funnel like str. that vibrate with sound wave.

b) Middle ear:

It is embedded in the cranial bone & is an air fill cavity. It consists of 2 parts:

1. Tympanic membrane: It is a border betn external & middle ear & produces the vibration.
2. Tympanic Cavity: It consist of 3 tiny bone ear ossicles that vibrate in response to the vibration of the eardrum. It also has a Eustachian tube communicating the cavity of middle ear to the pharynx. The 3 ear ossicles are:
   • 1) Malleus: It is hammer shaped & attached with incus on the other side.
   • 2) Incus: It is an anvil like & it is attached with stapes on other sides.
   • 3) Stapes: It is stirrup like & its foot plate is attached to the oval window or Fenestra ovalis. These 3 bone have a lever like action & intensify the vibration by 20 to 25 times & transfer its to cochlea of the inner ear through oval window.

* Eustachian tube: It is a short tube that connect the middle ear to the pharynx. It help in the equalising the ear pressure on both side of ear drum.

c) Inner ear:

[Diagram: Inner Ear Detail]
Labels: Posterior semicircular canal, lateral semicircular canal, Utriculus (macula utriculi), Ampula (cristae sensory cell), sacculus, ductus endolymphaticus, sacculus endolymphaticus, endolymph.

It is a fluid filled chamber that consist of a complex system of canal & cavities. Inner ear is made up of bony labyrinth & membranous labyrinth.

1. Bony labyrinth: It consist of bony cavities & is filled with endolymph whereas the cavity floats in perilymph. It consist of 3 parts:
   • Vestibule: It is an irregular shaped bony cavity. It consist of Utriculus that is upper layer part & sensory point called macula utriculi whereas Sacculus is the lower part of sensory point called macula sacculi. This macula helps to balance the body.
   • Semicircular Canal: 3 bony anterior, posterior, semicircular canal are present. The end of each canal is swollen & called ampulla. This ampulla contains sensory area called cristae (sensory hairs, coils & supporting cells). These canal are attached to Utricular of vestibule. It also helps in balancing & equilibrium of the body.
   • Cochlea: It is highly coiled tube about 25-30 mm long & is considered to sacculus through duct of reunion. It has 3 canal: the vestibular canal, the middle canal & the tympanic canal.

[Diagram: Cross-section of Cochlea]
Labels: Scala vestibuli, Oval window, Reissner's membrane, Perilymph, Endolymph, Tectorial membrane, Helicotrema, Scalamedia, Basilar membrane, Cochlear bond, auditory nerve, Organ of corti.

The Reissner's membrane separates the vestibuli & the middle canal while the basilar membrane separates the middle & the tympanic canal. The membranous part of cochlea is also called Scala media.

Both the vestibular & tympanic canal are filled with perilymph which are connected by a small hole called helicotrema & is connected to round window.

The middle canal is filled with a fluid called the endolymph & is separated by basilar membrane. On this membrane sensory hair cells rest & that are in contact with tectorial membrane above. So these sensory cells tectorial membrane + basilar membrane together form Organ of Corti which is responsible for sending the sound impulse to brain through auditory nerve.

2. Membranous labyrinth: It floats in the perilymph of bony labyrinth. It consist of number of membranous canal & sac which is filled with endolymph. The consist of:
   • a) Three membranous Semicircular Canal.
   • b) Two sac called Utricle & Saccule inside the vestibule.
   • c) A coiled membranous tube inside the cochlea called Scala media.

Mechanism of hearing:

As the sound waves enters the external ear, they pass through ear canal & strike the eardrum. The vibration of ear drum are transmitted to three ear ossicles which magnify them 20-25 times by their lever like action.

As the stapes fits into oval window, when it vibrates the fluid perilymph in the vestibular canal also vibrates & causes round window to move to & fro.

These vibration causes the movement of the basilar membrane which move the sensory hair attached to it against the tectorial membrane that creates the nerve impulse.

The sound waves converted to nerve impulse travel through the auditory nerve to the auditory cortex region of brain. The variation in the intensity & pitch are determined by amplitude by which basilar membrane vibrates the region of organ of corti that get stimulated.

Path: Sound waves → Auditory canal → Tympanic membrane → 3 ear ossicles → Oval window → Perilymph in vestibular canal → Reissner's membrane → Endolymph in Scala media → Basilar membrane → Perilymph in Tympanic canal → Round window.

Defects of Ear:

1. Labyrinthine disease: malfunction of inner ear.
2. Meniere's disease: loss of hearing duct to defect in cochlea.
3. Otitis media: Acute infection of middle ear.
4. Eustochitis: Inflammation of Eustachian tube.
5. Myringitis (Tympanitis): Inflammation of eardrum.
6. Otalgia: Earache (pain in ear).