SPECIAL SENSES
v The special senses of hearing,sight,smell and taste all have specialized sensory receptors (nerve endings) outside the brain.
v These are found in the ears,eyes,nose and mouth.
v The ear is also involved in the maintenance of balance.
v In the brain the incoming nerve impulses undergo complex processes of integration and coordination that result in perception of sensory information and a variety of responses inside and outside the body.
HEARING AND THE EAR
1.INTRODUCTION: The ear is the organ hearing.
2.SITUATION:The structures that form the ear are encased within the petrous portion of the temporal bone.
3.STRUCTURE:
The ear is divided into 3 parts.
1. Outer ear
2. Middle ear
3. Inner ear .
OUTER EAR :
It consists of the auricle (pinna) and the external acoustic meatus.
THE AURICLE (PINNA):
v The auricle is the expanded portion protecting from the side of the head.
v It is composed of fibroelastic cartilage covered with skin .
v It is deeply grooved and ridged and the most prominent outer ridge is the helix.
v The lobule (earlobe) is the soft pliable part at the lower extremity, composed of fibrous and adipose tissue richly supplied with blood.
EXTERNAL ACOUSTIC MEATUS (AUDITORY CANAL)
v This is slightly ‘S’ Shaped tube about 2.5 cm long extending from the auricle to the tympanic membrane(eardrum).
v The lateral third is cartilaginous and the remainder is a canal in the temporal bone.
v The meatus is lined with skin containing hairs continous with that of the auricle.
v There are numerous sebaceous and ceruminous glands in the skin of the lateral third.
v Ceruminous glands are modified sweat glands that secrete cerumen (wax).
CERUMEN (WAX):
Ø It is a sticky material containing lysozyme and immunoglobulins.
Ø e.g foreign materials….dust,insects and microbes are prevented from the reaching the tympanic membrane by wax,hairs and the curvature of the meatus.
Ø Movements of the temporomandibular joint during chewing and speaking ‘massage’, the cartilaginous meatus , moving the wax towards the exterior.
THE TYMPANIC MEMBRANE (EAR DRUM).
Ø It separates the external acoustic meatus from the middle ear.
Ø It is oval-shaped with the slightly broader edge upwards and is formed by three types of tissue.
Ø The outer covering of hairless skin , the middle layer of fibrous tissue and the inner lining of mucus membrane continous with that of the middle ear.
MIDDLE EAR (TYMPANIC CAVITY)
Ø This is an irregular –shaped air filled cavity within the petrous portion of the temporal bone.
Ø Its contents and the air sacs which open out of it are lined with either simple squamous (or) cuboidal epithelium.
Ø The lateral wall of the middle ear is formed by the tympanic membrane .
Ø The roof and floor are formed by the temporal bone with openings leading to the “mastoid antrum” through which are passess to the air cells within the mastoid process.
The medial wall is a thin layer of temporal bone, it has 2 openings.
1. Oval window
2. Round window
The oval window is occluded by part of a small bone called the “stapes” and the round window by a fine sheet of fibrous tissue.
Air reaches the cavity through the pharyngo tympanic(auditory or eustachian) tube. Which extends from the nasopharynx.
It is 4 cm long and lined with ciliated columnar epithelium .
The tympanic membrane is maintained by the pharyngotympanic tube and enables the membrane to when sound waves strike it.
AUDITORY OSSICLES:
These are 3 very small bones which are present in middle ear of tympanic membrane to the oval window.
They form a series of movable joints with each other. They are
1.THE MALLEUS
2.THE INCUS
3.THE STAPES
1.THE MALLEUS:-
ü It is the lateral hammer –shaped bone.
ü The handle is in contact with the tympanic membrane.
ü The head forms a movable joint with the incus.
2.THE INCUS
ü It is the middle anvil-shaped bone.
ü Its body articulates with the malleus,the long process with the stapes.
ü It is stabilized by the short process, fixed by fibrous tissue to the posterior wall of the tympanic cavity.
3.THE STAPES
ü It is the medial stirrup – shaped bone.
ü its head articulates with the incus and its foot plate fits into the oval window.
ü The three ossicles are held in portion by fine ligaments.
INNER EAR
The inner ear also called it has labyrinth.
It contains the organs of hearing and balance.
It is described into 2 parts.
1.Bony labyrinth
2. Membranous labyrinth.
1.BONY LABYRINTH :
§ It’s a cavity with in the temporal bone lined with periosteum.
§ It is a layer and encloses the membranous labyrinth of the same shape tube.
§ There is a layer between the bony and membranous labyrinth is called “perilymph”or watery fluid.
§ Within the membranous labyrinth there is a similarly watery fluid ‘’Endolymph’.
Bony labyrinth consists of
1. Vestibule
2. Cochlea
3. 3 semicircular canals.
1.Vestibule:
It is the extended part nearest the middle ear and contains the oval and round window in lateral wall.
The cochlea:
It’s a snails shape , it has a broad base where it continous with th vestibule and narrow apex.
3.semi circular canals:
These are three tubes arranged so that one is situated in each of the three planes of space .
They continous with the vestibules.
MEMBRANOUS LABYRINTH:
This contains endolymph and lies with in bony counter part. It has
1. The vestibule which contains the utricle and saccule.
2. The cochlea .
3. Three semicircular canals.
THE COCHLEA:
It contains 3 sections
1. The scala vestibule
2. The scala media or cochlear duct
3. The scala tympani.
The bony cochlea has 2 compartments contains ‘perilymph.
· i’e the scala vestibule
· scala tympani.
§ The cochlea duct is a part of the membranous labyrinth and it is in triangular shape.
§ The basilar membrane (or) base of the triangle there are supporting cells and specialized cochlear hair cells containing auditory receptors.
§ These cells form the spinal organ (of corti), the sensory organ that responds to vibration by initiating nerve impulses that are then perceived as hearing by the brain.
§ The auditory receptors are dendisites of efferent nerves that combine forming the cochlear (Auditory) part of the “Vestibulocochlear nerve” (8th cranial nerve).
§ 8th cranial nerve passes through a foramen in the temporal bone to reach the hearing area in the temporal lobe of the cerebrum.
EYE
1.INTRODUCTION:
The eye is the organ of the sense of sight or vision .
2.SITUATION:
It is situated in the orbital cavity.
3.MEASURMENTS:
SHAPE: - Spherical
DIAMETER - 2.5cm
v The space between eye and the orbital cavity is occupied by adipose tissue.
v The bony walls of the orbit and the fat help to protect the from injury.
4.ACCESSORY ORGANS OF THE EYE:
The eye is a delicate organ which is protected by several structures.
Eye brows
Eye lids and eye lashes
Lacrimial apparatus.
5.STRUCTURE:
There are 3 layers of tissue in the walls of the eye
1.The outer fibrous layer : sclera & cornea
2.the middle vascular layer (or) uveal tract :choroid ciliary body & iris.
3. the inner nervous tissue layer : retina.
4.structures inside the eyeball are the lens ,aqueous fluid (humour) & vitreous body (humour).
SCLERA & CORNEA:
THE SCLERA
- It is a white part of the eye.
- It forms the outermost layer of tissue of the posterior and lateral aspects of the eye ball,is continous anteriorly with the transparent cornea.
- It consists of a firm fibrous membrane that maintains the shape of the eye and gives attachment to the extraocular (or) extrinsic muscles of the eye.
- Anteriorly the sclera continues as a clear transparent epithelial membrane, the ‘cornea’.
- Light rays pass through the cornea to reach the retina.
- The cornea is convex anteriorly and is involved in refracting or bending light rays to focus them on the retina.
CHOROID:
- The choroid lines the posteriorly five-sixts of the inner surface of the sclera.
- It is very rich in blood vessels and is deep chocolate brown in colour.
- Light enters the eye through the pupil , stimulates the nerve endings in the retina and is then absorbed by the choroid.
CILIARY BODY:
- The ciliary body is the anterior contituation of the choroid consisting of ciliary muscle and secretory epithelial cells.
- It gives attachment to the sensory ligament which at its other end is attached to the capsule enclosing the lens.
- Contraction and relaxation of the ciliary muscle changes the thickness of the lens which bends or refracts light rays entering the to focus them on the retina.
- The epithelial cells secrete “Aqueos fluid “ into the anterior segment of the eye,I,e the space between the lens and the cornea (anterior & posterior compartment.
- The ciliary body is supplied by parasymphathetic branches of the oculo motor nerve(3rd cranial nerve).
- Stimulation causes contraction of the smooth muscle and accommodation of the eye.
IRIS
- The iris is the visible coloured part of the eye & extends anteriorly from the ciliary body,lying behind the cornea infront of the lens.
- The colour of the iris is genetically determined and depends on the number of pigment cells present.
- Albinos have no pigment cells and people with blue eyes have fewer then those with brown eyes.
- It divides the anterior segment of the eye into anterior and posterior chambers which contains “Aqueous fluid” secreted by the ciliary body.
- It is a circular body composed of pigment cells and two layers of smooth muscle fibres, one circular and other radiating.
- In the centre there is an aperture called the PUPIL.
- The iris is supplied by parasymphathetic and parasymphethetic nerves.
- Parasympathetic nerve stimulation constricts the pupil and sympathetic stimulation dilates the pupil.
LENS
- The lens is a highly elastic circular biconvex body,lying immediately behind the pupil.
- It consists of fibres enclosed within a capsule and it is suspended from the ciliary body by the suspensory ligament .
- The lens bends (refracts) light rays relected by objects in front of the eye.
- Lens is the structure in the eye that can vary its refractory power,achived by changing its thickness.
- When the ciliary muscle contracts, it moves forward,releasing its pull on the lens, increasing its thickness.
- The nearer is the object being viewed the thicker the lens becomes to allow focusing.
RETINA:
- The retina is the innermost layer of the wall of the eye.
- It is an extremely delicate structure and is especially adapted for stimulation by light rays.
- It is composed of several layers of nerve cell bodies and their axons, lying as a pigmented layer of epithelial cells which attach it to the choroid.
- The layer highly sensitive to light is the layer of sensory receptor cells :rods & cones.
- The retina lines about three-quarters of the eyeball.
- The eyeball is thickest at the back and things out anteriorly to end just behind the ciliary body.
- Near the center of the posterior part is the macculalutea, (or) yellow spot.
- In the centre of the area there is a little depression called “fovea centralis”. Consisting of only ‘cone-shaped cells.
- The rods and cones contain photosensitive pigments that convert light rays into nerve impulses.
- About 0.5 cm to the nasal side of the macula lutea all the nerve fibres of the retina converge to form the optic nerve.
- The small area of the retina where the optic nerve leaves the eye is the optic disc (or) blind spot.
- It has no light sensitive cells.
6.BLOOD SUPPLY:
ARTERIAL BLOOD SUPPLY:
- Cliary artery and thr central retinal artery these are the branches of ophthalmic artery and branches of the carotid artery.
VENOUS DRAINAGE:
- Central retinal vein which eventually empty into a deep venous sinus.
TONGUE
Tongue is a voluntary muscular structure that occupies the floor of the mouth.
It is attached by its base to the hyoid bone .
1.INTRODUCTION : It is a boneless muscle which is present in the mouth .
2. SITUAITION :Situaited at the base of the mouth .
3.STRUCTURE: 2/3rd portion in front & 1/3rd portion in the postrrior part of oral cavity has taste buds & papillae on its surface.
It is a mobile organ with a tip of body and base.
4.BLOOD SUPPLY:
Ø ARTERIAL BLOOD SUPPLY : Lingual artery branch of Carotid artery.
Ø VENOUS DRAINAGE: Lingual vein into internal jugular vein .
NERVE SUPPLY
MOTOR NERVE SUPPLY – Hypoglossal nerve.
SENSORY NERVE SUPPLY- Lingual nerve, glassopharyngeal nerve.
FUNCTIONS:
It helps for speech
It helps for Deglutition.
It helps to find the sense of taste.
It helps for mastication.
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