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15 Cards in this Set
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THE TONGUE The tongue is made up of muscles covered by mucous membranes. These muscles are attached to the lower jaw and hyoid bone, which is located just above the larynx. Hyoid bone anchors the muscles of the tongue (it is the only bone in the body that doesn't touch any other bone). The muscle fibers are supplied with nerves, so it can manipulate food in the mouth and place it between the teeth for chewing - without being bitten in the process. |
THE TASTE SYSTEM Anterior Surface: Consists of 3 types of taste papillae (bumps), on which some taste buds are located: (i) Filliform papillae: No taste buds here. (ii) Fungiform papillae: Mushroom shaped structures. Located at the front of the tongue. Each fungiform papillae contains 3-5 taste buds. (iii) Circumvallate papillae: Located at the back of the tongue. Contains more than 100 taste buds.Used for taste perception. Posterior Surface: No taste buds here. Contains lymph nodes called lingual tonsils. Inferior (Ventral) Surface: It is a smooth surface. Contains foliate papillae that have ridges and grooves and is at the sides of the tongue. |
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TASTE It is a special visceral afferent sensation. Taste is detected by 3000 taste buds mainly in the circumvallate papillae, as well as fungiform and foliate papillae on the tongue and epiglottis, pharynx and palate. Glands of von Ebner are serous glands that secrete fluid (solvent) into the circumvallate papillae to improve taste. Each taste bud contains ~50 cells. Some cells are supporting while others have gustatory role (sense of taste). There are different types of taste buds: •Bitter –detect and avoid toxins used by plants as defence mechanism. •Sour –(acid) disliked at high concentrations , discouraging eating of foods spoiled by acid producing bacteria – inverse indicator of sugar content. •Sweet and umami– indicates essential nutrients (carbohydrates) and calorie rich food – sweet taste is not precise. •Salt –required mineral for homeostasis, osmotic function within tissues (salt thirst) •Each taste is perceived greater in specific region of the tongue. •The structure of the taste buds appears similar! •Different tastes are perceived by different tongue receptors e.g Sour and salt (ion channels), Bitter, sweet & amino acids (G-coupled receptors) •Bitter and sour threshold is 10x less than sweet and salty because sweet and salty are more sensitive compared to bitter and sour. |
TASTE INNERVATION The tongue is innervated by different cranial nerves at its different parts: •Anterior 2/3 of the tongue (i) Lingual nerve (CN V Trigeminal n) carry general sensation e.g hot, cold, vibration. (ii) Chorda tympani (eardrum) branch of CN VII (Facial n) carries taste. •Posterior 1/3 of the tongue (i) CN IX (Glossopharyngeal n) carries general sensation and taste (including circumvallate papillae). •Pharynx and Epiglottis (I) CN X– Vagus carries taste. |
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TASTE PATHWAY When stimuli enters the oral cavity, they bind to taste cell membrane receptors, pass through specific channels, or activate ion channels. These processes trigger the taste cell to release neurotransmitters, sending a signal to the brain (synapse 1). The cell bodies in cranial nerves VII,IX, X (geniculate, petrosal and nodose ganglia) in the taste buds and oropharynx synapse at the medulla of the brainstem column – gustatory nucleus (in the rostral part of solitary tract). Then they ipsilaterate to the thalamus and go to the post-central gyrus of the somatosensory cortex located in the insula. The insula is an integrating centre and used for visceral sensation. In the cerebral cortex – conscious perception of taste in the frontal aperculum. Gustatory pathway is UNCROSSED i.e. IPSILATERAL Solitary tract gives visceral sensory information. Vestibucochlear nucleus is for somatic information (Hearing and balance) Trigeminal nucleus: For detecting hot, cold, itchness, dryness, vibration in the head and neck. It has 3 branches and it has a large area to cover. The three branches of the trigeminal nerve; Opthlamic nerve (V1), maxillary nerve (V2) & mandibular nerve (V3)- converge at the trigeminal ganglion located at Meckel's cave. The quality of taste is determined in the parabrachial pontine nucleus located in the limbic system (emotional brain) and hypothalamus. |
SMELL AND TASTE PERCEPTION Blocking the retro-nasal route between the oropharynx and nasal pharynx diminishes flavour sensation. •Age-related loss of sweet taste is less than salty bitter or sour •Genetic inheritance of bitter taste perception is part of survival mechanism against plant toxins •Gene for bitter taste has been identified |
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SMELL- THE NOSE The nose is the organ of smell, and serves in breathing and airway function. The nasal cavity lies between the base of the brain (skull base) and above the oral cavity and palate below. The nasal cavity is surrounded by paired, air-filled chambers on each side, and these lie towards the sides and above the nose (the “paranasal sinuses”). These paired sinuses are the maxillary (lying in the cheek bones), frontal (lying between the eyebrows), ethmoid (between the eyes) and the sphenoid sinuses (between the center of the skull base and the nose). The nasal cavity is divided by a vertical partition (the “nasal septum”) into a right and left side. The nasal septum is made up of cartilage (quadrangular cartilage) towards the front of the nose, and bone towards the back. Parts of four different bones make up the bony septum: the perpendicular plate of the ethmoid bone, vomer, and the nasal crest of the maxilla and palatine bones. The nasal septum is often not located in the midline, but pushed over to the right or left side in various parts. This shift is referred to as a “deviated nasal septum”. These deflections usually occur where the various parts of the nasal septum join together, due to overgrowth of one or more components. Sometimes, trauma to the nose can also create deflections and fractures. When this deviation is large or severe it can result in narrowing of one or both sides of the nasal cavity. In the front, the left and right sides of the nasal cavities form the right and left nares. The paranasal sinuses drain into the nose. Seven odours with distinct receptors' sites Pungent and putrid fits by charge and notshape. Hynosmia- diminished sense of smell. e.g. cold/flu Anosmia- loss in sense of smell. e.g. ageing. |
PARTS OF THE NOSE & NASAL PATHWAY Odours diffuse into specialised olfactory mucosal epithelium containing receptors of basal and supported ciliated cells. They open and depolarise Na+ and K+ channels and fire impulses. Unmyelinated axons pass the olfactory bulb (tract) to form bundles and make the olfactory nerve. Smell is perceived through part of the roof of the nasal cavity (olfactory cleft) that is just next to the part of the brain responsible for smell (olfactory bulb and fossa). This bony partition in this area (“cribriform” bone) is pierced by numerous nerves (“Olfactory” nerves) on each side. Smell goes through the top 2/3rds of the nose where the bipolar neurons and substenticular cells are. These nerves supply the top parts of the nasal septum, middle and inferior turbinates and carry smell sensations to the brain. In the nasal cavity, physical obstruction and injury, or inflammation to the olfactory cleft by tumors, polyps etc. can affect smell. They synapse at the telencephalon. Pathway is ipsilateral (2 olfactory tracts) with anterior commissure connecting left and right. The primary olfactory cortex is where you get the sensation of smell before you figure out what the smell is. The primary olfactory cortex is located at the medial aspect of the temporal lobe in the uncus/unicate gyrus. Smell directly passes through the Medial and dorsal nuclei of thalamus and neocortex (orbitofrontal cortex) (CONSCIOUS PERCEPTION). |
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CLINICAL SIGNIFICANCE •Age related loss of olfactory acuity is large and recognised to affect discrimination, memory, identification and intensity perception. Could affect safety (toxins, poisons, smoke). Greater in men than women. Decreased motivation to eat may lead to nutritional deficiencies, depression. •Environmental damage to epithelium is most common cause of age-related alteration. •Marked loss in olfaction in patients with Alzheimer’s Disease to age matched controls. Could this be a test of future cognitive decline or dementia. |
Questions: A lesion in which of the following areas of the CNS would present as loss of both taste and smell in a patient? a) Inferior orbital sulcus b) Primary somatosensory area c) Solitary nucleus d) Insula e) Uncus |
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THE EYE & ITS ACCESSORY STRUCTURES - The accessory structures of the eyes include the eyebrows, eyelids, eyelashes, lacriminal apparatus and the extrinsic eye muscles. - Two movable eyelids, the upper and lower eyelids (palpabrae), helps to shade the eyes during sleep, protect the light from excessive light and foregn objects, and spreads lubricating secretions over the eyeballs. - The upper eyelid moves more than the lower eyelid because of levator palpebrae superioris. - Palpebral fissure is the space between the upper and lower eyelids that exposes the eyeball. - The angles of the palpebral fissure and the medial and lateral commissures. - The broad lateral commissure/ lateral canthus is near the temporal bone. - The narrow medial commissure/ medial canthus is near the nasal bone. - Orbicularis oculi is a muscle that closes the eyelids. It is supplied by the temporal (orbital & palpebral) & Zygomatic (lacrimal) braches of the facial nerves (CN VII). - The lacrimal caruncle is the small, reddish elevation on the medial commissure that has sebaceous (oil) glands & sudoriferous (sweat) glands that secrete the whitish material seen sometimes on the medial commissure. - Tarsal plate is a thick connective tissue that gives form & support to the eyelids. - Tarsal glands in the tarsal plates prevent eyelids from sticking to each other by secreting oil. - Eyelashes which are at the border of each eyelid prevent dirt and sunlight from entering the eyes. Their sebaceous ciliary glands in each hair follicle secrete lubricating fluids. - Eyebrows which arch above the upper eyelid protect the eyes from sweat & foreign objects. - Conjunctiva is a thin protective mucous membrane composed of non-keratinised stratified squamous epithelium with many goblet cells supported by areolar connective tissue. - Palprebral conjunctiva is at the inner surface of the eyelids. - Bulbar conjunctiva is from the eyelids to the sclera at the surface of the eyeball. |
THE LACRIMAL APPARATUS & LACRIMAL GLANDS. - The lacrimal apparatus is a group of structures that produce and drains lacrimal fluid/tears in a process called lacrimation. - Lacrimal fluid is a watery solution containing salt, mucus and lysosome, a protective bactericidial enzyme. It protects, moistens and cleans the eyeball. - Lacrimation is a protective mechanism. - Lacrimal glands produce lacrimal fluid that drain into 6-12 lacrimal ducts and goes into the cornea and superior fornix of the conjunctiva to lubricate them. - The lacrimal gland is in the Superolateral part of the orbit in its own fossa (porthole),divided by the levator palpebrae superioris muscleinto superior orbital and inferior palpebral parts. - The nasolacrimal duct are openings on the upper end of the nasolacrimal canal. - The naso-lacrimal duct opens into the inferior meatus (nasal cavity) of the nose. - Lacrimal ducts convey fluid to the conjunctival sac. - The groove between the sphenoid and zygoamtic bones contains the lacrimal sac. -The lacrimal sac collects lacrimal fluid which has passed over the surface of the cornea. The lacrimal sac is the reservoir for drainage of lacrimal fluid. - The lacrimal glands are suppiled by the parasympathetic fibres of the facial nerve (CN VII). - Lacrimal puncta ---> lacrimal canaliculi ---> lacrimal sac. Path of drainage of lacrimal lake - Lacus lacrimalis is a lake that holds tears. |
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WALLS AND OPENINGS IN THE ORBIT - Optic nerve and ophthalmic artery pass through the optic canal. - There are two orbitalfissures: the superior orbital fissure and inferior orbital fissure where the ophthalmic vein passes through ("gateway"). - Superior orbital fissure: Lateral apex between greater and lesser wings of sphenoid bone. It transmits sympathetic nerve supply of oculomotor,trochlear, trigeminal and abducens, and ophthalmic vein - Inferior orbital fissure: Lateral wall and floor transmits infraorbital nerve and artery, orbital and zygomatic branches of the maxillary division of trigeminal, communication to pterygoid plexus. Sympathetic n. - Other openings include Supraorbital notch, Nasolacrimal duct, Ethmoidal foraminae |
VASCULATURE OF THE ORBIT • Blood supply is from the ophthalmic artery (from internal Carotid artery) and infraorbital artery (from External Carotid artery) •Central artery of the retina is a branch of the ophthalmic artery. - Central artery of the retina enters the eyeball at the optic disc and supplies the retina. - The branches of the central artery are end arteries i.e. they do not anastomose with other vessels. - Thus, if a branch of the central artery becomes blocked, the area of the retina which it supplies will die, so giving rise to a blind spot. - If the central artery itself is blocked the entire retina will die - causing blindness in the affected eye. • Superior and inferior ophthalmic veins enter the cavernous sinus via superior orbital fissure. |
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MUSCLES OF THE EYE 1 (EXTRA-OCULAR MUSCLES). - Superior & inferior recti produce unwanted medial rotations. - Recti muscles pull the orbit back. - There is a bundle of fibres adjacent to the midline which extends throughout the brainstem called the medial longitudinal fasciculus (MLF). - The majority of the fibres in the MLF originate in the vestibular nuclei. - The MLF also contains interneurons which link together the extra-ocular nuclei (i.e. the nuclei of the oculomotor, trochlear and abducens nerves). |
MUSCLES OF THE EYE 2 (EXTRA-OCULAR MUSLES). - They attach and work along the axis of the orbit - These muscles cannot work efficiently along their own part of the axis so they cannot produce pure movements. - Superior & inferior oblique muscles produce unwanted lateral rotations. - The superior oblique muscle loops through a fibrocartiligous pulley-like structure (the trochlea of superior oblique) and inserts into the sclera on the posterotemporal surface of the eyeball. It is the pulley system that gives superior oblique its actions, causing depression of the eyeball despite being inserted on the superior surface. - The unwanted medial rotations by the recti are "cancelled out" by the unwanted lateral rotations of the obliques. - Obliques pull the orbit forward. |
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Levator palpebrae superioris
- Elevates the upper eyelid. This is why the upper eyelid moves more than the lower eyelid. •Wide aponeurosis ---> orbital septum ----> orbicularis oculi m. andskin of the upper eyelid. - The superior tarsal muscle (Muller's muscle) is a smooth muscle that is next to and joined withthe levator palpebrae superioris via a tendonand inserts on the superior tarsal plate. -It is Innervated by the oculomotor nerve. |
NERVES OF THE ORBIT AND EYE - The globe of the eye, or bulbus oculi, is the eyeball apart from its appendages. - It is composed of a wall enclosing a cavity filled with fluid with three coats: the sclera, choroid, and the retina - The globe of the eye is innervated by the trigeminal nerve (sensory portion). (A) The Ophthalmic division (only a sensory part) of the trigeminal nerve is divided into 3 sub-divisions: (i) Lacrimal nerve: It is sensory and lateral to lacrimal gland, conjuctiva and skin. (ii) Frontal nerve: It supplies the upper eyelid & conjunctiva via supraorbital and supratrochlear branches at the forehead. (iii) Nasociliary, anterior ethmoidal and infratrochlear nerves: supplies the canthus, lacrimal sac, long ciliary to iris, cornea and ciliary muscle. (B) Maxillary division (sensory part). (i) The infraorbital nerve supplies the skin and conjunctiva of lower eyelid. - The nerve of special somatic afferent (SSA) in the orbit is the optic nerve. - The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit.It is supplied by the oculomotor nerve. - The oculomotor nerve coming into the ganglion contains preganglionic axons from the Edinger-Westphal nucleus (a part of the brainstem) which synapses with the ciliary neurons. - The short ciliary nerves comes from the ciliary ganglion and goes ---> back of the eyeball. - The short ciliary nerves contain postganglionic parasympathetic fibres destined for the constrictor pupillae muscle and ciliary body. - The short ciliary nerves also contain postganglionic sympathetic fibres which supply smooth muscle walls of the eye blood vessels, and somatic sensory fibres passing the back of the eyeball and cornea to join the nasociliary nerve. - The long ciliary nerves are branches of the nasociliary nerve which carry postganglionic sympathetic fibres to the eyeball. These fibres supply the dilator pupillae muscle. - So the sympathetic fibres are found in both the short and long ciliary nerves. |
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REFLEXES
PUPILLARY LIGHT REFLEX: The reflex where the pupil of both eyes constrict when a bright light is shone into one eye (direct). The other eye goes through the consensual light reflex. Light > retina > optic n. (II) > optic chiasma (partial decussation) > optic tract > pretectal area interneurons > E-W nucleus > GVE occulomotor output (III) > constriction of iris. ACCOMMODATION-CONVERGENCE REFLEX: The reflex where the focal length of the lens changes and the eyes converge to focus on an object near to the eye. This leads to the constriction of the pupil in response to bright light so that the amount of light entering the eyes is decreased - this sharpens the image of the object. Both the reflexes described above result in constriction of the pupil, but this is brought about by different pathways in each reflex. In the pupillary light reflex, the pretectal nucleus is involved, whereas in the accommodation-convergence reflex, there is involvement of the visual cortex and superior colliculus. In tabes dorsalis, a syphilitic disease of the central nervous system, the pupils constrict as part of the accommodation-convergence reflex, but it will NOT constrict in response to bright light. This is known as the Argyll Robertson pupil, a pupil which accommodates, but doesn’t react. This shows that the pathway for pupillary constriction in accommodation remains intact while the pupillary light reflex has been damaged by the disease process. CORNEAL REFLEX: Touching the cornea results in protective closure of the eyelids. The cornea is supplied by the ophthalmic part of the trigeminal nerve, which detects the stimulus of touch. From the sensory trigeminal nuclei, fibres pass to the motor nucleus of the facial nerve which sends out fibres to the orbicularis oculi muscle. This is one of the muscles of facial expression supplied by VII, and its contraction causes the eyelids to close. |
DISEASES OF THE EYE
HORNER’S SYNDROME: It is the damage caused by traumatic injury of the cervical part of the sympathetic nerve supply of the eye. Its symptoms include ptosis (drooping of the upper eyelid) caused by paralysis of levator palpebrae superioris due to the damage to the oculomotor nerve. Other symptoms are constriction of the pupil due to paralysis of dilator pupillae and the unopposed action of the constrictor pupillae, and slight enophthalmos (depression of the eyeball in the orbit). LOSS OF FUNCTION OF THE LACRIMAL GLAND: The lacrimal gland cannot produce lacrimal fluid. The conjunctiva and cornea will rapidly dry, and this causes severe pain, carried back to the brain via the ophthalmic division of the trigeminal nerve. Ulceration of the cornea may follow. This leads to areas of the cornea becoming opaque, and so affects the vision in that eye. Damage to ANY of the cranial nerves which supply extra-ocular muscles will cause the patient to experience diplopia (double vision). This is because normal binocular stereoscopic vision depends entirely on the fact that the two eyes are trained in exactly the same way on the object being viewed. If they are not, the images formed on the two retinae will not be the same, and when the brain tries to synthesise the two images into one they will not be perfectly superimposed - thus the person sees two images. OCULOMOTOR NERVE DAMAGE: It causes paralysis of all the extra-ocular muscles apart from superior oblique and lateral rectus. The patient will not be able to move the eye medially (medial rectus), upward (superior rectus) or downward (inferior rectus). The patient will have symptoms of lateral strabismus (= lateral squint). This is due to the fact the lateral rectus is functioning normally, but is not opposed by medial rectus. The patient will have ptosis - a drooping upper eyelid. This is becuase the levator palpebrae superioris is paralyzed. The constrictor pupillae muscle is also paralysed so the focal length of the lens cannot be changed. The dilated pupil (dilator pupillae works unopposed by the paralyzed constrictor pupillae. The pupil cannot constrict in response to increased light intensity, i.e. the pupillary light reflex is lost. Also absent is the accommodation reflex whereby the focal length of the lens is altered to focus on a near object. TROCHLEAR NERVE DAMAGE: Patient experiences diplopia cannot turn the eyeball downward and inward so it is difficult walking downstairs. This is because of the paralysis of the superior oblique muscle. ABDUCENS NERVE DAMAGE: Lateral rectus muscle is paralyzed. The patient cannot direct the eye laterally and has a medial strabismus (medial squint) because of the unopposed action of medial rectus. |
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ANATOMY OF HEARING AND BALANCE - The ear is used for hearing and balance. - The ear is divided into three (3) regions: (A) External ear: Collects sound waves and channels them inward. (B) Middle ear: Carries sound vibrations to the oval window. (C) Internal ear: Has the receptors for hearing and balance. |
EXTERNAL EAR - It consists of the auricle/pinna, the external auditory canal and the eardrum. - The pinna/auricle is a flap of elastic cartilage shaped like a trumpet and covered by skin. - The rim of the auricle is the helix. - The inferior end is the lobule/ear lobe. - Ligaments & muscles attach the auricle to the head. - The external auditory canal is a tube located on the temporal bone and goes to the eardrum. - The tympanic membrane/eardrum is a thin semi-transparent partition between the external auditory canal and the middle ear. - The eardrum is covered by epidermis and lined by simple cuboidal epithelium. - Between the epithelial layers in the simple cuboidal epithelium is a connective tissue that is made up of collagen, elastic fibres and fibroblasts. - The tympanic membrane has three layers. The outer layer is the thin skin lining the external acoustic meatus; the middle layer is a sheet of fibrous tissue, and the inner layer is mucous membrane continuous with the lining of the middle ear cavity. - The mucous membrane covering the inner layer receives its sensory supply via the glossopharyngeal nerve. - The lateral side of the tympanic membrane is CONCAVE. - The medial side of the tympanic membrane is CONVEX. - Under the light from the otoscope, the membrane appears a glistening pearly-grey colour. - The Pars flaccida is the small, triangular, flaccid portion of the tympanic membrane, or eardrum. - The pars tensa is the tense portion of the tympanic membrane and it is the main portion of the membrane. - The chorda tympani runs through the inner surface of the tympanic membrane. - Chorda tympani is a nerve that arises from the mastoid segment of the facial nerve carrying afferent taste sensation from the anterior two-thirds of the tongue via the lingual nerve and efferent parasympathetic secretomotor innervation to the submandibular and sublingual glands. - A vertical yellowish streak that extends from the centre of the tympanic membrane is the handle of the malleus. - The tip of the handle of the malleus marks the point of maximum convexity of the membrane (the umbo). - Reflection seen in otoscope forms a cone of light in the anterior-inferior quadrant. - The cone of light is the only part of the tympanic membrane that lies at right angles to the long axis of theexternal auditory canal and which therefore reflects the light of otoscope directly back towards the examiners eye. - Another vertical streak through the tympanic membrane, behind and parallel with the handle of the malleus is the long crus of the incus. - The mastoid antrum large irregular cavity is situated at the upper and front part of the temporal bone. It communicates with mastoid cells. Like the mastoid cells it is filled with air and lined by an extension of the mucous membrane of the tympanic cavity, with which it communicates with. - The mastoid cells are in the mastoid process of the temporal bone. - Perforated eardrum is the tearing of the eardrum. It may be due to pressure from a cotton swab, trauma or middle ear infection and heals within a month. - The outermost part of the external auditory canal contains few hairs and specialised sweat glands called ceruminous glands that secrete ear wax/cerumen. - The hairs and cerumen together prevent dust and foreign objects from entering the ear. - Accumulation of wax in the external acoustic meatus is the commonest cause of deafness. - It prevents the tympanic membrane from vibrating properly. - The wax can be removed by syringing the ears with warm water. - Otitis externa is the inflammation of the external acoustic meatus. Its symptoms are extreme pain because swelling of the lining of the meatus. |
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MIDDLE EAR CAVITY AND AUDITORY OSSICLES.
- It is a small-air filled cavity in the petrous portion of the temporal bone that is lined with epithelium. - It is separated from the external ear by the tympanic membrane. - It is separated from the internal ear by oval and round windows. - The malleus (hammer), incus (anvil) and stapes (stirrup) also known as auditory ossicles (smallest bones in the body) are attached to the middle ear via ligaments and synovial joints. - The tympanic cavity is a small cavity surrounding the bones of the middle ear. - The head of the malleus articulates with the body of the incus. - The body of the incus has two processes - the short crus and the long crus. - The incus articulates with the head of the stapes. - The base of the stapes articulates with the oval window. - The round window is enclosed by the secondary tympanic membrane. - Pharyngo-tympanic tube = auditory tube = Eustachian tube. - Eustacian tube connects the tympanic cavity at the middle ear to the nasopharynx. - The posterior part of the auditory tube is 1/3 bone - The anterior part of the auditory tube is 2/3 cartilage. - The auditory tube is closed at its medial (pharyngeal) end. - During swallowing and yawning, the pharnygeal end opens. This allows air to enter/leave the ear so pressure in the middle ear = atmospheric pressure. - If these pressures are not equal, intense pain, hearing impairment, ringing in the ears and vertigo occur. - The auditory tube is a path for pathogens to enter the ears from the throat or nose, causing otitis media (commonest type of ear infection). - Otitis media often follows on from the individual having a cold, or upper respiratory tract infection. - Pus accumulates in the middle ear cavity and causes deafness because the ossicles cannot move properly. - Children are particularly susceptible to both acute and chronic otitis media. - Chronic otitis media is often termed “glue ear”. It is difficult to treat because the normal secretions in the middle ear are not effectively drained away by the auditory tube. The secretions become thick and mucoid, causing deafness by interfering with the proper movements of the ossicles. - The treatment for Chronic otitis media is drainage of the tympanic cavity by inserting a very short plastic tube through the tympanic membrane called a grommet and this allows re-aeration of the tympanic cavity. -The vulnerability of children to these conditions is because their auditory tube are narrower and more horizontal in the immature head so it is not an efficient drainage pathway for the middle ear cavity. - Acute otitis media causes bulging and redness of the tympanic membrane, and can result in the membrane bursting - perforation of the tympanic membrane. - The tympanic membrane may be deliberately incised (myringotomy) to prevent a spontaneous perforation. - Acute otitis media could spread to the mastoid antrum and mastoid air cells if untrated, causing mastoiditis. - Acute mastoiditis is a dangerous condition because of the proximity of other structures to the mastoid antrum and air cells. - Above the mastoid antrum lies the middle cranial fossa where the thin plate of bone called the tegmen tympani separates the middle ear cavity from the middle cranial fossa, and the temporal lobe of the cerebral hemispheres covered by the meninges. - Spread of infection upwards through the tegmen tympani can lead to meningitis and the formation of a cerebral abscess in the temporal lobe. - Behind the antrum is the the sigmoid sinus. - Spread of infection through the posterior wall of the antrum can cause thrombosis of the sigmoid sinus. - The infected clot can disintegrate and particles be carried in the bloodstream to other parts of the body, e.g. the lungs, where they can become lodged in capillaries and cause abscesses. |
MUSCLES OF THE MIDDLE EAR. - The stapedius is the smallest skeletal muscle in the body. - The thin plate of bone called the tegmen tympani separates the middle ear cavity from the middle cranial fossa. - The trunk of the facial nerve is running downwards behind the posterior wall of the middle ear cavity to reach the stylomastoid foramen. - The tendon of stapedius goes from the apex of the pyramidal prominence and passes to attach to the stapes. - The stapedius decreases the sensitivity of hearing. - The paralysis of the stapedius muscle is called hyperacusia. - Hyperacusia is abnormally sensitive hearing. - The pyramidal prominence is a little bony prominence at the posterior wall of the midle ear cavity. |
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INNER EAR - The inner ear consists of a system of tunnels and chambers within the dense petrous temporal bone called the bony labyrinth. - INSIDE the bony labyrinth are a collection of tubes and sacs called the membranous labyrinth. - The bony labyrinth is filled with fluid called perilymph - The membranous labyrinth with fluid called endolymph. - In the tubes and sacs of the membranous labyrinth are the receptors for balance and hearing. - The parts of the BONY LABYRINTH are the cochlea (a coiled tunnel), the vestibule (a chamber) and the semicircular canals (semicircular tunnels). - The vestibule is a chamber in the petrous temporal bone. - The membranous labyrinth in the vestibule contains two parts (sacs) called the utricle and the saccule. - The semicircular canals are three tunnels in the petrous temporal bone. - Two of the canals are vertical while the other is horizontal so that turning of the head in all directions can be detected. - The superior semicircular canal is vertical and placed at RIGHT ANGLES to the long axis of the petrous temporal bone. - The posterior semicircular canal is vertical and placed PARALLEL to the long axis of the petrous temporal bone. - The lateral semicircular canal is horizontal. - The semicircular canals open into the vestibule by five openings because two of the canals are united at one of their ends. - The semicircular canals contain parts of the membranous labyrinth called the semicircular ducts. - The cochlea is a tunnel in the bone which is wound around a central pillar called the modiolus. - The tunnel makes two and a half turns around the modiolus, each turn tighter than the last, so that the cochlea has a broad base and a narrow apex. - The cochlea opens into the vestibule. - The cochlea contains the part of the membranous labyrinth called the cochlear duct. - The receptors for balance are in the utricle, saccule and semicircular ducts forming the vestibular apparatus. - The receptors in the utricle and saccule detect linear acceleration/deceleration of the head, i.e. the head moving forwards or backwards - The receptors in the semicircular ducts detect angular acceleration of the head, i.e. rotatory movements of the head. - They depend upon movement of hair cells caused by movement of the endolymph filling the sacs and ducts. -The mechanical energy of hair cells' movement is converted into electrical energy by the nerve fibres of the vestibular division of the vestibulocochlear nerve (cranial nerve VIII). - The hair cells in the utricle and the saccule are called the macula. - The surface (tip) of each macula is covered by a gelatinous membrane called the otolithic membrane which contains crystals of calcium carbonate -otoliths (“ear stones”). - The hair cells in the ampulla of the semicircular ducts form a structure called the crista. - The tips of the hair cells in the crista are in an overlying gelatinous mass called the cupula. - The receptors for hearing in the cochlear to form the Organ of Corti - The organ of Corti is on top of the basilar membrane of the cochlea. - Scala vestibuli is the upper bony passage of the cochlear. - The scala tympani is one of the perilymph-filled cavities in the human inner ear. - The cochlear is separated from the cochlear duct by the basilar membrane - After the round window, the cochlear continues as vestibular duct. |
THE PHYSIOLOGY OF HEARING 1) The auricle directs sound waves to the external auditory canal. 2) When the sound waves strike the tympanic membrane, the alternate waves of high and low pressures in the air causes the tympanic membrane to vibrate back & forth. The tympanic membrane vibrates slowly and quickly to low and high pitched frequencies respectively. 3) The central area of the tympanic membrane connects to the malleus and vibrates along with the tympanic membrane. The vibration is transmitted from the malleus ---> incus ---> stapes. 4) As the stapes vibrates, it is attached to the oval window via a ligament and the oval window vibrates as well. 5) The movement of stapes at the oval window causes fluid pressure waves in the perilymph of the cochlea. 6) As the oval window goes inward, it pushes the perilymph of the scala vestibuli causing pressure waves. 7) Pressure waves are transmitted from the scala vestibuli ---> scala tympani ---> round window ----> vestibular membrane --> endolymph inside the cochlear duct (9). 7) The pressure waves in the endolymph causes the basilar membrane to vibrate. This causes the hair cells in the cochlear to move against the tectorial membrane. This bends the stereocilia of the hair cells in the cochlea and this generates nerve impulses in the first-order neurons of the vestibulocochlear nerve. |
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THE AUDITORY PATHWAY Bending the stereocilia of the hair cells in the cochlea generates nerve impulses in the first-order neurons (axons) of the cochlear branch of the vestibulocochlear nerve. These axons synapse with the neurons in the cochlear nuclei in the medulla oblongata on the same side. Some of the axons in the cochlear nuclei either: (I) Decussate (cross-over to the opposite side) in the medulla and ascend to the lateral leminscus (a tract) and terminates in the inferior colliculus in the midbrain. OR (II) Go to the superior olivary nucleus in each pons. Axons from the superior olivary nucleus ascend to the lateral leminscus (a tract) at the opposite side and terminate in the inferior colliculus in the midbrain. Nerve impulses go from each inferior colliculus ----> medial geniculate nucleus in the thalamus ----> primary auditory area of the cerebral cortex. |
THE BALANCE PATHWAY Bending of the hair cells in the semicircular ducts, utricle and saccule generates nerve impluses in the axons in the vestibular branch of the vestibulocochlear nerve. The axons either: (I) synapse with the sensory neurons in the vestibular nuclei (integrating centers) of the pons and the medulla oblongata. The vestibular nuclei also collect nerve impulses (input) from the eyes and the proprioceptors in the neck and limb muscles that indicate the postion of the head and limbs. OR (II) Enter the inferior cerebellar peduncules in the cerebellum. Bidirectional nerve pathways connect the cerebellum and the vestibular nuclei in the pons and medull oblongata. The vestibular nuclei collects information fro the vestibular, somatic and visual pathways and sends commands to: (i) Oculomotor, trochlear and abducens nerves to control eye movements and focus on visual fields. (ii) Nuclei of accessory nerve to control head an neck movements. (iii) The vesibulospinal tract to transmit nerve impulses to the spinal cord to control muscle tone in skeletal muscle in the limbs. (iv) The ventral posterior nucleus in the thalamus and the vestibular area in the parietal lobe of the primary somatosensory area of the cerebral cortex to provide conscious awareness of the position and movements of head and limbs. |
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CLINICAL CONDITIONS OF THE EAR 1) Deafness- there are two types of deafness: a) conductive deafness - malfunction of the system by which vibrations of the tympanic membrane are conducted across the middle ear cavity; the causes lie in either the external or middle ear. b) sensorineural deafness- malfunction in the inner ear, or of the cochlear division of the vestibulocochlear nerve. CAUSES OF SENSORINEURAL DEAFNESS A. Presbyacusis- The hearing loss which occurs as the individual gets older due to the death of hair cells in the Organ of Corti near the base of the cochlea. The condition is progressive and probably begins quite early in life. B. High tone deafness, or Boilermaker’s Disease- This condition could also be termed occupational deafness because it results from the individual being exposed to very loud noise for a long period of time. It was called Boilermaker’s Disease because men who made boilers for steam trains and ships were particularly affected. Loud noise kills hair cells of the Organ of Corti. The hair cells most affected are those near the base of the cochlea because they are nearest to the point at which the pressure waves first enter the perilymph. C. Acoustic neuroma (vestibular schwannoma)- This is a tumour of the glial cells associated with the VIIIth cranial nerve. The commonest presenting symptom is unilateral deafness. The tumour usually starts in the glial cells of the vestibular division of the nerve and affects all the contents of the internal acoustic meatus, the canal in the skull that contains the proximal parts of both the vestibulocochlear nerve and the facial nerve. Owing to the involvement of the vestibular nerve, the patient may also complain of vertigo, nausea and other symptoms of vestibular dysfunction. Involvement of the facial nerve, which is motor to the muscles of facial expression, can cause facial palsy or paralysis on the affected side. If the tumour is not treated it can expand out of the cranial end of the internal acoustic meatus and into the cranial cavity and damage the trigeminal nerve, and pain in the face can become an added symptom and complication of the condition. Surgical removal of acoustic neuromas is usually quite successful, if they are promptly diagnosed. Drugs which can cause sensorineural deafness Aminoglycoside antibiotics are ototoxic (ear-poisoning) agents. They are very concentrated in the endolymph if administered for long periods of time (= days). Such high concentrations kill hair cells - both of the cochlea and of the vestibular apparatus. Other ototoxic drugs include quinine and aspirin (mild). This last is mildly ototoxic, but old people who use large amounts of aspirin to relieve, for example, the pain of arthritis, may by mistake be giving themselves ototoxic doses. Defects of the vestibular system The two principal symptoms associated with dysfunction of the vestibular apparatus are nystagmus and vertigo, but by far the most common affliction involving the inner ear is motion sickness (travel sickness/car sickness). I. Motion sickness occurs in susceptible individuals because the vestibular apparatus is overstimulated. Incoming impulses in the vestibular nerves reach visceral centres in the brain causing varying degrees of nausea, sweating and syncope (= light-headedness, faintness or actual fainting). II. NYSTAGMUS: It is caused by constant uncontrolled movement of the eyes caused by normal reflexes like the vestibulo-ocular reflex) and the optokinetic (railroad) reflex and abnormal reflexes like spontaneous nystagmus (basically an uncontrollable flickering of the eyes). The onset of spontaneous nystagmus is due to a lesion in the inner ear, in the vestibular nerve, in the brainstem or cerebellum. III. VERTIGO: Illusion of movement (dizziness), a sense of the “world spinning around me”. Additional symptoms are nausea, vomiting, tinnitus (= ringing in the ears) and deafness. Types of vertigo: I) Benign positional vertigo: Due to an otolith becoming detached from one of the otolithic membranes in the utricle or saccule. The otolith is free to float around in the endolymph and touches and interferes the cupulae of the semicircular ducts. The dizzy episodes will continue to occur until the otolith moves to a location where it cannot impact on the cupulae, or until it disperses. II) Meniere’s disease: This is vertigo accompanied by tinnitus, nausea, vomiting and hearing loss that may last several hours. This is due to an accumulation of endolymph which causes expansion of the membranous labyrinth. The labyrinth ruptures and endolymph is able to enter the perilymphatic space. This causes an ionic imbalance which disturbs the function of the neurons in the vestibular division of VIII. The attack of vertigo lasts until the rupture is healed and the ionic balance restored. It is thought likely that episodes of the condition also kill hair cells - so each attack leaves the vestibular apparatus and the cochlear duct (with which it communicates) more damaged. In patients who suffer the disease for a long time there will be progressive loss of hearing, resulting eventually in complete deafness and loss of vestibular function on the affected side. |
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