Special Senses Flashcards
What is the olfactory epithelium?
A specialised region of the mucous membrane covering the superior conchae at the roof of the nasal cavity
What is the olfactory lining epithelium?
Pseudostratified columnar
What are the 3 cell types found in the olfactory epithelium?
- Olfactory receptor
- Support/sustentacular
- Basal
Describe the morphology of basal cells.
Small, spherical/cone-shaped cells
Stem cells
Basally located nuclei
Describe support/sustentacular cells.
Columnar with broad, cylindrical apexes and narrower bases
Microvilli
Well-developed junctional complexes
Abundant ion channels
Describe the morphology of olfactory receptor cells.
Bipolar neurons
Knob-like swelling with basal bodies
Non-motile cilia on basal bodies
What is the lifespan of olfactory receptor cells?
1 month
What is the purpose of the rich vascular supply in the olfactory mucosa?
Warm the air
What are the Bowman’s/olfactory glands?
Large tubuloalveolar serous glands that secrete a watery fluid that dissolves odorants
What are the 3 concentric layers of the eye?
- Corneoscleral coat
- Vascular/uveal coat
- Retina
What is continuous with the sclera?
Meninges/dura
What is the composition of the corneosclera layer?
Sclera: tough external fibrous layer
Cornea: transparent anterior extension of the sclera
What does the vascular/uveal coat consist of?
Iris and ciliary body anteriorly
Vascularised choroid coat posteriorly
What does the retina consist of?
Layers of nerve cells, their processes, support cells (neural retina) and an outer layer of pigmented epithelium
How does the retina communicate with the brain?
Optic nerve
What is the name of the anterior edge of the neural retina?
Ora serrata
What is the lens?
A biconvex transparent structure held in place by zonular fibres from the ciliary body
What lies posterior to the lens?
Vitreous body
What is the name of the opaque pigmented expansion of the middle layer partly covering the anterior surface of the lens?
Iris
What are the 2 fluid-filled cavities in the eye?
Anterior chamber: space between cornea and iris
Posterior chamber: between iris, ciliary processes, zonular attachments and lens
What do the cavities in the eye contain?
Aqueous humor
What is the vitreous body?
Transparent, gelatinous mass of connective tissue
What is the conjunctiva?
A thin, transparent mucosa that covers the exposed anterior portion of the sclera and continues as the lining of the internal surface of the eyelids
What type of epithelium lines the conjunctiva?
Stratified columnar epithelium with numerous small cells resembling goblet cells
What makes up the anterior segment of the eye?
Cornea
Anterior and posterior chambers and their content
What are the 5 layers of the cornea?
- External stratified squamous epithelium
- Bowman’s membrane
- Stroma
- Descement’s membrane
- Inner simple squamous endothelium
What is the constitution of the corneal stroma?
Layers of parallel type I and V collagen bundles that align at right angles to each other
Between collagen bundles are cytoplasmic extensions of keratocytes
What is the corneal endothelium responsible for?
Maintaining a state of hydration
Protein synthesis to maintain Descemet’s membrane
What percentage of the cornea does the stroma account for?
85-90%
Describe keratocytes.
Modified fibroblasts
Star-shaped with numerous dendritic processes
Synthesise collagen and ECM components
Corneal crystallins in cytoplasm that reduce backscatter of light
What is the name for an accumulation of copper in Descemet’s membrane at the corneoscleral junction?
Kaiser-Fleischer ring
What is the limbus?
A transitional area where the transparent stroma merges with the opaque sclera
What sinus does the limbal stroma contain?
Canal of Schlemm which receives aqueous humor from an adjacent trabecular meshwork at the anterior chamber’s surface
What happens to Descemet’s membrane and its simple endothelium at the corneoscleral junction?
Replaced with trabecular meshwork that allows slow, continuous drainage of aqueous humor from the anterior chamber into the canal of Schlemm
Where is aqueous humor produced?
Ciliary processes
What is the circulation pathway of aqueous humor?
- Posterior chamber
- Around pupillary aperture
- Anterior chamber
- Iriodocorneal angle containing trabecular meshwork
Describe the trabecular meshwork.
Aka aqueous outflow apparatus
Loose C.T., flattened thin-walled endothelium channel containing collagen and elastic fibres
Encloses spaces of Fontana (communicate with anterior chamber)
Discontinuous basement membrane
Why is aqueous humor continuously drained?
Prevent increased intraocular pressure which causes glaucoma
Describe the choroid.
Highly vascular layer in the posterior 2/3 of the eye
Loose C.T. rich in collagen, elastic fibres, fibroblasts, melanocytes, macrophages, lymphocytes, mast cells and plasma cells
Inner region: choriocapillary lamina
Bruch’s membrane: thin amorphous hyaline sheet that separates choriocapillary lamina from retina
Where does Bruch’s membrane extend to and from?
Ora serrata to the optic nerve
Why does the choroid contain abundant melanocytes?
Produce pigment that block light from entering the eye except through the pupil
What is the ciliary epithelium?
Double layer of low columnar epithelial cells
Describe the morphology of the ciliary body.
Thick ring of loose C.T. containing melanocytes, smooth ciliary muscle, ciliary processes (numerous epithelium-covered extensions) and the ciliary zonule
What is the ciliary zonule?
A system of fibrillin-rich fibres that attach the capsule of the lens to the centre of the ciliary body
How is the shape of the lens changed to allow visual accommodation?
Changes in tension on the zonular fibres produced by contraction and relaxation of the ciliary muscles
What covers the anterior surface of the iris?
An irregular discontinuous layer of densely packed fibroblasts and melanocytes with interdigitating processes
What is the iris’ underlying epithelial layer composed of?
Myoepithelial cells
What do the radially extended processes of myoepithelial cells make up?
The thin dilator pupillae muscle along the posterior side of the iris
What is the sphincter pupillae muscle?
Smooth muscle bundles arranged in a circular array near the pupillary margin of this iris
Describe the lens.
Elastic, biconvex avascular structure
Between iris and vitreous body
Thick homogenous capsule (rich in proteoglycans and type IV collagen)
What is the germinal zone?
A layer of simple cuboidal epithelium over the anterior half of the lens
True or False: The cornea and lens depends on diffusion of aqueous humor for nutrition.
True
Describe lens fibres.
Elongated columnar epithelial cells with crystallins, filensin intermediate filaments and a degenerated nucleus
Lens fibres develop in successive waves from the embryonic period throughout adulthood from what region?
Equatorial region by deposition of new fibres in the peripheral cortex
Name the 2 layers of the retina.
Outer: retinal pigmented epithelium
Inner: neural retina (stratified and contains modified neurons)
Name the layers of the retina.
- retinal pigmented epithelium
- layer of rods and cones
- external limiting membrane
- outer nuclear layer
- outer plexiform layer
- inner nuclear layer
- inner plexiform layer
- ganglion cell layer
- nerve fibre layer
- internal limiting membrane
How many rod cells does the human retina have?
120 million
Describe rod cells.
Thin and elongated
2 segments: outer photosensitive and inner metabolic machinery with connecting stalk between
Describe the morphology of the photosensitive region of a rod cell.
600-1000 flattened membranous discs stacked and surrounded by a plasma membrane
Describe the morphology of the metabolic machinery segment of a rod cell.
Rich in glycogen and mitochondria near the base by the connecting stalk
Abundant polyribosomes in mitochondrial region
What is the function of the polyribosomes in the inner segment of a rod cell?
Produce proteins that are transported to the outer segment where they are incorporated into the membranous discs
What is rhodopsin?
Visual pigment which is bleached by light and initiates the visual stimulus
True or False: Rod cells are not sensitive to light and don’t allow vision in low light.
False
How are the membranous discs of rod cells formed?
Repetitive in-folding of the plasma membrane near the connecting stalk and then detach from the plasma membrane
How long does it take for the process of membranous disc replacement in rod cells take overall?
10 days
How much light is necessary for a rod cell to respond?
A single photon
How many cone cells are present in the human retina?
6-7 million
How are cone cells different from rod cells in terms of vision?
Less sensitive to light
Specialised for colour vision in bright light
There are 3 types of cone cells that contain variations of the visual pigment iodopsin. What are the colours they perceive?
Red, blue and green
Describe cone cells.
Elongated with 2 segments, a connecting stalk and an accumulation of mitochondria and polyribosomes
Outer segment is more conical and discs are continuous with plasma membrane
What is the retinal pigmented epithelium?
Low columnar cells with basal nuclei
Well-developed junctional complexes, gap junctions and numerous basal membrane invaginations with mitochondria
Apical processes that surround tips of photoreceptors with numerous melanin granules
What are the functions of the retinal pigmented epithelium?
- Blood-retina barrier
- Absorb light
- Phagocytose shed components
- Remove free radicals
What is the purpose of the ora serrata?
Creates a boundary between the neural and ciliary parts of the retina
Why is the optic disc referred to as the blind spot?
Lack any photoreceptors
What is the lamina cribosa?
A series of openings in the sclera that resemble a sieve that the fibres of the ganglion cell layer forming the optic nerve pass through
What is the fovea centralis?
A shallow depression having only cone cells at its centre (foveola) with bipolar and ganglion cells at the periphery
How are cone cells in the fovea centralis adapted?
Long and narrow that allows for closer packing and increases visual acuity
Define the macula lutea.
Oval yellowish avascular area surrounding the fovea near the centre of the retina in the eye
What is the contents of the first 5 retinal layer?
- RPE: 1 layer of melanin-rich cuboidal cells
- Rods and cones: photoreceptor cells in fibrillar layer
- ELM: line formed by junctional complexes between photoreceptors and Muller cells
- ONL: middle nucleated parts of photoreceptors
- OPL: lightly stained zone of synaptic areas between photoreceptors and bipolar cell dendrites
What is the contents of the last 5 retinal layers?
- INL: cell bodies of bipolar cells, associated neurons and Muller cell nuclei
- IPL: thick synaptic area between bipolar and ganglion cells
- GCL: cell bodies of multipolar ganglion cells
- NFL: about 1 million ganglion cell axons
- ILM: thin basal lamina of Muller cells
What are the functions of the 3 major parts of the ear?
External ear: Receive sound waves
Middle ear: sound waves are transmitted from air to fluids of the internal ear via set of small bones
Internal ear: fluid movements are transduced to nerve impulses that pass via the acoustic nerve to the CNS
What is the auricle/pinna?
An irregular funnel-shaped plate of elastic cartilage, covered by tightly adherent skin, that directs sound waves into the ear
Describe the external acoustic meatus.
A canal extending from the lateral surface of the head
What is the lining epithelium of the external acoustic meatus?
Stratified squamous epithelium
Continuous with skin of auricle
Has hair follicles, sebaceous glands and modified apocrine sweat glands called ceruminous glands in the submucosa
What is cerumen?
Oily or waxy, yellowish material that contains various proteins, saturated fatty acids and sloughed keratinocytes
How do sounds waves transmit energy to the middle and inner ear?
Vibrations of an epithelial sheet called the tympanic membrane
What is the tympanic annulus?
A rim of fibrocartilage that the edges of the tympanic membrane are attached to
What are the 3 inner ear bones (auditory ossicles)?
Malleus, stapes and incus
What covers the surfaces of the tympanic membrane?
Outer: Thin skin lacking hair follicles or glands
Inner: Simple cuboidal epithelium
What lies between the epithelial coverings of the tympanic membrane?
A thin sheet of fibrous C.T. consisting mainly of collagen with fibroblasts, small blood vessels and sensory nerves
What is the tympanic cavity?
An irregular, air-filled space in the middle ear between the tympanic membrane and the bony surface of the internal ear
What is the Eustachian tube?
Auditory tube that allows communication between tympanic cavity and the pharynx
What lines the tympanic cavity?
Simple cuboidal on a thin lamina propria that is strongly attached to the periosteum
How does the lining of the tympanic cavity change near the opening to the Eustacian tube?
Becomes ciliated pseudostratified columnar epithelium
What balances the air pressure in the middle ear?
The opening and closing of the auditory tube during swallowing
What is the name of the 2 membrane-covered regions devoid of bone in the medial bony wall of the middle ear?
Oval and round windows
What type of joint do the auditory ossicles articulate with?
Synovial
How is the movement of the ossicles restricted to protect from damage caused by extremely loud noises?
Insertion of 2 small skeletal muscles into the malleus and stapes
True or False: The stapes is attached to connective tissue of the tympanic membrane and the malleus is attached to the membrane of the oval window.
False, vice versa
What is the role of the auditory ossicles?
Transmit mechanical vibrations of the tympanic membrane to the inner ear
Where is the inner ear located?
Within the temporal bone
What is the bony labyrinth?
An intricate set of of interconnected spaces in the inner ear that house a continuous fluid-filled, epithelium-lined tubes and chambers that make up the smaller membranous labyrinth
What are the 2 divisions of the membranous labyrinth?
Vestibular labyrinth and cochlear labyrinth
What is the role of the vestibular labyrinth?
Mediates the sense of equilibrium
What does the vestibular labyrinth consist of?
2 connected sacs called the utricle and saccule, and 3 semicircular ducts arising from the utricle
What is the role of the cochlear labyrinth?
Contains the receptors for hearing
What does the cochlear labyrinth contain?
Cochlear duct that is connected to the saccule
Where are the large areas of columnar sensory mechanoreceptors in the epithelial lining of the vestibular and cochlear labyrinth located?
Vestibular: 2 maculae of the utricle and saccule as well as 3 cristae ampullaris in the enlarged ampullary regions of the semicircular ducts
Cochlear: spiral organ of Corti in the cochlear duct
What is the bony labyrinth filled with?
Perilymph
What is the membranous labyrinth filled with?
Endolymph
Define the modiolus.
The bony core of the cochlea that contains blood vessels and the large spiral/cochlear ganglia formed by the cochlear nerve
What are the 3 spaces of the cochlea?
Scala vestibuli
Scala media (cochlear duct)
Scala tympani
What is the helicotrema?
A small opening at the cochlear apex in which the scala tympani and vestibuli communicate
True or False: The cochlear duct is filled with endolymph while the other 2 scalae are filled with perilymph.
True
What separates the scala vestibuli from the cochlear duct?
A thin structure of a basement membrane covered by simple squamous epiuthelium called Reissner’s membrane
What is the stria vascularis?
A unique epithelium consisting of cells with many deep basal infoldings with mitochondria, responsible for production and maintenance of the endolymph
The spiral organ of Corti rests on a thick basal lamina called the basilar membrane. What are the 2 major hair cell types present?
Outer hair cells: 3 rows by the oval window increasing to 5 near the cochlear apex of curved row of longer stereocilia with the tallest embedded in the tectorial membrane
Inner hair cells: 1 linear array of short stereocilia
What is the tectorial membrane?
An acellular layer extending over the spiral organ from the modiolus that consists of fine bundles of collagen, associated proteoglycans and other proteins
Both of the hair cell types are innervated by afferent and efferent nerve endings but which type is more heavily innervated?
Inner hair cells
What are the 2 major types of columnar supporting cells associated with hair cells?
Pillar cells
Phalangeal cells
Describe pillar cells.
Columnar support cells
Stiffened by bundles of keratin
Outline a triangular, tunnel-like space between outer and inner hair cells
Describe phalangeal cells.
Surround and directly support inner and outer hair cells by completely enclosing IHC but only the basal ends of the OHC
What do the stereocilia of the cochlear hair cells detect?
Movement of the spiral organ
What does deformation of the stereocilia during vibrations do?
Opens ion channels that depolarise the hair cell which causes the release of neurotransmitters that causes action potential in the terminals of the spiral ganglion neurons
What do the receptive hair cells of the vestibular labyrinth detect?
Utricle and saccule: Head position with linear acceleration and gravity
Semicircular canals: Movement and responds to angular acceleration
Describe the maculae in the walls of the utricle and saccule.
Small areas of columnar neuroepithelial cells containing mechanosensitive hair cells, columnar support cells with basal nuclei and nerve endings
Innervated by branches of the vestibular nerve
Sense linear acceleration and gravity
What are the 2 types of hair cell found in the maculae of the utricle/saccule?
Type I: flask-shaped and innervated by cup-like afferent nerve terminal
Type II: elongated and in contact with several small punctate afferent and efferent nerve terminals
What features do type I and II cells of the maculae in the vestibular labyrinth share?
A single kinocilium at the apical end of each hair cell and many rigid stereocilia
What is the name of the thick, gelatinous layer of proteoglycans that the tips of the stereocilia and kinocilium are embedded in?
Otolithic membrane
What are otoliths/otoconia?
Calcified structures present in the outer part of the otolithic membrane
How many stereocilia project from the apical surface of vestibular hair cells?
40-80
How do the stereocilia of the vestibular hair cells trigger a nerve impulse?
By bending in the direction of the kinocilium which stimulates the impulse
What is the crista ampullaris?
A large ampulla in the semicircular ducts that has an elongated ridge-like area of mechanoreceptors
What is the cupola?
A proteoglycan ccap-shaped layer attached to the sensory cells’ hair bundles, extending completely across the ampulla
How is head rotation sensed by the hair cells in the semicircular ducts?
The movement of fluid displaces the cupola which bends the stereocilia
What does the resting output of the hair cells in the semicircular ducts mean for impulse output?
Bending the stereocilia one way increases output and the other decreases it
Describe the ultrastructure of the crista ampullaris.
Columnar hair cells sat in small crevised formed by the supporting cells
Apex of hair cells has a single non-motile kinocilium and many stereocilia
