Nervous System Lecture 7: The Ear (PDF)

Nervous System Lecture 7 In Block NUE-312 THE EAR Ass. Prof. Dr. Safaa Said Department of Histology &Cell Biology Assuit University  After this lecture, students should b...

Nervous System Lecture 7 In Block NUE-312 THE EAR Ass. Prof. Dr. Safaa Said Department of Histology &Cell Biology Assuit University  After this lecture, students should be able to: ▪Explain the histological structure of the ear. ▪Describe the ultrastructure of the retinal Learning photoreceptors’ rods, cones and pigmented objectives epithelium.. ▪Describe difference between rods and cones. ▪Describe the histological structure of accessory organs of the eye. The EAR  1-The external ear: Consists of the auricle, the external auditory meatus and the tympanic membrane. a- The Auricle: Is an irregular plate of elastic cartilage covered with thin skin. b-The external auditory meatus: Is a rigid tube extending from the auricle to the tympanic membrane. The tube is lined with thin skin which contains numerous hairs and sebaceous glands. There is a brown waxy secretion (cerumen) in the external auditory meatus which is combined secretion of ceruminous gland (a modified sweat gland) and sebaceous glands. Both hairs and cerumin have a protective function.  c-The tympanic membrane: Is formed of connective tissue core containing collagen, elastic fibers and fibroblasts. This core is covered with skin from outside and lined with low cubical or simple squamous epithelium from inside. -Sound waves causes vibration of tympanic membrane which transmits energy to the middle ear 2- The middle ear (Tympanic cavity)  2- The middle ear (Tympanic cavity) It is an air filled tympanic cavity that lies within the temporal bone. The middle ear is lined mostly with simple squamous epithelium or low cubical epithelium and columnar ciliated epithelium at the opening of the auditory tube. The middle ear cavity contains: -The ossicles: malleus (hammer), incus and stapes. The auditory (Eustachian) tube: It connects the middle ear with the nasopharynx. The lining epithelium is simple columnar ciliated near the tympanic cavity and pseudostratified columnar ciliated epithelium with goblet cells near the nasopharynx.  3- The inner ear (Labyrinth) It is present in the petreous part of the temporal bone and it is concerned with hearing and balance. It consists of a bony part called the "bony labyrinth" and a membranous part called the "membranous labyrinth" which is present inside the bony part. Bony labyrinth is filled with a fluid called perilymph. Membranous labyrinth is filled with endolymph which is characterized by a high potassium and low sodium content similar to intracellular fluids.  3- The inner ear (Labyrinth) The bony labyrinth:Is formed of: *the vestibule in the center *the semicircular canals on one side and *the cochlea on the other side. The semicircular canals open into the posterior part of the vestibule. Each canal has a dilatation called the ampulla.  3- The inner ear (Labyrinth) The membranous labyrinth: Is formed of three major divisions: The Utricle and saccule in the vestibule. The Semicircular ducts in the semicircular canal. The cochlear duct in the cochlea. *  The utricle and saccule;  The Macula Is formed of two main types of cells, sensory (hair cells) and supporting (sustentacular) cells. They are covered with a thick gelatinous glycoprotein layer on the surface of which calcium carbonate crystals( known as the otoliths) are deposited. I- The sensory hair cells: Also each hair cell has elongated villi (stereocilia) and one typical cilium (kinocilia) with its basal body. These microvilli and kinocilia are embedded in the gelatinous membrane. The hair cells are of two types; Type I and Type II.  The Macula Type I is goblet shaped and surrounded by a net of afferent nerve endings while Type II cell has a columnar shape and is surrounded by a net of both afferent and efferent nerve endings. The hair cells are stimulated by the change is position of the head in space resulting in increase or decrease in pressure on them by the otolithic membrane. II- The supporting cells:lie between hair cells and have a columnar shape with basal nuclei and microvilli on their surfaces. The Macula  The semicircular ducts *The semicircular ducts: take the same form of the bony part to which they are connected by connective tissue strands. They are lined with simple squamous epithelium. The crista ampullaris: is a neuroepithelial structure present in the ampulla although it simulates the maculae of the utricle and saccule in their structure. It differs in that the covering gelatinous membrane is called the "cupula" and there is no calcium carbonate on its surface. In the crista ampullaris, the hair cells are stimulated by movement of endolymph following angular acceleration of the head which cause movement of stereo cilia and kinocilia.  The cochlear duct The cochlear duct: is the part of membranous labyrinth present in the bony cochlea. The cochlea is conical in shape present anterior to the vestibule and formed of a spirally coiled tube which runs about 2.5 turns around a central bony axis called the modiolus (a spongy bone containing nerve fiber, vessels and spiral ganglion. The two and half turns in axial section form five circles of bony structure lateral to the modiolus. Each circle divides into 3 compartments by two membranes; (a) the vestibular (Reissner's) membrane above and (b) the basilar membrane below.  The cochlear duct The 3 compartments are: Scala vestibuli above which communicates with the vestibule. Scala tympani below which communicates with the tympanic cavity through the round window. The cochlear duct in between. This represent the membranous part of the cochlea. Each scala contains perilymph and communicate with each other at the apex of the cochlea via a small opening called the helicotrema while the cochlear duct contains endolymph.  The cochlear duct The cochlear duct is triangular in shape, its apex points towards the modiolus. The vestibular and basilar membranes form its two sides and the stria vascularis forms its base. The stria vascularis: is formed of a layer of stratified or pseudostratified epithelium covering a layer of vascular connective tissue which is in direct contact with the bony cochlea. The epithelium of the stria vascularis is rich in mitochondria and its cells have deep basal infoldings which are characteristic of ion and water transport cells. The stria vascularis secretes endolymph that fills the cochlear duct.  The organ of corti The organ of corti: It is the hearing receptor and formed of neuroepithelial structure resting on the basilar membrane and covered with thick gelatinous membrane called tectorial membrane. Like any neuroepihelial structure it is composed of two main types of cells, the hair (sensory) cells and supporting cells.  The organ of corti I- Supporting cells include: The inner and outer pillars. Phalangeal cells (outer and inner): these cells are tall columnar and contain concavities for holding the hair cells. Hensen's cells: are tall columnar cells adjacent to outer phalangeal cells. Cells of Claudius: they form the outer edge of organ of corti and they shorter than hensen's cells. Border cells: support the inner edge of organ of corti.  The organ of corti II- Sensory cells: outer hair cells that is present in three to five rows outer to the outer pillar cells. Inner hair cells present in one row inner to the inner pillar. The hair cells have the same structure described above in maculae and cristae except that they have no kinocilia and there are only stereocilia (The elongated microvilli). The hair cells rest on the concavities of the supporting (Phalangeal) cells. Their rounded bases are related to afferent and efferent nerve endings. The supporting cells are tall columnar cells having a thick fibrillar material and are joined to hair cells by zonulae occludens. The olfactory region It is present at the roof of the nasal cavity. It lined by olfactory mucosa (organ of smell) which consists of olfactory epithelium and thick lamina propria. a. olfactory epithelium: It is thick pseudostratified epithelium that is formed of 4 types of cells namely; supporting, olfactory, brush and basal cells. Olfactory receptor cells, are bipolar olfactory neurons that span the thickness of the epithelium and their axons enter the central nervous system. The olfactory region  The olfactory region. Supporting cells: They are tall columnar cells with fine yellowish granules in their cytoplasm that responsible for the yellowish color of the mucosa. Their free cell surface presents microvilli that covered with a thin film of serous fluid produced from the Bowman´s glands. They have a supporting and nourishment function for olfactory cells.  The olfactory region b. Olfactory (chemoreceptor) cells They are bipolar neurons which lie in-between the supporting cells. They have oval or rounded cell body contain spherical nucleus, apical dendrite and basal unmyelinated axons directed to the olfactory bulb. The nucleus is rounded and present in the widest part of the body. The apical dendrite is modified to form a roundish olfactory vesicle projecting above the surface of sustentacular cells..  The olfactory region Long, non motile (lacking the dynein) olfactory cilia (6-8 cilia) extend from the olfactory vesicles which act as receptors. The axon of olfactory cells penetrates the basal lamina and join together to form bundles of nerve fibers. The nerve fibers penetrate the cribriform plate of ethmoid in the roof of the nasal cavity to synapse with 2ry neurons in the olfactory bulb  The olfactory region.Basal (reserve or stem) cells They are small rounded cells located close to the basal lamina. It lie on the basement membrane forming a single layer at the base of the epithelium. They can be differentiated to form olfactory or sustentacular cells (during prenatal life). The neurons are not replaced at postnatal life. b. Lamina propria: ► Formed of dense fibro-elastic connective tissue that contains serous glands (Bowman´s glands). Their secretion covers the olfactory cilia to clear them and thus allow perception of new odorous substance  Taste buds Flask-shaped or onion-like structures present on tongue, oropharynx and soft palate. Each tast bud is formed of: Neuro-epithelial taste cells (gustatory cells): columnar pale cells with apical hairlets (microvilli) and basal sensory nerve fibers. Supporting cells (sustentacular cells): columnar dark cells present in the outer part. Basal cells: stem cells (for previous two types) present at the base. Taste buds REFERENCES: INTEGRATED SYSTEMS (PP. 55 ELSEVIER’S &56), THE NERVOUS INTEGRATED SYSTEM (PP. 32-36), HISTOLOGY (2007) BASIC HISTOLOGY (PP. 161-170). 1STEDITION.