6- Ear Microanatomy.docx

- **Ear: General Info** - The **functions** of the ear include: - Collecting auditory stimuli - Transducing auditory to mechanical and the nervous stimuli - Transmitting nerve impulses to the central nervous system (CNS) via the vestibulocochlear...

- **Ear: General Info** - The **functions** of the ear include: - Collecting auditory stimuli - Transducing auditory to mechanical and the nervous stimuli - Transmitting nerve impulses to the central nervous system (CNS) via the vestibulocochlear nerve (cranial nerve 3) - The ear is the organ of hearing and balance. - The inner ear is the organ for both hearing and equilibrium (art of the vestibular system), whereas the external and middle ear represent a sound collecting and conducting apparatus. - Organs of equilibrium and motion detection include the: utricle, saccule, and semicircular ducts. - Within these are specialized structures containing neuroepithelial cells: macula utriculi, macula saccule, and crista ampullaris (sensitive to angular acceleration/deceleration). - The facial nerve and vestibulocochlear nerve enter the ear together through the internal acoustic meatus. - The **lateral line system** in fish, larval, and adult aquatic amphibians uses **tactile sense organs**, which detect movements and pressure changes within the surrounding water. - **Jacobson's (vomeronasal) organ** is an example of a **chemoreception** mechanism within snakes that is utilized for tongue extension and tongue retraction. - Tongue extension: Odor particles are captured from the air by the tongue. - Tongue retraction: Odor particles mixed with fluids in the mouth are delivered to the ducts leading to the vomeronasal organs. - The manubrium of the **malleus** forms a very shallow arc and is centrally located in the tympanum. - In a dog, the **tympanum** is oval to comma-shaped, and the manubrium of the malleus is C-shaped. - In the horse, the tympanic membrane (tympanum) is more round than other species. - **Cochlea** - The organ of hearing is the **cochlea**. - The cochlea is a spiral structure that is divided into 3 chambers. - **Organ of corti** is known as the spiral organ within the cochlea. - Fluid waves traveling in the **Scala vestibuli** will result in deviation of the basilar membrane, within the cochlea. - This structure leads to deflection of the apical hair cell **stereocilia**. - The scala vestibuli and **scala tympani** are filled with perilymph and represent the bony labyrinth of the cochlea. - **Parts of the ear** - The ear can be divided into the external ear, middle ear, and inner ear. - The **external ear** contains the auricle or pinna and the external auditory meatus. - The **external acoustical meatus** contains **ceruminous glands**, which are a type of apocrine gland (varying from cuboidal to columnar) that secretes cerumen (ear wax). - Annular and **auricular cartilage** form the structure of the cartilaginous portion of the external acoustic meatus. - Dense, white, fibrous connective tissue attached the annular cartilage to the osseous rim of the external acoustic meatus. - The **middle ear** contains the tympanic cavity, tympanic membrane (ear drum), 3 auditory ossicles and their associated ligaments and muscles. - The middle ear connects to the pharynx through the auditory (**Eustachian**) tube. - The **guttural pouch** is a ventral expansion of the auditory tube within horses. - **Auditory ossicles** connect the tympanic membrane to the vestibular window membrane of the inner ear. - The **inner ear** is composed of a membranous labyrinth that is enclosed in the temporal bone. - **Endolymph** - Endolymph is a fluid within the membranous labyrinth of the inner ear that is high in potassium. - It bathes inner ear cells, allowing them to function normally. - Large diverticula (sacs) in reptiles are possibly involved in calcium homeostasis. - The endolymphatic system results in sac-like structures behind the eyes of amphibians and reptiles, which functions in controlling pressure, fluid, and ion (especially calcium) homeostasis. - The endolymphatic system, embryologically is derived from the neuroectoderm, like all sensory structures. - **Vestibular Apparatus** - Within the utricle and saccule, are maculae, that run perpendicular to one another. - **Otoliths** are located within a gelatinous membrane, and their movements bends the stereocilia of neurosensory hair cells. - The **vestibular apparatus** detects static equilibrium and linear acceleration, as well as rotational movement. - For the **Macule of Saccule**, otoliths are embedded in a gelatinous otolithic membrane, which lies on an epithelium consisting of sensory and supporting cells. - For the **Crista Ampullaris**, this ridge of sensory epithelium is supported by connective tissue, and protrudes into the ampulla of a semicircular duct, where it is oriented at right angles to the long axis of the duct. - The epithelium consists of sensory and supporting cells, similar to those found in the maculae. - A mass of gelatinous material, the cupula, covers the surface epithelium. - At the base of the semicircular ducts are ampullae, which contain crista ampullaris. - The **cupula** is gelatinous and detects movement by the stereocilia of neurosensory hair cells.