Objectives
- Identify characteristic histological structures of auricle and external auditory meatus.
- Describe function and relationships of important structures.
- Describe the detailed microscopic anatomy of the membranous labyrinth of the inner ear.
- Identify the organ of Corti and macula.
- Identify mechanoreceptors and supportive cells in organ of Corti and macula.
Auditory System Overview
The auditory system has three main components: 1) outer ear, 2) middle ear, and 3) inner ear. The outer ear includes the auricle (pinna) and external auditory meatus and extends to the tympanic membrane. The middle ear is called the tympanic cavity and extends from the tympanic membrane and spans 3 small bones, auditory ossicles, within the temporal bone. The tympanic cavity also contains the pharyngotympanic tube (also known as the auditory tube and Eustachian tube). The inner ear is located in the petrous, or dense part of the temporal bone. It includes the bony and membranous labyrinth.
The inner ear labyrinth consists of the cochlea (snail shell shape) and the vestibular apparatus (3 semicircular canals). The cochlea can be divided into 3 divisions or compartments that continue throughout the section as it wraps around. There are patches of neuroepithelium (6 total) found throughout the inner ear. This neuroepithelium includes the organ of Corti (1) which is located within the middle division, scala media, of the cochlea and is needed for sound perception. The vestibule contains a crista ampullaris for each ampulla of each semilunar canal (3 total) which detects changes in angular momentum of the head. The final patches of neuroepithelium are the maculae (2) of the urticle and saccule. Hair cells found in neuroepithelium have apical modifications including kinocilia (modified primary cilia) and stereocilia (modified microvilli). Hair cells contain a hair bundle formed by rows of sterocilia in the cochlea and rows of sterocilia plus a single kinocilium in the vestibular system, creating a mechanosensory structure. Kinocilia lack the inner dynein arms so they passively move due to vibrations and change in gravity. During development, kinocilia form before sterocilia to establish a polarity/arrangement of sterocilia. Hair cells of cochlea also contain a kinocilum initially but it is believed they degenerate and leave behind their basal body after development is complete.
Auditory System Slides
This slide is part of the external ear. You should be able to identify the different types of tissues that make up the external ear including skin with hair follicles and eccrine sweat glands, connective tissue, and elastic cartilage. To see an example stained with an elastin stain, see the “More Practice” below. You can also find muscle fibers extending from the scalp, can you wiggle your ears?
This specimen contains the external acoustic/auditory meatus (EAM). You should be able to identify both hyaline and elastic cartilage, thin skin and ceruminous glands. These modified apocrine sweat glands produce cerumen, or earwax. This substance helps remove desquamated cells of the skin to prevent buildup within the acoustic meatus. It also lubricates the skin and coats the hair to prevent entry of foreign particles while providing some antimicrobial protection.
This specimen contains the cochlea, a coiled tube that spirals 2 2/3 times around the modiolus, a central pillar of bone which detects vibration. Also in this specimen is a vestibular apparatus which detects acceleration, gravity direction, and static position. The cochlea is divided into compartments: the scala vestibuli (SV), the middle compartment is the scala media (SM), and the scala tympani (ST). The scala tympani and scala vestibuli communicate where they are joined at the helicotrema (center of the snail shell shape, see diagram above). The organ of Corti (OC) is the sensory compartment of the cochlea where sensory receptors of the cochlear nerve originate. The vestibular apparatus consists of 3 semicircular canals and 2 otolith organs, the utricle and saccule (see above diagram). There is a similar slide to this in the “More Practice” section where you can also identify layers of the macula utriculi.
More Practice
Use this slide to review structures of the external ear similar to the ear auricle slide above but with an elastin stain.
Use this slide of the inner ear to review structures of the cochlea and vestibule.
More Practice Answers
Slide Iowa External Ear
- A) white- hair follicle; B) green- sebaceous gland
- Elastic cartilage, black fibers- elastic fibers
- Thin skin- stratified squamous epithelium, keratinized
- White- nerve; Blue- vessels; Green- hair follicle; yellow- sebaceous gland
Slide GWU 217 Inner Ear
- Black- scala vestibuli, Blue- scala media, Green- scala tympani
- A) outer hair cells; B) inner hair cell; C) basilar membrane; D) tectoral membrane; E) outer phalageal cells
- A) Crista ampullaris; B) cupula; C) hair cells; D) supportive cells; E) nerve fibers
- Area of interest- review layers of the macula utriculi
