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Ear - Anatomy & Physiology (view source)
Revision as of 16:04, 14 August 2009
, 16:04, 14 August 2009→Links
*The shape of the pinna varies between breeds and species.
*The shape of the pinna varies between breeds and species.
*'''Fish''': to detect high frequency sound, some fish use the swim bladder as an acoustic detecror. It is connected to the '''lagena''' (the stumpy piscine cochlea) by three ''Weberian'' bones. These are derived from vertebrae.
*'''Fish''': to detect high frequency sound, some fish use the swim bladder as an acoustic detecror. It is connected to the '''lagena''' (the stumpy piscine cochlea) by three ''Weberian'' bones. These are derived from vertebrae.
==Content Requiring integration from 'The Ear' page==
==Function of the Ear==
The main function of the ear is to recieve auditory and vestibular input.
It locates the directional source of sound, collects sound waves and conducts them to the special organ of sense in the inner ear, where sound is converted to electrical impulses and transmitted to the brain.
On this page the structure of the ear is mainly discussed, to fully understand how the ear functions please look at the [[Ear - Anatomy & Physiology|Auditory]] section of the wiki.
The ear is composed of three separate but interconnected structural compartments: external, middle and inner ear.
==The External Ear==
[[image: Canine Auricular Cartilages.jpg|thumb|250px|right|The auricular cartilages of the left canine ear. Image by Rachael Wallace]]
This includes the '''pinna''' and the '''ear canal''' (''external auditory meatus'') down to the level of the '''tympanic membrane'''.
The pinna comprises the '''auricular [[Bones and Cartilage - Anatomy & Physiology#Structure and Function of Cartilage|cartilage]]''', which is flared distally into a flattened cone shape, covered by skin on both sides - more tightly on the medial or concave side than the convex side. The [[Hair - Anatomy & Physiology|hair]] covering on the convex side is usually similar to the rest of the body but the hair covering on the concave or medial aspect is variable. There are also irregularities on the concave surface with ridges and prominences, the '''medial and lateral crus of the helix''' on the medial aspect of the opening of the external ear canal, opposite the rectangular '''tragus''' on the lateral aspect.
[[image: Rabbit ears.jpg|thumb|250px|left|Two rabbits: one with wild-type erect pinnae, the other with pendulous pinnae for comparison]]
The functional shape of the pinna in wild-type mammals is erect, and the muscular connections at the base of the pinna allowing positional adjustments to efficiently collect sound. Many domesticated species, however, have non-erect, pendulous pinnae, as a result of selective breeding. This and other conformational changes may contribute to [[Special Senses - Pathology|aural disease]], particularly in the dog, but also other species including the lop-eared rabbit.
Proximally, the auricular cartilage is rolled into a funnel shape, known as the '''concha'''. The '''scutiform''' cartilage lies rostromedially in the lower ear canal and provides support. The '''annular''' cartilage lies between the proximal horizontal ear canal and the bony '''acoustic process''' of the [[Skull and Facial Muscles - Anatomy & Physiology#Temporal Bone (os temporale)|typmanic bulla]], connected by ligaments. The vertical ear canal lies in a rostroventral orientation before bending medially to become the horizontal canal.
===Innervation and Vascularisation of the Ear===
The vascular supply to the outer ear is provided by the '''great auricular arteries''' (ex internal carotid artery). Venous drainage is via satellite veins to the '''internal maxillary veins'''. Innervation is provided by the '''auriculopalpebral branches of cranial nerve VII''' to the auricular muscles and sensory supply by cervical nerve II and cranial nerve V.
In the tissues medial to the ear canal lie the auricular and superficial temporal arteries. Laterally is the '''parotid salivary gland''' and also a branch of the [[Nervous System - CNS - Anatomy & Physiology#Innervation - Cranial Nerves|facial nerve]], external maxillary vein and branches of the external carotid artery. The facial nerve exits the skull through the stylomastoid foramen behind the osseous bulla; it passes below the rostroventral aspect of the horizontal ear canal. The '''auriculo-temporal''' branch of the mandibular portion of the [[Nervous System - CNS - Anatomy & Physiology#Innervation - Cranial Nerves|trigeminal nerve]] (CN V) and branches of the facial nerve pass rostral to the vertical ear canal.
The cartilage and bony process of the external ear and the tympanic membrane are covered by skin. The skin of the normal ear has a thin stratified keratinising epidermis and a thin dermis containing adnexal structures – [[Hair - Anatomy & Physiology|hair follicles]], [[Skin - Anatomy & Physiology#Glands|sebaceous and apocrine glands]]. The density of hair follicles on the concave pinna is variable, as is the density and distribution of hair follicles in the ear canal itself. Some species are devoid of hair in the lower (proximal) ear canal (eg horse); hair follicle density in the dog is variable – usually simple but in some breeds compound hair follicles.
===Glands of the Ear Canal===
The apocrine glands in the ear canal are modified and known as ceruminous glands. The material secreted into the ear canal, '''cerumen''' (or wax), is compromised of exfoliated epithelial cells (squames) and glandular secretions. Sebaceous glands predominate distally and are largely responsible for the lipid component of cerumen. The density of ceruminous glands increases proximally, towards the tympanic membrane. The secretions of the ceruminous glands contain mucopolysaccharides and phospholipids. Immunoglobulins IgA, IgG and IgM have been identified in canine cerumen, predominantly IgG.
The epithelial cells in the [[Skin - Anatomy & Physiology#Epidermis|stratum granulosum]] of the tympanic membrane migrate centripetally, and this migration comprises the natural cleaning mechanism keeping the tympanum free of debris. The upward/distal migration of epithelial cells clears desquamated cells, secretions, debris and microbes. The more aqueous secretions of the ceruminous glands, present proximally, allow easier upward migration, whereas the higher lipid content of the distal secretions offers better barrier protection.
===Microclimate of the Ear Canal===
The microclimate of the external ear canal remains surprisingly stable in spite of marked changes in the ambient temperature and humidity of the surrounding environment. The temperature in the healthy ear canal is between 38.2 and 38.4 C, and 0.6 C below rectal temperature. The mean relative humidity in the ear canal is 80.4%, with mean pH of 6.1-6.2.
'''Otitis externa''' is associated with an increase in temperature, relative humidity and a rise in pH within the external ear canal. The nature of the secretions alters, with a decrease in the lipid content of cerumen.
==The Tympanic Membrane==
This epithelial structure separates the external from the middle ear. Below the stratified keratinising outer epithelium is a connective tissue lamina propria, with a cuboidal mucosal epithelium on the inner surface. The tympanic membrane is divided into the '''pars flaccid''' dorsorostrally, which lies next to the '''manubrium of the malleus''', and the '''pars tensa''' ventrally. The C-shaped manubrium inserts into the '''lamina propria''', extending towards the middle of the pars tensa. Foreign bodies and other damage (severe otitis externa, ascending pathology from middle ear) can result in rupture of the tympanic membrane in the pars tensa region. Defects of the tympanic membrane heal by epithelial migration bridging over the defect, then development of a granulation bed below.
==The Middle Ear==
The middle ear comprises the '''tympanic cavity''' bounded by the walls of the '''tympanic bulla''' and the '''medial tympanic membrane'''.
[[image: Canine Ear Canal.jpg|thumb|250px|right|The Canine Middle Ear]]
It has 3 compartments, the smallest dorsal or '''epitympanum''', conatining 2 of the auditory ossicles – the '''malleus''' and '''incus'''; the middle or '''mesotympanum''' bounded by the tympanic membrane laterally and containing the third auditory ossicle, '''stapes''', attached to the oval window and the ventral, '''hypotympamum''', or fundic cavity, which is the largest compartment. The auditory ossicles are supported by ligaments and muscles, which alter their position and influences the tension of the tympanic membrane. The ratio of the malleus:incus in dogs and cats is 2-3 times that of man, and may explain the increased acuity of hearing. The '''round window''' is on the caudomedial aspect of the mesotympanum and the opening of the auditory '''(Eustachian)''' tube opens on rostromedial aspect of the mesotympanum. Opposite the lateral tympanic membrane on the medial aspect of the cavity is a bony promontory. Associated structures close to or in channels in the wall of the tympanic cavity (bulla) are the '''facial nerve''', '''vagus nerve''' and branches of the '''carotid and lingual arteries'''. Post-ganglionic fibres of the '''cervical sympathetic trunk''' run in the region of the dorsomedial wall of the tympanic cavity.
The feline middle ear has an incomplete '''bony septum''' dividing the ventral chamber into a large ventromedial and small dorsolateral chamber, communicating caudomedially.
The middle ear is lined by cuboidal to columnar mucosal epithelium with scattered '''goblet cells'''. The auditory tube is lined by pseudostratified, ciliated columnar epithelium with scattered goblet cells. The goblet cells are more prominent at the tympanic cavity end, contributing to the surfactant nature of the secretions- containing '''lecithin''', lipids and '''mucopolysaccharides''' - that decreases surface tension and keeps the tube patent. The density of '''cilia''' increases as the tube runs dorsolaterally to open into the [[Nasopharynx - Anatomy & Physiology|nasopharynx]] behind the soft palate, facilitating movement and drainage of mucus and other material.
==The Inner Ear==
The inner ear contains the '''cochlea''', '''vestibule''' and '''semicircular canals''' in the bony labyrinth of the '''petrous temporal bone'''. These organs are responsible for the function of hearing and positional and gravitational relationships of the head at rest and during movement.
==Links==
==Links==
[[Special Senses - Pathology|Eye and Ear Pathology]]
[[Special Senses - Pathology|Eye and Ear Pathology]]