Ear - Anatomy & Physiology

Introduction

The ear is a paired sensory organ, that is involved in both hearing and balance. For this reason, the ear is known as the vestibulocochlear organ.

Structure

The Mammalian Ear - Copyright David Bainbridge

Anatomically, the ear can be looked at in three parts:

1. Outer ear - pinna and auditory canal

2. Middle ear - contains the malleus, incus and stapes bones - known as the ossicles

3. Inner ear - contains the membranous and bony labyrinths, and the cochlea

Outer Ear

Outer Ear - Copyright David Bainbridge

The pinna (or auricle) is defined as the outer projecting part of the ear, that is the part of the ear that can be seen. It consists of the auricular cartilage, surrounded by skin, which allows for flexibility and elasticity. The auricular cartilage connects to scutiform cartilage, which itself connects to annular cartilage, which allows for articulation. These cartilages fit into the bony passage of the ear canal - also called the external auditory meatus, which leads to the tympanic membrane. This is the deepest boundary of the outer ear.

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 follicles, 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.

Foreign bodies can become lodged in the external auditory meatus, but glands are present that produce wax which can trap these.

Muscles around the base of the ear that are attached to the skull allow movement of the pinna, so the ear can be directed to the source of sound:

Muscle	Origin	Innervation	Function
Preauricular	Deep temporal fascia	Auriculopalpebral branch of facial nerve (cranial nerve VII)	Moves the ear cranio-laterally, so the pinna is facing forwards
Ventroauricular	Laryngeal fascia	Retroauricular branch of facial nerve (cranial nerve VII)	Moves the ear laterally
Postauricular	Medial cervical raphe	Retroauricular branch of facial nerve (cranial nerve VII)	Moves the ear caudo-laterally, so the pinna is facing backwards

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 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 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.

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 typmanic bulla, connected by ligaments. The vertical ear canal lies in a rostroventral orientation before bending medially to become the horizontal canal.

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.

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 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.

Structures surrounding the Ear

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 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 trigeminal nerve (CN V) and branches of the facial nerve pass rostral to the vertical ear canal.

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.

Middle Ear

Middle Ear - Copyright David Bainbridge

• The middle ear consists of the tympanic cavity, the auditory ossicles and the eustachian tube. The boundary between the middle and inner ear is the oval window.
• The auditory ossicles are attached to the wall of the tympanic cavity by many ligaments and mucosal folds.
• The tympanic cavity is located within the petrous temporal bone, and can be divided into dorsal, middle and ventral parts:

- Dorsal: contains the auditory ossicles

- Middle: contians the tympanic membrane within its lateral wall, and opens rostrally into the nasopharynx via the eustachian tube

- Ventral: the tympanic bulla - a thin-walled, bulbous expansion of the temporal bone, which houses an extension of the tympanic cavity

• The oval window is positioned rostrodorsally, to which one end of the stapes is attached by an annular ligament. It functions to connect the tympanic cavity with the inner ear.
• The round window is positioned more caudally, and leads to the cavity of the cochlea.
• Sound vibrations are transmitted from the tympanic membrane, across the tympanic cavity, via the ossicles (malleus, incus, then stapes).
• The ossicles, as well as transmitting sound vibrations from the tympanic membrane, also magnify the vibrations by about 20 times. This is necessary for initiating waves in the endolymph of the cochlea. The magnification is achieved by the action of two muscles that are attached to the ossicles, and which act as antagonists of each other. These two muscles are the tensor tympani muscle and the stapedius muscle. The tensor tympani muscle originates within the tympanic cavity, and inserts on the malleus. The contraction of this muscle creates tension of the ossicles, and therefore also of the tympanic membrane, all of which results in greater sensitivity. The stapedius muscle originates from the wall of the tympanic cavity, and inserts on the stapes. Contraction of this muscle pulls the end of the stapes away from the oval window, thereby reducing the force of the transmission of sound vibrations.

• The eustachian tube connects the tympanic cavity to the nasopharynx.
• The eustachian tube functions to equalise pressure on either side of the tympanic cavity, by opening while yawning or swallowing, for example.

The middle ear comprises the tympanic cavity bounded by the walls of the tympanic bulla and the medial tympanic membrane.

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 behind the soft palate, facilitating movement and drainage of mucus and other material.

Inner Ear

Inner, Membranous Labyrinth - Copyright David Bainbridge

Outer, Bony Labyrinth - Copyright David Bainbridge

Section through a Cochlear Turn - Copyright David Bainbridge

• The inner ear is located within the petrous temporal bone.
• The inner ear contains the membranous labyrinth, which is surrounded by the bony labyrinth.
• The membranous labyrinth is an interconnected group of fluid-filled membranous sacs. The fluid is endolymph.
• It is the movement of the endolymph that stimulates the sensory cells within the membranous wall.

• The membranous labyrinth consists of:

- Vestibular labyrinth: contains the receptor organ involved with balance, containing the saccule, utricle and the semicircular ducts. The saccule and utricle contain sensory maculae within their walls, and there's a sensory crista within the ampullae of the semicircular ducts. The maculae and ampullae sense and conduct impulses concerned with balance via the vestibular nerve. The three semicircular ducts arise from the utricle, and the cochlear duct arises from the saccule.

- Cochlear labyrinth: contains the organ involved with hearing. It consists of the organ of Corti, within the cochlear duct. The cochlear duct is fluid-filled, the fluid being endolymph. The organ of Corti contains the receptor cells for hearing.

- Ductus reuniens: this is the duct through which the above two labyrinths communicate

• The bony labyrinth consists of:

- Vestibule: a chamber in the centre of the bony labyrinth, which communicates with both the cochlea and the semicircular canals. The oval and the round windows are both located in the lateral wall of the vestibule.

- Semicircular canals: contain the semicircular ducts, which have arisen from the utricle of the vestibular labyrinth. There are three semicircular canals, corresponding to the three dimensions in which you can move, so they are almost at right angles with each other. Each duct has two crura (leg-like parts). One crus of each duct has an ampulla, which is an expansion of the duct. Movement of endolymph stimulates receptor cells within the ampullae.

- Cochlea: forms a spiral around a central hollow core of bone, called the modiolus,which contains the cochlear nerve. The spiral lamina projects into the spiral canal, partially bisecting the lumen into two parts, which are called the scala tympani and the scala vestibuli. The scala media (the cochlear duct) is inbetween these two parts.

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.

Function - Hearing

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.

The external ear receives sounds, which cause vibrations of the tympanic membrane. These vibrations move along the ossicles of the middle ear, to be transmitted to the inner ear. The stapes is connected to the oval window, so when the stapes transmits vibrations, this causes movement of perilymph that is in the inner ear. The movement of the perilymph is transmitted via the scala vestibuli and the scala tympani, to the round window, where it induces movement of the secondary tympanic membrane. This results in the movement of the endolymph of the cochlear duct, causing pressure on the tectorial membrane, which then induces pressure on the sensory hairs, stimulating the receptor cells within the cochlear duct to send impulses to the spiral ganglion. The axons of the spiral ganglion form part of the vestibulocochlear nerve.

The Outer and Middle Ear

• Sound is transmitted from the tympanic membrane to the oval window, via first of all the ossicles (malleus, incus, stapes), then the middle ear wall, then the middle ear cavity:

tympanic membrane → ossicles → middle ear wall → middle ear cavity → oval window

• Amplification due to the bony lever is only 1.5 times, as the stapedius muscle prevents the stapes from vibrating too much.
• The tympanic membrane is 20 times larger than the oval window.

The Cochlea

• The entire basilar/Reissner's/ hair cells/scala media complex vibrates within the surrounding perilymph.
• It is not that a travelling wave passes along the cochlea, rather a standing wave is established within the resonant tube of the cochlea.
• Displacement of the basilar membrane during sound transmission is 200 times that of the tympanic membrane.
• This is due to the relative inertia of the tympanic membrane, so it remains relatively still as the basilar and hair cells move relative to it.

The Hair Cell Receptor

• Hair cells discharge in relation to the excursion (a range of movement regularly repeated in performance of a function) of the basilar membrane.
• The tips of the cilia are embedded in the tectorial membrane, and so are flexed by sound vibrations.
• Each cilium contains mechanically-gated non-selective cation channels. Potassium ions (K+) are the main cations in endolymph.

→ → → →

cilium flexes → gK+ ↑ → depolarisation → transmitter release ↑

← ← ← ←

cilium flexes → gK+ ↓ → hyperpolarisation → transmitter release ↓

This shows that if the cilia is pushed in one direction, it will be excited, but if pushed in the other direction it will be inhibited.

Hair Cell Resting Potentials

• Hair cells discharge at up to 300Hz and are very sensitive, due to the unique nature of endolymph encouraging K+ influx into hair cells.
• Cilia are in endolymph, but hair cell body is in perilymph.
• There is a high concentration of potassium ions (K+) in endolymph, which is maintained by ion pumps in the stria vascularis.

Cochlear Microphonics

• The importance of this is uncertain.
• It is the summated activity of hair cells, measurable at the round window.
• Non-nervous: no latency, refractoriness, and persists after death.
• Generated by hair cells.

Tone

1. The Place Principle: The physical properties of the basilar membrane change along its length. Dampening of sound waves by the basilar membrane is critical for determination of pitch:

- high frequencies are dampened out first in the basal, thinner/narrower/stiffer part

- low frequencies reach the apical thicker/wider/more pliant part

2. Lateral Inhibition: allows discrimination of two similar frequencies. It is an active process affected by olivocochlear efferents. It occurs by driving contractions of the outer hair cells which vibrate the membrane.

3. Phase Locking: allows extremely fine discrimination of pitch within the musical range. The action potential occurs at a particular point in the sound wave. The brain can determine frequency information from this.

Direction Discrimination

Outer ear:

1. Pinna shape (most mammals)

2. Pinna mobility (not primates)

Centrally:

1. Phase difference

2. Change in phase difference when head is moved

3. Head 'shadowing' of sound

Deafness

• Conduction Deafness: an interruption in the conduction of sound waves. May be caused by otitis, wax, tumours or tympanic rupture.
• Nerve Deafness: hearing loss due to a lesion to the auditory nerve within the central neural pathway. May be congenital or genetic, for example in white cats with blue eyes or in dalmatians, a problem created due to highly selective in-breeding. Histological examinations carried out on the organ of Corti from affected dogs shows the absence of sensory cells. Nerve deafness may also be due to age.

Central Auditory Pathways

Central Auditory Pathway - Copyright David Bainbridge

• The signal that has been created from the sound waves that were picked up by the ear, is carried to the brain by the vestibulocochlear nerve (CN VIII), which synapses in the cochlear nucleus.
• From here, the auditory information is then split.
• Those nerve fibres that travel to the ventral cochlear nuclear synapse on their target cells. The ventral cochlear nuclear cells then project to a group of cells within the medulla, called the superior olive nucleus. It is here that the timing and loudness of the sound that was picked up in each ear is compared, allowing determination of the direction that the sound came from. This information is then transferred via the lateral lemniscus to the inferior colliculus.
• The other nerve fibres start in the dorsal cochlear nucleus. It is here that the quality of sound is determined, as it compares the frequency differences. This pathway leads directly to the inferior colliculus, via the lateral lemniscus.
• Both of the above pathways are bilateral. This means that if there is a lesion at any point along the pathway, it usually has no effect on hearing. Deafness is only usually caused if there is damage to either the auditory nerve, the cochlea, or the middle ear.
• From the inferior colliculus, the information from both pathways is sent to the medial geniculate nucleus of the thalamus, which then leads on to the primary auditory cortex of the cerebral cortex.

Function - Balance

Vestibular Receptors and Balance - Copyright David Bainbridge

• The vestibular sense is rather more unconscious than that of hearing.
• The vestibular labyrinth, that is contained within the bony labyrinth of the inner ear is the part of the ear that is involved with the vestibular sense - balance.
• The vestibular labyrinth contains the saccule, the utricle and the semicircular ducts - the semicircular ducts being housed within the semicircular canals.
• There are sensory hair cells within the vestibular labyrinth, similar to those in the other regions of the inner ear, which detect movement.
• However, these sensory hair cells are lodged in the ampullary cupulae or in otoliths (minute calcareous particles), rather than in the tectorial membrane as in the rest of the ear.
• The ampulla is a swelling at the base of the semicircular ducts. The sensory hair cells project upwards from the ampulla into the cupula, which is a gelatinous mass. The ampullary cupulae detect flow around the semicircular canals, which are filled with endolymph, and there is an inertia of fluid for detection of angular acceleration. Angular acceleration is the detection of motion of the head in any direction.
• Otoliths are denser than endolymph - they are calcareous and crystalline. They are contained within the maculae, and detect gravity and linear acceleration. Linear acceleration is the detection of motion along a line, for example when you lean to one side.
• Movement of the sensory hair cells triggers impulses, which are carried by the vestibular portion of the vestibulocochlear nerve (CN VIII).

Unilateral Vestibular Signs

Unilateral Vestibular Signs - Copyright David Bainbridge

• The vestibular system is a common site for pathology. Brain infection, tumours and inflammation are often shown up by vestibular signs. These signs may include:

1. Head tilt towards the lesion

2. Fall towards the lesion

3. Turn towards the lesion

4. Vomiting - due to the connection to the vomiting centre of the brain

5. Nystagmus with slow phase to lesion - nystagmus is rapid, involuntary, oscillatory motion of the eyeball in any direction, and can be caused by a lack of coordination

Central Vestibular Pathways

Central Vestibular Pathways - Copyright David Bainbridge

• The sensory hair cells produce signals, which are carried by the vestibulocochlear nerve (CN VIII) first of all through the bipolar vestibular ganglion cells.
• Most nerve fibres that have come from the hair cells terminate in the vestibular nuclei, which are located in the fourth ventricle of the cerebral cortex.
• After entering the vestibular nuclei, some of the processes of the nerve fibres divide into ascending and descending branches. Some processes pass directly into the cerebellum.

Vasculature

• The medial, intermediate and lateral auricular rami supply the outer ear, all of which are branches of the great auricular artery, which itself is a branch of the external carotid artery.
• The tympanic membrane is highly vascularised.

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

Sensory innervation to the outer ear:

• Ce2 (second cervical spinal nerve) to medial pinna
• Auriculopalpebral branch of trigeminal nerve (CN V) to most of the lateral pinna
• Facial (CN VII) and vagus nerves (CN X) via small auricular branches to pinna

Sensory innervation to the inner parts of the ear is provided by the vestibulocochlear nerve (CN VIII).

Innervation to the muscles of the middle ear:

• The tensor tympani muscle is innervated by the pterygoid nerve, which is a branch of the mandibular nerve, itself being the third branch of the trigeminal nerve (CN V).
• The stapedius muscle is innervated by the facial nerve (CN VII).

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.

Histology

Section through Cochlea - Copyright David Bainbridge

• Tympanic Membrane: the outer surface is lined with an epithelium that is continuous with that of the external acoustic meatus. The inner surface is lined by the mucosa that lines the tympanic cavity. This inner mucosal layer is made up of simple squamous epithelium.
• Tympanic Cavity: lined by a single-layer of epithelium, and the underlying soft tissue has a rich vascular and nervous supply. The single-layered epithelium also covers the ossicles and the tympanic membrane.
• Saccule and Utricle: lined by simple squamous epithelium, underneath which is a layer of loose connective tissue.
• The connective tissue component of the organ of Corti is the basilar lamina.

Species Differences

• 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 aural disease, particularly in the dog, but also other species including the lop-eared rabbit

• The Cochlea: the spiral has 3 turns in carnivores, 2.5 turns in horses, 4 turns in the pig, and 3.5 turns in ruminants.

• 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.

• The guttural pouch is an anatomical structure that is only found in the horse and other perissodactyla.
  

  Equine Guttural Pouch - Copyright David Bainbridge

Two rabbits: one with wild-type erect pinnae, the other with pendulous pinnae for comparison

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