Equine Special Senses - Horse Anatomy

Ear

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. Sound waves that are transmitted into the ear provide a mechanical stimulus. These mechanical stimuli are then transferred into electrical signals by the cochlea. Neuroreceptors in the ear allow the horse to gain a perception of position and movement. 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 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 (or auricle) 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. This is the outer projecting part of the ear that can be seen. 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. The functional shape of the pinna in horses is erect, and the muscular connections at the base of the pinna allowing positional adjustments to efficiently collect sound. 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. 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.

Innervation and Vascularisation of the Outer 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 cranial 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 facial nerve (VII), 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 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. Horses are devoid of hair in the lower (proximal) ear canal.

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. 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.

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

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 auricular ossicle
• 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 fource of the transmission of sound vibrations.

The eustachian tube connects the tympanic cavity to the nasopharynx, which mark the beginning and end of the eustachian tube, respectively. The eustachian tube functions to equalise pressure on either side of the tympanic cavity, by opening while yawning or swallowing, for example. In the horse, the guttural pouch is a paired diverticulum of the eustacian tube that is unique to this species. This is of clinical importance.

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 contained within it is known as 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 between these two parts. In the horse, the cochlea makes 2.5 turns.

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.

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 cells 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 these 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.

Vestibular System and 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 the horse leans to one side. Movement of the sensory hair cells triggers impulses, which are carried by the vestibular portion of the vestibulocochlear nerve (CN VIII).

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.