The ear

• Divisions of the ear

  •External (outer) – auricle, external auditory meatus, external surface of the tympanic membrane

  •Middle – internal surface of the tympanic membrane, tympanic cavity, ossicles, pharyngotympanic tube

  •Internal (inner) – auditory apparatus, vestibular apparatus, internal auditory meatus, vestibulococchlear nerve (CN VIII), oval window, round window

  The tympanic membrane forms the boundary between the external and middle ears. The middle and inner ears communicate at the oval and round windows.

  
• Relations of the ear

  The external ear lies posterior to the superficial temporal vessels and auriculotemporal nerve. The middle ear is related to the internal carotid artery and the auditory tube. The internal ear sits surrounded by bone between the middle and posterior cranial fossae. The temporal lobes of the brain sit superior to the internal structures and the internal auditory (acoustic) meatus opens into the inner ear from inside the cranial cavity. This meatus carries the facial and vestibulocochlear nerves into the inner ear and petrous temporal bone.

  
• External ear - auricle

  •Elastic cartilage

  •Muscles of facial expression but vestigial in humans

  •Lymph to parotid and cervical lymph nodes

  The muscles that permit this are innervated by the facial nerve.

  
• Auricle - arterial

  The auricle is a living structure and therefore receives an arterial supply from branches of the external carotid artery. The blood drains into the corresponding veins into the external and internal jugular veins.

  
• Auricle - innervation

  The sensory innervation of the auricle is relatively complex for such a simple structure. Though it does guard the entrance to the more complex and delicate internal structures, so an awareness of a structure approaching the external auditory meatus is very useful.

  
• External ear – external auditory meatus

  •About 2.5cm long

  •Lateral third = elastic cartilage, hair follicles, sweat and sebaceous glands

  •Medial two-thirds = bone lined with stratified squamous epithelium

  •Blood from branches of external carotid (auricular)

  •Nerves – mainly auriculo-temporal from CN V3 but also auricular branches from CN VII and CN X

  The meatus conveys sound from the auricle to the tympanic membrane.

  
• Tympanic membrane

  •Roughly circular 8mm diameter

  •Outer surface = stratified squamous epithelium, inner surface covered by mucous membrane

  •Chorda tympani runs across the medial surface

  •Malleus attached to inner surface

  •Lateral surface = auriculotemporal nerve (CN V3) and auricular branch of vagus (CN X)

  •Medial  surface = tympanic branches of glossopharyngeal nerve (CN IX)

  
• Tympanic membrane

  The tympanic membrane vibrates in response to soundwaves hitting its external surface. The malleus ossicle bone attaches to the tympanic membrane, so as the membrane vibrates so does the malleus. This transmits the energy from the soundwave into a physical movement that can then be passed deeper into the middle ear.

  
• Tympanic cavity

  The medial (inner) surface of the tympanic membrane bulges into a space inside the petrous temporal bone called the tympanic cavity. This space houses the tiny ossicle bones that link the tympanic membrane to the oval window of the inner ear. The handle of the malleus bone sits on the tympanic membrane, the malleus articulates with the incus bone, which articulates with the stapes bone.

  
• These bones are the auditory ossicles, movement is transmitted from the tympanic membrane, through each bone until it reaches the base of the stapes. This part of the stapes rests against the oval window – a membrane covered hole that leads to the inner ear. The vibration passes from the footplate into the oval window and from there is carried deeper into the inner ear. The ossicles are sometimes called the hammer, anvil and stirrup The photograph shows a view of the middle ear - the circle lies on the tympanic membrane, the triangle on the head of the malleus and the square lies on the incus.

  
• Tympanic cavity

  The diagram shows the close relationship between the internal carotid artery and the middle ear. The middle ear also lies close to the wall of the internal jugular vein and the facial nerve . An aditus or opening in the middle ear communicates with the mastoid air cells. This creates a potential route for infection

  
• Muscles and neuro-vascular supply

  Sensory nerve supply to tympanic cavity (including medial surface of tympanic membrane) from CN IX

  Blood supply mostly via tympanic branches of posterior auricular, ascending pharyngeal and maxillary arteries (caroticotympanic from internal carotid also)

  There are also 2 tiny muscles within the middle ear. They both act to dampen the sound being passed from the ossicles to the more delicate structures of the inner ear. The pyramid of the middle ear is simply a small bulge of bone.

  
• Facial Nerve

  The facial nerve enters the temporal bone via the internal auditory meatus and travels through the petrous temporal bone until it exits the bone at the stylomastoid foramen. The facial nerve then passes into the parotid gland and divides into branches to supply the muscles of facial expression. As it passes through the temporal bone it also gives off several branches

  
• Facial Nerve

  The greater petrosal nerve passes to form the nerve of the pterygoid canal (with the deep petrosal nerve from the carotid plexus) to supply parasympathetic secretomotor fibres to the lacrimal gland and small glands of the nose and palate. The nerve to stapedius branches from the facial nerve to supply this muscle in the tympanic cavity. The chorda tympani branches from the facial nerve to cross the medial surface of the tympanic membrane.

  
• Chorda tympani

  •Crosses the medial surface of tympanic membrane and handle of  malleus

  •Leaves the tympanic cavity via petrotympanic fissure

  •Joins the lingual nerve

  •Parasympathetic = sub-lingual and sub-mandibular salivary glands

  •Special sense of taste for the anterior two-thirds of the tongue

  joins the lingual nerve (CNV3) in the infratemporal fossa.

  
• Auditory tube

  •Connects the nasopharynx with the tympanic cavity

  •Lateral  (1.2cm) = bony canal lined with mucosa

  •Medial (2.5cm) = cartilagenous and membranous tube

  •Normally closed, but when tensor veli palatini contract (swallow, yawn etc) lumen opens

  •Equalises pressure on both sides of the tympanic membrane

  •Sensory via CN IX

  
• Auditory tube

  The medial part is closest to the pharynx and has cartilage in its walls that provides attachment for salpingopharyngeus, and tensor and levator veli palatini. If the pressure on each side of the tympanic membrane is unequal, then the tympanic membrane cannot vibrate freely. If it cannot vibrate freely then an accurate transmission of sound cannot be passed into the inner ear.

  
• Middle ear arterial supply

  •Anterior tympanic (Glaserian) artery from 1st part of maxillary artery

  •Also contributions from ascending pharyngeal (inferior tympanic), middle meningeal (superior tympanic) posterior auricular (stylomastoid, posterior tympanic), artery of pterygoid canal and caroticotympanic branch of internal carotid

  
• Inner ear

  The inner ear houses the delicate vibration sensitive cells of hearing and balance. It is formed of several bony spaces that are filled with fluid. Tiny membranous structures hang suspended in this fluid. Vibration from the stapes at the oval window then creates waves of movement through the fluids of the bony and membranous labryrinths which reach the special sense of hearing and balance cells housed in the membranes.

  
•  Labyrinths

  BONY

  •Surrounded by the otic capsule

  •Otic capsule is very dense bone within the petrous temporal bone – NOT bony labyrinth but surrounds it

  •a system of canals filled with perilymph (similar to extra-cellular fluid)

  •Cochlea

  •Vestibule

  •Semicircular canals
•  Labyrinths

  MEMBRANOUS

  •Continuous system of ducts and sacs inside the bony labyrinth

  •Suspended in perilymph but CONTAINS endolymph (similar to K+ rich intra-cellular fluid)

  Perilymph and endolymph conduct sound vibrations and respond to mechanical forces (movement and acceleration)
• Cochlea

  Vibration is passed from the footplate of the stapes to the membrane of the oval window. The oval window vibrates in response, and a wave of movement passes into the perilymph of the scala vestibuli. The cochlea forms a shell-like shape so the scala vestibuli ascends to the top of the shell shape (helicotrema) and then the wave passes down the scala tympani until it reaches the base of the cochlea again, but this time at a second membranous structure – the round window.

  
• Cochlea

  The round window vibrates gently into the middle ear. Nothing is attached to this round window, so the vibration wave harmlessly dissipates into the middle ear. As the wave of vibration passes through the perilymph it presses on the membranous labyrinth (the cochlear duct). This deforms the membrane, and this deformation is detected by the special cells within the membrane.

  
• Cochlea

  how the cochlear duct lies between the scalae vestibuli and tympani. The cells in the spiral organ/organ of Corti detect movement and transform it into electrical energy (a nerve impulse) which is then passed to the cochlear nerve – part of cranial nerve VIII. This nerve carries the nerve impulses to the brain to be interpreted as different sounds by the auditory cortex.

  
• The ‘scala’ are named after stairs/steps – you ascend the vestibular stairs to the top of the cochlea and then descend via the tympanic staircase.

  
• Semi-circular canals

  •Lie posterior and lateral to the vestibule

  •Each canal is @ two-thirds of a circle and 1.5mm in diameter

  •Anterior, posterior and lateral

  •Ant and post set at right angles, lat = horizontal

  •Contain the semi-circular ducts (continuous with utricle)

  •Each duct swells at the end = ampullae

  
• Semi-circular canals

  The inner ear also detects movement of the head and therefore aids in the maintenance of balance. The vestibule is an area of bony labyrinth that the oval window opens into, it communicates with both the cochlea and semicircular canals. The semicircular canals of the bony labyrinth house the semi-circular ducts of the membranous labyrinth.

  
• Ampullae, utricle and saccule

  •Each ampulla houses equilibrium receptor called crista ampullaris which responds to angular (rotational) movements of the head

  •Vestibule = bony labyrinth @5mm long

  •Inside vestibule are 2 membranous sacs = utricle and saccule

  •House equilibrium receptors called maculae = respond to the pull of gravity and changes in head position

  •Otoliths

  
• Ampullae, utricle and saccule

  The nerve impulses pass into the vestibular part of cranial nerve XIII, which then passes into the brain for interpretation and for the correct postural movements to be made to maintain balance. Otoliths are minute pieces of bone that sit on the receptor cells and move in response to the movement of the endolymph.

  
• Vestibulocochlear nerve

  the pathway taken by the vestibulocochlear nerve from the inner ear to the brain.

  
• Inner ear arterial supply

  •Main supply from internal auditory (labyrinthine)

  •Internal auditory branches to form common cochlear and vestibular arteries

  •Internal auditory usually a branch of anteroinferior cerebellar artery (80+%) or basilar artery