Anatomy

Surface Anatomy:

Outer/External Ear

Function:

Directs sound waves inwards towards middle and inner ear

Anatomy: 

The outer ear extends from the auricle through to the eardrum (the tympanic membrane), via the external auditory canal.

The auricle (otherwise known as the pinna) is elastic cartilage covered by skin. Its rim is called the helix- where many people get piercings! It is connected to the rest of the head by ligaments and muscles.

The lower end is called the lobe/lobule of the ear.

The external auditory canal is a tube that sits in the temporal bone and terminates at the tympanic membrane. It contains ceruminous glands which secrete cerumen (earwax)

The tympanic membrane separates the external auditory canal from the middle ear. It is covered by epidermis and lined by simple cuboidal epithelium (1). Between these layers is connective tissue, which is made up of fibroblasts, collagen and elastic fibres.

The tympanic membrane can be seen and examined using an otoscope.

Middle Ear

Function:

Send vibrations to oval window

Anatomy:

The middle ear is a small, air-filled cavity which sits in the petrous part of the temporal bone.

It is separated by the tympanic membrane on one side, and a thin bony wall on the other. This wall contains the round and oval windows.

The middle ear contains the three auditory ossicles and their ligaments. These ossicles are connected to each other by synovial joints (1).

The three auditory ossicles are:

• Malleus (hammer)
  • The 'handle' attaches to internal surface of tympanic membrane.
  • The 'head' articulates with the body of incus.
• Incus (anvil)
  • Middle bone
  • Articulates with head of stapes
• Stapes (stirrup)
  • Fits into oval window (enclosed by secondary tympanic membrane)
  • This is the smallest bone in the body.

There are 2 main muscles which attach to these ossicles:

• Tensor tympani (innervated by CN.V3).
  • This limits the movement of and maintains tension in the tympanic membrane.
• Stapedius (innervated by CN.VII).
  • This is the smallest skeletal muscle in the body! It softens loud noises to protect the oval window.

The anterior wall of the middle ear connects with the auditory tube (also known as the eustachian tube)

• This is made up of bone and elastic cartilage.
• It opens into the nasopharynx to equalise pressure.
• Normally, it remains closed, but opens on yawning and swallowing.
• It is the route of infection for otitis media.

Inner Ear

Function:

Contains receptors for hearing and balance.

Anatomy:

The inner consists of many parts.

To start, there is the labyrinth, which sits in the petrous part of the temporal bone. This comprises of an outer bony labyrinth and an inner membranous labyrinth.

• The outer bony labyrinth is lines by periosteum and contains perilymph.
  • The perilymph sits in the space between the bony casing and the membranous labyrinth.
• The inner membranous labyrinth consists of epithelial sacs and tubes which house the receptors for hearing and balance.
  • It also contains endolymph, which contains an extremely high amount of potassium (K+) ions [This will become relevant later].

There are 3 key areas in the middle ear:

• The Semicircular canals
  • Function: balance
  • There is an anterior, posterior and lateral canal. The anterior and posterior sit vertically, and the lateral sits horizontally. Therefore, they all sit at right angles to each-other.
  • At one end of each canal, there is a swollen portion called an ampulla. 
  • The semicircular canals contain semicircular ducts.
    • These are the membranous labyrinth containing endolymph.
    • These join with the utricle which sits in the vestibule.
• The Vestibule
  • Function: balance
  • The vestibule is oval in shape.
  • Here, the membranous labyrinth contains 2 sacs: utricle + saccule
  • These are connected by a small duct
  • This area wits anteriorly and below the semi-circular canals.
• Cochlea (hearing)
  • Function: hearing
  • This is referred to as 'Snail-shaped'.
  • It consists of a bony spiral canal, which turns 3 times around a core called the modiolus.
  • Inside the cochlea, there are 3 channels:      
    • Scala vestibuli –
      • This is part of the bony labyrinth and contains perilymph.
      • It ends at the oval window.
      • The scala vestibuli opens up into vestibule and has continuous perilymph here.
    • Cochlear duct (otherwise known as scala media)
      • This is a continuation of the membranous labyrinth int the cochlea.
      • It contains endolymph.
    • Scala tympani – perilymph (bony labyrinth)
      • This is also part of the bony labyrinth and contains perilymph.
      • This ends at round window.

The scala tympani and scala vestibuli are separated by the cochlear duct (scala media), except at an apex called the helicotrema.

Membranes in the middle ear: 

• The vestibular membrane separates the scala media from the scala vestibuli. 
• The basilar membrane separates the scala media from the scala tympani.
  • The Spiral organ/organ of corti sits on this membrane (surrounded by scala media).

The Spiral Organ/Organ of Corti

• This consists of a sheet of epithelial cells, rolled up.
• These are called Hair cells.
  • There are 4 rows of hair cells:1 inner row and 3 outer rows.
  • Each hair cell has 100 actin-stiffened microvilli called stereocilia at their tips. These are the receptors for hearing. They protrude towards the tectorial membrane.
    • Note: The tectorial membrane covers the hair cells and separates them from the endolymph of the scala media.
• The hair cells receptors synapse with sensory and motor neurones, whose cell bodies lie in the spiral ganglion. 

Innervation

• Vestibulocochlear (VIII)- which splits into the vestibular (balance) and cochlear (hearing) branches.
• The vestibular branch splits further into the ampullary, utricular and saccular branches, with their cell bodies in the vestibular ganglia.

How do we Hear?

Sound Waves

Sound waves are created by a vibrating object.

They consist of a frequency and travel through a medium.

The frequency of a wave determines the pitch of the sound. The audible human frequency range is 20-20,000 Hertz (Hz) (1).

The intensity (amplitude) of the wave determines how loud the sound is. This is measured in Decibels (dB).

Physiology of Hearing:

The following is a step-by-step of the physiology of hearing:

1. The auricle directs the sound waves into the external auditory canal.
2. When these waves hit the tympanic membrane, it vibrates.
3. Centrally, the membrane is connected to malleus, so when it the membrane and malleus vibrate, this is transmitted to incus and then stapes.
4. The stapes vibrates and has an oval end. The oval end is attached to the oval window by a ligament. This causes the oval window to vibrate.
   • These vibrations are over 20x stronger than the tympanic membrane due to the transfer via the ossicles (larger surface area to a smaller surface area)
5. The oval window vibration causes waves in the perilymph of the scala vestibuli.
6. These waves transmit from the scala vestibuli perilymph, to the vestibular membrane, then to the endolymph of the scala media.
7. These waves in the scala media cause the basilar membrane to vibrate, causing the hair cells of the spiral organ to vibrate against the tectorial membrane.
8. The stereocilia then bend and this generates a nerve impulse in the cochlea nerve fibres. 
   1. Each section of the basilar emembrane will vibrate more/less at different frequencies (this is influenced by tighter/more flexible areas of the membrane).
9. The waves then go to the scala tympani and to the round window- which bulges. 

Transduction into Nerve Impulses

1. When the basilar membrane vibrates, the hair cell apexes bend. The stereocilia located here have have a protein that connects them to a mechanically gated ion channel (transduction channel)
2. The tug on bending causes them to open.
3. The channels allow the high concentration of potassium ions in the endolymph (of the scala media) to enter the hair cell cytoplasm. 
4. This depolarizes the cell membrane and opens voltage gated calcium ion channels in the base of the hair cell. 
5. This causes an influx of calcium ions into the cell and results in neurotransmitters being released via exocytosis of vesicles. 
6. These neurotransmitters generate nerve impulses in the sensory neurones.
7. Bending the other way closes these mechanically gated ion channels and allows hyperpolarization to occur, thus decreasing the frequency of the nerve impulses.

Auditory Pathway to the Brain

1. Cell bodies of the sensory neurones are located in the spiral ganglia. 
2. The nerve impulses pass along their axons into the vestibulocochlear nerve (VIII).
3. These axons synapse in the cochlea nuclei, situated in the medulla oblongata of the brain stem on the same side. 
   • Some will cross over in the medulla and can ascend along the lateral lemniscus on the contralateral side, ending in the inferior colliculus of the midbrain. 
   • Others will end in the superior olivary nucleus in the pons on each side of the brainstem. 
   • Delays in impulses entering the superior olivary nucleus from each ear allows us to establish the direction of the sound. 
4. From the inferior colliculus of both sides, nerve impulses travel to the medial geniculate nucleus in the thalamus and end in the primary auditory area of the cerebral cortex (temporal lobe of the cerebrum). 

Physiology of Balance (Equilibrium)

Introduction & Types of Balance

• Static equilibrium = the maintenance of one position in relation to gravity
  • This is affected by tilting the head, or linear movement (eg. travelling in a car)
• Dynamic equilibrium = the maintenance of body position during sudden movements.
  • Eg. Deceleration or rotation 

Physiology & Relevant Anatomy:

The vestibular apparatus consists of the receptor organs for balance. These are the otolithic organs located in the semi-circular ducts terminus, called the saccule and utricle.

These are 2 sacs with a small region that is thickened, called the macula.

Their maculae are situated perpendicular to one another and are the receptors for static balance.

• They detect sensory information on the position of the head to allow for posture and balance. 
  • They contain hair cells and supporting cells. 
  • The hair cells have stereocilia and kinociluium (the tallest sterecilia), which make up the hair bundle.
  • The supporting cells secrete otolithic membrane. This is a glycoprotein layer, which is covered by otoliths (calcium carbonate crystals). 
  • The otolithic membrane sits on top of macula – tilting causes it to be pulled by gravity. This causes bending of the stereocilia of the hair cells and hence, the generation of an impulse. 

The semi-circular ducts detect dynamic changes and are involved in dynamic equilibrium.

• They detect rotational acceleration or deceleration.
• Each ampulla has a small elevated area called the crista. 
• Each crista has hair cells and supporting cells.
• The crista are covered by a gelatinous material called the cupula. 
• On movement, attached semicircular ducts and hair cells move and drag along the stationary endolymph. This bends the hair bundles and causes the generation of an impulse. 

Balance Pathway to the Brain

1. Bending of the hair bundles in the ducts, saccule and utricle generates nerve impulses of the primary sensory neurones of the hair cells.
2. Their cell bodies are located in the vestibular ganglia. 
3. Impulses travel along the axons of the vestibular branch of VIII and synapse at the vestibular nuclei in the medulla oblongata and pons. 
   • These nuclei also receive input from the eyes and proprioception from the neck and limb muscles.
4. The axons then enter the cerebellum via inferior cerebella peduncles. 
5. Vestibular nuclei then send commands to the nuclei of III,IV (to maintain the visual field associated with this position) and XI (to control head and neck movements to maintain this position).
6. The vestibulospinal tract allows transmission of signals to the skeletal muscles of the body and help maintain muscle tone for this position. 
7. Some other areas that receive commands are: the ventral posterior nucleus in the thalamus and the vestibular area in the parietal lobe of cerebral cortex.

Clinical Relevance

Otitis Media

This is an infection of the middle ear. With effusion, many people know it as 'Glue Ear'.

This can be caused due to un-equalised pressure, resulting in build up / inflammation and fluid sitting behind the tympanic membrane. It is most common in children.

It is important to distinguish whether pain in the ear is related to a middle ear infection, referred from dental pain or related to TMJD.

Hearing Loss

There are different types of hearing loss. This is important to understand when treating patients who suffer from this:

• Conductive - sounds unable to pass freely into inner ear.
• Sensorineural - damage to hair cells within cochlea and/or vestibulocochlear nerve.
• Prespycussis = age-related hearing loss. This can be for a number of reasons:
  • Bilateral high-frequency hearing loss: difficulty processing auditory info
  • Sensory: caused by epithelial atrophy - loss of sensory hair cells and supporting cells of organ of corti
  • Neural: atrophy of nerve cells in cochlea and CN pathways
  • Metabolic: atrophy of stria vascularis - this maintains the chemical/bioelectric balance of cochlea
  • Mechanical: thickening and secondary stiffening of basilar membrane

Ototoxicity

This is the disturbance of cochlea function.

The mechanism is unclear but could be drug induced:

• Aminoglycosides, Macrolides
• Loop diuretics
• Chemotherapeutic drugs
• Antiseptics/ disinfectants

Cleft Lip & Palate

In some Cleft Lip & Palate cases, the eustachian tube can be involved and may lead to collection of fluid in the child's middle ear, which can result in hearing loss.

It can also affect their velopharyngeal capacity and the nasality of their speech.

Skin Cancer

A looser, but still important thing to consider is thoroughness of your extra-oral examinations. Some skin cancers can present around the ear and temple regions, so it is important to consider this and check thoroughly!