EAR - Anatomy, hearing & balance

A brief introduction to the ear's anatomy

The ear, like the eye, is a complex organ of sense and is responsible for hearing and balance. It is divided into three main parts: the outer, middle and inner ear.

Outer ear

The outer ear is made up of the auricle (or pinna), the part of the ear that is visible to the world, and the ear canal, which leads to the tympanic membrane (ear drum). The auricle acts as a receptacle, catching sound and transmitting it to the tympanic membran via the ear canal, causing it to vibrate. The ear canal is also responsible for cerumen (wax) production. Wax is important for the ear canal's health.

Problems that may occur in the outer ear include the following (click on one to read more):

 - Wax build-up or impaction

 - Otitis externa (infection of the ear canal - also called swimmer's ear and tropical ear)

 - Eczematous otitis externa (eczema of the ear canal)

 - Exostoses (bony growths in the ear canal due to cold water exposure - also called surfer's ear)

 - Keratitis obturans (excessive dead skin production in the ear canal)

Middle ear

The middle ear contains the ossicles, or hearing bones, the malleus, incus and stapes (also known as the hammer, anvil and stirrup respectively due to their shapes). Together they make up the ossicular chain. The malleus (hammer) is the outermost bone and is attached to the tympanic membrane, the incus (anvil) is the middle bone and the stapes (stirrup) is the innermost bone and is attached to the cochlea. 

 

The middle ear is separated from the outer ear by the tympanic membrane (ear drum) and connects to the nose via the Eustachian tube in front (you blow ear up this tube when you pop your ears). It is also connected to the mastoid bone behind, which is thought to act like a small lung to the middle ear and which you can feel behind your ear. The ossicles transmit sound waves from the vibrating tympanic membrane to the cochlea of the inner ear.

The middle ear is naturally filled with air and a functional Eustachian tube is important in maintaining this.

Problems that may occur in the middle ear include the following (click on one to read more):

 - Acute otitis media (middle ear infection)

 - Glue ear (fluid in the middle ear)

 - Mastoiditis (mastoid bone infection)

 - Tympanic membrane perforation (hole in the ear drum)

 - Tympanic membrane retraction pocket (retracted ear drum)

 - Cholesteatoma (skin cyst or trapped skin in the middle ear)

 - Otosclerosis (stiffened stapes)

 - Ossicular disruption (due to infection, erosion or trauma)

 - Eustachian tube dysfunction

 - Patulous Eustachian tube

Inner ear

The inner ear is made up of the snail shell-shaped cochlea (the hearing organ) and the vestibular system (balance organ), which itself is made up in part of the three semi-circular canals. The cochlea is responsible for converting sound waves it receives from the stapes of the ossicular chain into information that the brain can process. The vestibular system works in conjunction with the eyes, brain and musculoskeletal system to maintain a sense of balance.

The inner ear is naturally filled with fluid.

Problems that may occur in the inner ear include the following (click on one to read more):

 - Presbyacusis (age-related hearing loss)

 - Sudden sensorineural hearing loss (usually occurs in one ear)

 - Tinnitus (ringing or pulsating in the ear)

 - Labyrinthitis (viral infection of the inner ear)

 - Méniere's disease (excess fluid in the inner ear)

 - Benign paroxysmal positional vertigo (also called BPPV; caused by otolith (ear crystal) dislodgment)

 - Bell's palsy (facial weakness due to viral infection of the facial nerve as it passes through the ear)

 - Vestibular migraine (migraine affecting the inner ear and balance)

Hearing simplified
 

ENT surgeons generally divide the process of hearing into two types, conductive hearing and sensorineural hearing, depending on which parts of the ear are involved. Both types and necessary for you to be able to hear properly. 

Conductive hearing

This is the hearing that involves the outer and the middle ear and is so-called because these two parts work together to conduct sound waves to the inner ear where they can be processed. Conductive hearing works as follows:

Step 1: the auricle captures sound and directs it towards the ear canal.

Step 2: the sound travels down the ear canal to the end where it hits the tympanic membrane (ear drum), causing it to vibrate

Step 3: the vibrating tympanic membrane in turn causes the ossicular chain to vibrate, it being attached to the malleus (hammer)

Step 4: the vibrations travel from the malleus to the incus and then on to the stapes, which then transmits them into the cochlea

A normal auricle, an unblocked ear canal, a healthy ear drum and an air-filled middle ear are important for normal conductive hearing.

Sensorineural hearing

Once sound has been conducted from the outside world via the conductive hearing system, it is processed by the cochlea into electrical impulses that then travel up the auditory nerve and onward to the hearing centres in the temporal lobes of the brain. This part of the hearing process is called sensorineural hearing as it involves neurons (nerves) and the central nervous system. 

Balance simplified
 
 

Balance can be a complicated concept, even for ENT surgeons! What is important to remember is that balance is an orchestration of several senses working together to maintain stability. These are as follows:

1. The vestibular system of the inner ear - this provides the brain with information about the position of the head and is linked to eye movements via the vestibulo-ocular reflex

2. The eyes - this provides visual feedback and helps to focus on or track objects

3. The musculoskeletal system - this includes the spine, major joints (especially those of the lower limbs) and associated muscles; they work to provide the brain with postural and tactile information relating to the body's relation to its environment

4. The brain - this co-ordinates and acts upon all of the information provided by the other organs, in particular the cerebellum at the base of the brain 

When one or more of these organs malfunctions, a sense of imbalance may ensue. Vertigo, the sensation of spinning, is usually a sign that something is wrong with the vestibular system.