How Do We Hear?
Hearing is the sensing of sound vibrations in the air. When a sound occurs (people speaking, doorbell, birds chirping, music) it produces a vibration that travels through the air displacing it. The displaced air is called a sound wave. Sound waves are collected by the outer ear and delivered to the ear drum, which vibrates at different rates depending on the pitch of the sound. This vibration in turn sets the three tiny bones (the middle ear “ossicles”) into motion, transmitting the ear drum vibrations down to the inner ear (cochlea). The cochlea is a hollow bone, filled with fluid and tissue holding thousands of tiny “hair cells”. As vibrations enter the cochlea, the hair cells lining the inside of the cochlea move in response. When the hair cells “bend”, they send neural impulses (or electrical signals) to the hearing (auditory) nerve. The auditory nerve from both ears will travel up to the brainstem, and then finally tothe brain (in the auditory cortex).
We Hear with Our Brain!
Many people think we only need our ears to hear, but it is important to remember “we hear with our brains.” Our outer, middle, and inner ear provide a way for sound to be transmitted to the brain. The brain itself (from the auditory nerves, to the brainstem, and then up to the auditory cortex) does a lot of work when it comes to “processing” sounds, tohelp us understand what we have heard.
Parts of the Ear
Outer Ear – the external ear is the visible portion of the ear, which serves as a protective organ for the eardrum. It collects and guides sound waves into the middle ear. The outer ear consists of the following parts:
- Ear Flap (Pinna) – Sound waves enter the ear through this mechanism.
- Ear Canal (Meatus) – About 2cm in length, the ear canal amplifies sound waves and processes them through the middle ear. Sweat glands exist in the ear canal. They secrete earwax.
The middle ear is located between the outer and inner ear. It perceives sound waves from the outer ear in the form of pressure waves. The middle ear is filled with air. It has the following parts:
- Eardrum (Tympanic membrane) – A thin membrane that acts as a partition between the inner and outer ear, the eardrum vibrates as soon as it receives sound waves. It then transforms the sound energy into mechanical energy.
- Hammer (Malleus) – This tiny bone is located next to the eardrum. It vibrates when the eardrum vibrates.
- Anvil (Incus) – The anvil is a tiny bone next to the hammer. It vibrates in response to the hammer.
- Stirrup (Stapes) – Similar to the anvil and the hammer, this is a tiny bone in the middle ear. It eventually vibrates and passes the compressional waves to the inner ear.
Inner Ear (Labyrinth):
The inner ear is the innermost part of the ear. It is filled with a water-like substance and comprises both hearing and balancing organs. The inner ear is composed of the following:
- Cochlea – The cochlea, or spiral tube, is a rolled structure that stretches to about 3 cm. The membrane lining of the cochlea consists of numerous nerve cells. These hair-like nerve cells respond differently to various frequencies of vibrations, leading to the generation of electrical impulses.
- Semicircular Canals – These fluid-filled loops are attached to the cochlea and help maintain balance in human beings.
- Auditory Nerve – This part of the organ takes the electrical impulses from the cochlea, which are generated by the nerve cells, to the brain.
Damage to any parts of the ear can cause hearing loss.
There are THREE major types of hearing loss:
More information on them can be found on our Types of Hearing Loss page.
Balance is directly related to hearing. The semicircular canals, which are attached to the cochlea, in the inner ear help maintain balance. If they function incorrectly, your balance could be problematic.
Vertigo is a symptom of illusory movement. Almost everyone has experienced vertigo as the transient spinning dizziness immediately after turning around rapidly several times. Vertigo can also be a sense of swaying or tilting. Some perceive self-motion whereas others perceive motion of the environment.
Vertigo is a symptom, not a diagnosis. It arises because of asymmetry in the vestibular system due to damage to or dysfunction of the labyrinth, vestibular nerve, or central vestibular structures in the brainstem.
Vertigo is a troubling problem for many clinicians because it is symptomatic of a large range of diagnoses from benign to immediately life threatening. However, in most cases, the patient’s clinical history especially the tempo of the symptoms with examination findings distinguish between central and peripheral etiologies identify those patients that require urgent diagnostic evaluation.
Vertigo is only one type of dizziness. Other disorders that present with dizziness include presyncopal faintness, disequilibrium, and nonspecific or ill-defined light-headedness. The initial approach to the patient who complains of dizziness is to localize the cause of the symptom into one of these broad categories.
Dizziness is diagnosed mainly on history and a comprehensive physical but it is also diagnosed with the use of equipment that specifically tests the inner ear and neural pathways through which sound is processed in the brain. This testing is conducted by trained audiologists here at the AEC. It is important to avoid medications and substances that either stimulate the central nervous system (like caffeine) or inhibit the central nervous system (like Benadryl.) Other important procedures used to help in the diagnosis of vertigo include MRI, CT and MRA scans.