Special sense of Ear

The ear is the organ of hearing and balance. The parts of the ear include:

Ear

External or outer ear, consisting of:

• Pinna or auricle. This is the outside part of the ear.

• External auditory canal or tube. This is the tube that connects the outer ear to the inside or middle ear.

  

  Ear

External auditory canal- also called external auditory meatus, or external acoustic meatus, passageway that leads from the outside of the head to the tympanic membrane, or eardrum membrane, of each ear. ... The canal is nearly 1 inch (2.5 cm) in length and is lined with skin that extends to cover the tympanic membrane.

Pinna or auricle: The medical term for the outer ear is the auricle or pinna. The outer ear is made up of cartilage and skin. There are three different parts to the outer ear; the tragus, helix and the lobule. The ear canal starts at the outer ear and ends at the ear drum.

Middle ear (tympanic cavity), consisting of:

• Tympanic membrane (eardrum). The tympanic membrane divides the external ear from the middle ear.
• Ossicles. Three small bones that are connected and transmit the sound waves to the inner ear. The bones are called:

1. Malleus
2. Incus
3. Stapes

Tympanic membrane- also called eardrum, thin layer of tissue in the human ear that receives sound vibrations from the outer air and transmits them to the auditory ossicles, which are tiny bones in the tympanic (middle-ear) cavity.

Ossicles: The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss.

Inner ear, consisting of:

• Cochlea- This contains the nerves for hearing.
• Vestibule- This contains receptors for balance.
• Semicircular canals- This contains receptors for balance.
• Eustachian tube- A canal that links the middle ear with the back of the nose. The eustachian tube helps to equalize the pressure in the middle ear. Equalized pressure is needed for the proper transfer of sound waves. The eustachian tube is lined with mucous, just like the inside of the nose and throat.
• Vestibulocochlear nerve-
  

Eustachian tube, The Eustachian tube is a canal that connects the middle ear to the nasopharynx, which consists of the upper throat and the back of the nasal cavity. It controls the pressure within the middle ear, making it equal with the air pressure outside the body. Blocked Eustachian tubes can cause pain, hearing difficulties, and a feeling of fullness in the ears. Such a phenomenon is referred to as Eustachian tube dysfunction (ETD).

Vestibule- The vestibule is the central part of the bony labyrinth in the inner ear, and is situated medial to the eardrum (tympanic cavity), behind the cochlea, and in front of the three semicircular canals. The name comes from the Latin vestibulum, literally an entrance hall.

Cochlea:

• The cochlea is a portion of the inner ear that looks like a snail shell (cochlea is Greek for snail.) 
• The cochlea is made up of three canals wrapped around a bony axis, the modiolus. These canals are: the scala tympani , the scala vestibuli  and the scala media (or cochlear duct) .
• The cochlea is filled with fluid. Specifically, the scala vestibuli and scala tympani contain a fluid called perilymph, which is similar in composition to cerebrospinal fluid, and the scala media contains endolymph, which more resembles intracellular fluid in terms of its ionic concentrations.
• The cochlea contains the spiral organ of Corti, which is the receptor organ for hearing. It consists of tiny hair cells that translate the fluid vibration of sounds from its surrounding ducts into electrical impulses that are carried to the brain by sensory nerves.
• The auditory nerve is a bundle of nerve fibres that carry information between the cochlea in the inner ear and the brain. The function of the auditory nerve is to transmit signals from the internal ear to the brain.

Semicircular canals- Semicircular canals are three tiny, fluid-filled tubes in your inner ear that help you keep your balance. When your head moves around, the liquid inside the semicircular canals sloshes around and moves the tiny hairs that line each canal.

Cochlear Damage- It is the main organ of hearing and is part of your inner ear. Cochlear Damage means that all or part of your inner ear has been hurt. Damage to the cochlea typically causes permanent hearing loss. This is called sensori neural hearing loss (SNHL).

Mechanism of hearing:

Hearing starts with the outer ear. When a sound is made outside the outer ear, the sound waves, or vibrations, travel down the external auditory canal and strike the eardrum (tympanic membrane). The eardrum vibrates. The vibrations are then passed to 3 tiny bones in the middle ear called the ossicles. The ossicles amplify the sound. They send the sound waves to the inner ear and into the fluid-filled hearing organ (cochlea).

Once the sound waves reach the inner ear, they are converted into electrical impulses. The auditory nerve sends these impulses to the brain. The brain then translates these electrical impulses as sound.

• External ear "collects" the sound
• Ossicles vibrate in the air-filled environment, magnifying the sound and aiming it to the oval window
• The cochlea converts a mechanical stimulus into an electrical stimulus into an electrical one
• The message is transported to the brain by the VIII-th nerve

Any vibrating object causes waves of compression and rarefaction and is capable of producing sound.

Sound travels faster in liquids and solids than in air (roughly 344 m per second)

When sound energy has to pass from air to liquid, most of it is reflected because of impedance by the liquid

Mechanism of hearing can be broadly classified into:

1. Mechanical conduction of sound
2. Transduction of mechanical energy into electrical impulses.
3. Conduction of electrical impulses to brain.