MedifocusHealth

The human ear is a complex, sensory organ that is involved with two functions: hearing and maintaining balance. The ear consists of three parts:

• Outer (external) ear
• Middle ear
• Inner ear

Outer Ear

• Pinna - the part of the ear that we see. It is made of cartilage and enables us to localize the source of sound.
• Ear canal - the tube that directs sound waves towards the tympanic membrane
• Tympanic membrane (eardrum) - protects the middle ear from the outside environment and is also the first organ to respond to sound waves that are conducted through the ear canal.

Middle Ear

The middle ear is an air filled cavity known as the tympanic cavity that is carved out of the temporal bone at the base of the skull. There are two parts of the middle ear:

• Ossicles - convert the sound waves into mechanical vibrations when stimulated by vibration of the tympanic membrane. The ossicles consist of three small bones:

  • malleus (hammer) - is connected to the tympanic membrane, enabling the vibration of the tympanic membrane to set the ossicles into motion
  • incus (anvil) - is the middle bone and connects the malleus to the stapes
  • stapes (stirrup) - the base of the stapes fills the oval window, a membrane that leads to the inner ear

• Eustachian tube - connects the middle ear to the back of the nose and is essential for equalizing the pressure of the middle ear. ("Popping your ears" opens the Eustachian tube.)

Inner Ear

The inner ear contains the labyrinth which is a system of fluid filled passages that includes the organs for hearing and balance. It is located in the hollowed out area of the temporal bone (bony labyrinth) at the base of the skull. The labyrinth (the most relevant area to Meniere's disease) consists of three parts:

• Cochlea - This organ of hearing is coiled in the shape of a snail and is located towards the front of the inner ear. It contains the Organ of Corti which consists of hair cells that transmit nerve signals to the brain.
• Semicircular canals - Three loops located towards the rear of the inner ear that are organs of balance and movement

• Vestibule - This connects the cochlea and the semicircular canals and contains two membranous sacs involved with balance:

  • utricle - the larger of the sacs and arises from the semicircular canals.
  • saccule - the smaller of the sacs and is connected to the cochlear duct and to the utricle.

Within the cochlea, canals, and vestibule, there are two complex systems of sacs lined by membranes, ducts, tubes and nerves. One system contains fluid found throughout the inner ear called perilymph. Inside the system containing perilymph is another complex system of ducts. The fluid in this system is called endolymph and is chemically different than perilymph. The membrane that separates the endolymph-filled cochlear duct from the surrounding perilymph is called Reissner's membrane.

The organs of the inner ear are innervated by the vestibulocochlear nerve, also called the eighth cranial nerve. There are two branches of the nerve: the vestibular branch which carries signals from the vestibular organs (semicircular canals and vestibule) to the brain and the cochlear branch which carries signals from the auditory organ (cochlea) to the brain.