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In order to understand the various factors involved in vertigo, it is beneficial to be familiar with the structures that comprise the vestibular system and their role in our sense of balance and spatial orientation.

The ear consists of three parts, the outer (external) ear, middle ear, and inner ear (labyrinth).

Outer Ear

The parts that make up the outer (external) ear include:

• Pinna - the part of the ear that we see. It consists 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) - a membrane that protects the middle ear from the outside environment. The eardrum is 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, called the tympanic cavity that is carved out of the temporal bone. There are two structures that sit in the middle ear:

• Ossicles - these are small bones that convert the sound waves into mechanical vibrations when stimulated by vibration of the tympanic membrane.
• Eustachian tube - connects the middle ear to the throat and is essential for equalizing the pressure of the middle ear. "Popping the ears" opens the Eustachian tube.

Inner Ear

The organs of the inner ear, also called the labyrinth, form a system of fluid passages and are located in the hollowed-out area of the temporal bone at the base of the skull. The inner ear 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.

• Semicircular canals - Three loop-shaped structures located towards the rear of the inner ear which are oriented at right angles to each other. They are organs of balance that contain fluid and fine hair-like structures that function to maintain balance while turning and moving. They are referred to as the posterior, horizontal and anterior semicircular canals and each responds to different directions of movement or acceleration. Each semicircular canal contains:

  • Crista - a bulging area at the entrance to each semicircular canal which is covered by sensory hair cells. The cells connect to the nerve endings of the 8th cranial nerve, also known as the auditory nerve or the vestibulocochlear nerve.
  • Cupula - a gelatinous mass which sits atop the crista
  • Endolymph - a viscous fluid which is found throughout the vestibular system and which moves with the sway of motion

• Vestibule - the region between the semicircular canals and the cochlea. The semicircular canals converge at one end and the other end connects to the cochlea. The vestibule contains two membranous sacs called otolith organs that are both involved with balance. They include:

  • Utricle - this is the larger of the sacs and it communicates with the semicircular canals. The utricle contains tiny particles or stones made of calcium carbonate, called otoconia which are held together by a gel-like substance. The otoconia sit atop small hairs with sensors that help detect back-and-forth motion. The hairs are connected to nerve endings of the vestibulocochlear nerve. The utricle senses movement in the horizontal plane (i.e., forward-backward, left-right, and any combination thereof).