Equilibrium is the maintenance of balance. This is achieved mostly within the inner ear, though also within muscles, joints, and the eyes.

external image NSRW_Ear2.png
Clockwise from top: the respective inner ears of a reptile, ox, and bird commons.wikimedia.org

Deep within the ear are canals which contain fluid and hair cells. The hairs sense the movement of the fluid, and send the information to the brain, which can then interpret the way in which a person is moving. Impulses sent from the eyes also provide the brain visual cues in maintaining balance. Joints and muscles are also important, as sensory nerve fibers detect how muscles are moving and allow the brain further reinforcement on bodily positions and movements. The brain gathers all of this information and is then able to make appropriate decisions as to how a person can keep from falling.

1. Outer Ear external image 751px-Anatomy_of_the_Human_Ear_1_Intl.svg.png
2. Pinna
3. Ear Canal
4. Eardrum
5. Middle Ear
6. Ossicles
7. Malleus
8. Incus
9. Stapes
10. Tympanic Cavity
11. Temporal Bone
12. Eustachian tube
13. Inner Ear
14. Labyrinth
15. Semicircular Canal
16. Vestibule
17. Oval Window
18. Round Window
19. Cochlea
20. Vestibular Nerve
21. Cochlear Nerve
22. Internal Auditory Meatus
23. Vestibulocochlear Nerve

Vestibular Apparatus
This is the non-auditory part of the ear. It is involved in static equilibrium, which is the response to linear movements such as walking. It does three main things in maintaining equilibrium:

1. helps body sense motion and spatial orientation of the head
2. helps to adjust what muscles are doing to maintain a posture
3. helps to keep a steady fixation point for the eyes, so that retina has a stable image

The vestibular apparatus consists of one fluid filled tube, inside another fluid filled tube. Inside the apparatus are the saccule and utricle, two sac-like structures. On the walls of each of these sacs is a sensory organs called a macula. Each macula contains receptor cells, which are often called hair cells. Microvilli, cilia and stereocilia extend from the hair cells into a glycoprotein based gel, which forms the otholithic membrane. This membrane is full of Calcium Carbonate crystals (called otoliths), which cause the membrane to be quite dense and responsive to any changes in motion. When the body moves a different way, it sends the otholithic membrane moving either forward and backward in the utricle or up and down in the saccule. This movement of the membrane also causes the stereocilia to move, which then initiate graded potentials.

external image Otolith_organ_of_vestibular_system.jpg

Semicircular Canals
The canals keep dynamic equilibrium in check. Dynamic equilibrium is the response to rotational movements such as turning.
There are three canals, called the anterior, posterior, and lateral canals respectively. They lay at right angles to each other. At the base of each one is a bulbous expanse called an ampulla. Each ampulla contains sensory receptors called crista. The crista function rather like the maculae, as each holds many hair cells that extend into a gel. In this case, the gel is the cupula rather than the otholithic membrane, however the two are very similar. Whenever a body changes movement rotationally speaking, the cupula and its stereocilia move, initiating the same type
of graded potential.


external image Vestibular_system%27s_semicircular_canal-_a_cross-section.jpg
Graded Potential
The graded potentials stimulated within hair cells cause a change in how much neurotransmitter is secreted. This in turn causes action potentials to be created in the vestibular nerve. The impulse travel from there to the vestibulocochlear nerve, and then on the the pons and cerebellum.

external image 772px-Vestibular_neural_connections.jpg

Rough translation: Details of how to connect, contact, vestibular nerve, with large cells of the tangential nuclei of the
medulla of birds --- A, D, F, etc. are plates, pedicles, and vestibular terminals of that focus: a, b, c, etc. are axons of neurons.

One substance that effects the homeostasis of the sense of equilibrium is alcohol. When alcohol is consumed, one of its effects is impairment of balance. Alcohol dilutes the fluid in the inner ear which in turn inhibits the ability to be conscious of position and to have normal balance.

in this video is shown how ones balance is controlled with the inner ear and eyes. when they close their eyes, their
balance is dominantly controlled by the inner ear. When alcohol is added, this function is further impaired