Sense of Balance

The labyrinth of the inner ear consists, in addition to the cochlea, of two small sacs - round (sacculus) and oval (utriculus) - and three semicircular canals. These structures are filled with endolymph and surrounded on all sides by perilymph. Their destruction causes a significant loss of balance; a pigeon in which these organs have been removed is unable to fly.

Over time, however, he can relearn how to maintain balance using visual stimuli. In humans, in addition to these organs of the inner ear, balance depends on vision, proprioceptive stimulation and stimulation coming from cells located in the soles of the feet and sensitive to pressure. In some types of deafness, the balance organs in the inner ear, as well as the cochlea, do not function, but the sense of balance remains intact.

Sacculus and utriculus are small hollow sacs lined with sensitive cells with hairs and containing small ear pebbles, otoliths, consisting of calcium carbonate. Under normal conditions, gravity causes the otoliths to exert pressure on certain hair cells, which then send impulses to the brain along sensory nerve fibers coming from the bases of these cells. When the head is tilted, the otoliths put pressure on other cells and irritate them.

Many invertebrates, such as crayfish and lobsters, also have similar organs. The action of these cells in crayfish was demonstrated in an ingenious experiment; it was based on the fact that during molting, when the cancer sheds its old cover and grows a new, more spacious shell, it also develops new balance organs and the cancer introduces grains of sand into them, which it takes from the environment. By supplying molting crayfish with iron filings, experimenters forced them to respond to a magnet.

When a magnet was placed directly above the animal and it attracted iron filings, causing them to press on the upper cells of the organ of balance, the crayfish mistook “up” for “down”, turned over and floated belly up. In the ear labyrinths there are three semicircular canals, each of which is a tube curved in a semicircle, connected at both ends to an oval sac. The channels are arranged in such a way that each of them lies in a plane perpendicular to the planes of the other two channels.

When one of the ends of each canal flows into the oval sac, there is a small bulb-shaped extension (ampulla), containing a group of hair cells that are similar to the same cells of the oval and round sacs, but lack otoliths. These cells are excited by the movements of the fluid (endolymph) filling the channels. When the head turns, the movement of fluid in the channels lags behind this movement, so that in fact the hair cells move relative to the fluid and receive stimuli from its flow.

This stimulation causes not only a sensation of rotation, but also reflexive movements of the eyes and head in the direction opposite to the initial rotation. Because the three semicircular canals are located in three different planes, moving the head in any direction will cause fluid to move in at least one of these canals. By introducing warm or cold water into the external auditory canal, convection currents can be caused in the canal fluid without any movement of the head. There is a feeling of spinning and dizziness.

A person is accustomed to movements in the horizontal plane, irritating the semicircular canals in a certain way, but vertical movements parallel to the long axis of the body are unusual for him. Such movements (such as going up or down an elevator or sea motion) irritate the semicircular canals in an unusual way and can cause nausea and vomiting, as occurs with seasickness. If a person lies down, the movements will irritate the semicircular canals differently and the nausea will be less.