The cristae ampullares (CA) in the
semicircular ducts detect movement; the two maculae (one in the utricle and one in the saccule) are similar structures, designed to detect the effect of gravity on the animal. In the image to the right, the macula (Mac) located in the utricle is shown.
The utricle is membranous labyrinth, filled with endolymph. The vestibule (Vest) in which it's suspended is bony labyrinth, filled with perilymph.
Like the cristae and the organ of Corti, the maculae
have "hair cells" in contact with
afferent fibers from neurons of the spinal ganglion. The two maculae are arranged at right angles to each other, so that whether the animal is vertical or horizontal, one of them is always at right angles to the pull of gravity. By changing the angle of the two with respect to the gravitational axis, the brain is informed of the location of DOWN constantly. The importance of knowing where DOWN is can't be overstressed.
In order to detect the direction of gravitational pull, there has to be an independent reference to the field. this is supplied in the case of the maculae by otoliths. These are small granules of calcium salts that form in situ during embryonic development. The word translates from the Greek as "ear stones," an apt description. In this image you can see detail of the macula and the overlying otoliths that are immobilized on it by a glycoprotein coat.
The otoliths are subject to gravitational pull, independently of the animal itself. They have weight. The force exerted by this weight lies on the "hair cells" of the macula. A certain amount of weight (say, when the animal is vertical) is detected by the brain as the "normal" level of deformation of the "hair cells," and the relative input of the two maculae is weighed (ha, ha) and a decisin is made about what the body's position with respect to DOWN may be.
If the position of the individual changes, or if the force of gravity should be suddenly altered with respect to the normal (as for example, in orbit, or, to use a more mundane example, if you fall out of a tree) then the firing rate of the hair cells and their neurons is altered and the brain interprets it correctly. This happens because the animal and the otoliths are both in free fall at the same rate of acceleration; and thus the normal level of force is removed. It's possible to ignore this input if you want to. Skydivers and bungee-jumpers do it, at least temporarily, and astronauts do it for months at a time. The inability to ignore the conflicting inputs of the vestibular system and the visual system is the source of motion sickness in people on board ships or airplanes in violent motion. The drug Dramamine suppresses vestibular input to the brain, allowing the individual to ignore the signals more easily.
As is true of the hearing sense, all of these sensory areas are bathed in endolymph. If there is excess production or inadequate removal of endolymph the pressure builds up and interferes with things. Inner ear infections can cause this to happen, and so can a trauma to the side of the head. The results are hearing loss, ringing in the ears, and dizziness if the vestibular system is involved.
