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The inner ear, which plays an important role in the ear, has a complex structure as shown in the figure below. The inner ear is composed of two parts, the auditory organ "cochlea" and the equilibrium organ "vestibule".
The cochlea is a spiral-shaped organ that is filled with lymph. Inside the cochlea, there is a site called the organ of Corti, where many hair cells with small protrusions (sensory hair) like downy hair exist and extend into the lymph.
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Sound signals from the ear canal are transmitted from the eardrum through the three ossicles, the malleus, incus, and stapes, to the vestibular window of the inner ear, into the vestibule and cochlea, and transmitted to this lymph and hair cells.
Hair cells (sensory hairs) in the cochlea respond to vibrations of specific frequencies at each location and generate nerve impulses. The impulse signal is then transmitted to the brain via the cochlear nerve and recognized as sound.
On the other hand, in the vestibule, which is the organ responsible for the sense of balance, there is a "semicircular canal" that senses the rotation of the head and plays an important role in maintaining the balance of the body.
The semicircular canals are composed of three semicircular canals, "first half canal", "second half canal" and "outer semicircular canal", and have a structure that intersects at right angles to each other.
Although not shown in the figure above, the vestibule also has an "ear stone tool" consisting of parts called "utricles" and "saccules".
Ear stone tools have a structure in which many short hairs are densely grown and minute stones (otoliths) are placed on the tips. An otolith is a minute stone made of calcium carbonate, and when this minute otolith shakes, it senses gravity and linear acceleration.
When you move your head, the lymph fluid in the three semicircular canals also moves, along with which hair cells generate nerve impulses that signal to the brain.
The brain can recognize the rotation of the head because the movement of lymph in one semicircular canal corresponding to the direction of movement of the head is greater than the movement in the other two semicircular canals.
In this way, the functions of the vestibule and the semicircular canals allow the brain to recognize the rotation of the head, the linear motion of the body, and the acceleration, and to perform movements that maintain the balance of the body.
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