I’ve Got Rocks In My Head…
…and so do all of you. The image at the top is a colored scanning electron micrograph (SEM) of crystals of calcium carbonate on the surface of an otolith. They are the “balancing stones” of the inner ear and are found in our Acoustic Macula.
See more images of the Acoustic Macula
The acoustic macula is responsible for our static equilibrium (position of the head) and participates in dynamic equilibrium (recognition of the linear accelerations). Located at the level of the inner ear, the macula is composed of hair cells (in orange), constituting the sensorial receptors, and of supporting cells (in pink). Each hair cell possesses between 40 to 70 stereocilia and a single kinocilium.
See SEMs of Inner Ear Hair Cells
The supporting cells secrete a gelatinous substance forming the otolithic membrane, in which embed the stereocilia and kinocilia. This membrane is covered with a layer of those calcium carbonate crystals (shown at top). Each hair cell forms a synapse with a sensitive neuron (in yellow) and a motor neuron (in green) of the vestibular branch of the auditory nerve.
See more images of the Inner Ear
During a sharp acceleration leading the head forward (during the ascension in the roller coaster, for example), the inertia causes a sliding movement backwards of the otolithic membrane and the otoliths, that move the stereocilia and kinocilia with them. This leads to a stimulation of the vestibular nerve, enabling the recognition of the movement.
What would a trip to an amusement park be without your inner ear?!
All images © Science Source