Calcite crystals in the Cochlea’s Magic Ear Snail

Ears hear like microphones using calcite crystals in the cochlea in the inner ear. Then, the acoustic pressure is transformed to information your brain can read out! Truly we know little about the exact structure of the cochlea, as it is extremely hard to isolate. In vivo the pressure is calibrated very precisely. There is thick fluid, suspended crystals, and tiny hairs – with tons of sensitive substructures. Let’s journey into the inner ear and find calcite crystals and golden spirals.

Calcite Crystals in the Cochlea

There are calcite crystals in several of the ear’s medial and inner substructures. The calcite crystals suspended in the mid-ear’s fluid-filled vesicle are responsible for BPPV, a fairly common type of vertigo. These mid ear crystals communicate a sense of balance. Meanwhile, in the inner ear, we have a semi-hard shell with tiny hairs connected to calcite crystals. These are the calcite crystals in the cochlea, and they perform selective absorption of frequencies. The hairs pick up the sound and the calcite crystals in the cochlea wall help transform that into a signal the brain can read, as they connect to the 8th cranial nerve. Read on and we’ll find out how the crystals got there, how they’re arranged, and what their “purpose” is!

Note that there is so much anatomical jargon that it’s no wonder this hasn’t been discussed much outside of scientific papers before. So, if you are an ear anatomy expert, be aware that I studied the jargon and have simplified it in order to write this article where the reader does not need to know every single substructure name, while still getting an accurate impression.

Calcification of the Mid and Inner Ear – natural or nah?

In 1982, Johnson studied deposition of calcium salts in the cochlea. This phenomenon of dystrophic calcification also occurs in the pineal gland, urinary tract, and a number of other places. Often researchers look to the particular shape and placement of the calcium deposits to find out if they are endogenous (naturally forming in the body), or exogenous (picked up from some dietary choice or environment). For example, magnetite crystals in the brain are partially endogenous, while also being likely partially picked up from pollution. Read about magnetite crystals in the brain.

Particularly Johnson looked at mineral deposits that were roughly spherical in shape, outside of the cochlea in the ventricle preceding (called the otoconia, Greek roots being “ear dust”). Made of calcium phosphates and apatite, the question was if these concretizations were pathological or of biological origin. Apatite is similar to the material of bone and teeth. Apatite is also what forms many mollusks and other crystal-laden structures in the body like the pineal cone. Calcification happens in the human ear crystals similarly to how it does in the pineal gland. Read about calcite in the pineal gland’s bio-crystals.

Calcite specifically, is the trigonal polymorph of CaCO₃ (calcium carbonate). Vaterite is a hexagonal polymorph of calcium carbonate, and amongst apatite associates with degradation of hearing. We see calcium salt deposition in dead and degenerating tissues in many parts of the human body. Generally over time the otoconia reduce in mass and become skeleton versions of the turgid crystals they were in childhood.

Snail Shape of the Cochlea Spiral

Skipping forward to 2022, Bruss gives us a description of inner ear anatomy that takes us into the cochlea to find the crystals we seek. The mid ear is an air filled space with different shapes and sizes of caverns. The bones in the mid ear (ossicles) transmit vibration from the outer to inner ear. The inner ear space is the labyrinth. It’s called a labyrinth because it is like a maze, a tuba, or a French horn. A French horn takes the sound through a whirl to amplify/focus it, and the labyrinth’s shape does a similar function on a smaller more fine-tuned scale.

Now like a snail shaped onion, there are layers. Outermost is a bony, shell like layer, and innermost is membraneous, like the inside of your cheek. These zones are separated only by “perilymph”. Perilymph is high in sodium, and lower in potassium and calcium.

The membranous portion of the labyrinth contains “endolymph.” The endolymph is richer in potassium and low in calcium and sodium. Since the contrasting ion concentrations are on either side of a membrane, a concentration gradient is formed there. The concentration gradient gives potential energy to the membrane to pump ions across. This concentration gradient communicates the excitation of hair cells responsible for sound and vestibular transmission.

How the hairs work with the crystals to transform the sound for the brain is discussed below in the “Piezoelectric Transducers” section.

Now in Bosia’s paper also from 2022 we gather some details on the form of the cochlear spiral. As the sound travels through the “the “basilar membrane,” which separates the bony from soft tissue portions described above, the sound is focused like light through a lens. The calcite crystal is in the matrix of the wall of the cochlea.

The membrane is graded in stiffness and mass, meaning it is not homogenous, but physical changes in the cavern transform the sound (nonlinear effects). The magnitude increases and the wavelength decreases until it is able to be coded spatially, where the frequency components are sorted out. This is called “tonotopic” organization. Tones = the different frequency components. Topic = sorting them out.

Aside: “Frequency Components”

Imagine there is a drum beat with a certain frequency (pitch). Then one single guitar comes in with a different pitch. These sounds are played at the same time (music) and picked up by your ears. Consciously, you can pick apart the different sounds because of the tonotopic organization in your cochlear nerve. This is only recently being mapped. But besides the tonotopic organization, we can also pick out different voices in a room, of the same pitch even. In addition to frequency sorting, we also experience a higher level coherence sorting based on learned context. If this interests you you can read about time-ratios in waveforms here.

Often the research that goes into investigating how the frequency grading works to pass extremely dense information to the brain is funded for hearing implants and engineering biomimetic tech. Researchers that want to create topotonic organization (called rainbow sensors, or rainbow trappers) look not only to the stiffness gradient, but also the influence the geometric spiral has on the frequency transformation. They engineer these with “the aim is to separate different frequency components into different physical locations along the sensor.” We do know that as the sound waves travel through the cochlea, they collide elastically with its walls, meaning they bounce against the walls rather than be absorbed into the walls. Click here for a good picture of the frequency components along the cochlear spiral.

Piezoelectric Transducers in the Ear

Treuting 2018 tells us how the the lymph fluids contribute to the transduction. The researchers not that in humans the perilymph makes 2.75 turns across about 30-35 mm. The actual receptors for hearing are the hair cells, resting on the basilar membrane. The membrane separates the cochlear duct containing endolymph from the other cavern containing perilymph, both traveling along the spiral of the cochlea. “[N]erve endings at each hair cell’s base transmit impulses to the bipolar neurons of the spiral ganglion. These impulses are subsequently transmitted to the auditory cortex via the auditory branch of the eighth cranial nerve.” The role of the calcite crystal would be forming the cavity in which this takes place, processing collisions, or echoes, off of its surface.

They call them acoustic rainbow trappers, and they selectively filter different frequencies, as a result of the spiral configuration. Traps the acoustic wave pressure Click here to read about electrophonic hearing.

Acoustic Rainbow Trappers for Fake Hearing

Rainbow trappers are acoustic waveguides. Since the structures in the ear are so fragile, and the main funding opportunities are to create artificial ears, researchers try different forms to mimic the human ear. For example, researchers have made spiral rainbow trappers in the shape of Archimedes spiral to see the efficiency. “[T]he aim is to separate different frequency components into different physical locations along the sensor […] based on a set of Helmholtz resonators arranged at subwavelength intervals along a cochlear-inspired spiral tube […] to filter mechanical waves spectrally and spatially to reduce noise and interference in receivers.”

These experiments are based off 3D computer models grading a helicoidal membrane, leading to a “local resonant system with negative dynamic effective mass and stiffness.”

Hearing is magic? – The incomplete Story

The calcite crystals in the cochlea are considered mostly endogenous, forming during the same time as the rest of the ear structures. The calcite crystals make up the walls of the bony structures in the cochlea. This effect can be somewhat replicated by Helmholtz resonators in metematerial-like rainbow trappers. The dependence of the hearing functions on the dielectric properties of calcite crystals are really only just beginning to hit discussions. However, those who are familiar with music theory may be thinking of all kinds of patterns dealing with the logarithmic nature of musical scales. Could the logarithmic spiral have anything to do with the frequency coding functions? Later we will discuss the golden ratio’s role in tonotopic organization systems.

Sources:

Bosia, Federico, et al. “Optimized structures for vibration attenuation and sound control in nature: A review.” Matter 5.10 (2022): 3311-3340.

Zhao, Liuxian, and Shengxi Zhou. “Compact acoustic rainbow trapping in a bioinspired spiral array of graded locally resonant metamaterials.” Sensors 19.4 (2019): 788.

Treuting, P. M., Dintzis, S. M., & Sellers, R. (2018). Special Senses. Comparative Anatomy and Histology, 471–485. doi:10.1016/b978-0-12-802900-8.00022-1 

Johnsson, L.-G., Rouse, R. C., Wright, C. G., Henry, P. J., & Hawkins, J. E. (1982). Pathology of neuroepithelial suprastructures of the human inner ear. American Journal of Otolaryngology, 3(2), 77–90. doi:10.1016/s0196-0709(82)80037-9

Önerci, T. M. (2009). Ear Anatomy. Diagnosis in Otorhinolaryngology, 2–7. doi:10.1007/978-3-642-00499-5_1 

Bruss DM, Shohet JA. Neuroanatomy, Ear. In: StatPearls. StatPearls Publishing, Treasure Island (FL); 2022. PMID: 31869122.

Consulted

Deans, Michael R. “Conserved and divergent principles of planar polarity revealed by hair cell development and function.” Frontiers in Neuroscience 15 (2021): 742391.

COURT, N. (1992). Cochlea anatomy of Numidotherium koholense: auditory acuity in the oldest known proboscidean. Lethaia, 25(2), 211–215. doi:10.1111/j.1502-3931.1992.tb01385.x