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How a woman feels the effect of gravity.

Did you know that astronauts struggle to walk around corners after they’ve returned from a long space mission? Their perception of movement has changed, and this is partly because of how their inner ear perceives gravity.

In this section we will talk about the two structures in the ear, in charge of perceiving gravity: the Utricle and the Saccule. I will mention where they are and how they work.

Keep in mind that there are many structures in our body that help us perceive gravity, this is one of the more important ones.

Firstly, let’s talk about:

What is Gravity?

Gravity is the the pull the Earth has on matter that is around its mass. In this case we are subjected to Earth’s gravity. We know we are standing right side up because we can feel the soles of our feet are on the ground and our inner ears are telling our brains that gravity is pulling us from below our heads.

What are the structures in the ear that help us perceive gravity?

There are two structures inside of the inner ear that are in charge of perceiving gravity, or in more precise terms: linear movement. We do not sense gravity directly the same way we perceive our other senses. [1]

These structures, called the Utricle and the Saccule, are located in our inner ear. They perceive the linear movement which we interpret as gravity. Yet in outer space, where there is there is no gravity [scientifically called: microgravity, I will extend on this later] our ears will perceive the linear movement in the direction we push ourselves.

So on Earth we perceive the linear movement of the Earth as it pulls us towards its core. And in space it is dependant on the direction we move.

What is Linear Movement?

Linear, as in line. Moving, in a line. In this case it is about the movement over the vertical and the horizontal planes, or a combination of both.

The utricle will perceive movement on the horizontal plane. For simplicity let’s say it’s forward and backwards movement. But in reality it will be in all the directions of the horizontal plane.

On the other hand, vertical linear movement, such as up and down on a lift, will be perceived by the saccule.

If we are moving upwards and forwards, such as on a stair lift, our utricle will be perceiving the gradual forward motion, while the saccule will perceive the uppward motion. Our brains will interpret this array of messages as the familiar feeling of movement that we encounter when we hop onto a stair lift, and begin to go up. This also helps us keep our balance. [2]

Linear motion is in straight lines, while rotational movement is when we move our head in a circular motion, such as when we shake our heads, or spin around on one foot. I will talk about rotational movement in another article.

A plane flying level in a straight line is a simplistic example of linear motion forward. When the plane begins to descend, your saccule helps your brain understand that you are now loosing altitude.

How does the Utricle and the Saccule perceive linear movement?

When we move forwards, backwards, up or down, we are talking about linear movements. These are the same movements that cause small hair-like filaments located in the Utricle and the Saccule to move back and forth with the movement of our heads. Lets call these hair-like filaments: hair cells.

In the Utricle, the hair cells are standing straight up when we are not moving. When we move forwards, these filaments tilt backwards and send the message of movement to our brain. If we are not moving, we will not sense movement because the hairs on the hair cells will not be moving in any direction. [3]

Hair Cell Analogy

A hair cell analogy would be a group of people standing on a bus. With their feet firmly glued to the floor. When the bus moves forward all of the people will lean or fall in the opposite direction.

When our head move forward as we walk, the hair cells tilt backwards similar to the people on the bus. When this happens they send the message of movement to the brain in that direction.

There is a second analogy which I prefer. It is that of seaweed anchored to the bottom of a fish bowl. Hair cells are like the seaweed which are anchored and pointing straight up. If you were to shift the fishbowl in one direction, the water would move in the opposite direction, hauling the seaweed at the same time with it.

These hair cells tilt with the movements of our heads because they are within a thick membrane called the otolitic membrane. Swap the water in the fishbowl for syrup instead of water. The syrup would now represent the otolithic membrane. Said membrane is made of something called connective tissue, which is like a gelatinous flexible mesh, filling the space all around the hair cells.

The Utricle would be one fishbowl, and the Saccule would be another fishbowl.

We have two ‘fish bowls’ on each side of our head, and the combination of these organs will help our brains interpret where we are going. One ‘fishbowl is sat flat on the horizontal plane, the Utricle. And the other ‘fishbowl’ is on the vertical plane, the Saccule.

Floating on top of the substance that constitutes the Otolithic membrane are the otoliths or otoconia. These heavy rock like structures will add weight to the otolithic membranes. As we move our heads in different directions they will contribute to the nature of how the membrane responds to gravity and motion.

Putting it all together

Remember these are only two of the many organs involved in our perception of gravity and movement. In conjunction with our eyes, and the proprioception coming from our joints, our brain can interpret our bodies are standing on the ground, and understand that ‘up’ is above our heads and ‘down’ is below. [4]

When we are laying in bed: the messages from our skin sense the pressure of our bodies against the mattress. It is combined with our head position fed by the otolithic organs – the Utricle and the Saccule. On top of which our site confirms we are in fact facing the ceiling or the pillow. Finally our brain makes sense of this. We are now aware – as comfortable or as uncomfortable the mattress may be – that gravity is pulling us against our beds.

The same is also true about the empty feeling you feel inside inside when you are speeding down hill on a roller coaster. The rapid descent sends the message to your joints and skin that pressure is being relieved from the seat you are on, perhaps to a certain extent extent your legs or shoulders may be touching against other structures of the roller coaster cabin.

The otolithic organs – the Utricle and the Saccule – are perceiving the decent. Your eyes know what’s coming.

Gravity for a moment appears to have decreased. The same gravitational pull from the Earth is still present, ti has not changed. Your body however, in relation to the pull is now moving at a different speed. If you exaggerate this further you will have perceived no gravity at all. It will feel like there is no gravity even though you are still, within the Earths gravitational pull.

Finally for the astronaut that is in space. Where there is no downward gravitational pull. When someone in space pushes themselves in one direction or the other, the hairs cells move in the same linear fashion as they do on Earth, except that the constant pull from the Earths gravity is gone. If you close your eyes on Earth, you will know which way is down. In space, when you close your eyes, the hair cells in your Utricle and Saccule will not have been pulled in any direction, thus you will not be able to distinguish the position you are in if you were to rely on them alone. [5]

During the first days in space, astronauts suffer from bad motion sickness because the Utricle and the Saccule send the wrong messages to the brain. In my article about Motion Sickness I explain why this happens.


References

  1. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. The Otolith Organs: The Utricle and Sacculus.
  2. Kawakami O, Sudoh H, Watanabe S. Effects of linear movements on upright standing position. Environ Med. 1996 Dec;40(2):193-6. PMID: 12703539.
  3. Dakin CJ, Rosenberg A. Gravity estimation and verticality perception. Handb Clin Neurol. 2018;159:43-59. doi: 10.1016/B978-0-444-63916-5.00003-3. PMID: 30482332; PMCID: PMC6295197.
  4. Carriot J, Mackrous I, Cullen KE. Challenges to the Vestibular System in Space: How the Brain Responds and Adapts to Microgravity. Front Neural Circuits. 2021 Nov 3;15:760313. doi: 10.3389/fncir.2021.760313. PMID: 34803615; PMCID: PMC8595211.
  5. Kobel MJ, Wagner AR, Merfeld DM. Impact of gravity on the perception of linear motion. J Neurophysiol. 2021 Sep 1;126(3):875-887. doi: 10.1152/jn.00274.2021. Epub 2021 Jul 28. PMID: 34320866; PMCID: PMC8461827.

Photo by Ashley Bean on Unsplash

Next Review Date July 2024