Do Humans Have Gravity Receptors?

[jimmydasaint]

I have read about gravitropism in plants which

 

...dictates upward shoot growth to ensure a proper positioning of the leaves for efficient photosynthesis and gas exchange. It also directs roots to grow downward in soil, where they can reach out to take up the water and mineral ions required for plant growth and development.

 

http://www.plantphysiol.org/cgi/content/short/120/2/343

 

I then wondered if humans also had gravity receptors for movement of muscles or to maintain an upright posture? I presume that the gravity receptor function of the inner ear is upset by heavy drinking, affecting the balance.

 

However, are there receptors for gravity on other parts of the human body and are they upset by alcohol intake?

[Mokele]

Our sole sense of gravity comes from the inner ear. That sensory input is then integrated with visual input to determine our overall orientation in space (hence why disruption of either results in falling over and/or puking). That information is then integrated with current positions of body parts in the cerebral motor cortex and the cerebellum, which in turn send commands to the muscles to maintain posture and balance.

 

The muscles themselves have various receptors, including golgi tendon organs, intrafusal muscle fibers, and muscle spindles, but all deal with muscle tension, stretch, force generation, etc. None sense gravity directly, though of course the act of supporting oneself against gravity will result in feedback about the force, length, etc. of the muscles involved in doing so.

 

Interestingly, crustaceans have a gravity sense that acts in a similar mechanism to plants - a hollow sphere of touch sensors, with a grain of sand inside (acquired during molting).

[jimmydasaint]

Thanks for that Mokele. I had read about some Italian researcher who was investigating gravity receptors in humans but I can't trace the reference. Also, how does alcohol upset the inner ear receptors? I'll continue searching...

[Mokele]

I'm pretty sure that alcohol messes with the brain's interpretation of the signals, rather that the sensory system itself.

[D H]

I then wondered if humans also had gravity receptors for movement of muscles or to maintain an upright posture?

Yes and no.

 

The no answer first: There is no way to sense gravity, period. From the perspective of Newtonian mechanics, a gravity sensor would require a gravity shield. We don't know how to build a gravity shield, and neither do our bodies. From the perspective of general relativity, the answer is even easier: Gravity isn't a real force. It is instead a fictional force, like centrifugal force and coriolis force. Fictitious forces can't be measured because they aren't real.

 

The yes answer: We can build devices that measure the net non-gravitational acceleration of an object, and so can our bodies. Mechanical versions of these devices are called accelerometers. Our inner ears have otoliths -- the biological equivalents of an accelerometer.

[GDG]

If you ingest enough alcohol, it apparently affects the composition of the fluid in your semicircular canals (the position/gravity/inertial sensors).

 

As for the paper, here's one. PubMed is always a good source if you are looking for published research.

[SH3RL0CK]

Yes and no.

 

The no answer first: There is no way to sense gravity, period. From the perspective of Newtonian mechanics, a gravity sensor would require a gravity shield. We don't know how to build a gravity shield, and neither do our bodies. From the perspective of general relativity, the answer is even easier: Gravity isn't a real force. It is instead a fictional force, like centrifugal force and coriolis force. Fictitious forces can't be measured because they aren't real.

 

 

I'm confused. Doesn't the force of gravity diminish over distance? Then wouldn't a relatively large distance be similar to a shield in the sense that the gravity is less? Thus a gravity sensor might rely on a large distance (perhaps a couple satelites between two lagrangian points) as the "sheild"? Or is it more complicated than this?

[Mokele]

I'm confused. Doesn't the force of gravity diminish over distance? Then wouldn't a relatively large distance be similar to a shield in the sense that the gravity is less? Thus a gravity sensor might rely on a large distance (perhaps a couple satelites between two lagrangian points) as the "sheild"? Or is it more complicated than this?

 

Well, yes, but using such distances isn't really feasible for a biological organism.

[Sisyphus]

Well, yes, but using such distances isn't really feasible for a biological organism.

 

Unless you're an integral tree. Anyone?

[bascule]

Our sole sense of gravity comes from the inner ear.

 

There's also proprioception and kinesthesia. We can feel the effects gravity has on the movement of our body.

[Mokele]

There's also proprioception and kinesthesia. We can feel the effects gravity has on the movement of our body.

 

However, those don't actually 'sense gravity', only it's effects. You could also say those same senses detect water, because we can feel when we move through it, or that our eyes detect gravity, because we can see the direction objects fall.

[jimmydasaint]

Finally, I found something:

Psychophysical experiments show that the perception of posture is to a large degree affected by hitherto unknown graviceptors in the human trunk. By remote control subjects move themselves radially along their spinal axis over the horizontal platform of a rotating centrifuge until they feel horizontal. Normal subjects then set the centrifuge axis on average at 22–28 cm caudal of the meatus, neuromectomized subjects at 45–55 cm. Hence the mass centroid of these receptors should be situated near the last ribs

(emphasis placed there by jimmy)

 

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T4T-3VXH4TG-3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=9c75780e70abd2cc0169934602605087

 

This suggests from experimental data that there may be gravity receptors in the trunk of humans. I will read more about this but this is amazing stuff, if it is true.

[iNow]

If you ingest enough alcohol, it apparently affects the composition of the fluid in your semicircular canals (the position/gravity/inertial sensors).

Quite right. The way I've always thought about it is that the "viscosity" of the inner ear fluid changes, therefore it oscillates differently with movement, and hence causes the cilia against which it pushes to fire differently... which all results in the signal reaching the brain to seem scrambled and more chaotic. Since it's such a finely tuned system to begin with, even that slight change in viscosity causes the normal feedback systems to get all "wonky." That's the technical term...

[jimmydasaint]

This is off topic, and I apologise to the Mods in advance. However, whilst looking up proprioreceptors from bascule's post I found this interesting little effect, called the Pinocchio Illusion:

 

The Pinocchio illusion is an illusion that ones nose is growing longer, as happened to the literary character, Pinocchio when he told a lie. It is an illusion of proprioception, reviewed by Lackner (1988).

To experience the illusion, a vibrator is applied to the biceps tendon while one holds one's nose with the hand of that arm. The vibrator stimulates muscle spindles in the biceps that would normally be stimulated by the muscle's stretching, creating a kinesthetic illusion that the arm is moving away from the face. Because the fingers holding the nose are still giving tactile information of being in contact with the nose, it appears that the nose is moving away from the face too, in a form of perceptual capture.

 

http://en.wikipedia.org/wiki/Pinocchio_illusion

 

Back to the OP. It seems as if there are gravity receptors in humans, located in the trunk and seem to be discovered by tilting the subject at various angles from the horizontal, in a centrifuge. To the best of my understanding, 'normal' subjects tilted the centrifuge until they felt horizontal. Paraplegic subjects showed a different idea of being horizontal. It also seems to indicate that the kidneys (and possibly other surrounding organs) serve to act as gravity receptors, and that nephrectomy (removing kidneys) affects the gravity response.

[Mokele]

Huh, odd, but looks reasonable, though obviously not a primary sense like the inner ear.

 

What might make this actually interesting is if such visceral graviception is at play in insects, which lack any known graviception method.