Ask any first year medical student, what is the highest priority of signaling capable of influencing all sensory perception within the entire brain? The correct response, however counter intuitive, is the vestibular network. Imagine running an on-line App that demands the resources of the entire Internet to calculate an outcome. This astonishing observation will give you the utter primacy of sensing the pull of the Earth’s one gravity vector onto all the brain’s cellular communication. This gravity sensing influence is at evolution’s pivotal apex. Gravity has always been present, creating a massively elaborate hierarchical position sensing system yet mysteriously under-appreciated for its astounding impact onto all evolutionary biological signaling behavior. Constrained within our brain’s balance sensing organs, the otolith structures, are essentially multitudes of dynamically linked accelerometers, formed in the shape of sensing hair cells which orient all computed positional information within the 3 dimensions of our surroundings. Their combined output is uniquely capable to focus its command output to drive our entire autonomic nervous system. Our balance sensing paces our hearts pumping, our eyes seeing, and our brains cogitation relaying with an on-demand sense of, ‘where are we?’ each and every second of our life. Nowhere is this sense of gravity more evident than from the early space flights of astronauts, who discovered micro-gravity induced eye problems, space motion sickness, bone density changes, cardiovascular induced fainting spells, especially not even being able to support Earth’s gravity upon their return, by literally falling over as they attempted to stand up. Surprisingly the hypo-gravity effects are not dependent on lengthy space flight, they can be provoked during the parabolic flight profile of simulated gravity changes within a two-hour micro-gravity/hyper-gravity exposure.
First a little anatomy science, with the following quote which comes from: Neuroscience. 2nd edition.Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Sunderland (MA): Sinauer Associates; 2001. “Displacements and linear accelerations of the head, such as those induced by tilting or translational movements, are detected by the two otolith organs: the sacculus and the utricle. Both of these organs contain a sensory epithelium, the macula, which consists of hair cells and associated supporting cells. Overlying the hair cells and their hair bundles is a gelatinous layer, and above this is a fibrous structure, the otolithic membrane, in which are embedded crystals of calcium carbonate called otoconia. The crystals give the otolith organs their name (otolith is Greek for “ear stones”). The otoconia make the otolithic membrane considerably heavier than the structures and fluids surrounding it; thus, when the head tilts, gravity causes the membrane to shift relative to the sensory epithelium. The resulting shearing motion between the otolithic membrane and the macula displaces the hair bundles, which are embedded in the lower, gelatinous surface of the membrane. This displacement of the hair bundles generates a receptor potential in the hair cells. A shearing motion between the macula and the otolithic membrane also occurs when the head undergoes linear accelerations; the greater relative mass of the otolithic membrane causes it to lag behind the macula temporarily, leading to transient displacement of the hair bundle. The similar effects exerted on otolithic hair cells by certain head tilts and linear accelerations explains the perceptual equivalency of these different stimuli when visual feedback is absent, as occurs in the dark or when the eyes are closed.”
Where does emesis trigger, what part of the brain anatomy does the emesis begin? Receptors on the floor of the fourth ventricle of the brain represent a chemoreceptor zone, known as the area postrema, stimulation of which can lead to vomiting. The area postrema is a circumventricular organ and as such lies outside the blood-brain barrier; it can therefore be stimulated by blood-borne drugs that can stimulate vomiting or inhibit it. There are various sources of input to the vomiting center:
- The chemoreceptor trigger zone at the base of the fourth ventricle has numerous Dopamine D2 receptors, serotonin 5-HT3receptors, opioid receptors, acetylcholine receptors, and receptors for substance P. Stimulation of different receptors are involved in different pathways leading to emesis, in the final common pathway substance P appears involved. The vestibular system, which sends information to the brain via cranial nerve VIII (vestibulocochlear nerve), plays a major role in motion sickness, and is rich in muscarinic receptors and histamine H2 receptors.
- The Cranial nerve X (vagus nerve) is activated when the pharynx is irritated, leading to a gag reflex.
- The Vagal and enteric nervous system inputs transmit information regarding the state of the gastrointestinal system. Irritation of the GI mucosa by chemotherapy, radiation, distention, or acute infectious gastroenteritis activates the 5-HT3 receptors of these inputs.
- The CNS mediates vomiting that arises from psychiatric disorders and stress from higher brain centers.
But where do the otoliths come from, where do they originate in the time scale of evolutionary sensing, how old are they ? I found some of these answers on my own during the past few months but I also came across a web article written by Professor Robert Gauldie describing the rich ancient heritage of the otoliths. Here are some of his pertinent observations in his words. ” The first otoliths were devices that our forebears created that allowed a choice of direction. To choose my own direction we use as a metaphor for what we consider our most basic human rights: freedom of speech, freedom of movement and freedom of religion. Yet in it’s most literal, physiological sense, to choose my own direction dates back to the beginning of animal life.
The very essence of otoliths are still present in our oceans. They are called rhopalia. They are tiny little crystals in cells around the hoop-skirt shape, that is mimicking their billowing motion of the jellyfish dancing in water. “The rhopalia are balanced on a tiny mat of sensitive hair cells. Gravity, the ever-present gravity, means that when the extended umbrella shape of the jellyfish with its little crystal cells tilts to the orientation of the horizon, then the force of gravity on the hair cells will make the hair cells bend in a different direction. There you have the sense of direction! Nature creates a few bending-sensitive hair cells, add a bit of crystal, use the gravity vector, now even the lowly jellyfish knows up from down. The network sensing this gravity vector allows the jellyfish behaviourally to be aware that the directions up, down,and sideways now exist, and are maneuverable. The jellyfish has anima, it now has the essential character of an animal. “
” A half billion years ago places us back inside the Cambrian era. The strange creatures of the Cambrian are the oldest known direction orienting, directional capable animals. These Cambrian animals are the oldest known movers inside three dimensions capable to navigate in any particular controllable direction. The jellyfish are older, (by at least 300 million years best estimate) but their otolith apparatus of the rhopalia, proved to be a dead-end of development. The rhopalia have been successful enough to survive 500 million years, but their apparatus primitive sensing apparatus does not provide a flexible tool capable of progressing past knowing the difference between up from down. So the jellyfish was doomed to its slow pulsating existence, despite knowing up from down, aware of direction but not how to control its direction.”
” Going way, way back into the early Cambrian era, long before the fishes as we know them, the fossilized features are seen in the rocks as the preserved remains of Haikouichthys. The evidence is not from a single fossil but over five hundred known specimens of Haikouichthys have been discovered. The body parts from the remains of that era are so bizarre as to their shapes that we are still unsure how their separate fossilized parts fit together. The various combinations are so strange that all appear equally unlikely to our modern eyes, as if alien creatures. But to all intents and purposes Haikouichthys is a little vertebrate, surprisingly similar to what we might imagine what the very first fish looks like. Remember, this body design is at the beginning, before molluscs, before the crustaceans, before the worms, before even the spider-relatives, aeons before the insects. Here at the outset, at the edge of evolutions is a little vertebrate with paired otic capsules, the inner ears ! (With a left and right symmetry characteristic of all animal body plans). These paired otic capsules are located close to the primitive brain, also this is the first critical step towards the sense of hearing. Although we do not know how their paired ears worked, we know that the vertebrate otic apparatus distinguishes up from down. Since they are pairs, they know left from right. Situated at the front of the fish, they know front from back. These are the animals that can move in a certain direction . It is here that we can see the appearance of our first freedom: the freedom to choose our own direction.”
During parabolic flight the effect within the otolith structures sensing of acceleration or deceleration is like a slow motion movie of a concussion happening. Think of it, what happens during a concussion ? Incredibly when we try to respond to this question we don’t have the clear sequence yet in terms of a temporal event. What is the highest priority of change within the first few milliseconds of a brain concussion? It is the motion of the otoconia that react first, since they are meant to sense accurately positional changes of the head. Think of a brain concussion as a flow event. The flow of gravity sensing. This gravity sensing can be watched during parabolic flight affecting all the various networks within the brain, like I said before in slow motion. This is the advantage studying parabolic flight behaviour in humans how it teaches us how important those little ‘ear stones’ moving. The ear stones moving is the first event within a concussion that cascades rapidly into a stunning panoply of changes that are bewildering in their various effects on structures as shapes within the brain. So what I am going to do is within the slow motion aspect of the parabolic flight is try to capture some of those first few milliseconds of how our brain reacts to the sensing of the gravity continuum between acceleration and deceleration. Concussions are started during massive deceleration events, understanding a concussion means understanding how we sense gravity. Gravity in evolutionary terms has always been present going back billions of years. Nature used gravity as the first sensing network when motion became the challenge of progress for early life to change position from one place to another. At some point in time Nature took its first step. That first event takes 3 dimensional calculation, it is a huge step for Nature to accomplish on its own, first time.
“Parabolic flights can simulate microgravity. These parabolic manoeuvres, executed by modified commercial jets, give the opportunity to obtain short periods of free fall. On the upward arc, the thrust of the airplane is adjusted so that there is no lift: the plane is in free fall since nothing reduces the force of gravity. The plane remains in free fall over the top of the parabola and part of the downward arc. Microgravity conditions are achieved for time periods of 15 to 20 seconds, in which it is possible to conduct experiments. By free-floating experiments, low g-levels can be achieved for 5 to 8 seconds.” (Canadian Space Agency web site http://www.asc-csa.gc.ca)
Neuroscientist, Dr. Roberta Bondar, Canada’s first female astronaut studied parabolic flight effects extensively on human biological responses to micro-gravity/hyper-gravity. One of her studies concerned the rapid onset of space motion sickness that parabolic flight stimulates the vomit reflex, what in medical parlance is termed emesis triggering. The participants of these parabolic flights refer to the experience of this type of flying as, ” .. riding the vomit comet.” No one likes to throw up by loosing their entire stomach contents in a wrenching gut contraction of nausea, especially in front of your peers on the same parabolic flight. But what is intriguing of the emesis triggering is that it is also related to the temporary cardio-vascular changes that also are triggered following the parabolic flight, what is termed orthostatic intolerance, the inability to rise up from a lying down position to a vertical position of the body. It’s medical lingo for feinting.
Originally after longer space missions astronauts were suffering orthostatic intolerance following their space flights. Scientists thought it was the prolonged exposure to micro-gravity that was provoking the susceptibility toward feinting upon return to Earth’s one G gravity. Parabolic flight in as short as a couple of hours of exposure to micro-gravity/hyper-gravity will also provoke orthostatic intolerance. What happens inside our body as we transition from lying down to getting up, just like we do every morning ? What has to happen so we don’t faint?
Before Nature took its first step the essence of left and right was also present within the architecture of primal design, which is the building block of all life, DNA.
“Thanks to the speed of discrete potential simulations, we can explore timescales unavailable to traditional techniques. In one study, we looked at the folding behaviour of an idealised polymer (see figure below). Using DynamO, we were able to simulate millions of fold and unfolds of isolated polymers per day and fully map the effects of intra-polymer attraction and stiffness on the final structure.
This model is remarkable in its simplicity. Despite only including stiffness, self-attraction and excluded volume, it will still spontaneously fold into either a left or right-handed helix structure. All these properties are shared by most proteins and may explain why the helix structure is so prolific in nature. Future work on this system is examining the effect of crowding on the folding behaviour.” sourced from http://www.marcusbannerman.co.uk/index.php/research.html
You may be confused at this juncture as to why I am suddenly referring to left and right, which is the property of chirality. Again I emphasise a critical property that contributes to the gravity sensing. The hair cells are bending during the transition of micro-gravity to hyper-gravity during the parabolic flight profile. As part of the bending the word stiffness now comes into play because the stiffness of the hair cells is perturbed within the otolith structures. Just in case you didn’t catch the anatomy of the otolith: Overlying the hair cells and their hair bundles is a gelatinous layer, and above this is a fibrous structure, the otolithic membrane, in which are embedded crystals of calcium carbonate called otoconia. The crystals give the otolith organs their name (otolith is Greek for “ear stones”). The otoconia make the otolithic membrane considerably heavier than the structures and fluids surrounding it; thus, when the head tilts, gravity causes the membrane to shift relative to the sensory epithelium. The resulting shearing motion between the otolithic membrane and the macula displaces the hair bundles, which are embedded in the lower, gelatinous surface of the membrane. Inside the otolith during the first millisecond the key terms are shearing, which is bending, then bending under hyper-gravity or micro-gravity- which is left and right of the gravity sensing continuum. Do you get my point now why left and right, why chirality is so important?
“The term chirality was first used by Lord Kelvin in an address in 1904. In a lecture given at John Hopkins University on “Molecular Dynamics and the Wave Theory of Light” he stated:
“Human hands are perhaps the most universally recognized example of chirality: The left hand is a non-superimposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide. This difference in symmetry becomes obvious if someone attempts to shake the right hand of a person using his left hand, or if a left-handed glove is placed on a right hand. In mathematics chirality is the property of a figure that is not identical to its mirror image.” (Thank you Wikipedia). Notice that chirality has two distinct properties, that it is a mathematical entity plus a shape entity. Tensegrity also shares this duality of both shape design with mathematical coherence. It is no accident that Nature builds molecules into proteins with the grace of Snelson’s floating tension network, scalable within microfilaments into the cell’s cytoskeleton expandable to scaling up into describing an entire network of sensing such as the otolith structure. The design of the otolith is a function of its shape sensing employing the most elemental distortion (bending) that is respondent to the ever-present gravity field, acting as a vector, always referencing the down position. Except that is in outer space micro-gravity when down is now pointing everywhere. As my computer friends would say, “When you lose the code of direction orientation , the system crashes, the entire system goes down.” The first few milliseconds of a concussion happening is the entire system crashing as it tries to right itself.
So why is the triggering of emesis so significant for Dr Bondar? Let’s look at where emesis actually gets triggered. It triggers within the bottom of the fourth ventricle which is located within the medulla of the brain stem. We are still on a time frame of a few milliseconds so that limits the formation of specific transmitters to get into the blood to actually travel down to that position. According to the research article, Orthostatic intolerance and motion sickness after parabolic flight from Journal of Applied Physiology 90:67-82, 2001 Roberta Bondar was one of the reporting authors. The report stated, ” Most impressively, in military aviators, mere seconds of exposure to gravitational forces in the headward direction are sufficient to increase the risk of syncope (fainting) during subsequent exposure to gravitational forces in the footward direction, the so-called “push-pull effect”
The floor of the fourth ventricle is called the obex: represents the caudal tip of the fourth ventricle; the obex is also a marker for the level of the foramen magnum of the skull and therefore is a marker for the imaginary dividing line between the medulla and spinal cord. Now things for me get interesting. What possible force might get into this zone at this particular point to irritate the area postrema zone, the triggering zone for emesis ?
The abducens nerve leaves the brainstem at the of the pons and the medulla, medial to the facial nerve. In order to reach the eye, it runs upward (superiorly) and then bends forward (anteriorly). The nerve enters the subarachnoid space when it emerges from the brainstem. It runs upward between the pons and the clivus, and then pierces the dura mater to run between the dura and the skull through Dorello’s canal. At the tip of the petrous temporal bone it makes a sharp turn forward to enter the cavernous sinus. In the cavernous sinus it runs alongside the internal carotid artery. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye.
The long course of the abducens nerve between the brainstem and the eye makes it vulnerable to injury at many levels. For example, fractures of the petrous temporal bone can selectively damage the nerve, as can aneurysms of the intracavernous carotid artery. Mass lesions that push the brainstem downward can damage the nerve by stretching it between the point where it emerges from the pons and the point where it hooks over the petrous temporal bone.
Recall that returning astronauts have all reported varying degrees of visual problems. Astronaut John Glenn during his first extended spaceflights kept bumping into things while in space. So that all future missions had extra short-sighted spectacles added inside the spacecraft to counteract the ‘space eyes’ phenomena. But NASA researchers never went past this point, treating the nearsightedness as a minor anomaly. Disturbingly there have been additional more serious damage to the astronaut eyes in the form of distortions onto their visual nerve within the orbit holding the eye. The eye is separate from the brain in the anatomical sense that each eye is a mass tethered by the various rectus eye muscles which are the eye’s pivoting features for performing all eye motions. Incredibly the abducens cranial nerve with a moving tethered eye can pull onto the nerve stretching it at its insertion point into the medulla at the emesis triggering zone, which does not like to get stretched. There you have it: the first reporting of the first few milliseconds as the anatomy reveals itself through the slow motion sensing advantage of parabolic motion within the otolith’s structures as Nature reacts to the deceleration as a concussion starts.
I dedicate this essay to three special people who have inspired my beginning concussion research efforts toward a better understanding where concussion starts in our brain. I have been in contact with both astronauts Dr Roberta Bondar and Marc Garneau MP for Westmount/St-Henri plus Guy Laliberté OC CQ co-founder and current CEO of Cirque du Soleil. All three of these people have seen the Earth from space. All three have experienced ‘space eyes’. All three are courageous, special people. I dream one day I can fully explain to them with rigorous proof the significance of their experiences of ‘space eyes’ toward the boost of understanding how concussion affects the human brain. I am indebted to them to make this pledge happen during my life and hopefully their lives also. The sense of gravity is at the core of the very tipping point of a concussion happening, incredibly gravity also gives us the clue how to treat concussion brain damage too. I’ll write more about that possibility in a future essay in http://www.cerebrovortex.com