Questions about the human body

Post by **Chroelle** » Sun Apr 23, 2006 11:25

I', going for 18, but my back is busted no less, so it makes for a difficult count.

Zandrav Ibistenn · Post by **Zandrav Ibistenn** » Sun Apr 23, 2006 12:21

Dizi was closest.
There are 33 bones (vertebrae) in the spinal column, subdivided into regional segments sharing some similarities: 7 cervical, 12 thoracal, 5 lumbal, 5 sacral and 4 coccygeal. However, the sacral segments and the coccygeal segments are fused.

The interesting part is that the number of vertebral bones remains a constant in mammals (with few exceptions), indicating a common evolutionary root.

So, despite the disparity in length of the neck of a human and a giraffe, both have 7 cervical vertebrae!

Chillum · Post by **Chillum** » Mon Apr 24, 2006 10:21

Okay..

I have a new question:

Why do people yawn?

Post by **Chroelle** » Mon Apr 24, 2006 17:21

I always learned that it had something to do with the brain lacking oxygen, and hence makes the body take in a lot of air all at once... But I don't know if it is true...

mistergreen77 · Post by **mistergreen77** » Mon Apr 24, 2006 17:50

I thought it was something like stretching, similar to stretching arms and legs when you wake up or when you are tired.

Dizi · Post by **Dizi** » Mon Apr 24, 2006 21:25

When your tired you the body doesn't make as much effort to breathe. So the when the brain realises this it makes you yawn to get a large quantity of oxygen to make up for the shallow breathing.

When you see someone else yawn and it triggers you to yawn it is because your brain thinks that there isn't enough oxygen so thinks that you need to yawn too. (although this hasn't been proven officially its what a lot of people think)

mistergreen77 · Post by **mistergreen77** » Mon Apr 24, 2006 21:38

Sounds plausible so I googled. howstuffworks says about this theory: Robert Provine, a psychologist at the University of Maryland, Baltimore County, and a leading expert on yawning, has tested this theory. Giving people additional oxygen didn't decrease yawning and decreasing the amount of carbon dioxide in a subject's environment also didn't prevent yawning. My guess is that we are stretching breathing muscles to improve circulation. As to why auto-suggestion makes us yawn I am stumped. I am yawning just reading this thread!

Dizi · Post by **Dizi** » Mon Apr 24, 2006 22:14

Lol, so was I when I was typing up my opinion on yawning. My french teacher in school used to make us sit up stright as she said leaning down in your chairs caused you to yawn. If one person in the class would yawn she would tell them to sit up and for someone to open all the windows (even if it was really cold). So that we would all get enough oxygen to our brains so we could learn how to speak french.

Zandrav Ibistenn · Post by **Zandrav Ibistenn** » Fri Apr 28, 2006 21:05

I just want to add to yawning a not here mentioned social dimension:
As everybody knows yawning can be contagious; if somebody next to you yawns, then you are more likely to yawn too.
In evolutionary terms this make sense as it's practical for a herd to have its members sleep at the same time.

Yawning is definitely not about getting more oxygen, since that is simply an ineffective method - i.e. you don't yawn when exercising. If your blood CO2 rises, you breath faster, not necessarily deeper.

It may be about stretching, but it seems somewhat inconsequential. Still, a supressed yawn is also a dissatisfactory yawn - and that would seem to indicate that part of the function of yawning is stretching.

mistergreen77 · Post by **mistergreen77** » Fri May 05, 2006 14:51

That is what I meant about auto-suggestion. Just reading about the topic is enough to trigger it in some cases. It might be that in evolutionary terms the function of yawning evolved in primitive mammals for more than one purpose. Perhaps as a stretching exercise which then develops some sort of primitive psychological symbolism for communicating tiredness etc. I think the jury is still out on this question as there seem to be a few different theories but nothing conclusive.

Another mystery: why do we have an appendix?

Zandrav Ibistenn · Post by **Zandrav Ibistenn** » Fri May 05, 2006 18:44

The appendix vermiformis has no important functions, but it has been suggested that it may play a small role in the immune system.

Basically, it's a rudiment indicating some evolutionary ancestry with species of herbivores, that have had an additional abdominal sack for special digestive processes needed for their diet.

The appendix vermiformis remains because it does not affect fitness. Just as the vomeronasal organ remains as a primitive rudiment without any apparent function in humans. - In many animals it plays a role in their sense of smell, but we have no need of it.

- Or perhaps the functions of the organs mentioned have just not been discovered yet.

Chillum · Post by **Chillum** » Fri May 05, 2006 21:08

Time for another one:

Why, how and of what do we produce snot, and why does we sometimes have more of it than other times?

Post by **Chroelle** » Fri May 05, 2006 21:40

Actually I was going to post something very similar so maybe someone can answer this as well...

Why do I produce extra snot/mucus when my allergies hit. And why (just for grossness) is it more watery than a good old fashion winter cold mucus...

Zandrav Ibistenn · Post by **Zandrav Ibistenn** » Fri May 05, 2006 23:05

Nasal mucus is produced by specialised epithelium of the membranes lining the nasal cavity. The secretion is primarily water and protein called mucin. When mucin comes into contact with water it forms thick mucus. So, the more mucin in the secretion, the thicker the nasal mucus gets.
The function of nasal mucus is to clean the nose of particles that irritates it.

I'm unsure as to why the thickness of mucus varies, but if I have to take a guess...

If the secreting cells are very active, e.g. when the membranes are irritated (try pepper, dust or pollen), the secretion becomes more watery since the cells can easily secrete water, but cannot keep up with a corrosponding rate of mucin synthesis (it's a big molecule).

Ogre · Post by **Ogre** » Sat May 06, 2006 4:52

This is a question that I asked my lecturer in A & P class (who was a uni student at the time), and she couldn't give me a satisfactory answer- "Its one of the mysteries of the body.

When it comes time for more of a particular protein to be synthesised, a cell knows this through receptors in the cell wall (I think?...please correct me if that is wrong).
My question is: How does the request for that protein get transmitted from the cell wall to the nucleolus, and what is the mechanism that uncoils the chromosome at exactly the right spot to provide the correct code sequence for that protein?

Great thread by the way...anatomy & physiology is a fascinating subject!

Zandrav Ibistenn · Post by **Zandrav Ibistenn** » Sat May 06, 2006 12:35

Well now we're getting advanced - good thing I've passed my course in cellular biology...

There are several methods of signal conveyance between cells instructing them to either up or downregulate the synthesis of a specific protein.
One of them is via stereohormones (e.g. testosterone). Stereohormones are hydrophobic molecules, which means that they're soluble in lipids (fat) but not in water. They're thus able to diffuse over the lipid bilayer of the outer cellmembrane and bind to intracellular receptors to which they fit (like hand in glove). Once the receptor is activated it releases a messenger molecules which goes into the nucleus of the cell and interacts with the chromatin (=DNA), triggering another messenger (second messenger) as well as shutting down the primary response mechanism, so that more secondary messengers cannot be released - this prevents overstimulation and is a useful feedback mechanism. This intracellular signalling operate as molecular switches, which activate or deactivate certain proteins involved in protein synthesis as well as chemically modifying the DNA itself. Adding or removing functional chemical groups to or from the chromatin influences the proteins involved in protein synthesis.

But the chromatin is more than just DNA, it's also proteins called histones, which forms little octameric cylindrical histone-complexes made from different types of histone proteins. The chromatin fiber in inactive DNA is tightly coiled around these histone complexes to save space and prevent curling of the fiber itself. Before a cells can alter its protein synthesis it must first alter its chromatin by uncoiling the relevant part of the genome from the histone complexes, and this is accomplished by remodelling complexes that put acetyl-groups onto the histones to change their chemical properties so that they no longer bind chromatin so tightly.
Before a gene can be translated into a protein, the individual DNA strand must first be exposed, so that a copy of the gene can be transcribed into mRNA. Firstly the double helix structure must be destabilized locally near the gene. A protein called helicase does this "unzipping" of the double helix structure of DNA (which is only possible when the chromatin is not tightly bound to the histone complexes). The helicase is controlled by the so-called single strand binding protein (SSB), which "knows" where on the chromatin it must attach itself for the correct gene to be utilized because of the before mentioned chemical modification facilitated by the intracellular signals. Once the helicase has begun it work, a mess of other proteins arrive at the site to facilitate the synthesis of mRNA, which is then transportet to the rough endoplasmatic reticulum (sort of a big transport network), where the actual protein synthesis takes place through the ribosomal subunits, rRNA, which come from the nucleolus.

Depending on the function of the finished proteins, they are either transportet to the cell membrane via the Golgi-apparatus (if needed extracellularly or as part of the membrane) or left in the cell.

I hope this makes sense. Otherwise: ask.

Ogre · Post by **Ogre** » Sat May 06, 2006 14:25

It makes perfect sense Zandrav.
I knew about the process from the point of messenger RNA onward, through transfer etc...but there was a lot of gaps in the rest of the chain?

I did an intro to A&P course as part of a (dare I say it) natural medicine course I did some years ago. It was one of the most fascinating subjects I have ever studied. The workings of the human body are nothing short of miraculous.

Thanks for your answer.

mistergreen77 · Post by **mistergreen77** » Sun May 07, 2006 0:25

I had to read it twice to begin understanding but thats because I only have a very basic understanding of such things. Knowing the complexity of this and other natural processes gives me a sense of wonder that is the seems to border on religious.

Something that puzzles me is how the shape and form of the body is encoded by DNA? - or is it?

Ogre · Post by **Ogre** » Sun May 07, 2006 2:16

That is something I wonder about also MrG. I have read that each cell that contains DNA carries the 'recipe' for everything that can be made by our body, but I have no idea where the organising principle comes from.

I think stem cells are a part of the chain, giving instructions to the cells around them, but what tells the stem cells what they should be?

There must be some kind of intangible foundation that it is all built around, otherwise how would a cell know what it should differentiate into in the developmental stages.

I must admit that when pondering such things, I have a tendency to drift into the realm of spirituality to find the answers.

Zandrav Ibistenn · Post by **Zandrav Ibistenn** » Sun May 07, 2006 17:06

Every cell contain the entire genome, but all the information encoded in the DNA is not used; different cells activate different parts of the DNA.
Chromatin can be subdivided into euchromatin, which is actively used in the protein synthesis, and heterochromatin, which has been inactivated. Cells can be very diverse depending on what DNA becomes heterochromatin.

Progress in cellular differentiation is controlled by feedback mechanisms.
Cells produce A which initiates the production of B and turns off the synthesis of itself. B produces C and turns it own and A's production off. C, in the presence of A, modifies the DNA chemically to exclusively produce either D or E depending whether or not it receives F from a neighbouring cell. Such interplay quickly becomes complex, but allows many opportunities. Why does the first cell not produce F you could ask. Because one cell was first to produce A and thus B and C - and B goes to the second cell and turns it production of A and B off. Therefore C never appears in the second cell, which then produces F instead. Now you have precursor states for different tissues.
With feedback self-regulation the opportunities are endless.

In the embryo development is centered around axises, which are determined by the concentration and distribution of various proteins in the cell layers. For example a protein called, well, sonic hedgehog (I wonder what drugs its discoverers were on) is ventralizing, and bone morphogenic protein 4 (BMP-4) is dorsalizing. Together they establish the ventero-dorsal axis.

It's obvious that in a system this complicated something occasionally go wrong.
(DISCLAIMER: The links below lead to imagery some people may find disturbing. Future parents: Don't look...)
Some examples:
- Failure for the neural tube to close in the cranial direction leaves the primitive brain exposed to the amniotic fluid and causes the condition anencephaly; failure for the brain to develop.
- A loss of midline structures in the brain causes a fusion of the lateral ventricles. The eyes fuse into a central bulge in the middle of the face where the nose normally is. This condition is known as holoproencephaly.