Brain development - aging and newborns

How is brain changing during our lifes? What about its size, abilities, neurophysilogy? And what can we expect when getting older?

There is a huge increase in brain size during the prenatal period and a rapid neurogenesis (up to 250 000 new neurons/min). Cells are migrating and neurons are differentiating.

Weight of newborns' brain is 25% of adult brain, but the size of the brain is only few percent smaller. During this stage the breast feeding is important for neurons myelinization (myelin is important for transferring signal between neurons without lost) and crying and the response to it will enable newborns and the brain to learn the reaction to the stimuli. The process of myelinization is one of the most important during next few months and it is also the reason why the weight of brain is increased from 25% after birth to 75% of adult weight in 3 years. Myelinization is very important process and it durates from approx. 2 months to 5-10 years (in some areas it last longer - for example for prefrontal cortex it continues up to 10 years). Different brain arreas attain its peak cortical thickness in different time. Averaged thickness of cortex is the highest around 10 years of age.

During this stage synapses are also rapidly generated. In adulthood there are about 100mld neurons and each of them can have approximatelly 1000 synapses. Synaptogenesis in areas responsible for seeing/hearing lasts from prenatal period up to 5 years, on the other hand in areas responsible for higher cognitive functions synapses can be created up to approx 20 years. However experienced-dependent synapses can be created during whole life.

Fig. 1 Thompson RA, Nelson CA 2001 Developmental science and the media: Early brain development. American Psychologist 56(1):5-15

Have you ever heard that there are no new neurons after birth? It is actually not true. Neurogenesis in hippocampus (important part of brain for transforming memories from short-term memory to long-term memory) proceeds up to 30 years and maybe longer. On the other hand after birth neurons are overpopulated and lot of them must die (it is a controlled process).

Hm, weight of brain after birth is comparable for women and men but then the weight is more increased for men :-(.

And now what about aging? I am not speaking about any neurodegenerative diseases. I speak about normal aging. What is actually happening with the brain?

               Fig 2. (z webu: http://www.isagenixhealth.net/why-healthy-aging-starts-with-your-brain-webinar/)

The most important change is that cortex is getting thinner - primarily prefrontal cortex (planning, logical thinking etc.) and hippocampus (remembering). From the early adulthood the volume of the gray matter is lowered. This is very rapid in some areas while others seem to be nearly untouched until very high age (such as occipital lobe responsible for vision). White matter (composed of myelinated axons) is increased until 40 years and afterwards its volume is decreasing as well. Also some synapses are lost as well as plasticity (this is closely related to the usage of the brain). The reason for losing plasticity can be in problems with calcium regulation which is important for neural firing and transferring action potentials.

Morphology of neurons is also changed - they have smaller dendrites and their density and number of spins is 40% lower after 50 years compared to early adulthood. Whau thats a huge number, isn't it?

Another change - less number of neurotransmitters (dopamin, serotonin, glutamat...) which are important for communication among neurons. Also white matter is lost somewhere which I have mentioned above. Isn't it sad?

But it is not the end. Also aortas are thinner and less capillaries are created which changes blood supply to the brain (and less oxygen).
On the other hand the free radicals are accumulating (that are molecules which interacts easily with other molecules and are created during metabolism of mitochondrias when the oxygen is metabolised) which is called oxygenation stress. They can damage the cell membrane as well as DNA. This can be chain reaction (than it can cause Alzheimer disease).

And also DNA damages are accumulated.

Isn't it a sad story which is waiting for all of us? And I haven't mentioned several neurodegenerative diseases as Alzheimer, Parkinson, ALS etc.

Literatura

• http://www.zerotothree.org/child-development/brain-development/
• Stiles J, Jernigan TL. The Basics of Brain Development. Neuropsychology Review 2010;20(4):327-348. doi:10.1007/s11065-010- http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989000/
• Thompson RA, Nelson CA 2001 Developmental science and the media: Early brain development. American Psychologist 56(1):5-15
• The changing brain in healthy aging http://www.nia.nih.gov/alzheimers/publication/part-1-basics-healthy-brain/changing-brain-healthy-aging
• Aging brain - wiki http://en.wikipedia.org/wiki/Aging_brain
• Human brain development timeline - wiki http://en.wikipedia.org/wiki/Human_brain_development_timeline

Strabism and how big is the field of vision

During my presentation on university we have discussed the strabism and how big is the field of vision for different types of strabism.

Fig. Strabism (Esotropia - the upper image, Exotropia - lower image)

Strabism is the disease when both eyes cannot simultaneously gaze at the same fixation point. Therefore the image from both eyes cannot be combined in a single image and this can cause so called "double vision" = diplopia (Light from the visual input passess through lens to the retina but to not corresponding places - from one eye it reaches the fovea but in the other eye not - therefore the subsequent fusion of image is not possible). There are two main different kinds of strabism: esotropia (axis of eyes intersect before fixation point) and exotropia (axes of eyes intersect behing fixation point). Less frequent are hyper- and hypotropia which correspond to the deviation in vertical line.

Fig. Esotropia and Exotropia

But what is the cause of strabism?

Strabism can be caused by different length of extraocular muscles or by their lack of coordination. Secondly, strabism can be also caused by problems with ocular nerves and subsequent problems with neural interpretation of visual input (this is called lazy eye or amblyopia and brain will favor one of the eye and suppress the other). What I was not sure whether the strabism is somehow connected to the problems with eye lens. My collegue told me that the problem is not primarily caused by problems with lens but without any cure the problems with eye lens can appear (caused by using only one eye etc.).

How can people with strabism see and work?

This is very interesting. When you use only one eye, you cannot percieve the depth. When using both eyes they can perceive "double vision" which can be quite dangerous in normal life: people with this problem can fall from stairs or have problems with driving. Therefore they frequently cover one of their eyes (especially the ill one which can cause further problems which I will discuss beneath). The brain itself will supress deviated eye in the central visual field where we have binocular vision.

Fig. Diplopia (double vision)

Do they have bigger/smaller field of vision compared to healthy human?

You maybe know that the horizontal field of vision for normal human is something as 200° (each eye about 160°, binocullar vision ~ 120°).

We could expect that people with exotropia should have bigger field of vision than people with normal vision and similarly people with esotropia should have this field of vision smaller. Anyway, this is not exactly true. People with strabism have "bigger field of vision" when using only one eye (the second eye is covered), but while brain cannot work with "double vision", it accomodates and the central part of the field (approximatelly 20° is excluded). Sometimes one eye is completly suppressed.

What is even more interesting is that when you completelly cover one of your eyes (to improve your vision) and stop using it, the muscles which are used for accomodation of lens will loose their strength and afterwards it can also cause total "deth" of one of your eyes (also neural connection are not working anymore).

And...how big is the field of vision for birds?

Just interested, whether birds with bigger eyes (and lens) have also bigger field of vision. And the answer is YES! Yes, they have. Something as 300° compared to 200° which is the normal humens' field of vision. Furthermore they can perceive even small changes in light intensity (owl), big distances, binocularly and some birds see also colors.

OK! But how to cure strabism?

There are several methods. One of them is to cover the healthy eye which will cause that the "ill" eye will have to work more. Other methods are mainly focused on strengthening and training extraocular muscles. One of my collegue is prepares set of games which can be used for diagnosing and curing strabism: here (unfortunatelly just in Czech).

One problem with curing strabism can be that after cure, the brain is not able to supress the deviated eye in the area of binocular vision and the patient can again perceive "double vision" (dyplopia).