Development of the Nervous System-V

🎯 Objectives

To familiarize the students with:

• Various stages of neuronal development 🧬🔬
• Development of the brain 🧠: from the fertilization 🥚 to the various developmental stages in-utero 🤰, and postnatally 👶
• Cell differentiation 🧫, determination, migration 🚶, (inside-out) ↗️, cell competition ⚔️, cell death 💀, growth cones 🌱, Nerve Growth Factor and its role 🧪, influences in growth and development of the brain 📈
• Teratogenesis 🧬⚠️ - Genetic anomalies which affect development of the brain 🧠
• Plasticity of the Nervous system 🔄💪

🤔 Environment or Genetic Programming?

It is well researched and documented 📚 that during early brain development 👶🧠 the outside stimulation from the periphery 📡 becomes important as cells make connections with other cells 🔗 and with the muscles 💪. It is well established that when connections are made 🤝, the neurons with connections survive 💪, those who don't, don't survive 💀. Further, if the peripheral structure is removed ✂️, cells for that structure degenerate in the brain 🧠⬇️ and die off 💀.

👁️ Classic Visual System Experiments

There were a series of experiments carried out on the visual systems 👀 by the famous Nobel Laureates 🏆, Hubel and Wiesel 👨‍🔬👨‍🔬. In one of the experiments, one eye of the kitten 🐱 was removed ✂️👁️ during early development 🔬 and it was found that the striate cortex 🧠 (cortical area for vision 👁️) degenerates for the eye that has been removed ⬇️💀 and enlarges for the other eye ⬆️📈: i.e., for one it enlarges ⬆️ whereas the areas for other it diminishes ⬇️. The neurons from the non-deprived 💪 take over the areas vacated by the deprived 🎯.

🐀 Whisker Experiments on Rats

Similarly, experiments were carried out in which the whiskers 👃 (whiskers on the face of the rats 🐀) of the growing rat pup 🐁 were removed ✂️. These are in the form of barrels 🛢️. If the whiskers were removed ✂️ there was also degeneration of neurons 💀 which were responding to the whiskers 👃. However, the neighboring neurons get bigger 📈⬆️. If peripheral structure gets enlarged 📏⬆️ then fewer neurons die off 💀⬇️ as they get a site to connect with 🔗. If structure added such as another eye 👁️ or another limb 🦵 then that part also gets innervations 🔌 (Hollyday and Hamburger 1976).

🎯 Functionality of Peripheral Organs Correlates with Their Influence on Neural Development

An interesting series of experiments 🔬 by Merzenick were carried out to test out the influences of external stimulation 📡 on the development of brain areas 🧠. The third and fourth fingers 👆👇 of the monkey's hand 🐵✋ were joined together 🔗, and it was found that the area in brain also merged 🔄. However, if a finger is removed ✂️ then the neighboring fingers extend areas 📏 to use the "vacated area" 🎯 in the brain 🧠.

🎻 Brain Plasticity in Musicians

Using advanced technology 🔬 such as the MRI 🧲 this has been confirmed further ✅. Those who use a particular body area 💪 (or exercise it 🏋️), have enlarged corresponding brain area 🧠⬆️. The brain MRIs of violin players 🎻 were compared with the non-musicians 🚫🎵 for the finger areas representations 👆 - it was found that the violin players had larger areas 📈, these were the stimulated areas 📡, and were much larger than the ones which were not stimulated ⬇️.

❓ The Big Question

The question which is important: environmental stimulation 🌍 or genetic programming 🧬 keeps coming up again and again ❓ - is a major issue in developmental neurobiology 🧬🧠.

🤔 Two Key Questions:

• a. Is the Brain preprogrammed 🧬🎯 to grow in a particular manner and develop specific functions?
• b. Is the Brain affected by outside influences 🌍📡 and interference affects growth 📈?

✅ Evidence for Environmental Influence

Yes: In an intriguing experiment 🔬 Mriganka Sur (1988, personal communication) modified the embryonic neuronal connections 🔗 in such a way that fibers coming from the visual system 👁️ such as the optic nerves 🔌 and the eyes 👀 were connected to auditory cortex 👂, and vice-versa 🔄. The question was: would the auditory cortex start seeing 👀 or visual cortex start hearing 👂? Nothing of this sort happened ❌, the visual cortex developed in auditory areas 🧠, where the inputs were coming in 📡. Thus, these experiments showed the environmental stimulation was important 🌍✅ for brain areas to grow 📈.

✅ Evidence for Genetic Preprogramming

On the other hand, experiments carried out by Hubel and Wiesel 👨‍🔬, and Rizzolatti and colleagues 👨‍🔬 show that brain cells respond to only one kind of stimuli 📡 (they are preprogrammed 🧬). According to them, the brain is preprogrammed to develop and respond in a particular manner 🎯. The brain is not influenced by environmental influences 🌍❌ and no feedback can affect brain development 🧠. Experiments by Hamburger on chick embryos 🐣 show that there is an autonomous development 🧬 of the brain 🧠.

🐔 Hamburger's Chick Experiments

In Hamburger's experiments, differentiation 🔀 (messages do not get back towards the brain 🧠) at different stages of embryonic development 👶 was carried out. This means cutting spinal cord 🦴✂️ at different points without causing complete paralysis ⚠️. Decreasing the input from skin 👐 and other areas does not affect development of the brain 🧠 from day 3 embryonic ages 📅 to 3 days before hatching 🐣 (what if there was stimulation even before 3 embryonic days 🤔?). However, there was also no change in development of behavior patterns 🔄.

🧩 The Issues Still Remain!

The debate continues 🔄 - the mystery persists 🤔!

💡 Lesson to Take Away

Brain development 🧠 and outside influences 🌍 interact with each other 🤝. This is important for the development of the Nervous System 🔌.

🤔 Points to Ponder and Take Away

❓ What About Adults: Is Neurogenesis Possible?

When neurons die in the adult brain 🧠💀 do we grow new ones 🌱?

Earlier this was not thought to be possible ❌, and "once neurons die off 💀, they do not regenerate" was the standard ⚠️. Now, more evidence has come in showing that regeneration of neurons in adult brains is possible 💪✅.

🐦 Neurogenesis in Birds

In adult birds 🐦, areas responsible for songs 🎵 grow new cells 🌱 prior to the mating season 💕 (evolutionary benefit 🧬?)

🐀 Neurogenesis in Rats

Similarly, in the rat hippocampus 🧠 (area important for memory 💭) cells grow 🌱 (adult neurogenesis: new cells @ 2000 per hour ⏱️!) while rats are learning a task 📚🐭.

🧠 Neurogenesis in Primates and Humans

Similarly in the primates 🐵 and in humans 👤, there is growth of cells 🌱 in the association cortex 🧠 (higher order functioning 🎯). We have seen reorganization of cortical areas 🔄 due to experience 📚 in adult brain 🧠 (violin players 🎻, remember!).

🔬 Stem Cell Research

With the ongoing stem cell research 🧬, possibilities for regenerating 🔄 and repairing 🔧 damaged brains 🧠⚠️ using these cells may provide hope 🌟 for the brain damaged 💔.

📊 Early Embryonic Development: Quick Summary

❤️ Cardiovascular and Nervous System

👶 Post Natal Development and Growth: Quick Summary

🍼 New Born

• Brain weight 🧠⚖️: 350g – 10% of body weight 📊
• Brain layers 📋: 6 layers (as in adult brain 👤🧠) but very few connections or processes 🔗❌
• Cell density 🧫: Dense packing 📦, greater number of cells ⬆️
• Myelination 🧬: Little or no myelination ❌ (insulation sheath for the fibers 🔌)

👤 Adult

• Brain weight 🧠⚖️: 1400g, 2% of body weight 📊
• Brain layers 📋: Six Layers with many connections 🔗🔗🔗
• Cell density 🧫: Less dense packing 📦⬇️, decreased number of cells ⬇️, increased number of connections 🔗⬆️
• Myelination 🧬: Myelination of Axons and Dendrites ✅

📊 Comparison Summary

From the above comparison we can see that the neonate brain 👶🧠 has a long way to go 🛣️ before it can resemble the adult brain 👤🧠. The differences are contradictory 🔄: the cells number greater than the adult brain 🧫⬆️, but this is enclosed in a smaller sized "container" 📦 therefore it is very densely packed 📊. There are fewer connections 🔗⬇️ and smaller brain weight ⚖️⬇️. In the adult, the brain weight increases ⚖️⬆️ due to increases (in millions 📈) in cell connections 🔗🔗🔗.

📚 Next Lesson Preview

In the next lesson we would discuss the growth and development 📈 of the brain 🧠 as it progresses towards the adult size 👤 and functioning 🎯.

📚 References

• Kalat, J.W. (1998). Biological Psychology. Brooks/Cole Publishing Company.
• Carlson, N. R. (2005). Foundations of physiological psychology. Pearson Education New Zealand.
• Pinel, J. P. (2003). Biopsychology. (5th ed). Allyn & Bacon Singapore.
• Bloom, F., Nelson., & Lazerson. (2001), Behavioral Neuroscience: Brain, Mind and Behaviors. (3rd ed). Worth Publishers New York
• Bridgeman, B. (1988). The Biology of Behavior and Mind. John Wiley & Sons, New York
• Brown, T.S. & Wallace, P.S. (1980). Physiological Psychology. Academic Press, New York