Development of the Nervous System-VI

🎯 Objectives

These lessons would familiarize the students with:

• The stages of development of the brain 🧠 and the changes which takes place 🔄
• 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 🔄💪

🧠 Brain Development: Post Natal

Brain development is modulated by external influences 🌍, and the interaction with external stimulation 📡 is important for the growth of the brain 🧠📈. The development of behavior 🎯 matches the development of the Central Nervous System 🔌.

The brain development can be seen from the following table 📊:

📊 Table 1: Age and Brain Development and Functioning

🔄 Motor and Sensory Development: Interactive Process

Motor development 💪 (movement and coordination 🤸) and sensory development 👁️👂 are interactive 🤝. Therefore, motor stimulation 💪 inputs into the sensory 📡 and stimulates the formation of connections 🔗 and action ⚡. Similarly, the sensory input 📡 stimulates the motor connections 💪. This leads to the development of the brain 🧠 from the neonate's brain being subcortical 🔌 (functioning from the reflexive systems 🔄, not the cortex 🧠) at 1 month 📅.

👶 Developmental Milestones

From this the infant moves to:

• Grasp objects 🤲
• Raising head 👶⬆️
• Smiling at familiar faces 😊
• Focus on objects 👁️🎯
• Learn to roll 🔄
• Sit 🪑
• Crawl 🚼
• Pick small objects with fingers 👆📌
• Eventually stand 🧍 and walk 🚶

All within the space of a year 📅 (or a little more ⏰). It must be remembered that the development of Central Nervous Systems 🔌🧠 matches these behavioral and motor/sensory development 🎯.

🌍 Influences Which Affect Brain Development

The brain is vulnerable 🧠⚠️ to external influences during the period of rapid growth 📈 and development 🌱. We would be talking about some of these influences 🌍. One fact must be clearly understood: the fetus does not have any protective own mechanism 🛡️❌ (except for the placental barrier 🧬). It is at the mercy of the mother 🤰, as whatever it gets (nutrition 🍽️, blood 🩸, cleaning up of toxics 🧹) is through the mother (the umbilical cord 🔗 is the connection). However, there is a critical period ⏰ when the influences can affect the brain 🧠, and the areas of the growing brain that are most vulnerable ⚠️. The influences would not be effective either before ⏪ or after ⏩ the period.

⏰ Critical Sensitive Period

The periods within which the growth of the organism 👶 can be affected by influences 🌍 i.e., the period of development 🌱, differentiation 🔀, and proliferation 📈. The period prior ⏪ or subsequent ⏩ to the critical period is not important ❌. The influence/substances will not affect development before/after the critical periods ⏰. Brain most susceptible only during that period 🎯. Normal development can be affected by presence/absence ✅❌, increase/decrease ⬆️⬇️ in the required/normal levels ⚖️. Each of the following influences has its own critical/sensitive period ⏰, depending on where or what was required 🎯.

🌍 Major Influences:

• Nutrition 🍽️
• Hormones 🧬
• External stimulation 📡
• Oxygen levels 💨
• Motor stimulation 💪
• Viral and other diseases 🦠 such as measles 🤒
• Drugs 💊, radiation ☢️, etc.

🍽️ Nutrition

(ref Brown and Wallace, 1988). If the mother 🤰 and the embryo 👶 suffer from malnutrition 🍽️⚠️ during the period when brain cells are dividing rapidly 🧫⚡, then there is decreased division of cells ⬇️ leading to decreased number of cells in the growing brain 🧠⬇️. Cells need nutrition 🍽️ and material to divide and grow 🌱. If this is not available ❌, the cells would not divide further ⛔. The sensitive period is from 2nd trimester (gestation) to 1st year post-partum 📅 (after birth 🍼). This is extended as the neurons in different areas are developing at different times ⏰. The effect of malnutrition depends upon the time during which specific cells are dividing 🧫 (Winnick and Russo, 1975). The damage is permanent ⚠️, causing irreversible damage ❌ to the Nervous System 🔌. The multiplying neurons require proteins 🥩, NGF 🧬, other tropic factors 🔋, if this process is blocked ⛔; this leads to the irreversible damage ❌ that researchers have reported 📚.

⚠️ Types of Damage from Malnutrition:

1️⃣ Reduced Cell Size 📉

Cells do not develop to their normal size ⚖️. Histologically 🔬, we see small cells 🧫📉 (starved cells 😔) as compared to normal ✅. This effect is reversible 🔄 i.e., if proper nutrition 🍽️✅ is given the cells would grow to normal size 📈.

2️⃣ Reduced Cell Number 📊⬇️

Cell division is affected by malnutrition 🍽️⚠️ of cells. Proliferation would not take place 🧫❌, as without sufficient nutrition 🍽️, cells will not multiply ⛔. If nutrition is provided during the critical period ⏰ when cells are proliferating 🧫, the effects are reversible 🔄. However, once the period is over ⏰✅ and cellular proliferation ends ⛔, no change is possible ❌.

3️⃣ Reduced Cell Connections 🔗⬇️

Connections between cells 🔗 and of cells with other tissue 🧬 are being formed 🏗️. Therefore, there is sprouting of cell extensions 🌿. If enough nutrition is not available 🍽️❌ then there is decreased sprouting of dendrites 🌿⬇️. We must remember that in order to have an efficient communication system 📡, neurons need multiple connections 🔗🔗. These input connections are provided by the dendrites and their extensions 🌿 (dendritic spines 🔗). If there is an increased number of dendritic branching 🌿⬆️ (spines 🔗), there would be large number of connections available 🔗🔗🔗. Thus, this results in a more efficient ⚡ and quicker 🏃 system of communication 📡. This becomes a more intelligent 🧠✨ and alert ⚡ young brain 👶🧠 as it can get information from a large number of sources 📊 (it's like a computer 💻 with a larger connectivity 🌐). Intellectual functioning 🧠 is affected by reduced cell connections 🔗⬇️. However, this is a reversible effect 🔄 only if nutrition is provided within the critical period ⏰.

4️⃣ Myelination Problems 🧬⚠️

Myelination is a fatty sheath 🧬 for insulation of neuronal processes 🔌 (just as electrical wires have rubber covering 🔌 to insulate them ⚡). If the formation of myelin covering does not take place 🧬❌ it can affect the efficiency ⚡ of neuronal processing 🧠 and functioning 🎯. Myelin loss is not extensively reversed by nutritional rehabilitation ❌.

🥩 Severe Protein Deficiency

This is a selective deficiency of proteins 🥩⚠️ leading to:

• Decreased number of neurons 🧠⬇️
• Decreased glial cells 🧫⬇️
• Decreased dendrites 🌿⬇️
• Deficient myelination of the processes 🧬❌

Every part of the growing neurons is affected by protein deficiency 🥩⚠️.

😷 Kwashiorkor

Is the disorder where selective protein deficiency 🥩⚠️ or depletion has taken place. The symptoms are:

• Thin muscles 💪⬇️
• But fat is present on the body 🍔
• With edema on the feet 🦶💧 etc.

😔 Marasmus

Is the symptom of malnutrition 🍽️⚠️ (due to reduced caloric intake 📉) in the fetus 🤰. Even if the newborn appears physically healthy 👶✅, there is irreversible brain damage 🧠❌. There are:

• Decreases in brain capacity by 10-20% 📊⬇️ as a consequence of this decreased caloric intake 🍽️⬇️
• The brain weights are lesser than normal ⚖️⬇️
• The brain size is smaller than normal 🧠📉 (fewer cells 🧫⬇️)

There are also deficits due to decrease number of Vitamins 💊 and minerals (such as iron ⚙️ and potassium ⚡) on the brain development 🧠.

📊 Research on Nutrition and Intelligence

Monkeberg (1975) studied 500 preschool children 👶, relating nutrition 🍽️ and intellectual functioning 🧠. He reported that in the malnourished group 🍽️⚠️ there were 40% children below the IQ 80 📉. As compared to this in the normal group ✅ were only 3% below the IQ of 80 ✅.

Interesting experiment by Weiner (1977) shows that rat mothers 🐀 spend more time ⏰ with their malnourished young 🐁⚠️ as compared to the healthy one 💪.

🤔 Points to Ponder:

How does the mother know the young is weak 🤔?

🧬 Hormones: Effects on Development

Hormones 🧬 affect the developing brain 🧠 and body 👶 by the presence/absence ✅❌ or increases/decreases ⬆️⬇️ in normal amounts ⚖️. We are going to discuss the Sex Hormones 🧬: Androgens testosterone 💪 (male ♂️), Estrogens 🌸 (female ♀️), the thyroid hormones 🦋 and the stress hormones 😰 and their influences on growth 📈 (Cotman and McGaugh (700-705), Brown and Wallace 428-435).

1️⃣ Androgens (Male Hormones ♂️💪)

The hormones are important for developing neural substrates of male organs 🧬♂️ and male sexual behavior 💪. The release at the appropriate time ⏰ determines what reproductive organs would grow 🌱 and elaborate 📈, but the release also primes the way the brain is organized 🧠🎯. If prenatal androgens are there 🧬✅ then the organism would develop a hypothalamus 🧠 which directs pituitary to release gonadotrophin in a tonic fashion ⚡. In its absence ❌, the pituitary has a cyclic pattern 🔄 of release which is the female pattern ♀️.

If females are injected with male hormones 💉♂️ during early pregnancy 🤰, they would have more masculinized children 👶♂️, i.e., females with male characteristics – pseudohermaphrodites 🔀 (Male + female organs in females ♂️+♀️). On the other hand, if male hormones are injected at prenatal periods ⏰💉 and later in puberty 🧒 "male behaviors" 💪 develop. Perhaps tomboys 👧💪 are females with androgen exposure during early prenatal period ⏰. This could also explain Sexual preferences 💕, sexual identity disorders 🔀 such as transvestites and transsexuals 🔄.

2️⃣ Estrogens (Female Hormones ♀️🌸)

Do not appear to be as important for bringing about changes in the NS 🧠 or body 👶. (This is controversial ⚠️ as there is evidence of feminization of male fetus ♂️→♀️ and development of female organs upon repeated injections of estrogens 💉🌸). The young animal would develop into a male ♂️ or female ♀️ depending upon the stimulation of androgen 🧬. Those which are not exposed to androgen ❌ will emerge with brain responses only to estrogen 🌸 and not androgen 💪 and develop typical female behavior ♀️.

3️⃣ Cretinism (Thyroid Deficiency 🦋⚠️)

This is a disorder of the growing brain 🧠⚠️ caused by severe Thyroid hormone deficiency 🦋❌ and the child has below normal intellectual functioning 🧠⬇️. The sensitive period for responding to thyroid is around the last trimester 📅 (6-9 months of gestation 🤰). The deficiency or normal amounts of thyroid results in poorly developed cerebral cortex 🧠⬇️ shows thyroid is important in growth 📈, metabolic rate ⚡, glucose absorption 🍬 etc. Thus, thyroxine 🦋 is important in growth 🌱. The deficiency results in:

• Decreased brain size 🧠📉
• Decreased number of neurons 🧫⬇️
• Decreased axons 🔌⬇️, dendrites 🌿⬇️
• Decreased connections between axons and dendrites 🔗⬇️
• Decreased electrical activity ⚡⬇️

If thyroid is given within the first year of life 🍼📅 some damages can be repaired 🔧✅. There is a reversible effect 🔄 if hormone therapy given early enough ⏰ when presence of thyroxine important 🦋.

❓ Questions to Consider:

If Hypothyroidism is bad 🦋⚠️, is hyperthyroidism good 🦋✅? No ❌, early sprouting of growth 🌱⚡! Out of synchronization 🔄❌ with the body and brain development 🧠👶.

4️⃣ Stress (Stress Hormones 😰⚡)

Levine (1960) showed that early stress beneficial ✅💪. Stress was induced 😰 through handling 🤲 or mild shock ⚡ in rat young 🐁. It was found they:

• Matured earlier 📅⚡
• Explore more in novel environment 🌍🔍

These animals could cope with stress later 😰💪 (How is stress in rats measured 🤔? Through their emotional responses 😰). These rats remained undisturbed 😌 and did not defecate or urinate with fear 💩❌. The normal rats showed a typical emotional response 😱 and cannot cope with stress 😰 and also matured later ⏰.

The handling stress led to increased secretions of adrenocorticotropic hormones (ACTH) ⚡🧬 which leads to increased Adrenal secretion in stress 😰 and with quicker absorption in the body 📈 (stress immunization 💪✅). This leads to an earlier development of stress response ⚡, as the release of ACTH normally occurs at 16 days 📅 in rats, in the stressed rats this appears at 12 days 📅 (4 days earlier ⏰⚡).

⚠️ Stress Can Be Bad Too

As well as increasing the duration of stress 😰⏰. Ackerman, Hofer & Weiner (1978), compared adult rats 🐀 which had been separated from mothers at 15 days 📅) with adult rats separated from mothers at 22 days 📅. They report that early removal from mother 🤰➡️👶 leads to:

• High risk of ulcers 🤢⚠️
• Defective thermoregulatory systems 🌡️⚠️
• Could not survive in extreme cold 🥶 or heat 🥵 (cold and hot challenges ❄️🔥)

📚 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