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Neurological Basis of Behavior (PSY - 610)

Lesson22

DEVELOPMENT OF THE NERVOUS SYSTEM

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 differtiation, 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 behaviour matches the development of the

Central nervous System.

The brain development can be seen from the following tables

Table 1: Age and brain development and functioning

Age

Brain

Structural development and changes

Behaviours

weight

neonate 350 g

Dense cells, very few fibres

Sub cortical reflexes, moro

grasp,rooting

2-3 mos

500 g

Grey and white matter is poorly

Visuo sensory motor integrative

differentiated, not densely packed

functioning

neurons ( Nissyl staining)

-8 660 g

Cytoplasm more , and distinction

Higher Cognitive and cognitive

mos

between grey and white matter

functioning

8-12

925 g

Reduced cell number, increase in

Interaction

with

their

mos

neuronal/glial fibres ( for connections)

surroundings and show strange

year

anxiety

3 years

1080 g

Further reduction in cell number and

increases in cell connection

6-14

1350 g

Maturation and myelination of cells

years

continues

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 neonates brain being subcortical (functioning form the reflexive systems, not the cortex) at 1

month from this the infant moves to grasp objects, raising head, smiling at familiar faces, focus on

objects, then learn to roll, sit, crawl, pick small objects with fingers, and 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.

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Neurological Basis of Behavior (PSY - 610)

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 that the fetus does

not have any protective own mechanism of protection (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 wither 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.

1. Influences:

Nutrition

Hormones

External stimulation,

Oxygen levels e) 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, and 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. The damage can be

categorized as follows:

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.

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.

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

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Neurological Basis of Behavior (PSY - 610)

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.

Myelination: Myeliantion 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, glial cells, dendrites, and 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, and the brain size is smaller than normal (fewer cells)

There are also deficits due to decrease amount of. Vitamins and minerals (such as iron and potassium)

on the brain development.

Monkeberg (1975) studied 500 pre school 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 alnourished

young as compared to the healthy one.

POINTS TO PONDER:

How does the mother know the young is weak?

Hormones: 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)

1. Androgens: 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 preganancy, they would

have more masculinized children, i.e. females with male characteristics psuedohermaphrodites

(Male+female organs in females). On the other hand if male hormones are injected at prenatal

periods and later in puberty"male behaviors". 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.

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Neurological Basis of Behavior (PSY - 610)

2. Estrogens 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.

3. Cretinism: 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, 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:

if Hypothyroidism is bad, is hyperthyroidism good? No, early sprouting of growth! Out of

synchronization with the body and brain development

4. Stress: Levine (1960) showed that early stress beneficial. Stress was induced through handling

or mild shock in rat young. Wit 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 lead 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 a high risk of ulcers, these animals also

had defective thermoregulatory systems. They could not survive in extreme cold or heat (cold

and hot challenges).

References

1. Kalat J.W (1998) Biological Psychology Brooks/ Cole Publishing

2. Carlson N.R. (2005) Foundations of Physiological Psychology Allyn and Bacon, Boston

3. Pinel, John P.J. (2003) Biopsychology (5th edition) Allyn and Bacon Singapore

4 Bloom F, Nelson and Lazerson (2001), Behavioral Neuroscience: Brain, Mind and Behaviors (3rd

edition) Worth Publishers New York

5. Bridgeman, B (1988) The Biology of Behaviour and Mind. John Wiley and Sons New York

6. Brown,T.S. And Wallace. (1980) P.M Physiological Psychology

Academic Press New York