DEVELOPMENT OF THE NERVOUS SYSTEM:The issues still remain

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Neurological Basis of Behavior (PSY - 610)

Lesson21

DEVELOPMENT OF THE NERVOUS SYSTEM

Objective

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 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.

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. There were a series of experiments carried out on the visual systems by the famous

Nobel Laureates, Hubel and Weisel. 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

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 neighbouring

neurons get bigger. If peripheral structure gets enlarged then fewer neurons die off as they get a site to

connect with normal die or degenerate. 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 the 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. 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 question which is important environmental stimulation or genetic programming keeps coming up

again and again is a major issue in developmental neurobiology.

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?

Neurological Basis of Behavior (PSY - 610)

Yes: In an intriguing experiment Mriganka Sur (1988, personal communication) modified the

embryonic neuronal connections in such a way that fibers coming form 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.

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. 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!

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. In adult birds, areas responsible for songs grow new cells prior to the mating season

(evolutionary benefit?). Similarly, the in the rat hippocampus (area important for memory) cells grow

(adult neurogenesis: new cells @ 2000 per hour!) while rats are learning a task. 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!)

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

Heart------- ----------3 weeks

1st NS reflex---------8 weeks

Swallowing-----------11 weeks

Stretching movements 22 weeks

Sucking development ---29 weeks

20,000 neurons per minute

Neurological Basis of Behavior (PSY - 610)

Post natal development and growth: quick summary

New Born

Brain weight: 350 g 10% of body weight.

Brain layers: 6 layers, ( as in adult brain) but very few connections or processes

Cells density: dense packing, greater number of cells

Little or no myelination (insulation sheath for the fibres)

Adult

Brain weight: 1400 g, 2% of body weight

Brain layers: Six Layers with many connections

Cells density: less dense packing, decrease number of cells, increase number of connections

Myelination of axons and dendrites

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 increase due to increases (in

millions) in cell connections.

In the next lesson we would discuss the growth and development of the brain as it progresses towards

the adult size and functioning.

References

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

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

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

edition) Worth Publishers New York

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

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

Academic Press New York

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