Chapter 10

Question 0

How does these genes build this intricate system?

Answer 0

The first set of genes aims at generate the neurons and help them specialise. A second set of genes help them wire up in the intricate manner.

Question 1

How many genes does most mammals have? How much is devoted to building and maintaining the brains? How many neurons are there?

Answer 1

There are 25000 genes in most mammals, and half of these are primarily devoted the building and maintaining the brain. The human brain had 86 billion neurons, therefore the genes just provide a basic sketch rather than a specific blueprint.

Question 2

Write about the formation of the neural tube?

Answer 2

The neural tube is formed from the neural plate. The mesoderm underneath develops into the notochord which secretes certain factors which causes the neural plate to sink. The sinking in the middle cause two edges to come together. The edges are the cells of neural crest. As soon as the neural plate makes a complete cylinder, the cells of the neural crest forms the ganglia of the autonomic nervous system and dorsal root ganglion of all spinal nerves. They also form melanocytes of the skin, and the bones and muscles of the face.

Question 3

How does the neural tube form into the brain?

Answer 3

The neural tube grows three swellings on the rostral end. The most rostral swelling is known as the forebrain vesicle, the second swelling as the midbrain vesicle and the last swelling as the hindbrain vesicle. As development continues, the forebrain vesicle forms two addition vesicles laterally to itself. These are known as the lateral vesicles. The lateral vesicles develop into the cerebral cortex and the basal ganglia. The first part of the cortex to develop is the hippocampus, olfactory areas and later the neocortex.

The forebrain vesicle develops into the rest of the cerebral structures- diencephalon and the hypothalamus. The midbrain vesicle forms two cell groups that are like bumps in the dorsal side. This is the tectum but more commonly referred to as the superior and inferior olive. The most rostral part has the diamond shaped fourth ventricle. Posterior to it, two outgrowth forms to later develop into the cerebellum.

Question 4

What can be said about the multiplication about the neurons in the post natal life?

Answer 4

The few hundred cells quickly multiplies and specialises into the different types of the neuron to form the intricate neural system of 90 billion neurons. During first two years of life, there are times when upto a quarter of million cells are produced in a minute.

Question 5

How do the cells in the cortex migrate?

Answer 5

Most cells are produced in the linings of the ventricles. The cells travel radially to the cortex in which they reside, most of which is the future pyramidal cells. However there is a smaller group of GABAergic neurons in the ganglionic eminence that travels tangentially across the cortex. The ganglionic eminence also produces the GABAergic cells of the striatum.

Question 6

What did Pasko Racik discover?

Answer 6

He discovered the tangential migration of the GABAergic neurons in the rodents’ brain. He questions the significance of tangential migration in the human brains.

Question 7

Where is the rhombic lip located?

Answer 7

The rhombic lip is located at the rostral end of the hindbrain called the rhombic lip. It produces the cells that make up the hindbrain and the cerebellum. Since the cerebellum accounts for 70% of the neurons in the brain, the rhombic lip is the most active site of cell generation in the developing nervous system

Question 8

What are proneural genes?

Answer 8

Proneural genes guide the cells to differentiate into different functional cells. In the embryo there are ten proneural genes that are known as basic Helix Loop Helix(bHLH) genes. It is believed that the bHLH genes work with additional factors to produce 100 different neuronal clusters. It is closely related to Notch/delta signalling pathway. For example, Ngn will inhibit neighbouring uncommitted cells into becoming neurons.

Question 9

What are some proneurals?

Answer 9

Math1/Atoh1
Mash1
Ngn
Tcf4

Question 10

How is the future forebrain segmented during development

Answer 10

The cortex is clearly separated from the non cortical regions. The diencephalon is split into three segments called prosomeres.

Question 11

How is the hindbrain segmented?

Answer 11

The most rostral part of the hindbrain is called the isthmus, and the following eleven segments are rhombomeres. First there was thought to be only seven rhombomeres. The genes responsible for segmentation in the hindbrain is due the homeobox genes(hox). There are 13 hox genes in insects and four sets of these genes in vertebrates. Hox 1-4 are expressed in the hindbrain and the rest in the spinal cord, limbs, trunk and the tail.

Question 12

What is not solely controlled by hox genes in the hindbrain?

Answer 12

The isthmus and the first rhombomeres. Their the areas that gives rise to the cerebellum too.

Question 13

What can the be said about the segmentation process in the brain?

Answer 13

The segmentation process is similar to the segments of the lobster. The lobster is segmented similarly, except their specialised due to their projections likes legs, antenna and claws. The hox genes helps in the development of the hindbrain segments and the associated facial bones and muscles.

Question 14

What has powerful effects on the rostrocaudal development of the brain?

Answer 14

The isthmus organiser in the junction of the midbrain and hindbrain.

Question 15

What are some genes that have further development of the midbrain and the hindbrain, including the cerebellum?

Answer 15

A number of genes including wnt1, en1, and fgf8 have strong influence over the development of the brainstem and the cerebellum.

Question 16

What is the sonic hedgehog gene?

Answer 16

Sonic hedgehog plays an important role in the early role in the dorsoventral pattern of the neural tube. The sonic hedgehog is released in the ventral side and uses gradient concentration to influence cells. The lowest concentration of sonic hedgehog is induced to differentiate into motor neurons, this is the reason why the dorsal spinal cord contains motor functions.

Question 17

How was the importance of the isthmus organiser proven?

Answer 17

It was first demonstrated by grafting experiments in developing birds and then was proven in mammals too.

Question 18

What is an organising centre and give examples?

Answer 18

The organising structures are structures that are responsible for the development of the surrounding structures. For example, the isthmic organisers is partially responsible for the organising the brainstem and the cerebellum. Similarly, the notochord is an organising centre for the spinal cord.

Question 19

How does motor neurons find their right target muscle?

Answer 19

Motor neurons and the target neuron expresses the same gene. This allows it to find the right target with an accuracy over 95%

Question 20

What are the most important connection which are layed down first?

Answer 20

The most important connections are of the respiratory system and swallowing, since they are needed for survival. The body cannot afford to make a mistake in these connections, therefore these connections are formed according to a strict program.

Question 21

When are the connections with muscles done?

Answer 21

In mice, it is 12 days after fatal development and in humans it is 12 weeks after feral development.

Question 22

What is the second second set of connections?

Answer 22

The second set of connections are formed at the end of fetal development and extends throughout childhood. These connections are not made through rigid programs, instead is formed by the interaction with the stimuli. It was thought the brain will not make new connections after a certain period, however it was learnt the new connections are made throughout our life but probably not as actively during childhood.

Question 23

What is the main mechanism of axon guidance?

Answer 23

Pairing of axons and targets that express the same gene. It is believed that the axons just follows a normal pathway until it begins to sense faint signals from its targets. This is found in the brainstem nuclei connecting to certain cells in the cerebellum, and motor neurons connecting to their target.

Question 24

What is the role of the growth cone?

Answer 24

The growth cone is the tip of a growing axon, that moves through the tissue seeking for faint signals that leads to its correct target or steers it away from unwanted connections. The signals are like signposts and is called guidance molecules.

Question 25

What are some guidance molecules?

Answer 25

Netrins, semaphorins and Slit proteins.

Question 26

How does the guidance molecules work?

Answer 26

For example, an area expresses the ephrin protein or molecule in different gradients. The growth cone has Eph receptors in varying numbers. So the growth cone with low number of ephrin receptors need to travel through the tissue until it reaches an area of high ephrin concentration, whilst growth cones with high ephrin receptors will stop immediately at the contact with tissue that has low ephrin concentration itself.

Question 27

What is use it or lose it theory?

Answer 27

The use it or lose it theory is the idea of losing neurons that are not stimulated for a long time. Neurons that are active keep the transport protein KIF4 active, which suppresses a regulatory enzyme. When the neuron is not stimulated for an amount of time, the unsuppressed enzyme cause the cell to shut down and disassemble its protein, a process called apoptosis

Question 28

Which neurons are pruned?

Answer 28

The neurons that shows evidence of poor connections or are unnecessary are pruned. This allows the brain to be more efficient.

Question 29

How does the environment influence gene expression?

Answer 29

The genes are responsible for the development of the brain during prenatal stages. However, after birth, complex interactions between the organism and the immediate surroundings or environment helps to shape the brain accordingly. The brain is constantly being shaped to adapt to the world around it. This is done by environmental factors changing or turning on and off of gene expression.

Question 30

What is critical period?

Answer 30

The critical period is a period of time, usually around 6 years after birth, where the brain development relies heavily on stimuli from the environment. If deprived of the stimuli, the capability to react to the stimuli is not developed.

Question 31

What is the difference between the theory of critical period 60 years ago and now?

Answer 31

The period of time was applied rigidly and it was thought that it was not possible to recover that development after that period. However, now the period of time is applied loosely and now known delayed inputs can be still effective.

Question 32

What is sensitive periods and how does it differ from critical periods?

Answer 32

Sensitive periods are elongated periods in which the nervous system can still develop if provided with the stimuli.

Question 33

What are some sensitive periods for acquiring certain skills?

Answer 33

Language acquisition is from nine months to five years
Emotional control is from birth to two years.
Motor control is from birth to eight years.

It is important to recognise this as the most optimal time to learn and not the only time to learn these skills.

Question 34

What are neural tube defects and its effect?

Answer 34

Neural tube defect or Spina bifida is a disruption in the closure of the neural tube during very early stages of development. Some people are genetically predisposed to this condition, but the most common cause is insufficient folate in the mother. The membrane of the spinal cord does not cover properly and bulges out of the back of the baby. In severe cases, the lower limbs, bladder and bowel are paralysed. It is usually treated by artificially covering the spinal cord.

Question 35

What happens when the embryo is affected by alcohol?

Answer 35

Alcohol consumption by the mother can cause fetal alcohol syndrome.

Question 36

What are the effects of infections like rubella on the fetal development?

Answer 36

Infections like rubella, can cause severe brain abnormalities, including blindness and deafness.

Question 37

What effect does iodine deficiency have on fetal development?

Answer 37

It is the most common cause of mental retardation in millions of children in Asia and South-East Asia. It can be easily remedied by the intake of iodine supplements such as salt and oil.

Question 38

What is the critical period for attachment?

Answer 38

A period of time in which the baby has to development attachment to the primary caregiver for the healthy emotional development of the child. It extends over for the first two years of our life but the first year is crucially important. Failure in attachment in this period has reflected aggressive behaviour when they are teenagers and adults.

Question 39

What does iodine deficiency result in?

Answer 39

It results in hyperthyroidism, where the thyroid gland enlarges to compensate for the deficiency. The enlargement is called goiter. This results in physical and mental growth retardation in fetal developement and this is called cretinism.