Chapter 5 - Development And Plasticity Of The Brain

Question 0

What is proliferation?

Answer 0

It is the production of new cells. Early in development, the cells lining the ventricles of the brain divide. Some cells remain where they are (stem cells) others become neurons and glia that begin migrating to other locations.

Question 1

What are the processes involved in the development of neurons?

Answer 1

Proliferation, migration, differentiation, myelination, and synaptogenesis.

Question 2

What is migration?

Answer 2

Some neurons migrate faster than others, and a few of the slowest don’t reach their final destination until adulthood.

Some neurons move radially (from inside of the brain to the outside), some move tangentially (along the surface of the brain), and some do both.

Chemicals known as immunoglobulins and chemokines guide neuron migration. A deficit in these chemicals leads to impaired migration, decreased brain size, decreased axon growth, and mental retardation.

Question 3

Explain differentiation.

Answer 3

The axons grow first. After the migrating neuron reaches its destination, its dendrites begin to form.

Question 4

Describe myelination.

Answer 4

This is a later and slower stage. Myelin forms first in the spinal cord and then in the hindbrain, midbrain, and forebrain. This process continues gradually for decades.

Question 5

What is synaptogenesis?

Answer 5

This is the final step in the development of neurons. The formation of synapses. This continues throughout life but it generally slows in older people as does the formation of new dendritic branches.

Question 6

Can adult brains create new neurons?

Answer 6

Originally they thought the answers was no but later research found that stem cells in adults also split and become neurons within the brain.

Question 7

Describe the replacement of olfactory receptors.

Answer 7

Because the olfactory receptors are exposed to the outside world and its toxic chemicals, they have a half life of only 90days. Stem cells in the nose remain immature throughout life. Periodically they divide, with one cell remaining immature while the other differentiates to replace a dying olfactory receptor. It grows its axon back to the appropriate site in the brain.

There is also a population of stem cells in the interior of the brain. They sometimes divide to form “daughter”cells that migrate to the olfactory bulb and transform into glia cells or neurons. This is necessary to maintain the olfactory bulb. Without this gradual shrinkage will occur.

Question 8

Describe the formation of new neurons in the hippocampus.

Answer 8

Stem cells differentiate into neurons in adult mammals. The hippocampus is an important area for memory formation. Blocking the formation of new neurons impairs new memories.

In general they learn easily when they are young. As they grow older, their neurons become less changeable. More newly formed neurons survive during times of new learning. A supply of new neurons keeps the hippocampus young for learning new tasks.

Question 9

Describe the creation of other new neurons.

Answer 9

Research found that all of your skin cells are less than a year old. Skeletal muscles are replaced slowly, making the average cell 15 years old. Cells of the heart are, on average almost as old as the person (body replaces no more than 1% per year). And the mama, cerebral cortex forms few or no new neurons after birth under normal circumstances. New neurons do seem to form after a stroke, etc.

Question 10

Describe Sperry’s experiment.

Answer 10

1. Sperry’s cut the optic nerves of some newts.
2. The damaged optic nerve grew back and connected with the animals main visual area thereby reestablishing normal vision.
3. This time he also rotated the eye 180degrees.
4. The axons grew back to their original parts meaning that the newt now saw the world upside down and backwards.

Question 11

How do axons find the correct targets?

Answer 11

A growing axon follows a path of cell-surface molecules, attracted by some chemicals and repelled by others, in a process that steers the axon in the correct direction. Eventually, axons sort themselves over the surface of their target area by following a gradient of chemicals

Question 12

The axons may find their way to the general destination. How do they connect to the right synapses?

Answer 12

Each axon synapses onto many cells in the approximate correct location, and each target cell receives synapses from many axons. Over time, each postsynaptic cell strengthens some synapses and eliminates others.

Question 13

What is neural Darwinism?

Answer 13

In the development of the nervous system, we start with more neurons and synapses than we can keep. Synapses form with only approximate accuracy and then a selection process keeps some and rejects others. The most successful axons and combinations survive and the others fail.

Question 14

What is nerve growth factor (NGF)?

Answer 14

Initially, the sympathetic nervous system forms far more neurons than it needs. When one of its neurons forms a synapse onto a muscle, that muscle delivers a protein called nerve growth factor that promotes the survival and growth of the axon. An axon that does not receive NGF degenerates and its cell body dies. NGf. A cells apoptosis.

Question 15

What is apoptosis?

Answer 15

If an axon does not make contact with an appropriate postsynaptic cell by a certain age, the neuron kills itself through a process called apoptosis, a programmed mechanism of cell death.

Question 16

What is the purpose of apoptosis?

Answer 16

The brain system overproduced neurons and then applies apoptosis to ensure that the CNS can match the number of incoming axons to the number of receiving cells.

The sympathetic system doesn’t know the exact size of muscles and glands so it makes lots of axons and discards the excess.

In fact, all areas of the developing nervous system and brain had extra axons and got rid of the excess.

The visual cortex is thicker than average in people born blind. It has been unable to prune inappropriately synapses.

Question 17

What is a neurotrophic?

Answer 17

NGF is a neurotrophic, meaning a chemical that promotes the survival and activity of neurons. Neurotrophins are not necessary for survival of brain neurons but they are essential for growth of axons and dendrites, formation of new synapses and learning.

For an immature neuron to avoid apoptosis and survive, it needs to receive neurotrophins not only from its target cells but also from incoming axons.

When neurons release neurotransmitters, they also release neurotrophins. Neurons that fail to receive neurotransmitters also fail to receive neurotrophins, and so they die.

Question 18

Why is gastrulation important?

Answer 18

If you mess up early development, you have problems from then on. The early stages of brain development are critical. The early brain is highly vulnerable to malnutrition, toxic chemicals, and infections.

A fever is an annoyance for an adult but impairs neuron proliferation in a fetus. Low blood glucose decreases an adult’s pep but before birth impairs brain development.

Question 19

How do drugs impact on developing brains?

Answer 19

Fetal alcohol syndrome (hyperactivity, impulsiveness, difficulty maintaining attention, mental retardation, motor problems, heart defects, facial abnormalities)
Exposed to prenatal alcohol(impairments in learning, memory, language and attention)
Alcohol suppresses the release of glutamate, an excitatory neurotransmitter, and enhances activity of GABA, the main inhibitory neurotransmitter. Thus many neurons receive less excitation and neurotrophins than normal and they undergo apoptosis.

Cocaine or cigarettes (ADHD, behavioural deficits)
Antidepressants (increased risk of heart problems)
The immature brain is highly responsive to influences from the mother eg stress and fear.

Question 20

Explain differentiation of the cortex.

Answer 20

Immature neurons experimentally transplanted from oneart of the developing cortex to another develop the properties characteristic of their new location. However, neurons transplanted at a slightly later stage develop some new properties while retaining some old ones. Eg. Older children retain accents.

Question 21

Do axons and dendrites continue to modify their structure throughout life?

Answer 21

Yes. Some dendritic branches extend, others retract and disappear. The gain or loss of spines means a turnover of synapses, which relates to learning. Experiences guide neuronal change.

Eg a rat in more stimulating environment developed a thicker cortex, more dendritic branching and improved learning. Much of the enhancement produced by the enriched environment is due to physical activity. Activity improves learning and memory.

Question 22

What is far transfer?

Answer 22

Near transfer is training on one task and finding improvement on a very similar task. Far transfer is to teach one thing and hope students get smarter in other ways. It is weak and hard to demonstrate. Your brain is not like a muscle where you could exercise it to be bigger and stronger.

Many people advises older people to do crossword puzzles to exercise their brains. Correlation all studies show that people who engage in these activities do stay alert longer but studies also suggest that it does not help people to remember where they left their keys.

The best way to maintain intelligence in old age is physical activity. Physical activity enhances both cognitive processes and brain anatomy.

Question 23

How does the brain adapt in blind people?

Answer 23

Being blind does not change the touch receptors in the fingers or the receptors in the ears. However, it increases attention to touch and sound and the brain adapts to that attention.

PET and fMRI scans indicate substantial activity in the occipital cortex of blind people while they were reading Braille writing. In other studies they found auditory stimuli also produce increased responses in visual areas of the cortex.

Similar changes can occur in a person loses their sight in adulthood but to a limited extent.

Question 24

What is the difference between the brain of these who learned how to read in adulthood and those who do not?

Answer 24

Those who learned to read had more grey matter in five gyri of the cerebral cortex, and greater thickness in part of the corpus callosum.

Question 25

How does music training change the brain structure?

Answer 25

According to a study using MRI, grey matter of several brain areas was thicker in professional musicians. These areas strongly affected areas related to hand control and vision.

A related study on string players found that a larger than normal section of the postcentral gyrus in the right hemisphere was devoted to representing the fingers of the left hand (used to control the strings). These results suggest that practicing a skill reorganises the brain to maximise performance of that skill.

Question 26

What is focal hand dystonia?

Answer 26

Normally, if a skill is practiced it will cover a bigger area of the somatosensory cortex. Sometimes, this doesn’t work and instead of spreading over more of the cortexs surface, it overlaps. Eg. If you can’t feel the difference between one finger and another, it is difficult for them to move independently. This can also be called musicians cramp.

People who spend all day writing develop the same problem, known as writers cramp. They once thought the problem was in the hand but now they know that the issue is in the organisation of the brain. They are still looking for a solution

Question 27

Discuss the brains of adolescents.

Answer 27

Impulsiveness is a problem if it leads to risky driving, etc. remember that the prefrontal cortex matures slowly. Adolescents are more likely to choose an immediate reward. Adolescents are not equally impulsive in all situations. Peer pressure impacts on their decisions. Most research reports that adolescent prefrontal cortex is relatively inactive in certain situations.

Question 28

How does the brain change in old age?

Answer 28

People’s memory and reasoning fade beyond age 60. Neurons alter their synapses more slowly, the thickness of the temporal cortex shrinks, the volume of the hippocampus also gradually declines and certain aspects of memory declines, the frontal cortex begins thinning at age 30.

Much research underestimates older people. Some people show little signs of deterioration. Also, as they age they may be slower in many intellectual activities but they have greater base of knowledge and experience. Lastly, older people find ways to compensate for any losses. High performing older adults activate more brain areas to make up for less efficient activity.

Question 29

What are the possible causes of brain damage?

Answer 29

Tumours, infections, exposure to radiation, toxic substances, and degenerative conditions

Question 30

What causes most head injuries in young people?

Answer 30

Closed head injury. A sharp blow to the head resulting from an accident or assault that does not puncture the brain.

Most mild blows (falling off a bike) are recoverable within days. Severe head injuries recovery is slow and often incomplete.

Causes of damage as the result of closed head injury are 1. The rotational forces that drive brain tissue against the inside of the skull, 2. Blood clots that interrupt blood flow to the brain.

Question 31

What is a cerebrovascular accident?

Answer 31

It is a stroke, which is a common cause of brain damage in older people. It is a temporary interruption of normal blood flow to a brain area.

Question 32

Describe the 2 kinds of strokes.

Answer 32

Ischemia- this is the more common type. The result of a blood clot or other obstruction in an artery. The neurons are deprived of blood lose much of their oxygen and glucose supplies.

Hemorrhage- the result of a ruptured artery. Neurons are flooded with blood and excess oxygen, calcium, and other chemicals

Both ischemia and hemorrhage lead to many of the same problems:

• edema- the accumulation of fluid. This increases pressure on the brain and the probability of additional strokes.
• impair the sodium-potassium pump, leading to an accumulation of sodium inside neurons.
• the combination of edema and excess sodium provokes excess release of the neurotransmitter glutamate, which overstimulates neurons. Sodium and other ions enter the neurons faster than the sodium-potassium pump can remove them. The excess positive ions block metabolism in the mitochondria and kill the neurons.
• neurons die, microglia cells proliferate, removing the products of dead neurons and supplying neurotrophins that promote survival of the remaining neurons.

Question 33

What is tissue plasminogen activator (tPA)?

Answer 33

It breaks up blood clots. For the best benefits the patient should receive it within 3 hours of having a stroke. It is difficult to determine the type of stroke a person has had. tPA is useful for ischemia but will make things worse for hemorrhage patients. Doctors often decide to give it as hemorrhage is less common and usually fatal anyway.

Question 34

What other treatments might be useful for stroke victims?

Answer 34

1. Decrease stimulation by blocking glutamate synapses, blocking calcium
2. The most effective in lab animals is to cool the brain. This slows processes, additional research is happening.
3. Another method used on lab animals is exposure to cannabinoids (found in marijuana). This minimises the damage caused in animals. Cannabinoids put the brakes on the glutamate and has anti-inflammatory effects. It is best used when given to the animal just before the stroke.

Question 35

What is diaschisis?

Answer 35

After damage to any brain area, other areas that have lost part of their normal input become less active. It can impact on parts of the opposite hemisphere. Diaschisis refers to the decreased activity of surviving neurons after damage to other neurons. Recovery from a stroke depended largely on increasing activity for the opposite side of the brain.

Question 36

Does brain stimulation help stroke victims?

Answer 36

If diaschisis contributes to behavioural deficits then increased stimulation should help. Electrical stimulation (used on humans) and stimulant drugs (used on lab animals) have been shown to help. A new idea is to use drugs that block the release of GABA (an inhibitory).

Question 37

Can axons regrow?

Answer 37

Axons can grow back under certain circumstances. A crushes axon grows back towards the periphery following its myelin sheath yo the original target. If the axon is cut through the myelin on the two sides if the cut may not line up correctly, and the regenerating axon may not have a sure path to follow. In this case, a motor nerve may attach to the wrong muscle.

Question 38

What limits the axon regeneration in mammals?

Answer 38

1. a cut in the nervous system causes a scar to form creating a mechanical barrier. This blocks regrow to of axons later.
2. Neurons on the two sides of the cut pull apart.
3. The glia cells that react to CNS damage release chemicals that inhibit axon growth

Question 39

What methods are researchers developing to help axons to regrow?

Answer 39

1. They developed a way to build a protein bridge providing a path for axons to regenerate across a scar filled gap.(lab animals)
2. Injecting neurotrophins at appropriate locations helps axons grow and establish normal synapses. (Lab animals)
3. Infant axons grow under the influence of a protein called mTOR. As the individual matures, mTOR levels decrease and axons in the spinal cord lose their capacity for re growth. Deleting a gene responsible for inhibiting mTOR enables re growth of axons in adult spinal cord (lab animals)

Question 40

What are collateral sprouts?

Answer 40

New branches on axons

Question 41

Can axons create new synapses?

Answer 41

The surface of dendrites and cell bodies is covered with synapses and a vacant spot doesn’t stay vacant for long. After a cell loses input from an axon it secretes neurotrophins that induce other axons to form new branches or collateral sprouts that take over the vacant synapse. In areas of damage, new synapses form at a high rate.

Question 42

Is collateral sprouting helpful or harmful?

Answer 42

It depends on whether the sprouting axons convey information similar to those that they replace.

Question 43

What is denervation supersensitivity?

Answer 43

Neurons make adjustments to maintain a nearly constant level of arousal. Eg. If most of the axons that transmit dopamine to some brain area die or become inactive, the remaining dopamine synapses become more responsive, more easily stimulated. This is also called receptor supersensitivity.

This can have unpleasant side effects like chronic pain.

Question 44

If a brain loses a set of incoming axons, what happens?

Answer 44

We could expect denervation supersensitivity and collateral sprouting. We can also expect changes in the somatosensory cortex.

Question 45

How does the somatosensory cortex change after amputations?

Answer 45

The cortical cells that previously responded to information from finger 3 lost their input. Soon various cells became more responsive to finger 2, finger 4, or part of the palm. The cortical area does not stay silent after the loss of an arm. Gradually to face moved across into the vacant area. This reorganisation also occurs in other areas like the spinal cord, brain stem and thalamus.

Question 46

What is a phantom limb?

Answer 46

The phantom sensation can feel like an occasional tingling or intense pain. It can last for days to a lifetime.

Phantom limbs develop only if the relevant portion of the somatosensory cortex reorganises and becomes responsive to alternative inputs. Eg a touch on the face now produces a facial sensation but it also produces a sensation in the phantom hand.

Question 47

Is there any way to relieve a painful phantom sensation?

Answer 47

In some cases yes. Amputees who learn to use an artificial arm report that their phantom sensations gradually disappear. They start attributing sensations to the artificial arm and in doing so they displace abnormal connections from the face. Evidently the brain areas that start off as hand areas etc. retain those properties even after decades without normal input.

Question 48

What is deafferented mean?

Answer 48

The animal no longer feels the limb, although the motor nerves still connected to the muscles. We say a limb was deafferented because it has lost its afferent (sensory) input.

Research found that animals didn’t use the deafferented limb because they were not trying to. When they had no choice and had to use the limb suddenly they used it. They say that the damage does not destroy the memory trace but merely impairs the rats ability to find it. As the animal recovers it regains access to misplaced memories. Therefore therapy for people with brain damage focuses on encouraging them to practise skills that are Impaired but not lost. Therapists get their best results if they start soon after a patients stroke. The brain has increased plasticity during the first days after damage. One important note is that behaviour recovered after brain damage is effort full and its recovery is precarious.

Question 49

Is all brain recovery a matter of wait and see what the brain manages to do?

Answer 49

No. Much recovery from brain damage depends on learning to make better use of the abilities that were spared.