Chapter 5- Development And Plasticity Of The Brain

Question 1

The human central nervous system begins to form when the embryo is about __ weeks old. The dorsal surface thickens and then long thin lips rise, curl, and merge, forming a _____ ____ that surrounds a fluid-filled cavity. As the tube sinks under the surface of the skin, the forward and enlarges and differentiates into the ______, ______, and______. The rest becomes the spinal cord. The fluid-filled cavity within the neural tube becomes the central canal of the spinal cord and the four ventricles of the brain, containing the cerebrospinal fluid.

Answer 1

Two; neural tube; hindbrain, midbrain, and forebrain

Question 2

Neuroscientists distinguish these 5 processes in the development of neurons:

Answer 2

Proliferation, migration, differentiation, myelination, and synaptogenesis

Question 3

Production of new cells

Answer 3

Proliferation

Early in development, the cells lining the ventricles of the brain divide. Some cells remain where they are as stem cells, continuing to divide. Others become primitive neurons and glia that begin migrating to other locations.

Question 4

Movement of brain neurons or glia

Answer 4

Migration

Some neurons migrate much faster than others, and a few of the slowest don’t reach their final destinations until adulthood. Chemicals known as immunoglobulins and chemokines guide neuron migration

Question 5

To develop the axon and dendrites that give a neuron its distinctive properties

Answer 5

Differentiate

The axon grows first. In many cases, a migrating neuron tows it’s growing axon along like a tail, allowing it’s tip to remain at or near its target. In other cases, the axon needs to grow toward its target, finding its way through a jungle of other cells and fibres. After the migrating neuron reaches its destination, it’s dendrites begin to form

Question 6

Process by which glia produce the insulating fatty sheaths that accelerate transmission in many vertebrate axons

Answer 6

Myelination

Myelin forms first in the spinal cord and then in the hindbrain, midbrain, and forebrain. Unlike the rapid proliferation and migration of neurons, myelination continues gradually for decades.

Question 7

Formation of synapses

Answer 7

Synaptogenesis

Although this process begins before birth, it continues throughout life, as neurons form new synapses and discard old ones. The process generally slows in older people, as does the formation of new dendritic branches

Question 8

Which develops first, a neurons axon or it’s dendrites?

Answer 8

The axon forms first

Question 9

Undifferentiated cells that divide and produce daughter cells that develop more specialized properties

Answer 9

Stem cells

Question 10

In which brain areas do new neurons form in adults?

Answer 10

Olfactory receptors, neurons in the hippocampus, and neurons in the song-producing areas of certain bird species

Question 11

What evidence indicated that new neurons seldom or never form in the adult cerebral cortex?

Answer 11

The 14C concentration in the DNA of cerebral cortex neurons corresponds to the level during the year the person was born, indicating that all or nearly all of those neurons are as old as the person is.
The concentration of 14 C in the atmosphere, compared to other isotopes of carbon, was nearly constant overtime until the era of nuclear bomb testing, which released much radioactivity. The concentration of 14 C peaked in 1963, and started to decline with the test ban treaty of 1963

Question 12

What did the famous biologist Paul Weiss conclude about chemical pathfinding by axons after conducting experiments where he grafted an extra leg to a salamander and then waited for axons to grow into it?

Answer 12

After the axons reach to the muscles, the extra leg moved in synchrony with the normal leg next to it. Weiss dismissed the idea that each axon found its way to exactly the correct muscle in the extra limb. He suggested instead that the nerves attach to muscles at random and then send a variety of messages, each one tuned to a different muscle. Each muscle received many signals but responded to only one

Question 13

What was Roger Sperrys evidence that axons grow to a specific target instead of attaching at random?

Answer 13

Sperry found that if he cut a newts eye and inverted it, axons grew back to their original targets, even though the connections were inappropriate to their new positions on the eye.
A growing axon follows the path of cell-surface molecules, attracted by some chemicals and repelled by others, and 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 14

If all cells in an amphibians tectum produced the same amount of TOPdv, what would be the effect on the attachment of axons?

Answer 14

Axons would attach haphazardly instead of arranging themselves according to their dorsoventral position on the retina

Question 15

Principle of competition among axons

Answer 15

Neural Darwinism

In the development of the nervous system, we start with more neurons and synapses then 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 16

A protein that promotes the survival and growth of axons in the sympathetic nervous system and certain axons in the brain

Answer 16

Nerve growth factor NGF

Question 17

A programmed mechanism of cell death

Answer 17

Apoptosis

Question 18

A chemical that promotes the survival and activity of neurons

Answer 18

Neurotrophin

Question 19

Rita Levi-Montalcini discovered that the muscles do not determine how many axons form; they determine how many ______

Answer 19

Survive

Question 20

Describe the formation of neurons in the sympathetic nervous system and understand why there is initial overproduction of neurons

Answer 20

Initially, the sympathetic nervous system forms far more neurons that it needs. When one of its neurons forms a synapse onto a muscle, that muscle delivers a protein called nerve growth factor NGF that promotes the survival and growth of the axon. An axon that does not receive NGF degenerates, and its cell body dies. The neuron kills itself through a process called apoptosis. NGF cancels the program for apoptosis.
The brains system of over producing neurons and then applying apoptosis enables the CNS to match the number of incoming axons to the number of receiving cells. When the sympathetic nervous system begins sending axons toward the muscles and glands, it doesn’t know the exact size of the muscles or glands. It makes more neurons the necessary and discard the excess. In fact, all areas of the developing nervous system make far more neurons than will survive into adulthood. This loss of cells is a natural part of development.

Question 21

Nerve growth factor is a _______, meaning a chemical that promotes the survival and activity of neurons.

Answer 21

Neurotrophin

In addition to NGF, the nervous system responds to brain derived neurotrophic factor BDNF and several other neurotrophins. 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

Question 22

What process ensures that the spinal cord has the right number of axons to innervate all the muscle cells?

Answer 22

The nervous system builds far more neurons then it needs and discards through apoptosis those that do not make lasting synapses

Question 23

What class of chemicals prevents apoptosis?

Answer 23

Neurotrophins, such as nerve growth factor

Question 24

At what age does a person have the greatest number of neurons – before birth, during childhood, during adolescence, or during adulthood?

Answer 24

The neuron number is greatest before birth

Question 25

A condition resulting from prenatal exposure to alcohol and marked by hyperactivity, impulsiveness, decreased alertness, varying degrees of mental retardation, motor problems, heart defects, and facial abnormalities

Answer 25

Fetal alcohol syndrome

The mechanism of fetal alcohol syndrome relates partly to apoptosis: Alcohol suppresses the release of glutamate, the brains main excitatory transmitter, and enhances activity of GABA, the main inhibitory transmitter. Consequently, many neurons receive less excitation and neurotrophins than normal, and they undergo apoptosis

Question 26

Anaesthetic drugs increase inhibition of neurons, blocking most action potentials. Why would we predict that exposure to anaesthetics might be dangerous to the brain of a fetus?

Answer 26

Prolonged exposure to anaesthetics might produce effects similar to foetal alcohol syndrome. Foetal alcohol syndrome occurs because alcohol increases inhibition and therefore increases apoptosis of developing neurons

Question 27

In the ferret to study, how did the experimenters determine that visual input to the auditory portions of the brain actually produced a visual sensation?

Answer 27

They trained the ferrets to respond to stimuli on the normal side, turning One Direction in response to sound and the other direction to lights. Then they presented light on the rewired side and saw that the ferret again turned in the direction it had associated with light

Question 28

An enriched environment promotes growth of axons and dendrites. What is known to be one important reason for this effect?

Answer 28

Animals in an enriched environment are more active, and their exercise enhances growth of axons and dendrites

Question 29

Name two kinds of evidence indicating that touch information from the fingers activates the occipital cortex of people blind since birth

Answer 29

First, brain scans indicate increased activity in the occipital cortex while blind people perform tasks such as feeling two objects and saying whether they are the same or different. Second, temporary inactivation of the occipital cortex blocks blind people’s ability to perform that task, without affecting the ability of sighted people

Question 30

Under what circumstance would the occipital cortex of a sighted adult become responsive to touch?

Answer 30

A sighted person who practises tactile discrimination for a few days, such as learning braille while blindfolded, begins to use the occipital cortex for touch

Question 31

Extensive practice of a skill expands the brains representation of sensory and motor information relevant to that skill. For example, the representation of _______ expands in people who regularly practice musical instruments

Answer 31

Fingers

Question 32

Which brain area shows expanded representation of the left-hand in people who began practising stringed instruments in childhood and continued for many years?

Answer 32

Post central gyrus or somatosensory cortex of the right hemisphere

Question 33

Musicians cramp; a disorder where one or more fingers is in constant contraction or where moving one finger independently of others is difficult

Answer 33

Focal hand dystonia

Question 34

What change in the brain is responsible for musicians cramp?

Answer 34

Extensive practice of violin, piano, or other instruments causes expanded representation of the fingers in the somatosensory cortex, as well as displacement of representation of one or more fingers in the motor cortex. If the sensory representation of two fingers overlaps too much, the person cannot feel them separately or move them separately

Question 35

What procedure is most promising for treating musicians cramp?

Answer 35

The most promising treatment so far is trading the person to attend to specific sensations in the hand. The training is intended to re-organize the brain representation.
Researchers gave periodic bursts of vibration stimuli to various hand muscles, in random sequence, instructing people with musicians cramp to attend carefully to the stimuli and any changes in their vibration frequency.

Question 36

A voluntary eye movement away from the normal direction

Answer 36

Antisaccade task

Example: before age 5 to 7 years, most children find it almost impossible to look away from a wiggling finger when hands are held up to the left and right of their head. They are “impulsive” in that they do not inhibit their strong tendency to look at a moving object. Ability to perform this task improves sharply between ages 7 to 11, and then gradually improves during the teenage years

Question 37

In addition to being more impulsive than adults, adolescents and children tend to “discount the future”, preferring a smaller pleasure _____ over a larger one _____

Answer 37

Now; later

Question 38

Under what circumstances are adolescents most likely to make an impulsive decision?

Answer 38

Adolescents are most likely to make an impulsive decision when they have to decide quickly in the presence of peer pressure

Question 39

When people claim that adolescents make risky decisions because of a lack of inhibition, which brain area do they point to as being responsible for inhibition?

Answer 39

The prefrontal cortex

Question 40

Many studies confirm that, on average, peoples memory and reasoning fade beyond age 60, if not sooner. In old age, neurons alter their synapses more ______.

Answer 40

Slowly

The thickness of the temporal cortex shrinks by about half a percent per year on average. The volume of the hippocampus also gradually declines, and certain aspects of memory decline in proportion to the loss of hippocampus. The frontal cortex begins thinning at age 30

Question 41

What are three ways in which older adults compensate for less efficient brain functioning?

Answer 41

1. People vary. Some people deteriorate markedly, but others show little sign of loss in either behaviour or brain anatomy well into old age.
2. As people grow older, they may be slower in many intellectual activities, but they have a greater base of knowledge and experience. On certain kinds of questions, older people do significantly better than younger people.
3. Many older people find ways to compensate for any losses. Many of them compensate by activating additional brain areas. For example, a certain memory task activated the right prefrontal cortex in young adults and in older adults who did poorly on the task. For older adults who did well, the task activated the prefrontal cortex of both hemispheres. High-performing older adults activate more brain areas to make up for less efficient activity

Question 42

A result of a sharp a blow to the head that does not puncture the brain

Answer 42

Closed head injury

Question 43

A temporary loss of normal blood flow to a brain area

Answer 43

Stroke

Also known as a cerebrovascular accident

A common cause of brain damage, especially in older people

Question 44

Type of stroke resulting from a blood clot or other obstruction in an artery

Answer 44

Ischemia

The more common type of stroke

Question 45

Type of stroke resulting from a ruptured artery

Answer 45

Hemorrhage

The less common type of stroke

Question 46

Accumulation of fluid which increases pressure on the brain

Answer 46

Edema

Question 47

Describe the processes by which strokes damage the brain

Answer 47

In ischemia, the neurons deprived of blood lose much of their oxygen and glucose supplies

In hemorrhage, they are flooded with blood and excess oxygen, calcium, and other chemicals.

Both lead to many of the same problems, including edema or the accumulation of fluid, which increases pressure on the brain and the probability of additional strokes.
Both also 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 transmitter 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. As neurons die, microglia cells proliferate, removing the products of dead neurons and supplying neurotrophins that promote the survival of the remaining neurons

Question 48

Drug that breaks up blood clots

Answer 48

Tissue plasminogen activator tPA

Question 49

Describe several means of lessening the damage caused by strokes

Answer 49

1. With ischemia, a drug called tissue plasminogen activator or TPA breaks up blood clots. To get significant benefit, a patient should receive TPA within three hours after a stroke, although slight benefits are possible during the next several hours
   It is difficult to determine whether someone has had an ischaemic or haemorrhagic stroke. Given that TPA is useful for ischaemia but could only make matters worse in a hemorrhage, and an MRI scan takes too much time, the usual decision is to give TPA because haemorrhage is less common and usually fatal anyway.
2. Decrease stimulation by blocking glutamate synapses, blocking calcium entry, or other means. Many such techniques have shown benefits in laboratory animals, but so far none has produced much benefit in humans which may be due to the fact that treatments have not been the same as in the laboratory.
3. The most effective known method of preventing brain damage after strokes in laboratory animals is to cool the brain. Cooling slows a variety of harmful processes.
4. Exposure to cannabinoids minimize the damage caused by strokes in laboratory animals. Cannabinoids decrease the release of glutamate. If excessive glutamate is one of the reasons for cell loss, then cannabinoids might be helpful. Cannabinoids also exert anti-inflammatory effects and alter brain chemistry in other ways that might protect against damage

Question 50

What are the two kinds of stroke, and what causes each kind?

Answer 50

The more common form, ischemia, is the result of an occlusion of an artery. The other form, hemorrhage, is the result of a ruptured artery

Question 51

Why is tPA not helpful in cases of hemorrhage?

Answer 51

The drug tPA breaks up blood clots, and hemorrhage results from a ruptured blood vessel, not a blood clot

Question 52

If one of your relatives has a stroke and a well-meaning person offers a blanket, what should you do?

Answer 52

Refuse the blanket. Recovery will be best if the stroke victim remains cold

Question 53

Describe the regrowth of axons

Answer 53

Although a destroyed cell body cannot be replaced, damaged axons do grow back under certain circumstances.

A neuron of the peripheral nervous system has it cell body in the spinal cord or in a ganglion near the spinal cord. In either case, the axon extends into one of the limbs. It crashed axon grows back toward the periphery at a rate of about 1 mm per day, following it’s myelin sheath to the original target. If the axon is cut instead of crushed, the myelin on the two sides of the cut may not line up correctly, and the regenerating axon may not have a sure path to follow. In that case, a motor nerve may attach to the wrong muscle

Within a mature mammalian brain or spinal cord, damaged axons do not regenerate, or do so only slightly. However, in many kinds of fish, axons do regenerate across a cut in the spinal cord and restore nearly normal functioning.

Question 54

Why do damaged CNS axons regenerate so much better in fish then in mammals?

Answer 54

Several problems limit axon regeneration in mammals:

A cut in the nervous system causes a scar to form which is thicker in mammals that in fish, creating a mechanical barrier. This blocks regrowth of axons later.

Neurons on the two sides of the cut pull apart

The glia cells that react to CNS damage release chemicals that inhibit axon growth

Hope: researchers developed a way to build a protein bridge, providing a path for axons to regenerate across a scar-filled gap. Also, injecting neurotrophins at the appropriate locations helps axons grow and establish normal synapses. Also, infant axons grow under the influence of a protein called mTOR. Deleting a gene responsible for inhibiting mTOR enables regrowth of axons in the adult spinal cord

Question 55

Newly formed branches of an axon

Answer 55

Collateral sprouts

After a cell loses input from an axon it secretes neurotrophins that induce other axons to form new branches that take over the vacant synapses. In the area near the damage, new synapses form at a high rate, especially for the first two weeks

Question 56

Receptor supersensitivity. Increased sensitivity to neurotransmitters by a postsynaptic neuron after loss of input

Answer 56

Denervation supersensitivity

Neurons make adjustments to maintain a nearly constant level of arousal. In brain damage, 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 through denervation supersensitivity.

Denervation supersensitivity helps compensate for decreased input. In some cases, it enables people to maintain nearly normal behaviour even after losing most of the axons in some pathway. Can also have unpleasant consequences, such as chronic pain

Question 57

Is collateral sprouting helpful or harmful?

Answer 57

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

Example: the hippocampus receives much input from an area called the entorhinal cortex. If the entorhinal is damaged in one hemisphere, then axons from the entorhinal cortex of the other hemisphere sprout, take over the vacant synapses, and largely restore behavior.
However, if the entorhinal cortex is damaged in both hemispheres, then axons from other locations sprout into the vacant synapses, conveying different information. Under those conditions, the sprouting interferes with behaviour and prevents recovery

Question 58

Is collateral sprouting a change in axons or dendritic receptors?

Answer 58

Axons

Question 59

Is denervation supersensitivity a change in axons or dendritic receptors?

Answer 59

Dendritic receptors

Question 60

Many people with schizophrenia take drugs that block dopamine synapsis. After prolonged use, the side effects include frequent involuntary movements. What is one possible explanation?

Answer 60

Denervation supersensitivity. The decreased input may have led to hyperresponsive receptors

Question 61

A continuing sensation of an amputated body part

Answer 61

Phantom limb

Question 62

What is responsible for the phantom limb experience?

Answer 62

Synapses that used to receive input from the now-amputated part become vacant. Axons representing another part of the body take over those synapses. Now stimulation of this other part activates the synapses associated with the amputated area, but that stimulation feels like the amputated area.

example: the part of the cortex responsive to the feet is adjacent to the part responsive to the genitals. Two patients with foot amputations felt a phantom foot during sexual arousal.

Question 63

To remove or disable the sensory nerves from a body part

Answer 63

Deafferent

For example, an animal that incurred damage to the sensory nerves linking a forelimb to the spinal cord no longer feels the limb, although the motor nerves still connect to the muscles. We say the limb is deafferented because it has lost its afferent or sensory input.

A monkey with a deafferented Lim does not spontaneously use it for walking, picking up objects, or any other voluntary behaviors. However, if you cut the afferent nerves of both for limbs, the monkey will use both deafferented limbs to walk, climb, and pick up food. Apparently, a monkey fails to use a deafferented for lamb only because walking on three limbs is easier than using an impaired limb. When it has no choice but to use its deafferented limbs, it does.

Question 64

Therapy for people with brain damage focusses on encouraging them to practice skills that are:

Answer 64

Impaired but not lost

It is important to note that the brain has increased plasticity during the first days after damage

Question 65

Suppose someone has suffered a spinal cord injury that interrupts all sensation from the left arm. Now he or she uses only the right arm. Of the following, which is the most promising therapy; electrically stimulate the skin of the left arm, tie the right arm behind the persons back, or blindfold the person?

Answer 65

Tyler right arm behind the back to force the person to use the impaired arm instead of only the normal arm. Stimulating the skin of the left arm would accomplish nothing, as the sensory receptors have no input to the CNS. Blindfolding would be either irrelevant or harmful by decreasing the visual feedback from left-hand movements