Chapter 5- Development And Plasticity Of The Brain Flashcards
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.
Two; neural tube; hindbrain, midbrain, and forebrain
Neuroscientists distinguish these 5 processes in the development of neurons:
Proliferation, migration, differentiation, myelination, and synaptogenesis
Production of new cells
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.
Movement of brain neurons or glia
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
To develop the axon and dendrites that give a neuron its distinctive properties
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
Process by which glia produce the insulating fatty sheaths that accelerate transmission in many vertebrate axons
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.
Formation of synapses
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
Which develops first, a neurons axon or it’s dendrites?
The axon forms first
Undifferentiated cells that divide and produce daughter cells that develop more specialized properties
Stem cells
In which brain areas do new neurons form in adults?
Olfactory receptors, neurons in the hippocampus, and neurons in the song-producing areas of certain bird species
What evidence indicated that new neurons seldom or never form in the adult cerebral cortex?
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
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?
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
What was Roger Sperrys evidence that axons grow to a specific target instead of attaching at random?
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.
If all cells in an amphibians tectum produced the same amount of TOPdv, what would be the effect on the attachment of axons?
Axons would attach haphazardly instead of arranging themselves according to their dorsoventral position on the retina
Principle of competition among axons
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.
A protein that promotes the survival and growth of axons in the sympathetic nervous system and certain axons in the brain
Nerve growth factor NGF
A programmed mechanism of cell death
Apoptosis
A chemical that promotes the survival and activity of neurons
Neurotrophin
Rita Levi-Montalcini discovered that the muscles do not determine how many axons form; they determine how many ______
Survive
Describe the formation of neurons in the sympathetic nervous system and understand why there is initial overproduction of neurons
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.
Nerve growth factor is a _______, meaning a chemical that promotes the survival and activity of neurons.
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
What process ensures that the spinal cord has the right number of axons to innervate all the muscle cells?
The nervous system builds far more neurons then it needs and discards through apoptosis those that do not make lasting synapses
What class of chemicals prevents apoptosis?
Neurotrophins, such as nerve growth factor
At what age does a person have the greatest number of neurons – before birth, during childhood, during adolescence, or during adulthood?
The neuron number is greatest before birth
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
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
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?
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
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?
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
An enriched environment promotes growth of axons and dendrites. What is known to be one important reason for this effect?
Animals in an enriched environment are more active, and their exercise enhances growth of axons and dendrites
Name two kinds of evidence indicating that touch information from the fingers activates the occipital cortex of people blind since birth
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
Under what circumstance would the occipital cortex of a sighted adult become responsive to touch?
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
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
Fingers
Which brain area shows expanded representation of the left-hand in people who began practising stringed instruments in childhood and continued for many years?
Post central gyrus or somatosensory cortex of the right hemisphere
Musicians cramp; a disorder where one or more fingers is in constant contraction or where moving one finger independently of others is difficult
Focal hand dystonia
What change in the brain is responsible for musicians cramp?
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
What procedure is most promising for treating musicians cramp?
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.
A voluntary eye movement away from the normal direction
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
In addition to being more impulsive than adults, adolescents and children tend to “discount the future”, preferring a smaller pleasure _____ over a larger one _____
Now; later
Under what circumstances are adolescents most likely to make an impulsive decision?
Adolescents are most likely to make an impulsive decision when they have to decide quickly in the presence of peer pressure
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?
The prefrontal cortex
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 ______.
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
What are three ways in which older adults compensate for less efficient brain functioning?
- People vary. Some people deteriorate markedly, but others show little sign of loss in either behaviour or brain anatomy well into old age.
- 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.
- 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
A result of a sharp a blow to the head that does not puncture the brain
Closed head injury
A temporary loss of normal blood flow to a brain area
Stroke
Also known as a cerebrovascular accident
A common cause of brain damage, especially in older people
Type of stroke resulting from a blood clot or other obstruction in an artery
Ischemia
The more common type of stroke
Type of stroke resulting from a ruptured artery
Hemorrhage
The less common type of stroke
Accumulation of fluid which increases pressure on the brain
Edema
Describe the processes by which strokes damage the brain
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
Drug that breaks up blood clots
Tissue plasminogen activator tPA
Describe several means of lessening the damage caused by strokes
- 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. - 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.
- 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.
- 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
What are the two kinds of stroke, and what causes each kind?
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
Why is tPA not helpful in cases of hemorrhage?
The drug tPA breaks up blood clots, and hemorrhage results from a ruptured blood vessel, not a blood clot
If one of your relatives has a stroke and a well-meaning person offers a blanket, what should you do?
Refuse the blanket. Recovery will be best if the stroke victim remains cold
Describe the regrowth of axons
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.
Why do damaged CNS axons regenerate so much better in fish then in mammals?
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
Newly formed branches of an axon
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
Receptor supersensitivity. Increased sensitivity to neurotransmitters by a postsynaptic neuron after loss of input
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
Is collateral sprouting helpful or harmful?
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
Is collateral sprouting a change in axons or dendritic receptors?
Axons
Is denervation supersensitivity a change in axons or dendritic receptors?
Dendritic receptors
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?
Denervation supersensitivity. The decreased input may have led to hyperresponsive receptors
A continuing sensation of an amputated body part
Phantom limb
What is responsible for the phantom limb experience?
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.
To remove or disable the sensory nerves from a body part
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.
Therapy for people with brain damage focusses on encouraging them to practice skills that are:
Impaired but not lost
It is important to note that the brain has increased plasticity during the first days after damage
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?
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
