Chapter 12.2: Development and Aging- Brain Substrates

what changes occur in the prefrontal cortex during adolescence?

-large numbers of synapses are decreased until the number plateaus at the adult level -neurons show much more complicated branching patterns that probably allow more complex patterns of information flow -development of myelin sheaths

what happened with Elizabeth Gould injected adult monkeys (a mammal compared to the previous bird experiment) with a synthetic form of thymidine called BrdU?

1-3 weeks later, the researchers found traces of BrdU in prefrontal, inferior temporal, and parietal cortex, suggesting that these areas contained neurons that had been born in the last few weeks

what are the two kinds of dendritic spines?

1. the majority (60-75%) are small, thin spines that are capable of growing and retracting and are believed to play a role in encoding new information 2. the minority are large, mushroom-shaped spines that tend to stay in place and are believed to be the locus of long-term memories

how many synapses does the average adult brain have?

10^14 synapses

the number of dendritic spines on prefrontal neurons decreases ____% overall in aged monkeys?

33%

BDNF gene is only one of many that appear to influence learning and memory behavior. what are the others?

5-HT2AR gene, WWC1 (or KIBRA) gene, SNC1A gene

what connection do canary birds and neurogenesis have?

Canaries are life-long learners who alter their songs from year to year. Their song is especially important in the spring, when males sing to defend their territory and to attract mates. This seasonal variation in signing is mirrored in the canaries' brains. Their high vocal center HVC doubles in volume during the spring relative to its size in the fall, meaning their brains undergo neurogenesis during the spring

what is an example of study done to evaluate the effects of estrogen on learning and memory using transsexuals before and after than have begun hormone therapy?

In one study, male to female transsexuals on estrogen therapy scored higher on a paired associate task than a similar group who had not yet started estrogen therapy. On a list learning task, control females recalled the most words, control males recalled the fewest, and male to female transsexuals on estrogen scored in the middle

what is one explanation as for why testosterone makes boy brains grow differently?

In the brain, "male" hormones such as testosterone are converted into estradiol, a form of the "female" hormone estrogen, and so it may be the high level of estradiol in young males that actually makes their brains develop differently from female brains

what happened to the LTP in the old rat between session 1 and session 2 of Barnes' experiment?

LTP of the old rats hippocampal neurons was unstable and didn't last over the interval, so no learning occurred

true or false: older adults have a smaller volume in the prefrontal cortex

TRUE! this accounts for why working memory declines with age

what happened when Fernando Nottehohm and his colleagues injected radioactive thymidine into adult canaries?

Thymidine is a compound that is taken up by cells undergoing mitotic division; it therefore can be used as a marker for newly born cells. The researchers found traces of thymidine in the birds' HVC as well as in the forebrain, suggesting that new neurons were either being born in these brain areas in the adult bird or migrating there soon after being formed.

what is an example of how epigenetics play a key role in learning and memory?

We already know that memories can be encoded by long-term structural changes in neuron, as when the creation of new synapses is triggered by the activation and inactivation of genes such as CREB-1 and CREB-2; this is an example of epigenetic modulation.

one estimate suggested that in the adult macaque monkey brain, how many new neurons might be born daily in the hippocampus alone?

a few thousand new neurons each day in the hippocampus

what is an example of how epigenetic changes can be passed from parent to offspring in mice?

a mouse that experiences early-life stress will show epigenetic changes- and its own offspring may have the same epigenetic pattern, even if the offspring never even meet their biological parents

define spine

a tiny protrusion on a dendrite where synapses may form

through apoptosis, about how many of all the neurons produced prenatally will be die?

about 1/3

what percent of people inherit at least one copy of a different version, Met allele, which produces slightly less effective form of BDNF?

about 1/3

there are about ____________ spines per neuron

about 100,000 spines per neuron

about how many weeks after conception are the majority of the human fetus's neurons in place?

about 25 weeks after conception

what percent of all synapses in the cortex of humans may be pruned during childhood and adolescence?

about 42%

the shrinkage of the human brain begins in adolscence and continues thereafter, so by the age of 80, what percent of brain weight has been lost?

about 5% of the brain has lost its weight

we know estrogen may improve verbal learning, but what evidence is there that testosterone improves spatial learning?

adult male rats normally outperform females on learning to swim to an escape platform hidden just below the surface in a pool filled with cloudy water, and in another study, male rats given testosterone injections performed better than males without testosterone injections. not all studies agree with this theory though

the instability of the old rats hippocampal LTP could contribute to what?

age-related deficits in spatial learning as well as in other kinds of learning and memory that depends on the hippocampus, such as episodic memory, which required remembering the context in which the event occurred

what does the SNC1A gene do?

appears to govern how action potentials propagate down the axon, determining whether the message gets passed on to the next neuron

what does the WWC1 (or KIBRA) gene do?

appears to help modify synaptic plasticity and may be associated with the rate at which we forget new information

in what species was adult neurogenesis was first reliably observed?

birds

men are often better than women at navigating through space, what brain areas does this depend on?

brain areas that process visual and spatial information, "angular gyrus" and "visual cortex", which tend to be larger in men than women

when is the bulk of synaptogenesis completed in the visual cortex?

by about 3 or 4 months of age

how does BDNF affect learning and memory?

by enhancing long-term potentiation LTP

what is the most common allele version of BDNF?

called the Val allele

myelin sheaths are produced by glia. what is glia?

cells in the brain that provide various support functions to neurons and that are as critical to proper brain function as the neurons themselves

define neurogenesis

creation of new neurons in the brain

define synaptogenesis

creation of new synapses

a dramatic increase in what neurotransmitter occurs in the prefrontal cortex of adolescents?

dopamine

genes may set up the basic blueprint of an individual's learning and memory abilities, but the final outcome reflects the interaction of what two things?

environment and heredity

what determines which individual spines survive?

experience and learning

what happens when a neuron is derived of neurotrophic factors?

genes become active that cause the neuron to die

what was the experiment psychologist Robert Tyron did regarding the genetics of learning and memory?

he trained a large group of rats in a complex maze, labeling the rats that quickly learned the maze "maze bright" and the others "maze dull". He then paired the bright rats to breed with each other, and the dull rats to breed with each other. He continued to make sure each offspring was either "maze bright" or "maze dull" depending on who their parents were. By the 7th generation of offspring, there was nearly no overlap between the "maze bright" and "maze dull" rats

age-related declines in episodic and semantic memory may be due to reduction in the ability of the ________________________ to encode new information

hippocampus

which twins, identical or fraternal, showed more similarity on tests of learning and memory measures, including tests of working memory and memory for verbal and picture information?

identical, suggesting that genes play a strong role in determining our learning and memory abilities

how does apoptosis implement the brain's version of Darwinian natural selection?

if many neurons are competing for a limited amount of neurotrophic factor, only some can survive

what is an example of how epigenetic changes can be passed from parent to offspring in humans?

in humans also, epigenetic changes can sometimes be inherited, so that individuals subjected to extreme stress (as were survivors of the Holocaust or Rwandan genocide) resulting in epigenetic changes may pass those changes along to their children

what is an example of studies that suggest that epigenetics can even provide a mechanism for passing specific learning across generations?

in one particular study, mice were classically conditioned to associate a particular odor with footshock; these mice developed a fear response to that odor and also showed cortical remapping that resulted in an increased number of olfactory neurons responding to that odor. The offspring of these mice also showed enlargement of these cortical regions, along with enhanced ability to learn about that specific odor

some of the most profound and obvious changes in the adolescent brain occur where?

in the prefrontal cortex, which appears not to become fully mature until after teenage years

what is the time frame of myelination of neurons in the human cortex?

it doesn't start until after birth and it continues throughout the first 18 years of life

what does the 5-HT2AR gene do?

it encodes the instructions for building a particular kind of receptor for the neurotransmitter serotonin and therefore may help determine the effectiveness of neural transmission

what is the role of the BDNF gene on human chromosome 11?

it regulates production of brain-derived neurotrophic factor (BDNF), a protein that is vital for the health of neurons

what happens to synaptogenesis after birth?

it speeds up, with as many as 40,000 synapses being created per second in the infant macaque monkey

is learning ability determined by multiple genes or one single gene?

like all facets of intelligence, it's determined by multiple genes interacting

synapse strength is increased by ........

long-term potentiation LTP, the process whereby conjoint activity in two neurons strengthens the synaptic connection between them

what is an example of how epigenetic mechanisms are flexible, reacting to outside factors such as diet, environmental chemicals, and stress?

many forms of cancer are caused by genes that are normally suppressed by methylation; if methylation is decreased, these genes may be released from inhibition, triggering cancer.

what are some reasons we lose brain weight with age?

maybe a neurodegenerative disorder like Alzheimer's, maybe an injury, or simply old age leading to shrinkage

what are some anatomical differences in mens brains when compared to womens?

mens are about 100 grams heavier & contain about 4 billion more neurons compared to womens

one is one mechanism of epigenetics?

methylation: occurs when a certain group of atoms (specifically, a methyl group) attaches to a gene, "tagging" that gene and altering (usually, inhibiting) its activity

what does cancer research have to do with epigenetics?

much of modern day cancer research involves drugs that target epigenetic processes such as methylation, to restore proper balance of inhibited and activated genes

define apoptosis

natural cell death, as opposed to cell death caused by accident or disease

how does estrogen affect emotional memory?

naturally cycling women tend to remember more details of an emotional story than women using hormonal contraception

define allele

naturally occurring variant of a gene

what is an example of age-related decreases in the connectivity between existing neurons?

neurons in the cerebral cortex of aged monkeys tend to show fewer dendrites and less complex branching than neurons of a young monkey. this means there is a reduction in the ability to receive signals from other neurons and could be a reason as to why some cortical areas shrink with age

what happens if LTP occurs, but then fades away?

new learning will not occur

is the process of neurogenesis uniform throughout the brain?

no!! For example, Purkinje cells in the cerebellum are one class of neuron that form relatively early in gestation, explaining why classical eyeblink conditioning is possible for young infants. BUT, the cerebellum continues to develop after birth too, which is why conditioning is faster later in life

other than genes, what else causes variation in an individuals' genetic makeup?

nongenetic factors: health, stress, living conditions, and social interactions

a previously mentioned experiment studied infant cats and monkeys who had one eye sewn shut during a sensitive period of infancy and ended up "bling" in that eye when it was opened. How did this occur?

normally, visual stimulation activates sensory neurons in the retina of the eye that project to neurons in the primary visual cortex (V1). But if one eye is deprived of sight, there is no activity along the pathway from that eye to V1 and the synapses will be weakened and eliminated

how is a sensitive period beneficial to the developing brain?

once a sensitive period ends, the brain must not change dramatically with each new experience or it would risk overwriting critical older information. so if there wasn't a sensitive period, it would be harm to maintain the most important information

what are some reasons as for why LTP would become less stable with age?

one reason could be epigenetics: major mechanisms of epigenetic control change with age, such as methylation

what simple daily tasks can increase neurogenesis?

physical exercise and environmental enrichment

what do researchers believe is the reason for why several kinds of learning are different in males than females?

since most of these differences don't emerge until puberty, they believe these kinds of learning and memory are strongly affected by circulating estrogen and testosterone in sexually mature adults

in what way is the BDNF gene subject to epigenetic influence?

specifically, both acute and chronic stress can lead to reduction of BDNF activation, which in turn may be one way that stress can impair memory

how might the genes of the rats that Tyron produced be modifying learning indirectly rather than directly?

the "maze bright" rats may be less emotional, or more prone to explore, or more active overall-- and any of these characteristics could translate into better performance in a mze

Carol Barnes and colleagues suggested that the total number of hippocampal neurons and synapses does not significantly decline with age, but what does change in the hippocampus with age?

the ability to maintain changes in synapse strength

while aged monkeys have about 45% fewer thin spines than young monkeys, the number of mushroom spines remain approximately constant. what does this account for?

the age-related reduction in thin spines could be one reason aged individuals often show impairments in their ability to learn new information but little loss of old, well-established memories

how is your brain's sensitive period similar to a canvas on which an artist is painting?

the artist's early broad strokes define the picture being painted; later, small details can be added or changed, but the overall layout of the picture is fixed

what is another region where neurons are lost with age?

the cerebellum- eyeblinking conditioning declines with age

estrogen reliably stimulates neuronal growth and synaptic plasticity (LTP) in rats, so why don't women learn better at times in their menstrual cycle when circulating estrogen is at it's highest?

the fluctuations in women's estrogen levels at different points in the menstrual cycle are very small compared to the difference in estrogen levels between men and women

which two brain regions are the only human brain regions in which adult neurogenesis has been documented?

the hippocampus and the basal ganglia

what happens after the prenatal flurry of neuogenesis?

the infant brain undergoes a period of reduction in the number of neurons

what is different in the prefrontal cortex of an elderly monkey?

the prefrontal cortex shows nearly a third fewer neurons compared to what it did during adolescence

what are some brain regions that do not appear to lose appreciable numbers of neurons or synapses in old age?

the primary visual cortex and the hippocampus

define epigenetics

the study of mechanisms by which gene expression can be modified (without modifying the genes themselves)

how are synapses subject to a kind of Darwinian natural selection?

the synapses that are frequently used (therefore presumably important to the neuron's function) are strengthened; those that are rarely used (therefore less important) may be weakened and may die away altogether

what is a downside of neurogenesis?

the vast majority of newly born neurons appear to die off within a few weeks after formation

where do neurons receive neurotrophic factors from?

their neighbors! if they are densely connected to their neighbors, they likely play vital roles in brain function, and will obtain neurotrophic factor and not die

what are testosterone levels like after birth?

there is a surge of testosterone after birth in both males and females, but it is greater in males. Levels then decline during the first year of life in both males and females, and they remain low until puberty

what happens to people who carry one or two copies of the Met allele?

they are slightly worse at learning and memory tasks than people carrying two copies of the Val allele

What was the experiment Barnes and colleagues did to test the strength of synaptic connections with age?

they placed rats in a squared-off 8 maze, and each time a rat wandered into a new area, a place cell would fire. different place cells fired for each spatial location. When the young rats reentered the maze, the same place cells fired for the same locations at the same strength. But when the older rat was returned to the maze for session 2, the neurons did not always fire in the same places as before, so the old rat lost the previous session information much quicker than the younger rats did

dopamine is important for learning about reward, so the sudden increase of it during adolescence accounts for what?

this could account for why teenagers tend to be impulsive and risk-taking, and why adolescents humans (and adolescent rats and monkeys) become easily addicted to drugs

the motor cortex and sensory cortex complete myelination early, but the frontal cortex isn't fully myelinated until late adolescence, making neurons functional, but weak and slow. what does this account for?

this could account for why working memory is among the last types of learning and memory to fully mature

the hippocampus and language areas of the brain are larger in women: what does this mean for learning?

this may help explain why women often outperform men on list learning, which requires both hippocampal-dependent episodic memory and language skill

rats undergoing new hippocampal-dependent learning tend to show higher rates of neurogenesis: what does this mean?

this means that new neurons might be created on demand to encode the new information

the lateral frontal cortex is larger in women than it is in men: what does this mean for learning?

this might be why women usually outperform men on working-memory tasks

true or false: the overall number of spines on a dendrite remains approximately constant

true!

when does synaptogenesis continue to in the prefrontal cortex?

until about 6 years of age

what is the result of classical conditioning in women who are taking oral contraceptives?

women taking OC (which typically raise levels of estrogen and progesterone) show faster classical conditioning than women not taking OC, and both groups learn faster than males

what are myelin sheaths?

wrappings of fatty substance called myelin that insulate the electrical signals traveling down the axon, speeding neuronal transmission

what happened when Peter Eriksson gave BrdU to human cancer patients in an attempt to quantify the progress of their disease by tagging proliferating cancer cells?

BrdU labeled not only cancer cells but also neurons in the basal ganglia and in the hippocampus