CHAPTER 4
Genetics, Evolution, Development, and Plasticity
A. Genetics and Evolution of Behavior
Before the work of the late 19th-century monk Gregor Mendel, scientists believed
that inheritance involved a blending process in which the traits of the sperm and the egg
merely mixed, much like two colors of paint. Mendel showed that genes, units of heredity
that retain their structural identity from generation to generation, are the means by which
inheritance takes place. Genes typically appear in pairs because they are arranged along
chromosomes, which are made up of pairs of genes. The fact that a male mammal has
unpaired X and Y chromosomes with distinct genes makes an exception to this rule. A
gene has traditionally been thought of as a segment of a chromosome made up of the
double-stranded DNA molecule. But many genes don't actually have the distinct
locations we once thought they did. On a section of chromosome, several genes can
occasionally overlap. A genetic result can occasionally be influenced by regions on two
or more chromosomes. In many instances, a portion of a chromosome that does not code
for any proteins of its own influences the expression of another portion of the
chromosome that does.
The creation of ribonucleic acid (RNA) molecules, a single-strand chemical, uses
a DNA strand as a template (model). Messenger RNA is one kind of RNA molecule that
acts as a template for the creation of protein molecules. The four "bases" that make up
DNA are adenine, guanine, cytosine, and thymine. The positions of these bases—
adenine, guanine, cytosine, and uracil—determine the positions of the equivalent bases
along an RNA molecule. The arrangement of amino acids that make up a protein is in
turn determined by the arrangement of bases along an RNA molecule. For instance, the
protein adds the amino acid glutamine if the three RNA bases are cytosine, adenine, and
guanine. Uracil, Guanine, and Guanine make up the following three RNA bases, making
tryptophan the following amino acid on the protein. Any protein is made up of 20 amino
acids, which are combined in a way that depends on the arrangement of the bases in the
DNA and RNA. Considering the complexity of the resulting body structures and
functions, it is an incredibly simple code.
Genes might be intermediate, recessive, or dominant. Whether homozygous or
heterozygous, a dominant gene exhibits a substantial influence. Only when homozygotes
are present do the consequences of a recessive gene manifest. A gene for brown eyes, for
instance, is dominant, whereas a gene for blue eyes is recessive. Brown eyes originate
from having one gene for brown eyes and one gene for blue eyes. Phenylthiocarbamide
(PTC) taste sensitivity genes are recessive for low sensitivity and dominant for high
sensitivity. The only person who has difficulty tasting it has two recessive genes.
Sex-linked genes are those found on the sex chromosomes, or X and Y in animals.
The genes on all other chromosomes are referred to as autosomal genes, and they are all
autosomal chromosomes. Male mammals have an X and a Y chromosome, whereas
female mammals have two X chromosomes. The X chromosome is always contributed by
the female during reproduction, while the X or Y chromosome is always contributed by
the male. The offspring is female if he provides an X, and male if he contributes a Y.
(There may be rare, but plausible, exceptions to this rule.) X-linked genes are often meant
when scientists refer to sex-linked genes. The Y chromosome is tiny and contains just a
limited number of genes on its own, but it also contains locations that affect how genes
on other chromosomes function.
Genes can alter in various ways. A genetically transmitted alteration in a DNA
molecule is called a mutation. The mutant gene will code for a protein with a different
amino acid at one place in the molecule if just one base in the DNA is changed to any of
the other three kinds. A mutation is seldom helpful since evolution has had millennia to
choose the best gene composition for each gene. However, those uncommon exceptions
are significant. Only two bases separate the human FOXP2 gene from the chimpanzee
version of that gene, but those two mutations altered the human brain and vocal chords in
a number of ways that promote language development.
The study of epigenetics includes changes in gene expression in addition to
mutations that alter genes permanently. Except for your red blood cells, which lack DNA,
every cell in your body shares the same DNA. A gene's level of activity can change,
though. The genes that are most active in your brain are not the same genes that are most
active in your lungs or kidneys, nor are they the same genes that are most active in
different regions of your brain. After birth, many genes that are crucial for a fetus'
development become less active, while others that had little impact on the fetus become
significant. Some previously almost silent genes become much more active during
puberty. In some people, a gene may be active while inactive in others. Monozygotic
("identical") twins can occasionally have different handednesses, mental health
conditions, or other characteristics.
Most often, three types of evidence are used by researchers. They first contrast
dizygotic ("from two eggs") and monozygotic ("from one egg") twins. Monozygotic
twins are frequently referred to as "identical" twins, yet this phrase is inaccurate because
occasionally, they have significant differences. Some individuals, for instance, are mirror
versions of one another, with one being right-handed and the other left-handed. They
share the same genes, in contrast to dizygotic twins, who do not. When monozygotic
twins are more alike than dizygotic twins, this points to a genetic component. That
evidence is not entirely conclusive, though, because how you look affects how people
treat you and, in turn, how you act. Researchers occasionally also look at "virtual
twins"—children who were adopted into the same family at the same time and are the
same age. They share the same environment from birth, but they are genetically distinct
individuals. Any behavioral similarities suggest impacts from the environment. However,
the behavioral variations—many of which are significant—indicate genetic influences.
Environmental interventions can change even highly heritable traits.
Phenylketonuria, often known as PKU, is a hereditary disorder that prevents people from
properly metabolizing the amino acid phenylalanine. Phenylalanine builds up to
hazardous levels if PKU is left untreated, affecting brain development and leaving a kid
with mental retardation, restlessness, and irritability. A recessive gene for PKU is present
in about 1% of Europeans. Almost no Africans and fewer Asians are carriers of the gene.
A gene cannot act independently of other genes. A gene results in a protein that
communicates with both the environment and the rest of the body's chemistry. The
precise mechanism by which a gene may affect behavior is a complicated question with
several possible solutions. Even if a gene is not expressed in the brain, it still has the
potential to affect your behavior. Let's say you have extraordinarily gorgeous genes. As a
consequence, you receive smiles from complete strangers and interest in you is high. If
their responses to your looks have an impact on your personality, the environment has
changed due to the genes that impacted your conduct.
The creator of evolutionary theory, Charles Darwin, disliked the word
"evolution." To emphasize the concept of changes without necessarily indicating
betterment, he chose descent with modification. Evolution is a shift in a population's
frequency of different genes over many generations. We separate two inquiries
concerning evolution: How do species evolve and how did some species arise? To
question what developed from what is to inquire how a species did evolve. Biologists
assume that humans and chimpanzees shared an ancestor because of our resemblance to
them more than to other animals. Fossils also shed light on historical changes. Biologists
occasionally alter their beliefs on the evolutionary relationship between one species and
another when new information becomes available.
Evolutionary psychology looks at how behavior has changed through time. The focus is
on functional and evolutionary reasons, which include how our genes are similar to those
of our ancestors and why natural selection may have preferred genes that encourage
particular behaviors. Any behavior a species possesses is presumed to have evolved
through natural selection and likely offered some benefit at least during prehistoric times.
Many human actions are illogical outside of the context of evolution. For instance, when
a person is chilly or scared, their hair erections, especially on their arms and shoulders,
are known as "goose bumps". Humans seldom profit from goose bumps since our arm
and shoulder hairs are short and frequently covered by clothes. However, erected hairs in
most other mammals give a scared animal a bigger, more menacing appearance.
B. Development of the Brain
Across species, the earliest stages of development are strikingly similar.
Homeobox genes are a group of genes that govern the expression of other genes and the
beginning of anatomical development, including things like which end is the front and
which is the back. They are present in vertebrates, animals, plants, even fungi and yeast.
A lengthy sequence of DNA nucleotides is shared by all of these genes. Insects can
develop additional wings or legs where their antennae should be due to a mutation in one
of the homeobox genes. Homeobox gene mutations in humans have been associated with
a variety of neurological conditions, such as mental retardation, as well as physical
malformations.
Multiple stages in the development of neurons are identified by neuroscientists.
The creation of new cells is known as proliferation. The brain's ventricles' lining cells
divide in the early stages of development. While some cells migrate to different regions
of the nervous system, others remain where they are as stem cells and continue to divide.
The majority of migration in humans happens before birth, while a tiny percentage does
so during the first several months following birth. The fact that human neurons continue
to proliferate for a longer period of time is one of the main distinctions between human
and chimpanzee brains. Since most neurons develop during the first 28 weeks of
pregnancy, premature birth before that point prevents neuron development.
The basic cells, which are not yet distinguishable as neurons or glia, start to
migrate (move) early in development. Neuron migration is regulated by chemokines and
immunoglobulins. Mental retardation, reduced brain growth, and hindered migration are
all caused by a lack of these substances. Immunoglobulin and chemokine diversity in the
brain is a reflection of the complexity of brain development. A cell starts to generate its
dendrites, axon, and synapses as it develops into a neuron. The process of creating
synapses, or synaptogenesis, starts long before birth and continues throughout life as
neurons create new synapses and eliminate old ones. Both the procedure and the growth
of new dendritic branches often slow down as people age. Myelination, the process by
which glia generate the insulating fatty sheaths that speed up transmission in many
vertebrate axons, is a later and slower stage of neuronal development. Prior to developing
in the forebrain, midbrain, and hindbrain, myelin initially develops in the spinal cord.
Learning a new motor skill leads to an increase in myelination, which lasts for decades
gradually.
The olfactory receptors, which are exposed to the outer world and its harmful
compounds, were the first exceptions. The stem cells of the nose are immature and stay
so throughout life. They divide periodically, with one cell staying immature and the other
differentiating to take the place of an aging olfactory receptor. It regenerates its axon at
the proper location in the brain. Olfactory receptors transmit axons to the olfactory bulb,
and later studies also showed that many species' olfactory bulbs produce new neurons.
However, after the first year or so of life, human beings do not develop new neurons in
this region.
Paul Weiss, a well-known scientist, performed an experiment in which he grafted
a salamander with an additional leg and watched for axons to develop into it. In contrast
to mammals, salamanders and other amphibians grow new axon branches to the
additional limbs after receiving transplants of extra limbs. Finding the appropriate species
to examine needs a lot of investigation. The additional leg coordinated its movement with
the neighboring regular leg once the axons had reached the muscles. Weiss rejected the
notion that every neuron in the additional limb traveled to the appropriate muscle.
Instead, he proposed that the nerves randomly joined to muscles before sending several
messages, each of which was tailored to a distinct muscle.
Later evidence confirmed the theory Weiss had rejected: the extra leg of the
salamander moved in unison with its neighbor because each axon correctly located the
muscle. Weiss' old student Roger Sperry conducted a well-known experiment that
demonstrated how sense axons locate their intended destinations. The idea behind how
axons go to muscles is the same. Sperry first severed several newts' visual nerves. (Note
the significance of selecting the correct species: In amphibians, a severed optic nerve
grows back, but not in mammals or birds.) Amphibians' major visual region, the tectum,
was attached to the injured optic nerve as it healed back, restoring normal vision.
In order to move in the right direction, a developing axon follows a route of cell
surface molecules that are drawn to some substances and drawn away from others. Axons
eventually organize themselves over the surface of their target area by adhering to a
chemical gradient. A protein in the frog tectum is 30 times more concentrated in the
dorsal retina's axons than it is in the ventral retina, and the ventral tectum is 10 times
more concentrated than the dorsal tectum. The retinal axons with the highest
concentration of this substance link to the tectal cells with the highest concentration as
they extend toward the tectum. The tectal cells that have the lowest concentration of
axons are connected to them. The axons are aligned along the anterior-posterior axis by a
comparable gradient of a different protein. You may imagine males lines up from tallest
to shortest, along with women queuing up from tallest to shortest, as an analogy.
It's unlikely that axons achieve perfect accuracy when they first reach their
targets, but chemical gradients guide them to the general area where they should be.
Instead, numerous cells are synaptically connected to each axon in roughly the right
places, and numerous axons connect to each target cell. Each postsynaptic cell gradually
strengthens the best connections while removing others over time. The pattern of
information from the arriving axons determines how this correction is made. One area of
the thalamus, for instance, receives input from several retinal fibers. Long before the
embryo is exposed to light for the first time, the retina experiences repeated waves of
spontaneous activity that move from one side to the other. As a result, axons from nearby
regions of the retina communicate with the thalamus virtually simultaneously. Each
thalamic neuron chooses a set of axons that are active at the same time. It locates
receptors from nearby retinal regions in this way.
The early years of Levi-Montalcini's life would seem to be very unsuitable for a
career in science. During the Nazi era, she was a young Italian Jew. The Italian economy
was wrecked by World War II, and at the time, nearly no one encouraged women to
pursue careers in science or medicine. During the war, she had to spend a number of
years in hiding, but she used those years to carry out research on the nervous system's
development, as she outlined in her autobiography and a subsequent conversation with
Moses Chao. She acquired a passion for learning and eventually learned that the muscles
do not control the number of axons that grow; they control the number that survive. The
sympathetic nervous system produces significantly more neurons than it need at first. The
muscle that one of its neurons connects to releases a protein called nerve growth factor
(NGF), which aids in the survival and expansion of the axon. Without NGF, an axon
degenerates, and the cell body dies. That is, every neuron has a "suicide program" when it
is first formed: Apoptosis, a predetermined method of cell death, is the process by which
a neuron kills itself if its axon does not make contact with the proper postsynaptic cell by
a specific age. (Necrosis, which is death brought on by an injury or a toxin, is different
from apoptosis.)
The CNS is able to match the number of axons to the number of receiving cells
thanks to the sympathetic nervous system's method of creating too many neurons and
subsequently enacting apoptosis. The sympathetic nervous system doesn't know the
precise size of the muscles or glands when it starts sending axons toward them. It
produces more neurons than is required and then throws away the extras. In actuality,
more neurons are produced throughout the entire developing nervous system than will
live to adulthood. Every part of the brain experiences a phase of intense cell death during
which dead and dying cells are everywhere. This cell death is a normal process of
development. In fact, cell death in a specific brain region frequently signals maturation.
Successful cells mature at the same time as unsuccessful ones die off. A neurotrophin is a
substance that aids in the survival and activity of neurons, and nerve growth factor falls
into this category. (The term "trophin" comes from a Greek word meaning
"nourishment.") The brain-derived neurotrophic factor (BDNF) and a number of other
neurotrophins have an impact on the nervous system in addition to NGF. The
development of new synapses, the expansion of axons and dendrites, and learning all
depend on neurotrophins. Keep the phrase BDNF in mind since it will come up again
when discussing depression.
Alcohol can seriously harm a baby's developing brain. Fetal alcohol syndrome,
which is characterized by hyperactivity, impulsivity, difficulties focusing, varied degrees
of mental impairment, motor issues, heart defects, and facial deformities, is a disorder
that affects children of pregnant women who consume large amounts of alcohol.
Drinking during pregnancy causes cerebral cortical thinning that lasts into adulthood.
Even modest drinking has a detectable impact, but more drinking leads to bigger deficits.
Neurons vary in size, chemistry, and form. How and when does a neuron "decide"
what kind of neuron it will be? It is not an impulsive choice. Experimentally moved
immature neurons take on the characteristics of the area of the developing cortex from
where they were transplanted. However, neurons that are transplanted at a later stage
develop some new characteristics while keeping others that are already present. It is
comparable to how immigrant children speak: Younger children who immigrate to a
country learn the correct pronunciation, whereas older children still have an accent.
Researchers once questioned whether mature neurons underwent significant shape
change. The outer branches of a dendrite are flexible throughout life, even if the
dendritic's inner structure becomes fixed by adolescence. Injecting a dye allowed Dale
Purves and R. D. Hadley to observe the anatomy of a living mouse neuron over the
course of several days or weeks. They discovered that some dendritic branches continued
to grow between observations, while others shrank or vanished. Within a month, about
6% of dendritic spines appear or vanish. A turnover of synapses, which is related to
learning, results from the growth or decline of spines.
Remember the ferret experiment where the visual system's axons, unable to reach
their regular targets, instead joined to the brain regions usually devoted to hearing and
managed to transform them into more or less successful visual areas? Could something
similar occur in the brains of those who are born blind or deaf? persons frequently claim
that blind persons develop enhanced touch and hearing abilities. In some ways, the
remark is accurate, but we need to be more explicit. Blind persons gain practice-based
attention to touch and sound. Researchers discovered that blind persons, particularly
those who read Braille and thus regularly hone their finger sensitivity, had stronger than
average levels of touch sensitivity. Lips are the only area where touch sensitivity does not
increase at all, and blind persons are no more sensitive to touch than anyone else.
People who become experts in any field practice for a very long time, usually
starting in childhood, so it makes sense to look for corresponding changes in their brains.
Which of the several types of knowledge would you wish to research? Musicians have
been the preferred choice of researchers for two reasons. First, we know exactly where to
search for alterations in the brain—the regions in charge of hearing and finger movement.
Second, it's simple to locate and find many serious musicians. The majority of colleges
and almost all major cities both have orchestras.
Both long-term musical training and being born blind or deaf cause changes to
brain architecture. Could shorter experiences affect the brain's structure as well? The
response is, in a way, "Yes, of course." There must be some impact on the brain from
what you learn. Your brain's molecules have already been changed by reading this line.
The question is whether a relatively brief encounter has a significant enough impact that
it may be picked up by MRI or other imaging techniques.
In comparison to adults, adolescents are frequently thought of as impulsive and
inclined to seek out instant gratification. If impulsiveness results in unsafe driving,
drinking, sex, shopping binges, and other behaviors, it is a problem. Numerous studies
have shown that memory and reasoning generally deteriorate as people age. Many
neurons lose some of their synapses, and the synapses that are still there respond to
stimuli more slowly than before. On average, the temporal cortex loses half a percent of
its thickness each year. At age 30, the frontal cortex starts to thin. In old age, the
hippocampus' volume also gradually decreases, and several components of memory
deteriorate proportionately to the loss of the hippocampus. After an illness or accident,
elderly adults are more likely to decrease quickly due to brain inflammation.
C. Plasticity after Brain Damage
Various etiologies can lead to brain damage, including neoplastic growths,
infectious agents, exposure to radiation or noxious agents, and neurodegenerative
disorders such as Parkinson's and Alzheimer's diseases. Closed head injury is identified
as the primary etiology among young individuals, which refers to a forceful impact to the
head that does not result in brain penetration. The impact of closed head injury is
contingent upon the degree and frequency of occurrence. A considerable number of
juveniles and adolescents experience at least one minor impact to the cranium resulting
from incidents such as a tumble, a collision involving a bicycle or automobile, or a
sports-related trauma. The majority of individuals recuperate without intervention, albeit
potentially experiencing intermittent headaches subsequently. Approximately 0.7-0.8% of
young individuals necessitate hospitalization, with 20% of this cohort experiencing
enduring disabilities. The duration of a period of confusion and loss of recent memory
resulting from a head injury is a robust indicator of potential long-term complications.
Temporary interruption of normal blood flow to a brain area during a
cerebrovascular accident, commonly referred to as a stroke, is a prevalent cause of brain
damage, particularly among the elderly. Ischemia, which is caused by a blood clot or
obstruction in an artery, is the more prevalent form of stroke. Hemorrhage, which occurs
due to the rupture of an artery, is a relatively infrequent type. The impact of strokes can
range from imperceptible to instantaneous fatality. During ischemia, the neurons
experience a reduction in their oxygen and glucose supplies due to the deprivation of
blood. Hemorrhage is characterized by an influx of blood, as well as an excess of oxygen,
calcium, and other chemical substances. Ischemia and hemorrhage share similar
consequences, such as the occurrence of edema, resulting in elevated intracranial pressure
and a heightened likelihood of subsequent cerebrovascular events. Both ischemia and
hemorrhage have been observed to hinder the function of the sodium-potassium pump,
resulting in the accumulation of sodium within neurons. The co-occurrence of edema and
surplus sodium induces an overabundance of the neurotransmitter glutamate.
Until the 1980s, stroke patients had limited access to medical interventions in
hospital settings. Currently, the prognosis for ischemia appears favorable provided that
medical professionals take prompt action. Tissue plasminogen activator (tPA) is a
pharmacological agent utilized for the purpose of thrombolysis, which involves the
dissolution of blood clots. In order to derive maximum benefit, it is recommended that
patients receive tPA within a time frame of 4.5 hours following the onset of a stroke. The
response times of emergency wards have shown improvement; however, a significant
constraint remains in the form of delayed hospital arrival of stroke patients, often due to a
lack of awareness regarding their condition.
The manifestation of impaired behavior subsequent to brain injury is indicative of
a complex interplay of factors beyond mere cellular death. Following injury to a specific
region of the brain, there is a reduction in neural activity within other regions that have
experienced a partial loss of their typical input. Following an instance of brain damage in
one hemisphere, there is a subsequent reduction in the input to the other hemisphere,
resulting in observable deficits in the latter as well. Diaschisis is a phenomenon
characterized by a reduction in the functional activity of viable neurons following the
impairment of neighboring neurons. The term originates from the Greek language, where
it denotes the concept of "shocking throughout." If diaschisis is a contributing factor to
behavioral impairments subsequent to brain injury, then heightened stimulation may be
beneficial. D. M. Feeney and colleagues conducted a series of experiments to evaluate the
behavioral consequences of cortical damage in rats and cats. The animals exhibited
deficiencies in mobility or visual acuity contingent upon the site of the inflicted injury.
The administration of amphetamine resulted in a significant improvement in both
behaviors. Furthermore, subjects that performed the behaviors while under the influence
of amphetamine exhibited enduring advantages. The administration of a drug aimed at
inhibiting dopamine synapses resulted in a hindrance of the restoration of normal
behavior.
The impairment of the brain or spinal cord results in the damage of numerous
axons of neurons that have managed to endure the injury. Facilitating axonal regeneration
and appropriate synaptic connectivity could yield significant advantages. The possibility
of axon regrowth appears to be theoretically feasible. Axonal regeneration in the
peripheral nervous system occurs at a rate of approximately 1 mm per day, with the
damaged axons utilizing their myelin sheath as a guide to reach their original target. The
regeneration of damaged axons in the spinal cord of fish is facilitated by a gene that is
active in glia cells. Axonal regeneration in the mammalian brain or spinal cord is limited
and insufficient to yield any discernible advantage. Numerous endeavors have been
undertaken to explore strategies for enhancing axon regeneration in mammalian
organisms.
Typically, the dendritic and cellular surfaces are adorned with synapses, and any
unoccupied location is swiftly filled. Following the loss of input from an axon, the cell
initiates the secretion of neurotrophins which stimulate the formation of collateral sprouts
by other axons. These sprouts subsequently occupy the unoccupied synapses. In the
vicinity of the affected region, there is a notable increase in the formation of fresh
synapses, particularly during the initial fortnight.
Neurons engage in regulatory mechanisms to sustain a state of homeostasis in
arousal levels. Upon acquiring new knowledge, a particular group of synapses undergoes
strengthening while other synapses experience a weakening effect. In the absence of this
occurrence, the acquisition of knowledge would result in a progressive increase in
cerebral stimulation. On the other hand, in the event that a particular group of synapses
ceases to be active, potentially due to impairment in other regions of the brain, the
remaining synapses exhibit heightened responsiveness and increased susceptibility to
stimulation. The phenomenon of denervation supersensitivity or receptor supersensitivity,
which refers to an augmented response to a stimulus, has primarily been observed in
dopamine synapses.
In certain cases, individuals or animals who have sustained brain damage may
exhibit apparent inability to perform a task, when in reality they are not exerting effort
towards completing it. Let us consider an animal that has suffered damage to the sensory
nerves extending from a forelimb to the spinal cord. Despite the presence of motor nerves
connecting to the muscles, the animal experiences a lack of sensation in the limb. The
term "deafferented" is used to describe a limb that has experienced a loss of afferent, or
sensory, input. When a monkey experiences deafferentation in one of its limbs, it does
not exhibit voluntary behaviors such as walking or picking up objects using the affected
limb. Initially, researchers postulated that the primate was incapable of utilizing the
deafferented appendage. Subsequently, in a subsequent experiment, the afferent nerves of
both forelimbs were severed. Notwithstanding the significant harm incurred, the primate
utilized its deafferented appendages to ambulate, ascend, and grasp sustenance. It has
been observed that a primate is unable to utilize a deafferented forelimb due to the ease of
walking on three limbs as opposed to utilizing a compromised limb. In circumstances
where the utilization of its deafferented limbs is the only option available, the organism
proceeds to do so. An intervention commonly employed in stroke rehabilitation involves
constraining the usage of the unaffected limb, thereby compelling patients to utilize the
weaker limb.
