3.6 Summary
The Brain
Theme 1
Brain Basics
Literature 1 – Evolution of the Brain and Behavior (Breedlove & Watson)
Evolution
- Evolution: the process by which a population of interbreeding individuals changes over long
periods of time.
- Evolution by natural selection: the Darwinian theory that evolution proceeds by differential
success in reproduction.
- Evolution continues today.
Adaptation
- Adaptation: a trait that increases the probability that an individual will leave offspring in generations.
- Individuals better suited to the prevailing conditions enjoy more success in reproduction.
- Sexual selection: A form of evolution through natural selection in which members of one sex favor specific
heritable traits in other sex when choosing a reproductive partner.
Similar Situations of Evolution
- Homoplasy: A physical resemblance between physical or behavioral characteristics due to convergent evolution
(i.e., similar body forms of tuna and dolphins) – No common ancestor (Convergent evolution)
- Homology: a physical resemblance that is based on common ancestry, such as the similarity in forelimb structures
of different mammals (both being mammals) – Common ancestor (Divergent evolution)
- Analogy: similarity of function, although the structures of interest may look different (i.e., the human hand and an
elephant's trunk are analogous features) They may look different, but they have a similar function.
Modern Evolutionary Theory
- Darwin’s theory suffered from uncertainty about two important mechanisms.
1. The mechanism by which an individual inherits its characteristics from its parents.
2. The source of individual variation upon which natural selection acts.
- Depending on how it modifies the individual, a given mutation can be harmful, neutral or beneficial.
- Epigenetics: the study of factors that affect gene expression without making any changes in the nucleotide
sequence of the genes themselves.
- Evolution has no endpoint or goals. It is a continual remodeling of organisms in response to their environments,
driven by differential reproductive success.
- Phylogeny: the evolutionary history of a particular group of organisms.
Chromosomes and Genes
- Chromosomes are the supercoiled lengths of DNA, found within the cell nucleus that contains genes that encode
the tens of thousands of proteins that make up the body.
- Kingdom – phylum – class – order – family – genus – species.
Taxonomy
- Taxonomy: classification, makes use of our understanding of genetics to reconstruct phylogeny.
- The proportion of differences between DNA samples from two species can be used as a molecular clock to
estimate how long ago they diverged from a common ancestor.
- Ecological niche: the assortment of environmental opportunities and challenges to which each organism is
adapted.
Studying Other Species
- Old-fashion reason for comparing species was based on unfounded assumption that humans were the pinnacle
achievement or evolution. However, this is not true.
- Linear descent: evolution had proceeded along a single trajectory from simple to complex culminating in humans.
- Outstanding features, convenience, comparison, preservation, economic importance, and treatment of disease.
Brain Sizes and Different Species
- Different species used to obtain food are correlated with brain size and structure.
- Selection pressures favored increased size of the forebrain, allowing these species to cope with environmental
challenges and opportunities in new flexible ways.
- Birds in families that store bits of food for later use have a larger hippocampus relative to the forebrain and to body
weight than do birds in families that do not store food.
Vertebrate Brains
, - Sizes, proportions, and anatomical locations of these brain regions have been subject to evolutionary modification
as a species have adapted to their unique ecological niches.
- All vertebrate nervous systems share certain main features but differ in others
1. Development from a hollow dorsal neural tube: the head of the embryonic neural tube goes on to form the
major subdivisions of the brain.
2. Bilateral symmetry: the cerebral hemispheres are almost mirror images.
3. Segmentation: pairs of spinal nerves extend from each level of spinal cord.
4. Hierarchical control: the cerebral hemispheres control or modulate the activity of the spinal cord.
5. Separate systems: the CNS is clearly separate from the peripheral nervous system.
6. Localization of function: Certain functions are controlled by certain locations in the CNS.
Evolution of Vertebrate Brains
- Brain can be examined by two methods.
1. Endocasts: a cast of the cranial cavity of a skull.
2. Study present-day animals, choosing species that show various degrees of similarity to the ancestral species.
Encephalization Factor
- A measure of brain size relative to body size.
- Residual: how much each mammal deviate from this expectation.
- Encephalization factor: the greater the encephalization factors for species, that is the highest value is above the
diagonal line. The more exaggerated the brain sizes for that species relative to the mathematical prediction for a
model of its size.
- Encephalization factor is greatest for humans and quite a bit less for chimpanzees, despite our evolutionary
closeness.
- This pattern suggests that the cortex has grown disproportionately over the course of human evolution.
- Human medulla is fully developed at birth, but cortex continues adding neurons throughout childhood.
- Later-added outer layers of cortex have enlarged more in primates than the innermost layers.
- The size of the human brain now appears to be at a plateau.
Factors Led to the Rapid Evolution of a Large Cortex
- Growth of a large brain involves a long and burdensome gestation period and difficult birthing.
- Prolonged dependence on the infant and prolonged parental care.
- Most of the post-natal brain growth is due to expansion of the cortex.
- Fitness advantage: any change in an organ during evolution is assumed to confer a fitness advantage and increase
likelihood that the individual will survive and reproduce.
- Social brain hypothesis: A larger cortex is needed to handle the complex cognitive task of maintaining social
relationships with other large-brained individuals.
Brain Size Predicts Success in Novel Environments
- Species with larger brains relative to body size tended to be more successful in establishing themselves in novel
environments.
Sexual Selection
- Natural selection to obtain food and shelter is not likely to account completely for the large brain and complex
intelligence of Homo sapiens.
- Miller: much creativity along with related brain growth is due to sexual selection for abilities to attract attention,
stimulate and surprise of potential mate.
Primate Species Differ in Gene Expression
- Humans and their closest relatives can differ on a genetic basis in two principle ways
1. DNA sequences of the specific genes may vary in important ways between the species.
2. Humans and their nonhuman relatives may also differ in how those genes are expressed to construct a
complex brain.
- Humans differ considerably from other primates in patterns of gene expressions in the brain.
- The pattern of gene expression in the brain has changed and accelerated in the human lineages, presumably under
selection pressure since the time we shared a common ancestor with chimpanzees.
- Even a small change in gene expression can cause a dramatic difference in brain development.
- Single-nucleotide polymorphism (SNPs): a minor variation within a gene, or neighboring noncoding DNA where
one nucleotide has been substituted for another.
- Allele: one of two or more different forms of gene or genetic locus.
- SNPs result in significantly different versions of genes.
- If one particular allele confers a slight reproductive advantage on those who possess it, it will come under natural
selection pressure.
- Genes thus selected will gradually spread throughout subsequent generations until most people possess the same
allele.
Literature 2 – The Nervous System and Behavior (Breedlove & Watson)
, - Glial cells: provide various forms of support and make additional contributions to information processing.
Theories
- Cajal proposed that although neurons come very close to one another they are not quite continuous with one
another.
Neuron Doctrine
- The brain is composed of separate neurons and other cells that are independent structurally, metabolically, and
functionally.
- Information it is transmitted from cell to cell across tiny gaps.
Neurons
- Have 3 organelles.
1. Mitochondria: produces energy.
2. Cell nucleus: contains genetic instructions.
3. Ribosomes: structures in the cell body where genetic information is translated to produce
proteins.
Major Parts of the Neuron
- Cellular extensions called dendrites serve as an input zone, receiving information from other
neurons.
- A cell body (soma) which contains the cell’s nucleus. In most types of neurons inputs are combined
and transformed in the cell body so this region acts as an integration zone.
- A single extension (axon) leads away from the cell body and serves as a conduction zone,
transmitting the cell’s output information in the form of electrical impulses away from the cell body.
- Specialized swellings at the end of the axon (axon terminals) are a functional output zone.
Classification of Neurons (by shape)
1. Multipolar neurons: have many dendrites and a single axon, and they are the most common type of
neuron.
2. Bipolar neurons: have a single dendrite at one end of the cell and a single axon at the other end. This type of
neuron is especially common in sensory systems.
3. Unipolar neurons: have a single extension, usually thought of as an axon, that branches in two directions after
leaving the cell body. One end is the input zone with branches like dendrites, the other, the output zone. Such cells
transmit touch information from the body into the spinal cord.
Classification of Neurons (by function)
1. Motoneurons (motor neurons): the neurons that govern movements, have long axons reaching out to synapse on
muscles, causing them to contract in response to commands from the brain.
2. Sensory neurons: have long axons, carry messages from the periphery back to the spinal cord and brain. Sensory
neurons have diverse shapes, depending on whether they detect light, sound or touch.
3. Interneurons: neurons that receive information from other neurons, process it and pass the integrated information
to other neurons.
- Larger neurons have more complex inputs, cover greater distances and or convey information more rapidly than
smaller neurons.
Synapses
- At each synapse, information is transmitted from the axon terminal of the presynaptic neuron to the receptive
surface of the postsynaptic neuron.
- A synapse has three principal components
1. The presynaptic membrane of the axon terminal of the presynaptic neuron.
2. A specialized postsynaptic membrane on the surface of the dendrite or cell body of the postsynaptic neuron.
3. A synaptic cleft, the gap of about 20-40 nm that separates the presynaptic and postsynaptic membranes.
- Neurons with elaborate dendrites tend to have more synaptic inputs.
- Neural plasticity: the ability of the nervous system to change in response to experience or the environment.
Axons
- Axon collateral: a branch of an axon from a single neuron.
- Axonal transport: the transportation of materials from the neuronal cell body to
distant regions in the dendrites and axons and from the axon terminals back to the
cell body.
- Axon hillock: a cone-shaped projection of the cell body that binds with the soma
and dendrites.
- The axon hillock is the neuron’s integrations zone, gathering and integrating
information from all the synapses on the neuron’s dendrites and soma, then
converting the processed information into a code of electrical impulses that carries
the neurons message down the axon towards its targets.
, Glial Cells
- Glial cells can communicate with each other and with neurons, and they directly affect neuronal functioning by
providing neurons with raw material and chemical signals that alter neuronal structure and excitability.
1- Astrocytes
- Receive synapses directly from neurons and surround and monitor the activity of nearby neuronal synapses.
- Involved in the formation of new synapses.
- They regulate blood flow.
2- Microglial cells
- Very small.
- Remarkably active, continually extending and withdrawing very fine processes that, when they contact the site of
damage, form a spherical containment zone around the injury.
- Migrate two sites of injury or disease in the nervous system to remove debris from injured or dead cells.
- Key components of neuronal pain systems.
3- Oligodendrocytes and Schwann cells
- Responsible for myelination.
- All along the axons of many neurons, these glial cells wrap sections of the axon and multiple layers of myelin (a
fatty insulating substance giving the axon the appearance of a string of slender beads).
- Nodes of Ranvier: between adjacent beads, small uninsulated patches of axonal membrane.
- Within the brain and spinal cord, myelinization is provided by oligodendrocytes. In the rest of the body, Schwann
cells do the myelinization.
- Myelinization improves the speed of conduction of nerve impulses.
Literature 3 – Cellular Mechanisms and Cognition (Gazzaniga)
Ion Channels
- Proteins.
- They have the shape of an inverted V.
- At the wider end of the ion channel is a region serving as a filter that imparts permeability only to the ion for
which it's selective.
Proteins
- Primary structure is simply the order of the amino acids.
- The secondary structure refers so how the long chain of amino acids coiled to
form characteristic patterns.
- The tertiary structure of proteins reflects the fact that the long strands of
coiled amino acids can fold on themselves to form complex three-dimensional
structures.
- Quaternary structure: many large proteins are composed of subunits that
create the final structure.
Gated and Nongated Ion Channels
- Ion channels are either passive (nongated) or active (gated by electrical,
chemical or physical stimuli).
- The Na+ and K+ channels involved in the generation of the action potential are
voltage-gated channels.
- Voltage-gated channels exist for Na+, K+, Cl+ and Ca2+.
- Changes in the transmembrane potential influence the molecular structure of
the proteins in the channels, changing the three-dimensional configuration of the pore region.
Receptors
- Receptors are specialized ion channels that mediate signals at synapses.
1. Directly coupled receptors (ionotropic): chemically interact with or bind, neurotransmitters on their
extracellular surfaces. This binding leads to structural changes in the receptor that permit the passage of ions.
2. Indirectly coupled receptors (mesotrophic): do not open themselves to create a pore. Instead, the binding of
neurotransmitters leads to the generation of a signal inside the postsynaptic cell that then activates ion
channels in the membrane, causing them to open (G proteins and second messengers).
- One advantage of second-messenger system is that they can permit signal amplification by triggering enzymatic
cascades. These cascades exponentially increase the final signal via several intermediate biochemical steps,
allowing a relatively small neurotransmitter signal to lead to a large postsynaptic response.
Synaptic Transmission
- An action potential must arrive at the axon terminals where the synapses are located. The invasion of the axon
terminals by the action potential leads to depolarization of the terminals, initiating an influx of Ca 2+ ions into the
terminal region.
