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Theme 1 – Brain Basics
1) Evolution of the Brain and Behaviour
HOW DID THE ENORMOUS VARIETY OF SPECIES ARISE ON EARTH?
-through the process of evolution, multiple specifies derived due to single species changing/losing/gaining some
features over time -> different evolutionary ideas:
1. evolution by natural selection -> individuals better suited to the prevailing conditions enjoy more success in
reproduction => guarantees adaptations (traits that increase the probability of successful reproduction) becoming
predominate in a population ➔ over time, this natural selection mechanism can significantly change a species
*convergent evolution -> adaptation process by which responses to similar ecological features bring about
similarities in behaviour or structure among animals that are only distantly related => 3 different types:
1) homoplasy = resemblance between physical/behavioural characteristics due to convergent evolution
-> example: similar body form of dolphins & tuna for efficient swimming
2) homology = resemblance between physical/behavioural characteristics due to common ancestry
-> example: similar limb structures in different mammals
3) analogy = similarity of function, although the structures of interest may look different
-> example: human hand & elephant's trunk are analogous features
2. modern evolutionary theories combine natural selection & genetic elements -> genetic basic information
provided an explanation for vaguely phrased statements by Darwin => heritable mutations were discovered
➔ they could either provide normal variation (good) OR be harmful (which made them to be selected out through
not being passed on) → ADDITIONALLY: chromosomes containing a cell nucleus with genes were discovered
-common classification system for species: phylogenetic approach -> uses family trees that shows which species may
have given rise to others => allows to look at history of body, brain & behaviour in certain species
=> NOTE: more modern approach determines common ancestors based on the current DNA-variation between 2
species based on the assumption that DNA-variation between 2 species increases at a steady rate
-support for the evolutionary approach: relative size of a brain region is a rough guide to the importance of the
function of that region for the adaptations of the species to the environmental pressures faced over time
-> example: frontal lobes are larger in species that have to find creative ways to obtain access for food
-another reason for studying non-human species: invertebrate (no backbone) nervous systems are simpler versions
of human nervous systems -> they can act as theoretical models, guiding understanding of cognitive functions
ALL VERTEBRATE BRAINS SHARE THE BASIC STRUCTURES
-all mammals share the same set of brain regions being devoted to visual, auditory & somatosensory processing
-> HOWEVER: the sizes of the brain regions vary across mammals due to different evolutionary pressures
-other similarities across vertebrates in their main brain structures:
1) bilateral symmetry
2) development - starts with a hollow dorsal neural tube
3) segmentation – spinal nerves extend from each level of the spinal cord
4) hierarchical control – cerebral hemispheres control & modulate activity of the spinal cord
5) separation – central NS & peripheral NS are clearly separated
6) localization of function – certain functions are controlled by certain brain regions
=> vertebrates (including humans) all share these features due to a common ancestor
EVOLUTION OF VERTEBRATE BRAINS REFLECTS CHANGES IN BEHAVIOUR
-general tendency: brain size increased over time in vertebrates -> IMPORTANTLY: size increase differs across species
=> HOWEVER: relationship between brain size & body weight has been found ➔ when plotting values for all
,mammals in one diagram, a linear function (representing positive correlation) results → vertical distance from that
regression line for a particular species = k (encephalization factor) →→ NOTE: largest k for humans
-another tendency: despite basic brain structures being shared (midbrain, forebrain, hindbrain, spinal cord,
diencephalon & telencephalon) & all vertebrates possessing a neocortex (consisting of 6 layers), brain organization
(relative size of single brain areas) differs across species as well => different evolutionary challenges
-fact: medulla becomes proportionally smaller relative to brain weight, the cerebellum keeps pace with overall brain
weight, and the cortex becomes proportionally larger than any other part -> suggests that the most expanded brain
regions are the ones that develop later in life & serve more complex functions (cortex)
MANY FACTORS LED TO THE RAPID EVOLUTION OF A LARGE CORTEX IN PRIMATES
-human brain actually enlarged rapidly throughout the last 2.5 million years – probably through new behaviour
-> NOTE: current brain size seems to have hit a plateau in homo sapiens => reason: large brains also come at the cost
of long gestation periods, troublesome births & longer dependence on parents
-rapid brain-development in humans is attributed to the increase in social group size during that time
-> social brain hypothesis: the larger the cortex, the more someone is able to maintain social relationships
-alternative explanation attributes rapid brain-development in humans to 3 factors:
1) innovations in behaviour; 2) use of tools & 3) social (observational) learning
NOTE: even small genetic differences (in gene expression or through DNA variations) can cause differences in brain
size -> especially genetic expression in the brain of humans, compared to other primates, is different
NOTE 2: Analysis of patterns of reproduction as well as genetic studies of the frequencies of single-nucleotide
polymorphisms (SNPs) in specific chromosomal loci indicate that, while cultural influences have an important
impact, evolution through natural selection continues in humans to this day.
2) Functional Neuroanatomy
CELLULAR COMPOSITION OF THE BRAIN
-nervous system (including brain) consists of 100-150 billion nerve cells (neurons), which are involved in information
processing -> they are supported structurally & nutritionally by a comparable number of glial cells
=> current perspective: neuron doctrine ➔ brain is composed of separate neurons & other cells, which transmit info
across tiny gaps (synaptic cleft) in between them
NEURONS HAVE 4 STRUCTURAL DIVISIONS SPECIALIZED FOR INFORMATION PROCESSING
-neurons contain of multiple elements: 1) mitochondria (produce energy); 2) cell nucleus
(contains genetic instructions (chromosomes)); 3) ribosomes (translate genetic instructions
into proteins) + 4 structures, which are directly involved in info processing:
*dendrites = cellular extension of cell body, receiving info from other neurons -> input zone
*cell body (soma) = contains nucleus & dendrite inputs are combined/transformed here
-> integration zone
*axon = single extension which transmits cell’s info output as electrical signal away from
cell -> conduction zone
*axon terminals (synaptic buttons) = specialized swellings at the end of the axon, which
transmit the output signal to other neurons at the synapses -> output zone
NEURONS CAN BE CLASSIFIED BY SHAPE, SIZE OR FUNCTION
-neuronal shape allows classification into 3 types:
1) multipolar neurons -> many dendrites & single axon (most common)
, 2) bipolar neurons -> single dendrite & single axon at the other end
(common in sensory neurons)
3) unipolar neurons -> single extension that immediately branches
into 2 directions (1 end = dendrites & other end = axon terminals/
synaptic buttons) => often transport info from body to spinal cord
-functional classification into 3 types:
1) motor neurons -> they have long axons & carry motor messages
from the spinal cord to the muscles, causing them to contract
=> NOTE: some motor neurons control organs & glands
2) sensory neurons -> they are directly affected by environmental
changes (e.g. light or touch) and transfer this perceptual info to the
spinal cord
3) interneurons (majority) -> receive info from other neurons, transform it & pass it onto other neurons
NEURONAL CELL BODY & DENDRITES RECEIVE INFO ACROSS SYNAPSES
-arborization = tree-like complex arrangement of
neuronal dendrites
-info is transmitted across synapses -> 3 components:
1) presynaptic membrane
2) synaptic cleft
3) postsynaptic membrane
-the presynaptic axon terminal contains a lot of
vesicles filled with NTs -> these will be released into
the synaptic cleft, where they will bind to receptors
at the postsynaptic membrane, eliciting electrical changes in the postsynaptic neuron
=> NOTE: this influences the chance that the postsynaptic neuron will release its own NTs
-neural plasticity = configuration of dendrites & synapses constantly changes in response to new input
-dendritic spines = outgrowths on dendrites, which are meant to enlarge the surface area of the dendrites, giving the
synapses more space to attach to
AXONS INTEGRATE & TRANSMIT INFO
-typical regions of an axon:
*axon hillock -> attached to the cell body & the axon arises from here => holds as integration zone by 1) gathering &
combining info from all synapses on the neuron’s dendrites & soma + 2) converting the processed info into a code of
electrical impulses, which is carried down along the axon
*axon collaterals -> neurons most often only have 1 axon (often myelinated), BUT this single axon often divides into
several branches => allows the neuron to communicate with multiple postsynaptic cells
*axon terminals -> ending of axons (ELABORATED ON ABOVE)
-cell nucleus generates multiple important materials: DNA, enzymes & proteins -> 2 types of axonal transport:
1) axonal transport of these materials from cell body to axon terminals is rapid (function: transmission of signals)
2) axonal transport of materials used at the axon terminals back to the cell body is slow (function: recycling)
GLIAL CELLS SUPPORT & ENHANCE NEURAL ACTIVITY
-glial cells directly affect neuronal functioning by providing neurons with raw materials & chemical signals, which
then alter neuronal structure -> 4 types of glial cells:
,1) astrocyte = star-shaped cell with multiple extensions in all directions -> monitor the activity of nearby synapses &
provide neurons with nutrition once they are active
2) microglial cells = small cells that remove debris from injured/dead cells -> considered as the clean-up crew
-ion channels = proteins consisting of amino acids -> NOTE: any 2 amino acids
together can form a peptide BUT a chain of amino acids constitutes a protein
-proteins have 3 levels of structure:
1) primary structure level -> sequence/order of amino acids
2) secondary structure level -> the way amino acids form into a helix
3) tertiary structure level -> folding of the helix itself
NOTE: many tertiary structures can group together to form a quaternary
structure -> makes up structure of ion channels
*MORE SPECIFICALLY: ion channels are formed like a “V” & created through
the arrangement of 4 tertiary structures
-> the wider opening contains a selective ion filter (e.g. only for K+)
=> once the appropriate ion has made it through the filter, it moves on into a
wider space filled with water, which is connected with the cytoplasm
➔ the chemical environment created by the quaternary structure enables
the ion to cross the membrane down the corresponding (K+) concentration
gradient
-ion channels can either be gated (active) or non-gated (passive)
-> example: N+ & K+ ion channels involved in AP generation are voltage-gated => EPSPs can depolarize to reach a
threshold that makes ion channels opening
*another example: chemically gated ion channels can serve as receptors for NTs (ligands)
-> receptors = specialized ion channels that post-synaptically mediate signals => 2 classes of postsynaptic receptors:
1) directly coupled receptors -> they bind NTs on their extracellular surface, which elicits structural changes in the
receptor (they open), allowing passage of ions
2) indirectly coupled receptors -> these receptors are no ion channels themselves & THUS don’t open themselves
=> INSTEAD: binding a NT elicits signal within the postsynaptic cell, which causes ion channels in the cell membrane
to open ➔ signal involves action of G-protein & generation of second messengers, which strengthen the signal
SYNAPTIC TRANSMISSION
-synaptic transmission = interneuronal communication (sending signals) -> takes place at 2 types of synapses:
1) chemical synapses (ELABORATED ON HERE) & 2) electrical synapses => NOTE: they use different mechanisms
, CHEMICAL TRANSMISSION
-4 step process of chemical transmission:
*initial situation: AP reaches presynaptic axon terminal -> 4 steps:
1) depolarization of axon terminal causes voltage-gated CA+ channels
to open & THUS CA+ influx
2) CA+ influx causes vesicles (containing NTs) to bind to presynaptic
membrane
3) NTs are released into
the synaptic cleft
4) NTs diffuse through
synaptic cleft & bind to
receptors at
postsynaptic membrane
-binding process of NT to receptor at postsynaptic membrane may either elicit an 1) inhibitory postsynaptic
potential (IPSP -> hyperpolarization) or an 2) excitatory postsynaptic potential (EPSP -> depolarization)
=> new AP is only created if the arriving IPSPs & EPSPs sum up to reach the depolarized threshold of -65Vm
➔ NOTE: resting state potential = -70Vm
NEUROTRANSMITTERS (involved in chemical transmission)
-neurotransmitters = molecules, that are synthesized by presynaptic neuron, transported to axon terminal, stored in
vesicles & eventually released into synaptic cleft -> NOTE: after binding to receptor at postsynaptic membrane, they
are degraded or picked up through enzymatic action => 4 criteria for obtaining NT status:
1) NTs should be synthesized by & localized within the presynaptic neuron + stored in presynaptic terminal
2) they must be released by presynaptic neuron when APs invade & depolarize the axon terminal (mediated by CA+)
3) postsynaptic neurons must contain specific receptors for them
4) when artificially applied to postsynaptic cell, they should elicit the same response that stimulating the presynaptic
neuron would
3) neuropeptides (chains of amino acids) -> 5 classes:
*tachykinins -> peptides in brain & gut
*neurohypophyseal hormones -> oxytocin & vasopressin
*hypothalamic releasing hormones -> released by hypothalamus
*opioid peptides -> similar to opiate drugs
*other peptides -> any non-classified peptide
*most NTs either have excitatory OR inhibitory effects on postsynaptic neuron (excitatory NTs vs inhibitory NTs)
-> NOTE: the minority of NTs only have an effect in the presence of other factors (conditional NTs)
-synthesis of NTs:
*large NTs (peptides) are synthesized in the cell body -> after synthesis of peptides in the cell body, they are directly
packed into vesicles & transported at a fast pace (400mm/day) to the axon terminal
*small NTs are synthesized in axon terminals BUT the required enzymes to do so stem from cell body
-> after synthesis of enzymes in the cell body, they are slowly (1mm/day) transported to the axon terminal
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