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Brain (chap 3) textbook and lecture notes (1).doc

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University of Toronto St. George
Ashley Waggoner Denton

Notes chapter 3, inextricably entwined with lecture notes. Nature and nurture are inextricably entwined. The brain reflects genetic inheritance, experience, and together these variables determine the size, function, and structure of the brain. What makes human brains ‘better’ than animals? – It’s doesn’t just depend on the size/body mass ratio, but the complexity and organization of its neural circuits (how it’s put together). Within nearly every cell is the entire genome making up the entire organism (you). The genome is the master blueprint of where the nose goes, how much the ears droop etc. Whether a cell becomes a part of the nose, or the droopy ears, all depends on what genes are turned on or off in the cell. This is determined outside the cell. Chromosomes are structures of genes, which are again components of DNA. DNA consists of two strands of molecules, intertwining, providing codes for instructions of the building of proteins, which are the basic chemicals that make up the structure of the cells and their activities. Note that red blood cells don’t contain chromosomes, and sperm and egg cells don’t have 23 pairs, but rather 23 ‘lone’ chromosomes. All cells contain the same DNA, but as mentioned, different genes are turned on (segments along the DNA (think This is called gene expression – the genes that are ‘activated’ or ‘on’. Note that it’s the gene expression that makes us fall in love, learn etc. Genotype – an organism’s genetic constitution, the genetic makeup determined at the moment of conception. The organisms full heredity overview, even the qualities that aren’t visible (recessive), Phenotype – only the observable visible characteristics, which result from environmental influences. (read p 86) Polygenic effects – when a population has a large range of different characteristics (height, weight, skin colour), the characteristic is polygenic. It shows the characteristics of multiple genes, not just one or two. Most cells in the body contain 23 pairs of chromosomes, among them sex chromosomes. Men have XY, women XX. Note that when a zygote (fertilized cell) is formed, there are 8 million combinations of chromosomes that are possible. This is the reason there is genetic variation of human species. The zygote grows through cell division. The chromosomes duplicate, then the cell splits into two new cells with an identical chromosome structure. Sometimes errors occur  mutation. Most mutations have no large impact on the organism, but some do. Recessive genes do usually not interfere with people’s health, unless obtained from both parents. Note that the recessive gene still can be passed along. The dominant gene disorders are dominant, so the carriers usually die and don’t survive (pass along) the gene pool. Behavioural genetics – It is how genes and environment interact to affect psychological activity. Provided breakthroughs how genes and environment affect our brain, mind and behaviour. Most studies point towards the conclusion of genes being the groundwork for human personality traits, which may lead to say divorce, intelligence, learning ability. Two studies to assess the degree of traits passed along: Twin studies – similarities between twins to determine genetic basis and specific traits. Monozygotic twins (en eggede) – share a zygote, hence same chromosomes and genes on chromosome. However, it is believed that the genes aren’t identical due to different bonding between the mothers and fathers genes. Mutations might also play a part in why they are different when growing up, apart from the environment. Dyzigotic twins (fraternal/non identical twins) – two zygotes. No more genetically similar than any other sibling. Adaption studies – Compare the similarities between biological relatives and adoptive relatives. Adopted children share environment (to some extent), but not genes. (Hence similarities between adopted and bio is most likely due to environment) Some way of doing a behavioural genetic study is to compare monozytogic twins have were raised together, with those were raised apart, looking at phenotypes, intelligence, well-being/happiness, achievement, aggression. Jim-twins (adoptive background) vs Oskar Stohre and Jack Yufe (raised in Trinidad vs. Germany). Similar traits such as behaviour, intelligence, shyness (other personality traits). Heritability – the variation of a genetic trait in a population/group of people. Not heredity – shows the variation of a people, can then be used to compare say genetic variation of siblings compared to random people. Estimates of heritability only consider how people differ in their genetic make-up within that group/population. On the bottom – read up on genetic inheritance – passing on diseases etc. Neuron structure and communication The nervous system is the body’s electrochemical communication circuitry. 99% of our nerve cells is in the central nervous system (CNS) Peripheral nervous system (PNS)– all the other nerve cells in the body (tues lecture) The somatic nervous system – has to do with influences from external world Autonomic nervous system – inside – your hungry Sympathetic nervous system Neurons are the basic unit of nervous system – operate through electrical impulses, communicate through chemical signals. Sensory (afferent) neurons- info from external world, sent to the brain through spine usually. Motor (efferent) neurons – sends info from brain to muscles (SAME- sen – aff, mot – eff) Interneutrons – most in the brain, sends info between motor and sensory Neuron structure – Dendrite part – branchlike part – receive messages Connected to the cell body – takes all the info from the dendrite, integrates it and figres out what to do with it. Axon – coming from cell body, sending info from neuron to neuron. They’re how the neurons send the messages Terminal buttons – the end, it has neurotransmitters (chemical signals being passed) which transfers messages to other dendrites. When do neurons fire? – send chemical signals – excitory signals. Increase the likelihood that the neuron will fire (send signal). Inhibitory signals decrease likelihood. They do this by the polarization of cell. READ UP. Neurons have a memory potential. The electrical charge is different inside and outside (more neg inside). There is a potential for change. When fired, negative ions are released, and the positive charges go inside! So now the polarization switches. Then repolarization (ending up back where you started) occurs. The neuron never fires a little bit – it sends the full message (all or none principle). Can differ in frequency it fires. Note that an inhibitory neurotransmitter (GABA) will make the inside of the neutron more negative. How does the CNS react differently to more pain? – MORE neurons fire, and MORE repeatedly. Action potential – how it sends signal. The impulse that passes along the axon, causing it to release signal to pass on chemical/signals. Look up, polarization. Neurotransmitters – chemicals substances are substances in the end of the neurons that carry signals from one to another. Stored in vessels. Then fused to the end membrane – then released, then accepted by specific receptors. It only binds to certain dendrites, so message goes to right place. USE ZAPS – synaptic transmission Dfferent neurotransmitters – read up, have a general sense of what EACH does – easy point on exam Epinephrine – responsbiel for energy (adrenaline) Dopamine – reward system of the brain. Gets activated when something is pleasureable. How drugs work – they influence how the nerurostransmitters worka Agonists – enhance neurotransmitters action by - increasing the amount of neurot - blocking the re-uptake of neurot (they linger longer) - mimicking a neurot and activating a postsynaptic receptor Eq – cocaine, methamphetamine – increase/blasts the release of dopamine. Making it addictive, you want to feel good Antagonists – they decrease/inhibit neurotrans actions by - by blocking the release of neutros to be released to the synapse - they can destroy the neurotrans in the synapse - they can mimick the neurotrans by binding to the post synaptic receptor instead of the neurotrans Eg – beta-blockers, botox (prevents acetocholine from working, paralyses particular muscles) Synethesticia – brain centers overlap and sensory experiences are mixed. (own poor definition). It’s a perceuptual condition – blending of the senses. Automatic associations between things like letters and colours, words and tastes. (spell check with colour – eric=earwax taste). Some genetic component to it. The sensation areas in the brain are close – some of them are crossed. 1 theory : There is more communication between neurons, connecting say touch and taste together. An overabundance of neurons. Can also be linked to phantom limbs (phantom smerter). Cross talk of neurons in the sections of the brain. Book notes again, p 93 Genes predispose people to certain behaviours, those behaviours create different responses, and these subsequent interactions (aka social contexts) shape the phenotype. Social contexts and genes affect phenotype. Hence nature vs. nurture. It is hard to separate genes and social contexts, as they are so intertwined (nat vs. nurt). Gene manipulation – altering genes of organisms. Often done by Knockouts – disabling a certain gene related to a specific function. Then compare to the manipulated species, looking at behaviour, thoughts and feelings. A change in a single gene can have massive effects on behaviour. Mouse example – knocking out a social gene, caused the mice to forget other mice they met. This has large effects on his total behaviour pattern. So human behaviour is influenced by genetic processes. Neurons communicate through electrical impulses (ZAPS) internally, and sending out neurotransmitters, which are chemical substances that get sent from one neurons terminal button, to another neurons dendrite. (see lecture notes of how it works, just up a little). Internurons – send messages within the brain – often between motor neurons and sensory neurons. Motor neurons – efferent neurons - the neurons sending messages to contract or relax muscles, movement. Sensory neurons – afferent neurons– the neurons that deal with the outside environment, interpreting the information our senses give us and carrying that information to our brain. Motor neurons and sensory neurons control movement together. When you want to do push hang ups, the brain sends messages to the motor neurons to take care of this action. The sensory neurons feel how tight you must grasp the bar to not fall off, and this is again sent to the finger muscle controlling motor neurons. For neuron structure, refer to lecture notes above, and USE PAGE 97 for illustrations. Dendrite – branchlike part of neuron, around the cell body that increases the neurons receptive field, and detects chemical signals from other neurons. It is then sent to the cell body. Once the incoming information has been transmitted, electrical impulses are sent down the axon by polarization (see movie – Youtube whatever). The axon is polar with it’s negative ions outside, but when an electrical impulse is sent, it switches, positive out, negative in, and this creates a ‘wave’-like motion, sending the impulse down the axon, to the terminal buttons. The terminal buttons have chemical substances that are released when they receive these electrical impulses, sending the chemicals signals into the synapse/synaptic cleft (area between neurons containing extracellular fluid. ), where the chemical signals are picked up by another neurons dendrites. Note that on the axon, the cells are covered by a fatty myelin sheath, allowing rapid transportation of electrical impulses. This myelin sheath covers cells (imagine picture) called glial cells. Between these glial cells there are small gaps of exposed axon called the nodes of Ranvier. These parts of the axon have ion channels, allowing the electric impulse travel as the polarity switches (positive up, negative down. Then repolarisation). This polarity is known as the–resting membrane potential-. The resting membrane potential is negative for the neuron (negative inside, positive outside) - When the charges differ and slide down the axon, this is known as firing the neuron. Sodium and potassium ions pass through the membrane (still on the axon) through these ion channels (1 channel fits for Na, other for K). These channels (think of them as gates) can open and close, regulating the polarity. There is also a ‘sodium potassium pump’ inside the neuron, which increases potassium and decreases sodium inside the neuron. Action potential – neural firing – the charging electrically of a neuron, then passing along the signals to other neurons by letting the charge travel down the axon in a wave-like form known as propagation, and cause the release of chemicals into the synapse, letting other neurons connect to these, hence receiving a signal/information. Whether the neuron fires or not, depends on the signal that the neuron received through their dendrites from other neurons. - Two types of signals that arrive at the dendrites – o Excitatory – these signals depolarize the cell membrane, increasing chance of neural firing. If the input from another neuron has more excitatory input that the neurons threshold, an action potential is created. o Inhibitory – these signals hyperpolarize the cell, decreasing chance of neural firing. When the neuron fires – the sodium gates open, allowing the sodium to rush into the neuron. The sodium is more positive than potassium (??), so the inside of the neuron becomes more positive. Right after, the potassium is let out of the channels, creating negative outside. So the electrical charge inside the cell, is first negative, then positive, then slowly becomes negative again (through channels, and pump repolarization). - Multiple sclerosis – problems with the myelin sheaths – causing slower neural impulses. This leads to worsened motor skills (movement and coordination) and sensational abilities. Keep in mind the all-or-none principle – either the neuron fires, or it doesn’t. It also fires with the same potency/strength every time. It isn’t one signal or so that makes the neuron fire, as the dendrites pick up thousands of signals, and it is the combination and frequency of these signals that determine whether it fires or not. These signals must be frequent enough to build up big enough electrical potential that exceeds the neurons firing threshold. How often the neuron fires depends on the strength of the stimulation (such as physical stimulation). Consider the firing of missiles in a video game, the harder I press doesn’t matter, but if I keep the finger down on the button, missiles fire unlimited. Compare to touching table with thumb and pressing thumb down for several seconds. Neurotransmitters – chemical substances found in small vessels inside the terminal buttons. – bind to other neuron dendrites across the synapse. The action potential causes neurons to release chemicals (neurotransmitters) from the terminal buttons. The vessels containing the neurotransmitters fuse with the membrane of the presynaptic neuron, being released into the synaptic cleft. The neuron that sends the message is called presynaptic, the one who receives it is postsynaptic. Note that the neurotransmitters can only bind with certain receptors (specialized protein molecules on postsynaptic neuron – when binded with neurotransmitter, an excitatory or inhibitory signal is sent to cell body) on the postsynaptic neuron’s dendrite – consider it a key in a lock, only fits certain places. They continue to fill up the receptors, until terminated by either - Reuptake – the neurotransmitters are taken back to the presynaptic neurons terminal buttons. This happens over and over (recycling). - Enzyme deactivation – an enzyme destroys the neurotransmitters in the synaptic cleft. Different enzymes destroy different neurotransmitters. - Autoreception – they monitor how much neurotransmitters have been released to the synapse. When excess is detected, it reports to the presynaptic neuron to stop releasing the neurotransmitters. All neurotransmitters trigger or inhibit action potentials. Note that whether the neurotransmitters inhibit or trigger action potentials depends on the receptors on the postsynaptic neurons. Drugs and toxins can alter the neurotransmitters actions in many ways – can lower/raise amount of neurotransmitters released to the synapse , or alter the way they are synthesized. - antagonists – drugs that inhibits the action of a certain neurotransmitter. - Agonist – drugs that enhances the action of a certain neurotransmitter. Drugs that have similar chemical structure as neurotransmitters can fool the receptors on the dendrites, hence we feel the effect (think of key in lock – a lock cannot distinguish a forgery) Drugs can effect behaviour as well – if there is a certain part of the brain the encourages laughing – injecting agonist drugs into that brain region should enhance laughter, as it acts like a neurotransmitter. Neurotransmitters (more than 60 known): - Acetylcholine (Ach) – responsible for motor control between nerves and muscles. Released by terminal buttons, and binds with re
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