Psychobiology Chapter 4
Psychobiology Chapter 4 PSYC2070
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This 6 page Class Notes was uploaded by Karly Lord on Thursday February 4, 2016. The Class Notes belongs to PSYC2070 at University of Cincinnati taught by Dr. Kenneth King in Fall 2016. Since its upload, it has received 28 views. For similar materials see Psychobiology in Psychlogy at University of Cincinnati.
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Date Created: 02/04/16
Chapter 4 book Wednesday, February 3, 2016 2:49 PM Genetics and Evolution of Behavior Mendelian Genetics • Mendel first to demonstrateinheritance through genes ○ Genes - units of heredity that maintain their structural identity from one generation to another ○ Genes comein pairs of chromosomes ○ Gene is defined as part of a chromosomecomposedof deoxyribonucleicacid (DNA) • DNA serves as a template for synthesis of ribonucleic acid (RNA) ○ Messenger RNA - serves as a templatefor the synthesis of protein molecules • DNA contains 4 bases - adenine, guanine, cytosine, and thymine ○ RNA contains adenine, guanine, cytosine, and uracil • Homozygous - identical pair of genes on 2 chromosomes;heterozygous - unmatched pair of genes • Dominant - strong effect in either homozygousor heterozygous condition; recessive - only shows its effects in homozygousconditions Sex-Linked and Sex-Limited Genes • Sex-linked - genes on the sex chromosomes(X and Y) ○ All other chromosomesare known as autosomalgenes ○ Females have 2 X chromosomeswhile males have an X and Y ○ Sex-linked genes usually means X-linked genes • Sex-limited - present in both sexes but active mainly in one sex Genetic Changes • Mutation - heritable change in DNA • Duplication or deletion - part of a chromosomemight appear twice or not at all Epigenetics • Field of epigenetics deals with changes in gene expression ○ Which genes are active or inactive • Drug addiction can produce epigenetic changes in the brain • The experience of feeling socially isolated or rejected alters the activity of hundreds of genes • DNA contains a protein called histones that bind the DNA into a shape like a string wound around a ball, chemical groups attach to the loose ends of the histone ○ To activatea gene, the DNA must partially unwind from the histones Heredity and Environment • Heritability - the degree to which variation in a characteristic depends on genetic variations in a given population ○ If a variation depends largely on genetic differences, that characteristic has high heritability • Researchers rely on 3 kinds of evidence to determine heritability ○ Compare monozygotic(from one egg) and dizygotic (from two eggs) twins Twins with the same genes that look alike suggests a genetic contribution ○ Studies of adopted children Any tendency for adopted children to resemble their biological parents suggests a hereditary influence ○ Identify specific genes linked to some behavior Examine all the genes of people with the same condition and look for any gene that is more commonthan in the rest of the population EnvironmentalModification • Phenylketonuria(PKU) - a genetic inability to metabolize the amino acid phenylalanine • Phenylketonuria(PKU) - a genetic inability to metabolize the amino acid phenylalanine ○ If not treated, phenylalanine accumulates to toxic levels Impairs brain developmentleaving a child mentally retarded, restless, and irritable ○ Approximately1% of Europeans carry the recessive gene for PKU, fewer Asians and almost no Africans. ○ Environmentalinterventions can modify it Low phenylalanine diet to protect the brain □ The success of this diet shows heritable does not mean unmodifiable How genes affect behavior • Some genes control brain chemicals and others affect behavior indirectly ○ Genes make you unusually attractive and a result strangers smile at you and people want to get to know you The Evolution of Behavior • Evolution is a change over generations in the frequencies of various genes in a population • Artificial selection - plant and animal breeders choose individuals with a desired trait and make them the parents of the next generation Commonmisunderstandings • Does the use or disuse of some structure or behavior cause an evolutionaryincrease or decrease in that feature? ○ Lamarckian evolution- theory of evolution through the inheritance of acquired characteristics ○ biologists have found no mechanisms for Lamarckian evolutionto occur and no evidence that it does • Have humans stopped evolving? ○ If people with certain genes have more than the average number of children, their genes will spread in the population • Does evolutionmean improvement? ○ Evolution improvesfitness - the number of copies of one's genes that endure in later generations ○ Genes that increase fitness at one time and place might be disadvantageous after a change in the environment • Does evolutionbenefit the individual or the species? ○ Neither - it benefits the genes Brain Evolution • Humans have evolvedsuch big brains because of our diet ○ Cooked food is easier to digest ○ Hunting in groups brings back more food EvolutionaryPsychology • Concerns how behaviors evolved ○ Goosebumpsdo little for humans but a lot for other mammalswith more hair • Altruistic behavior - action that benefits someoneother than the actor ○ A gene that encourages altruistic behavior would help other individuals survive and spread their genes ○ Altruism is rare in nonhumans • Kin selection- selection for a gene that benefits the individual's relatives • Reciprocal altruism - idea that individuals help those who will return the favor Development of the Brain Maturation of the Vertebrate Brain • Human central nervous system begins to form when the embryo is about 2 weeks old • First muscle movementsstare at age 7.5 weeks - only accomplishmentis to stretch the muscles • At birth the average brain weighs about 350 grams - by the end of the first year it wears 1000 • At birth the average brain weighs about 350 grams - by the end of the first year it wears 1000 grams, close to the adult weight of 1200to 1400 grams Growth and Developmentof Neurons • Proliferation- the production of new cells ○ Early in development,cells lining the ventricles of the brain divide ○ Some cells remain where they are as stem cells and continue to divide ○ Others becomeprimitive neurons and glia that migrate to other locations Some migrate faster than others - the slowest don’t reach destination until adulthood • Primitiveneurons differentiate, forming axon and dendrites ○ Axon grows first, migrating neurons tow its growing axon along like a tail • Myelination - later and slower stage of neuronal development ○ The process by which glia produce the insulating fatty sheaths that acceleratetransmission in many vertebrate axons • Final stage is synaptogenesis- formationof synapses New neurons later in life • Neurons can modifytheir shape but the brain cannot develop new neurons after initial development ○ Olfactory receptorshave a half-life of only 90 days ○ Certain type of brain damage leads to production of new neurons in the cerebral cortex Pathfinding by Axons Chemical pathfinding by axons • Paul Weiss conducted an experiment in which her grafted an extra leg to a salamander and then waited for axons to grow into it ○ After the axons reached the muscles, the extra leg moved in synchrony with the normal leg next to it ○ He suggested the nerves attached to muscles at random and then sent a variety of messages, each one tuned to a different muscles • A growing axon followsa path of cell surface molecules,attracted by somechemicals and repelled by others Competitionamong Axons as a General Principle • Chemical gradients steer axons to approximatelytheir correctlocation • Carla J. Shatz found adult precision of neuron circuits is sculpted from an early imprecise pattern by a process in which connectionsare verified by the functioning of the neurons themselves ○ The developing brain is not simply a mini version of the adult brain • Neuron Darwinsim - principle of competitionamong axons ○ In developmentwe start with moreneurons and synapses than we can keep ○ Selection process keeps some and rejects others Determinantsof Neuronal Survival • Levi-Montalcinifound: ○ When a neuron in the sympatheticnervous system forms a synapse into a muscle, that muscles delivers a protein called nerve growth factor (NGF) - promotesthe survival and growth of the axon An axon that does not receive NGF degenerates and dies The neuron kills itself through a process called apoptosis - a programmedmechanism of cell death • NGF is a neurotrophin - a chemical that promotesthe survival and activity of neurons The Vulnerable Developing Brain • Earliest stages of developmentare similar across species ○ Homeoboxgenes regulate the expression of other genes and control the start of anatomical development • During early developmentthe brain is highly vulnerable to malnutrition, toxic chemicals, and infections that would produce only mild problems at later ages infections that would produce only mild problems at later ages • Fetal alcohol syndrome - a condition marked by hyperactivity,impulsiveness, difficulty maintaining attention, varying degrees of mental retardation, motorproblems, heart defects, and facial abnormalities ○ Leads to thinning of the cerebral cortex that persists into adulthood ○ Interferes with neuron proliferation at earliest stages of pregnancy Later it impairs neuron migration and differentiation Later it impairs synaptic transmission ○ Alcohol kills neurons partly by apoptosis • An immaturebrain is highly responsive to influences from the mother Differentiation of the Cortex • Immature neurons experimentallytransplanted from one part of the developing cortex to another develop the properties characteristic of their new location • Ferret experiment ○ Researchers damage the superior colliculus and the occipital cortex and the auditory input As a result the optic nerve attached to what is usually the auditory area of the thalamus ○ Result - the auditory thalamus and cortex reorganized, developing some by not all of the characteristic appearance of the visual areas The rewired temporalcortex, receiving input from the optic nerve, produced visual responses Fine-Tuning by Experience • Brains have evolved the ability to remodelthemselvesin response to experience Experience and Dendritic Branching • The central structure of a dendrite becomes stable by adolescence but the peripheral branches remain flexible throughout life • Experiences guide the neuronal changes ○ An enriched environmentpromotesgrowth of axons and dendrites Effects of Special Experiences • Brain adaptions in people blind since infancy ○ Blind people improvetheir attention to touch and sound based on practice ○ Touch informationactivated a cortical area which is ordinarily devoted to vision alone in blind people ○ The occipital lobe also responds to auditory information because of strong connectionsfrom the temporal cortex to the occipital cortex • Music training ○ In responses of the auditory cortex to pure tones were about twice as strong as those in nonmusicians ○ One area of the temporal cortex in the right hemisphere is about 30% larger in musicians ○ These brain changes help musicians attend to key sounds in tonal languages ○ Focal hand dystonia - musicians cramp - a disorder where one or more fingers is in constant contraction or where moving one finger independently of others is difficult Extensive practice causes expanded representation of the fingers in the somatosensorycortex, as well as displacement of representationof one or more fingers in the motorcortex If the sensory representationof two fingers overlaps too much, the person cannot feel them separately or move them separately Brain Development and Behavioral Development Adolescence • Adolescent humans show stronger brain responses than older adults do when anticipating rewards, and weaker responses in the areas of the prefrontal cortex responsible for inhibiting behaviors • Adolescents more likely to many an impulsive decision in the presence of peer pressure • Adolescents more likely to many an impulsive decision in the presence of peer pressure Old age • Neurons lose someof their synapses and the remaining ones change more slowly than before in response to experience • Neurons in the prefrontal cortex become less able to maintain a high level of firing while storing a working memory • The thickness of the temporal cortex shrinks by about half a percent per year • Volume of hippocampus gradually declines • On average, everyoneis decaying a little each year • Many old people find ways to compensate - activating more widespread brain areas to compensatefor decreased arousal in one or two areas Plasticityafter Brain Damage Brian Damage and Short-Term Recovery • Closed head injury - a sharp blow to the head that does not puncture the brain ○ Effects depend on severity and frequency ○ After a severehead injury, recoveryis slow and often incomplete • Cause of damage after closed head injury ○ Rotational forces that drive brain tissue against the inside of the skull ○ Blood clots Reducing the harm from a stroke • Stroke - temporaryinterruption of normal blood flow to a brain area, also known as cerebrovascularaccident ○ More commontype of stroke is ischemia - result of a blood clot or other obstruction in an artery Neurons deprived of blood lose much of their oxygen and glucose supplies ○ Less commonis hemorrhage - result of a ruptured artery Neurons are flooded with blood and excess oxygen, calcium, and other chemicals ○ Effects can be barely noticeable to immediatelyfatal ○ Edema - accumulation of fluid • Immediatetreatmentsof stroke ○ Tissue plasminogen activator (tPA) - breaks up blood clots Not helpful in hemorrhagebecause tPA breaks up blood clots and hemorrhages are ruptured blood vessels ○ Decreasestimulation by blocking glutamate synapses or blocking calcium entry ○ Cool the brain - reduces overstimulation,apoptosis, and inflammation ○ Exposure to cannabinoids - minimizes the damage caused by strokesin lab animals Later Mechanisms of Recovery Increased Brain Stimulation • Diaschisis - refers to the decreased activity of surviving neurons after damage to other neurons ○ If it contributes to behavioral deficits following brain damage, then increased stimulation could help Regrowth of Axons • Damaged axons grow back under certain circumstances Axon Sprouting • Collateral sprouts - after a cell loses input from an axon, it secretes neurotrophins that induce other axons to form new branches ○ Take over the vacant synapses ○ Changes in axons DenervationSupersensitivity • Has been demonstratedmostlywith dopamine synapses • Helps compensate for decreased input • Changes in dendritic receptors Reorganized sensory representationsand the phantom limb Reorganized sensory representationsand the phantom limb • If a brain loses a set of incoming axons, we can expect some combination of increased response by the surviving axons and collateral sprouting by axons that ordinarily attach to someother target • After loss of sensory input from a removedlimb, the axons representing the limb degenerate, leaving vacant synaptic sites at several levels of the CNS • Phantom limb - continuing sensation of an amputated body part ○ Synapses that used to receive input from the now amputated part becomevacant ○ Axons representing another part of the body take over those synapses so stimulation of this other part activated the synapses associated with the amputated area, but that stimulation feels like the amputated area Learned adjustments in behavior • Recoveryfrom brain damage depends on learning to make better use of the abilities that were spared • A limb is deafferented when it has lost is afferent (sensory)input ○ A monkeywith a deafferented limb does not use it for walking, picking up objects, or other voluntary behaviors - at first investigators assumed the monkeycannot use it In later experimentsthey found the monkeyfails to use the deafferented limb because it is easier walking on 3 limbs rather than using the impaired one but if it has no choice but to use its deafferented limb then it will • A person with brain damage who appears to be functioning normally is working harder than usual ○ Recoveredbehavior deterioratesafter drinking alcohol, physical exhaustion, or other kinds of stress; also deterioratesin old age
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