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Chapter 3

Chapter 3 - Biological Foundations.doc

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Dan Dolderman

Chapter 3 - Biological Foundations WHAT IS THE GENETIC BASIS OF PSYCHOLOGICAL SCIENCE? • environment affects our genes, can turn them on an off; biology and environment mutually influence each other • gene: unit of heredity that determines a particular characteristic in an organism; the components of DNA • chromosomes: structures within the cell body that are made up of genes HEREDITY INVOLVES PASSING ALONG GENES THROUGH REPRODUCTION • Mendelian genetics: ◦ dominant gene: expressed in offspring whenever it is present ◦ recessive gene: expressed only when it is matched with a similar gene from the other parent • genotype: the genetic constitution determined at the moment of conception • phenotype: observable physical characteristics that result from both genetic and environmental influences • polygenic: influenced by many genes; hair colour, height, etc. GENETIC VARIATION IS CREATED BY SEXUAL REPRODUCTION GENES AFFECT BEHAVIOUR • twin studies compare similarities btwn diff type sof twins to determine genetic ba- sis of specific traits • monozygotic: identical twins • dizygotic: fraternal twins • adoption studies compare similarities between biological relatives and adoptive relatives • Thomas Bouchard (The Minnestoa Twin Project) studied identical and nonidentical twins, some raised together and some raised apart ◦ general finding was that identical twins, raised together or not, were likely to be similar ◦ "Jim twins": famous case study, both named Jim but also shared many other similarities and had the same traits ◦ Oskar Stohr and Jack Yufe: had very different backgrounds but displayed similar mannerisms and odd habits • twins raised together may exhibit more differences bc parents encouraged individu- ality • heredity: transmission of characteristics from parents to offspring by means of genes • heritability: statistical estimate of the variation, caused by differences in heredity, in a trait within a population; depends on variation SOCIALAND ENVIRONMENTAL CONTEXTS INFLUENCE GENETIC EXPRESSION • Avshalom Caspi: studied 1000 children from birth to adulthood to determine if low levels of MAO cause aggressive behaviours; result was that those who had gene for low MAO activity were more likely to have been convicted of violent crimes if they had been maltreated as children; effects of maltreatment had less influence on those with the high MAO gene Chapter 3 - Biological Foundations • Sandra Scarr: early environments influence young children, but children's genes also influence the experiences they receive; as they become older they can choose their social situations GENETIC EXPRESSION CAN BE MODIFIED • knockouts: transgenic mice who have had certain genes rendered inactive via re- moval or disruption within genome • experimental gene manipulations have created mice with dramatically altered be- haviour • there may not be specific genes for certain behaviours, but rather that changing one gene's expression leads to expression of a series of other genes, resulting in even more complex behaviours • genes seldom work in isolation to influence mind and behaviour; rather, complex in- teraction among thousands of genes gives rise to the complexity of human expe- rience HOW DOES THE NERVOUS SYSTEM OPERATE? NEURONS ARE SPECIALIZED FOR COMMUNICATION • neurons: basic unit of nervous system; it operates through electrical impulses, which communicate with other neurones through chemical signals; neurons re- ceive (reception), integrate (conduction), and pass information (transmis- sion) in the nervous system TYPES OF NEURONS • somasensatory: general term for sensations experienced from within the body • afferent: carry information to the brain • efferent: carry information from the brain (to muscles throughout the body) • sensory neurons: afferent neurons that detect information from the physical world and pass that information along to the brain • motor neurons: efferent neurons that direct muscles to contract or relax, thereby producing movement • interneurons: communicate only with other neurons, typically in a specific brain region within local or short-distance circuits; integrate neural activity within a single area rather than transmitting • complex networks of thousands of neurons sending and receiving signals are func- tional basis of all psychological activity • neurons communicate selectively with other neurons to form circuits, or neural networks NEURON STRUCTURE (pg. 97) • four structural regions: • dendrites: short branchlike extension of the neuron that detects chemical signals/information from other neurons • cell body: in the neuron, where information from thousands of other neurons is collected, processed, and integrated • axon: a long narrow outgrowth of a neuron by which information is transmitted to other neurons; vary in length Chapter 3 - Biological Foundations ◦ nerve: bundle of axons that carry information between the brain and other places in the body • terminal buttons: small nodules at the ends of axons, that receive electrical im- pulses and release chemical signals from the neuron to the synapse • synapse/synaptic cleft: site for chemical communication between neurons, which contains extracellular fluid • myelin sheath: fatty material, made up of glial cells, that insulates the axon and allows for the rapid movement of electrical impulses along the axon; grows along axon in short segments • nodes of Ranvier: small gaps of exposed axon, between the segments of myelin sheath, where action potentials are transmitted; contain ion channels which allow negatively and positively charged ions to pass in and out of cell when neuron transmits signals down the axon THE RESTING MEMBRANE POTENTIAL IS NEGATIVELY CHARGED • resting membrane potential: the electrical charge of a neuron when it is not ac- tive due to greater ratio of negative to positive ions inside than outside the neu- ron • thus electrical charge inside the neuron is slightly more negative than the one outside • polarization: creates electrical energy used to power neural firing by changing electrical charge inside and outside the neuron THE ROLES OF SODIUM AND POTASSIUM IONS • ions pass through cell membrane at ion channels: specialized pores located at the nodes of Ranvier • each channel matches specific type of ion (sodium channel allows sodium but not potassium and vice versa) and flow of ions is controlled by gating mechanism • ion flow also affected by cell membrane's selective permeability - some types of ions cross more easily than others • more potassium (-) is inside the neuron than sodium, contributing to polarization • sodium-potassium pump also creates polarization by increasing potassium and de- creasing sodium inside the neuron ACTION POTENTIALS CAUSE NEURAL COMMUNICATION • neural communication depends on neuron's ability to respond to incoming stimula- tion by changing electrically and then passing along signals to other neurons • action potential: also called neural firing; the neural impulse that passes along the axon and subsequently causes the release of chemicals from the terminal buttons CHANGES IN ELECTRIC POTENTIAL LEAD TO ACTION • a neuron receives signals from nearby neurons through its dendrites, and these signals tell the neuron whether to fire • two types: • excitatory: depolarizes (makes more positive) the cell membrane, increasing the likelihood that the neuron will fire • inhibitory: hyperpolarizes (makes more negative) the cell membrane, decreasing the likelihood that the neuron will fire • when a neuron fires: Chapter 3 - Biological Foundations ◦ 1. sodium gates in cell membrane open, sodium ions (+) outside the cell rush in; inside of neuron becomes more positively charged ◦ 2. potassium channels then immediately open, potassium ions (-) in- side the cell rush out; inside of neuron goes from negative to positive, creating an action potential ◦ 3. channels close, inside of neuron returns to original, slightly neg- ative resting state ACTION POTENTIALS SPREAD ALONG THE AXON • when the neuron fires, the cell membrane's depolarization moves along the axon like a wave: this is propagation • as sodium ions rush through their ion channels adjacent sodium channels open, moving down the axon away from cell body to the terminal buttons • myelin sheath allows action potential to skip quickly along the axon, only pausing to briefly be recharged at each node of Ranvier • multiple sclerosis: deterioration of myelin sheath, leading to slowing down of neural impulses ALL-OR-NONE PRINCIPLE • the principle whereby a neuron fires with the same potency each time, although fre- quency can vary • if sum of excitatory and inhibitory signals leads to a positive change in voltage that exceed's neuron's firing threshold, an action potential is generated; it either fires or not - it cannot partially fire NEUROTRANSMITTERS BIND TO RECEPTORS ACROSS THE SYNAPSE • neurotransmitter: a chemical substance that carries signals from one neuron to another; can bind only with its particular type of receptor • see figure 3.16 on pg. 101 ◦ 1. neurons do not touch each other; communicate by sending neurotrans- mitters from axon of "sending" neuron across synaptic gap to dendrites of "receiving" neuron ◦ 2. neurotransmitters are stored in vesicles ◦ 3. action potentials cause vesicles to fuse to presynaptic membrane and release their contents into the synapse ◦ 4. neurotransmitters then spread across synaptic cleft and bind to recep- tors: specialized protein molecules located on postsynaptic membrane; binding produces an excitatory or inhibitory signal that encourages or dis- courages neural firing ◦ 5. neurotransmission is terminated by reuptake, enzyme deactivation, or auto reception NEUROTRANSMITTERS BIND WITH SPECIFIC RECEPTORS • once neurotransmitters are released into synapse, they continue to fill and stimu- late receptor, blocking new signals until their influence is terminated by one of three events: • reuptake: process whereby a neurotransmitter is taken back into the presynaptic terminal buttons; process of reuptake and release repeats continuously • enzyme deactivation: enzyme destroys transmitter substance in synaptic cleft; different enzymes break down different neurotransmitters Chapter 3 - Biological Foundations • autoreception: neurotransmitters can also bind to presynaptic neuron; these au- toreceptors monitor how much neurotransmitter has been released into synapse, when excess is detected they signal presynaptic neuron to stop releas- ing the neurotransmitter • all neurotransmitters trigger or inhibit action potentials; the same neurotransmitter can send excitatory of inhibitory postsynaptic signals depending on the receptor's properties NEUROTRANSMITTERS INFLUENCE MIND AND BEHAVIOUR • drugs and toxins can alter neurotransmitters' actions in several ways: alter how neurotransmitters are synthesized, raise or lower amount released from terminal buttons, blocking reuptake, change the away they are deactivated in synaptic cleft, mimic neurotransmitters and bind to their receptors as if they were the real thing • agonist: drug that enhances the actions of a specific neurotransmitter ◦ increase the release of neurotransmitters ◦ block the reuptake of neurotransmitters ◦ mimic a particular neurotransmitter, binding to postsynaptic receptors and either activating them or increasing neurotransmitter's effects • antagonist: drug that inhibits the action of a specific neurotransmitter ◦ block the release of neurotransmitters ◦ destroy neurotransmitters in the synapse ◦ mimic a particular neurotransmitter, binding to postsynaptic receptors enough to block neurotransmitter binding • addictive drugs such as heroin and cocaine are chemically similar to naturally oc- curring neurotransmitters, receptors cannot tell the difference TYPES OF NEUROTRANSMITTERS • acetylcholine (ACh): responsible for motor control at the junction between nerves and muscles; also involved in mental processes such as learning, memory, sleeping, and dreaming ◦ binds with receptors on muscle cells to make them contract or relax de- pending on receptors, ex. excites skeletal muscles and inhibits heart mus- cles ◦ Botox, or botulism, inhibits release of ACh leading to paralyzation of mus- cles ◦ ACh antagonists can cause temporary amnesia; associated with Alzheimers • monoamines: major functions are to regulate states of arousal and effect (feel- ings) and to motivate behaviour; include these four transmitters: • epinephrine: responsible for adrenaline rushes, bursts of energy caused by its re- lease throughout the body • norepinephrine: involved in states of arousal and awareness; important for vigi- lance, fine-tuning clarity of attention, inhibits responsiveness to weak synaptic in- puts and strengthens/maintains responsiveness to strong synaptic inputs • serotonin: important for wide range of psychological activity including emotional states, impulse control, and dreaming; low levels associated w sadness, anxiety, food cravings, and aggression; ex. of serotonin antagonist: selective serotonin Chapter 3 - Biological Foundations reuptake inhibitors (SSRIs): block serotonin reuptake, used to treat OCD, eat- ing disorders, depression (Prozac) • dopamine: involved in reward motivation (eating when hungry, having sex when aroused), motor control; lack of is cause of Parkinston's disease (PD): neuro- logical disorder marked by muscular rigidity, tremors, difficulty initiating voluntary action • monoamine oxidase (MAO) is an enzyme that interrupts the activity of all monoamines • GABA (gamma-aminobutryic acid): primary inhibitory transmitter in nervous system ◦ lack of leads to synaptic excitation getting out of control - epileptic seizures ◦ GABA agonists used to treat anxiety; benzodiazepines such as Valium used for relaxation ◦ alcohol has similar effects on GABA, producing relaxation and interference with motor coordination • glutamate: primary excitatory transmitter in nervous system ◦ opens sodium gates in postsynaptic membranes; involved in fast-acting neural transmission throughout the brain ◦ aid learning and memory by strengthening synaptic connections ◦ excess glutamate may act as a neurotoxin, killing neurons by overexciting them • endorphins: involved in natural pain reduction and reward ◦ include opiates such as heroin and morphine ◦ morphine alters way pain is experienced rather than blocking nerves that t
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