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McGill University
PSYC 100
Daniel J Levitin

CHAPTER 3- Biological Foundations • Genetic Basis of Psychological Science o Genetics describes how characteristics are passed along to offspring through inheritance o The environment affects our genes; how they are expressed and therefore how they influence our behavior o Every cell contains the genome; a ‘master blueprint’ o Certain genes are turned on or off depending on where that cell is o Every cell has chromosomes, made up of genes (which are segments of DNA) o Typical humans have 23 pairs of chromosomes, with half of each pair coming from each parent o Genes are components of DNA, the sequence of the molecules along the DNA specifies how to make a specific protein o Proteins make up the structure of cells and direct their activities o Human Genome Project started by mapping out the complete structure of DNA o Humans have between 20,000 and 30,000 genes which is less than an ear of corn (50,000)- the point being that it is not the number of genes that we possess that makes humans complex, but the subtleties in how we express and regulate these genes o Gregor Medel first discovered heredity in 1866, selective breeding in plants o Dominant and recessive genes – genotype is the combination of genes, the genetic constitution (bb), phenotype is what is expressed or the observable characteristics (blue eyes) o Environmental influence has an effect on appearance and character traits as well as one’s genes o Polygenic effects (when one trait is influenced by many genes) o Sexual reproduction mixes up the genotype by making a new set of 23 chromosomes called a zygote (a fertilized cell) o The zygote grows through cell division, multiplying o Errors here lead to mutations, most have no effect but some can be either adaptive or maladaptive (helpful or detrimental in terms of survival and reproduction) o Industrial melanism= darker butterflies and moths now live in polluted areas, as it helps them escape predators o Ethics: should parents be allowed to choose the sex of their child? This could lead to an unbalanced male-female ratio o A recessive sickle-cell gene helps protect against malaria (this is why it is common in Africa), but two causes sickle-cell disease. Disease-causing genes won’t disappear through natural selection if they are recessive (since they can be passed on without being expressed) o Genes affect behavior – behavioral genetics studies this, the extent to which biology influences mind, brain and behavior o Twin studies are often used to study behavior: Monozygotic twins are identical (one zygote that splits in two, so they have the exact same chromosomes), Dizygotic twins are not (two separate eggs that happen to develop at the same time) o Adoption studies observe children that are raised by non-biological parents to study the effect of nurture on their upbringings o Identical twins, whether raised together or apart will have similar traits (famous “Jim twins” – separated at birth and had extremely specific similarities later in life) o Controversy: any two people raised in similar societies will exhibit some similarities (though it is accepted that some traits like shyness are hereditary) o Heritability is “a statistical estimate of the variation, caused by difference in heredity, in a trait within a population”, not to be confused with heredity which is the transmission of characteristics to offspring through genes. o Heritability example: if height has a heritability of 0.6 means 60% of height variation within the population is genetic, and 40% is due to environmental factors o Social and environmental contexts influence genetic expression: Low-MAO gene found to increases susceptibility in children to the effects of maltreatment and later violent crime- genes and environment o Genes predispose people to certain behaviors, those behaviors elicit particular responses and those subsequent interactions then shape the phenotype o Since there is so much interaction between genes and social contexts, some people say their respective effects are inseparable o We can modify genetic expression, knocking out certain genes and/or enhancing others (knocking out one gene allows the study that gene’s function based on behavior differences in its absence) • How Does the Nervous System Operate o Neurons: basic unit of nervous system  Cells that specialize in communication  Excitable through electrical impulses  Functions: • Reception (taking in information from neighboring neurons) • Conduction (integrating the signals) • Transmission (passing the signal to other neurons)  Sensory neurons • Detect information from physical interactions • Called “afferent”, meaning they carry information to the brain (usually done via the spinal cord) • The neurons that provide information from muscles are called somatosensory (sensations experienced from within the body)  Motor neurons • Tell muscles to contract or relax, producing movement • Efferent neurons- send signals from the brain to muscles  Interneurons • Integrate neural activity in a single area  You touch something sharp with your finger and a signal goes from: • Sensory receptors in skin to • Afferent (sensory neurons) to • Interneurons (CNS) to • Efferent (motor neurons) to • Muscle contraction to raise forearm  Signals go back and forth extremely rapidly in order for you to know how much pressure to apply to holding a pen, or do anything that involves physical input and output  Billions of neurons form neural networks by selective communication with othe neurons  Structure • Dendrites (short branches that increase receptive field and detect incoming chemical signals from other neurons) • Cell body (collects and integrates information from other neurons) • Axon (long narrow outgrowth that transmits electrical signals – nerves are bundles of axons that carry information between the brain and other parts of the body) • Terminal buttons (“small nodules at the ends of axons that receive the electrical impulses and release chemical signals from the neuron to the synapse or synaptic cleft- the site for chemical communication between neurons”)  Neurons communicate by sending chemical signals into the small gaps between the axons of the sending neuron and the dendrites of the receiving one (these spaces are called synapses)- diagram on p.97  Myelin sheath • Fatty • Encases and insulates axon and allows electrical signals to move across faster • Grows along an axon in short segments • Small gaps between these segments are called nodes of Ranvier (Ion channels are found here)  Resting membrane potential of a neuron is negatively charged (more negative ions inside). Polarization gives the neuron the energy it needs to send impulses (fire)  Sodium and potassium ions are most important in contributing to resting potential – more potassium inside than sodium (because of selective permeability)  Sodium-potassium pump works to increase concentration of potassium and decrease that of sodium, helps polarization o Neural communication  An action potential/neural firing is the electrical signal that passes along the axon and causes the release of chemicals (the ones that signal other neurons)  Excitatory signals depolarize membrane, makes cell more likely to fire  Inhibitor signals hyperpolarize the cell, decreasing its likelihood to fire  If the excitatory input from the other neurons surpasses the receiving neuron’s threshold, an action potential is generated  When a neuron fires, sodium gates in the cell membrane open, allowing sodium ions into the neuron, causing the inside to become more positive than the outside. Potassium channels open immediately to allow the potassium ions in the cell to rush out- causing a change in charge and action potential  The cell membrane’s depolarization moves across the axon when a neuron fires, this is called propagation. Sodium channels open, causing more to open, and this moves down along the axon to the terminal buttons  The myelin sheath offers insulation and allows the action potential to move quickly, it only stops to recharge at each node of Ranvier- takes about 1 millisecond  MS (multiple sclerosis) is caused by deterioration in the myelin sheath- signals weaken  All-or-none principle: a neuron either fires or doesn’t, regardless of how much impulse its getting, there is no ‘partial firing’  Terminal buttons have vesicles which contain neurotransmitters (diagram p.101) once the action potential reaches them, they release their neurotransmitters into the synaptic cleft- the neurotransmitters travel across and bind to receptors in the postsynaptic neuron’s membrane. This binding is what creates and excitatory or inhibitory signal for the postsynaptic neuron  Once a neurotransmitter fills a receptor (there are different types for different receptors), it blocks any others from entering it. Three events that terminate the influence of neurotransmitters in the s
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