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WEEK 1 NOTES The global issues of biological psychology Psychobiology = new and fascinating Elements: o Anatomy o Biochemistry o Biology o Endocrinology o Neurology o Psychopharmacology Psychobiology Nervous system Sensor and motor processes Motivation and emotion Names for psychobiology: o Physiological psychology o Biological psychology o Neuropsychology o Biopsychology Relationships between behavior and the nervous system Underlying the basic assumption in all fields of psychobiology is the belief that a great deal of abnormal behavior can ultimately be traced to our neuroanatomy and biochemistry For every behavioral event (every action, feeling, and thought) there is a corresponding physical event or events taking place in the body ultimately involving the chemical and electrical properties of the nervous system Psychobiology is a behavioral science The emphasis on behavior is important because it serves to differentiate the psych biologists from scientists working in such fields as physiology, biology, and biochemistry Exploring the mind and body (brain) problem The notion that unexplainable invisible spirits are the controlling forces of life – animism (souls) It took a very long time for scientific principles to replace the animistic view of the mind The principal theory (pneuma theory) to emerge during the rise of the Greek Civilization Attributed the function of mind to invisible spirts known as pneuma – the Spirit of God Greek physician Hippocrates theorized that the brain was the controlling mechanism of all mental and emotional faculties Greek physician Galen severed the nerves leading to a pig’s larynx and offered the pig’s inability to vocalize as proof of a relationship between the nervous system and behavior Both did not use their findings to challenge the pneuma theory Dualism – people view the mind and body in dualistic terms with the mind traditionally seen as an entity distinct from the physical world of matter and governed by an entirely separate set of principles Greek philosopher Anaxagoras states that all living organisms possess a power called nous (mind or intellect) o Mind with although distinct from the body is an infinite self-ruling entity that exercises complete control of the life processes Plato maintained that the mind, because of its spiritual nature cannot be governed by the same physical laws that rule the body Contrary to Plato’s conception, it is now generally believed that mental events are governed not by separate laws but by the same basic laws that apply to physical events, a way of thinking known as monism The impact of Descartes (interactionism) Considered the father of physiological psychology and was a 17 th century French philosopher and mathematician He rejected the notion that the mind and body operated in separate, unrelated spheres, and theorized that many behaviors formerly thought to be investigation could be explained mechanistically He thus made the first true break from dualism, but he never really attacked the pneuma theory directly nor did he ever propose that all human behavior could be reduced entirely to mechanics His view of the mind and body is called the interactionism The nature and nurture issues British empiricists John Locke, David Hume, and George Berkeley were responsible for taking the study of mind-body relationships completely from the realm of the metaphysical and the religions into the mainstream of experimental scientific inquiry Rene Descartes and Immanuel Kant, believed that we are born with a priori knowledge o A set of innate ideas that exists at birth and develops naturally as we mature o This is called the nativism that emphasizes the importance of genetics John Locke believed that at birth a human is nothing more than a tabula rasa – a blank slate – on which life experiences make their marks, this is called the empiricism that emphasizes the influences of environmental factors Nature = genetics Human behavior or action is a complex interaction of nature and nurture Genes make proteins which make enzymes which are catalysts that get molecules to interact o PKU – missing a specific gene, missing an enzyme which makes you unable to digest certain things (changes some foods to an acid which kills brain cells and causes mental retardation) o This is an effect of NATURE but can be controlled by NURTURE (not eating certain foods) The current view of nature and nurture issues is that human behavior is a result of a complex interactions between genetic and environmental factors The study of brain relations: Franz Joseph Gall linked the sizes of different parts of brains which he determined by measuring bumps (protuberances) along the surface of the skull with personality traits This is called the phrenology Whether specific areas in the brain control specific behavior events – known as the specific-localization thesis – or – whether the brain operates more of less as a whole in its control of behavior – known as the doctrine of anti-localization Paul Broca – surgeon and anthropologist performed an autopsy on a dead mute patient and found damage to a section of the frontal lobe of the left hemisphere – Broca’s area (motor speech) and concluded that Broca’s area controls speech, it supports the specific-localization thesis as Gall did Later, Karl Wernicke found an area in the temporal lobe of the left hemisphere, Wernike’s area (sensory speech) Wernicke’s aphasia is a condition marked by poor language comprehension and great difficulty remembering the names of objects Wernicke opposed the notion of specific-localization thesis Brain parts have to work together to achieve a goal o Motor and sensory speech areas, you need both to listen, respond, and understand properly o Localization and the amount of tissue that is damaged – able to test what you are affecting, speech, memory, etc. o Localization, memory and learning work together Karl Lashley refuted the localization view First, he trained rats to learn their way through a maze, then damaged specific parts of the rat’s brain and observed the behavior consequences He found that a rat’s ability to learn and remember depended on how large an area was affected and not the location of the damage He concluded that while there are localized portions of the brain that control sensory and motor behaviors, the control of higher-order behaviors, such as learning and memory, are not localized, but is controlled by the brain as a unit WEEK 2 NOTES Both Wernicke and Lashley supported the doctrine of anti-localization The modern view of how the brain operates to control behavior is really a combination of the anti-localization and localization views That is, specific behaviors are controlled more by one area than by others, but no behavior is exclusively controlled by any one area 2 supporters of localization (one part of the brain controls one thing, not true) 2 people against specific localization Center means localized In the early 1960’s, the Spanish Scientist Jose Delgado, entered a bull ring with a cape and a radio transmitter to tame a raging bull by stimulating a part of the bull’s brain Studies of electrical stimulation of the brain provide strong evidence that the brain is responsible for mental activity Sometimes, it is hard to imagine how evolution might have favored the behavior, such as altruistic behavior (kin selection theory) The ethics of animal experimentation has become controversial VMH – ventral (bottom) medial (middle) hypothalamus o Satiety center o Stimulation – feel full o Damage – never feel full, over eating LH – lateral (side) hypothalamus o Hunger and thirst center o Stimulation – always hungry o Damage – never hungry There are 4 kinds of explanations of an observed phenomenon, such as bird song Why do birds sing? Among songbirds, adult males generally do most of the singing, they sing vigorously in spring and early summer Why do particular kinds of birds sing at the particular times as they do? A physiological explanation of bird song: o Bird song depends on two areas of the brain, the caudal nucleus of the hyperstriatum ventrale and the robust nucleus of the archistriatum o These two areas are well developed in songbirds, such as sparrows and finches o They are small or absent in birds with only simple vocalizations such as chickens and pigeons If the testes are surgically removed from a male songbird, his testosterone level drops, the two brain areas decrease in size, and he does not sing A functional explanation of bird song: The song of a male bird serves two functions: o It attracts females for mating and it defends its territory Mendelian genetics Gregor Mendel is the father of genetics Before 2001, scientists estimated 100,000 genes in each human being Between 2001 and 2004, the estimate was about 30,000 After October 2004, the new estimate is 20,000 to 25,000 genes By comparison, C elegans, a dirt worm that is a favorite research subject has around 19,500 genes Another lab favorite, a mustard plant called Arabidopsis, has about 27,000 genes So how can humans be so complex with relative few genes? In comparison to similar organisms, humans benefit more from genes Each gene manufactures multiple proteins (3-10, possible more) rather than one (like a fruit fly) The proteins from human genes are far more complex and do more than one job o Recessive and dominant genes o Autosomes chromosomes 1-22 o Sex-linked chromosome 23 o Chromosomes = genes o Human genes are responsible to produce proteins – amino acids – proteins make enzymes o We have complex proteins Homozygous dominant – having two dominant alleles of the same gene Homozygous recessive – having two recessive alleles of the same gene Heterozygous – possessing two different forms of a particular gene, one inherited from each parent Every normal human has 23 pairs of chromosomes 23 from mother and 23 from father st Extra 21 chromosome will cause Down’s Syndrome (Mongolism) (1/700) o A disorder includes mental retardation, heart defects, and other physical abnormalities Genes do two things: o They instruct cells on how to produce enzymes to sustain life o They pass these instructions to the next generation to reproduce life Genes are chemicals found in the nuclei of cells that determine traits Every living organism has genes, and genes by the hundreds of thousands are linked together in every human being to form chromosomes Alleles are the different forms of a gene Some genes are dominant and some genes are recessive There are two possible states of a gene for a diploid organism Each gene is made up of two representative alleles – one inherited from the maternal source (mother) and the other inherited from the parental source (father) WEEK 3 NOTES Homozygous dominant – having two dominant alleles of the same gene Homozygous recessive – having two recessive alleles of the same gene Heterozygous – possessing two different forms of a particular gene, one inherited from each parent Every normal human has 23 pairs of chromosomes: o 23 from mother and 23 from father Extra 21 chromosome will cause Down’s Syndrome (Mongolism) (1/700) A disorder includes mental retardation, heart defects, and other physical abnormalities Genes on the first 22 chromosomes (autosomes) are called autosomal rd genes and genes on the 23 chromosomes (sex chromosomes, either X or Y) are called sex-linked genes The Y chromosome is small, in humans, it has genes for only 27 proteins. However, the Y chromosome also has many sites that influence the functioning of genes on other chromosomes The X chromosome has genes for about 1,500 proteins Thus when biologists speak of sex-linked genes, they usually mean X- linked genes Females have two X’s and makes have one X and one Y Generally, on autosomal chromosomes, but active mainly in one sex (activated by sex hormones) Examples include the genes that control the amount of chest hair in men or breast size in women Since, typically the X chromosome is longer, it bears a lot of genes not found on the Y chromosome Thus, most sex-linked genes are X-linked genes One example of a sex-linked gene is color blindness Genes change in several ways, one way is by mutation, a heritable change in a DNA molecule The human FOXP2 gene differs from the chimpanzee version of that gene in just two babes, but those two mutations modified the human brain and vocal apparatus in several ways that facilitate language development Another kind of mutation is a duplication (micro-duplication) or deletion (micro-deletion) some tiny part of a chromosome during the process of reproduction Many researches believe that micro-duplications and micro-deletions of brain-relevant genes are a possible explanation without modification of the DNA sequence Rat pups with a low depress of maternal care early in life after the expression of certain genes in the brain are called hippocampus, resulting in high vulnerability to emotional stress reactions later in life Changes in gene expression are also central to learning and memory and to brain changes resulting from drug addiction Epigenetics is a new, growing field that will almost certainly play an increasing important role in our understanding of behavior For example, when monozygotic twins differ in the psychiatric or other medical conditions, epigenetics differences are a likely explanation Experiences act by altering the activity of genes (interaction between nature and nurture) Chemical substance that makes up the gene is called deoxyribonucleic acid (DNA) DNA is make up chemically of 4 nucleotide bases, A (adenine), T (thymine), G (guanine), and C (cytosine) James Watson and Francis Crick suggested that DNA consists of long, linear strands of chemicals twisted together in the approximate form of a helix For every trait in every species, there is a specific sequence of nucleotide bases along the helical strand It is this helical arrangement of DNA that confers on genes, their 2 functions: o 1 reproduction o 2 the production of enzymes (chemicals of life) The DNA in an existing cell separates, or unzips, down the middle, and each half attracts the appropriate chemical bases from its environment to form a new DNA chain (A-T and C-G) Thus, each of the two new double chains is identical to the original Estimated base pairs in human (homo sapiens) is 3.5 billion Fruit fly has 4 pairs of chromosomes (13,338 genes) and has 61% of coding genes shared with humans There are two types of cell division: o Mitosis occurs in nearly all cells o Meiosis is reserved for organisms that engage in sexual reproduction and it takes place only in reproductive cells (gametes: eggs and sperms) A zygote is formed from a fertilization of an egg by a sperm and grows into a new organism Errors in mitotic division known as nondisjunction, frequently result in abnormal behavior, including a form of mental retardation – Down’s Syndrome Genes regulate enzyme production by regulating the way in which amino acids align to form different proteins Genes (DNA) act in the nucleus but enzymes are produced in the cytoplasm, so an intermediary is needed That intermediary is messenger ribonucleic acid (mRNA) RNA resembles DNA, but it consists of a single strand of nucleotide babes and contains the nucleotide base U (uracil) instead of T (thymine) DNA determines the sequence of nucleotide bases in RNA (U-A and C- G) The new RNA then travels to the cytoplasm where it regulates the production of enzymes by determining the alignment of their amino acids (NH2) There are 22 (20-21) kinds of amino acids A typical protein may contain as many as 200 amino acids linked together in a specific sequence Sometimes the makeup of a gene is altered, causing a mutant form This error in duplication is thought to occur in DNA after it up-zips during reproduction Mutations may cause: o Lesch-Nyhan syndrome – deficiency in the enzyme guanine monophosphate pyrophasphorylase (GMP) as it causes self- mutilating behavior, biting off finger tips and others o Phenylketonuria (PKU) (1/10,000 to 1/20,000) – the lack of a specific enzyme (phenylalanine hydroxylase), it may cause mental retardation, the basis of PKU lies in the metabolism of an amino acid called phenylalanine, under normal conditions, this enzyme is taken into the body through the diet and broken down into another amino acid known as tyrosine, this breakdown calls for a specific enzyme, phenylalanine hydroxylase. If that enzyme is missing as in PKU, the phenylalanine is converted to phenylpyruvic acid (an odorous toxin) rather than tyrosine. The acid may cause neural damage and produce mental retardation PKU can be detected in the urine of newborns Every state now screens the blood phenylalanine level of all newborns at about 3 days of age Usually, a few drops of blood are obtained by a small prick of heel, placed on a card, and then sent for measurement Newborn screening allows early identification and early implementation of treatment The goal of PKU treatment is to maintain the blood level of phenylalanine between 2 and 10 mg/dl Some phenylalanine is needed for normal growth This requires a diet that has some phenylalanine but in much lower amounts than normal Many biologists contend that we human beings are the products of millions and millions of mutations that began at the foot of the evolutionary ladder with the primitive single-celled organism Charles Darwin stated that if a species becomes too fixed in its genetic makeup, it cannot adjust to environmental change He used the term natural selection to describe the process that results in the survival of individuals or groups best adjusted to the environment (survival of the fittest) Two factors are constantly at work in most species to produce and maintain genetic variation: mutation and sexual reproduction (evolution) The environment indirectly but substantially affects the makeup of the gene pool Chinese One Child Policy has a negative impact on Chinese evolution because of the stagnation of the gene pool Evolution requires reproduction Nerve Cells and Nerve Impulses All organisms are built of the same biological materials, cells Cells are powered by the same biological processes: o The production of energy and o The regulation of this cellular activity through the genes Humans interact with their environment, and this interaction must be adaptive Adaptive behavior is composed of 3 elements: o The detection of stimuli in the environment o The responses we make to these stimuli o The ability to coordinate responses with stimuli in an adaptive manner Structure of a cell: a typical cell is composed of protoplasm: an all encompassing term that refers to a variety of intricately organized substances – proteins, fats, sugars, and inorganic salts in a 95% water solution The protoplasm is separated from the environment by the cell membrane (plasma membrane) which is composed of 2 layers of lipids (fat molecules) and proteins The cell membrane has two basic functions: o To act as a passive mold to give the protoplasm form o To regulate the flow of energy-rich material into the cell and of waste material out of the cell Inside the cells is a second membrane called the nuclear membrane that separates the nucleus from the rest of the cell (cytoplasm) Red blood cells don’t have a nucleus, so red blood cells contain no genes The nucleus is comprised primarily of chromosomes whose purpose is to regulate two functions: o The reproduction of the cell and o The activity of the cytoplasm The cytoplasm contains a number of different structures known collectively as organelles (cell organs) most of which help to process nutrients into energy A complex system of membranes – endoplasmic reticulum – form a network of tubules (thin tubes) that course across the cytoplasm to link the outer cell membrane with the inner nuclear membrane The endoplasmic reticulum performs two jobs: o It functions both as a passive skeleton to give the cytoplasm some structural organization and o As a transport system carrying energy-rick material from the external environment to points within the cytoplasm and nucleus Ribosomes are the sites at which the cell synthesizes new protein molecules Ribosomes attach to endoplasmic reticulum Lysosomes contain enzymes Golgi complex (Golgi apparatus) is a network of vesicles preparing hormones and other products for secretion The energy-processing cycle within each cell begins outside it in the sunlight Through the process of photosynthesis, the energy of sunlight is captured by green plants in the form of chemical bonds that gold carbon and hydrogen atoms together within organic compounds As this state the energy is in stored form In order for an animal to make use of this energy, it must not only eat the plant, but also convert the stored energy into an active form which involved the cellular process known as respiration Respiration is the process by which energy entering the cell in the form: carbon bond is broken down and packaged in a form that can be utilized by the cell Respiration is accomplished mainly by the mitochondrion – which is the power plant of the cell – the place where energy is extracted from nutrients and converted into a form of suitable use by the cell ADP + P + E = ATP in the mitochondrion Adenosine diphosphate (ADP); Adenosine triphosphate (ATP) ATP leaves the mitochondrion, ATP = ADP + P + E, release E to do work and ADP returns to the mitochondrion, and the energy cycle renews Energy cannot be released randomly In order to be used, it must be released at very specific sites within the cells Energy must reach the cell nucleus for reproduction of cells Must reach the cytoplasm to build new proteins Must reach the mitochondrion to spur the formation of ATP Enzymes (the chemicals of life) channel energy to the appropriate areas of the cell The human nervous system consists of as many as 200 billion working parts, all of them so interconnected that the number of combinations approaches infinity In humans and other animals, the nervous system is divided into 2 major parts: o The central nervous system (CNS) o The peripheral nervous system (PNS) The neuron is the basic unit of the nervous system and is also called the nerve cell There may be as many as 150 billion neurons in the human nervous system The majority (about 70-100 billion) of them are located in the brain, the rest are distributed throughout the spinal cord and PNS Each neuron is connected functionally (not physically) with others, often with thousands of others There are non-neural cells in the nervous system, known as glial cells of glia (neuroglial cells) Glial cells are not directly involved in the transmission of information They hold neurons in place and regulate the flow of energy-related materials to the neuron and of waste matter out of them Glia play a crucial role in guiding the growth of neurons during embryonic development and during their regeneration after they have been damaged Gila are the chief source of tumors and other forms of disease in the nervous system Such as multiple sclerosis (MS) The term of glia is derived from a Greek word meaning glue The size of a glial cell is about 1/10 of that or a neuron, but there are about 10 times as many as neurons, as the result that glia and neurons occupy the equal space in the nervous system In general glia cells can be divided into 2 major groups: o Those found in the CNS The oligodendrocytes (from myelin sheath) The astrocytes (astroglia, radial glia) from part of the blood brain barrier o Those found in the PNS The Schwann cells (from the myelin sheath) The satellite cells (mechanical support) WEEK 4 NOTES Early sensory experiences can have a profound effect on the organization of the nervous system Mice are highly dependent on their whiskers for survival as they are a major source of tactile stimulation and their whiskers are well represented by neurons in the brain If some of the whiskers are removed a few days after birth so that their corresponding neurons are not stimulated, the neurons fail to develop Depriving cats immediately after birth of visual stimulation by wearing goggles results in similar effects, the neurons in the visual system fail to develop Neurons die from not being stimulated during the critical period early in life, aging, injuries, and diseases Neurons, like all other cells, have a limited life span Neurons begin to die from the moment of birth Dead neurons are not replaced Neurons do not go through mitosis and they cannot reproduce Neurons need stimulation especially during the critical period, if not they die off Axonal sprouting – when damaged neurons work and bind to healthy neurons to “pick up the slack” Rehab helps other neurons to take over the jobs of dead neurons Interneuron – neurons in the CNS (brain and spinal cord) Sensory and motor neurons – neurons in the PNS Neurons have a unique capacity, the capacity to compensate for the loss of neighboring neurons by taking over their functions through a process known as the axonal sprouting We are born with far more neurons (70-100 billion in the brain) than we need But even the surplus pool can run out and it is possibly this situation that produces senility among old people When an axon is severed, the first part to go is its detached end Deprived of chemicals produced by Nissl substance, the detached end has no way of maintaining itself and soon dies This stage is known as Wallerian degeneration The next part of the neuron to break down is the part of the axon that remains attached to the soma This breakdown, known as retrograde degeneration usually coincides with the deterioration of Nissl substances in the soma and dendrites – a process known as chromatolysis 3 stages of degeneration: o Wallerian degeneration o Retrograde degeneration o Chromatolysis Retrograde degeneration and chromatolysis cannot be reversed in the CNS, but they can be reversed in the PNS with glial (Schwann) cell Astrocytes reside in the blood brain barrier Damage to interneurons in the CNS appears to be irreversible; damage to motor and sensory neurons in the PNS, the retrograde degeneration and chromatolysis can be reversed, and the axon can grow back to the original site The differences are thought to be related to differences in glial cells, rather than in the neurons themselves Neurons have the capacity to regenerate in both the CNS and the PNS In the CNS, they meet resistance from scars formed by the astrocytes In the PNS, they encounter no such resistance from glial cells, the Schwann cells can guide the regenerating axon back to its original connection Voltage allows electrical impulses to occur, the membrane separates + and – ions Ions – charged particles Neural impulse travels away from the soma Refractory period – inhibition to fire, a break between “fires” A neural impulse is slower than electricity and is not controlled by electrical impulses Stages of neural impulse: o Resting state – polarized (all balanced) 70 mili-volts o Firing state – depolarized o Refractory Period Absolute refractory period 1 mili-second (1/1000) Relative refractory period 2-4 mili-seconds Neuron may be able to re-fire now if the incoming stimulation is very strong (skips recovery state) o Recovery state – repolarized Ion channels – specific sites for different ions (K+, Na+, Cl-, Ca+, NaCl) Outside the membrane – Na+ (some K+) Inside the membrane – K+ (some Na+) Concentration gradient – Na+ wants to come in and K+ wants to go out via their specific ion channels Ion channels: o Passive (always open) some channels for each Na+ and K+ o Gated channel (active channel) – closed during resting state, controlled by voltage specific channels for each ion There are selective channels in favor of K+ o For every 1 Na+ ion comes in, 50-75 K+ ions come in Neuron is stabilized by the Na+ - K+ (sodium-potassium) pump which maintains the neuron at 70 mili-volts The neuron in the resting state registers a potential of -70 mili-volts (mv) A D-cell battery, by comparison have 1.5 volts = 1,500 mv The charge outside the neuron is (+) with respect to the charge inside the neuron (-) When a membrane of the neuron is in the resting state, it is said to be polarized A neuron generates impulses through a temporary breakdown in the membrane The initial flow of current that follows this breakdown is known as firing Once the neuron has fired, we say that it is depolarized After current is produced, a neuron has the capacity to separate the charges and to recharge itself o After the firing, the neuron is able to repolarize This sequence of electrical events – firing and repolarization is known as the action potential The 3 basic conditions that govern electrical activity in the neuron are: o 1. The resting state – the neuron is polarized o 2. The firing state – the neuron is depolarized; and o 3. The recovery state – the neuron is repolarized WEEK 5 NOTES Immediately after stimulation, when the neuron fires, the membrane becomes more permeable to Na+ and less permeable to K+ The Na+ ions accumulate inside the neuron to such a degree that the electrical charge is reversed It becomes positive on the inside and negative on the outside The potential now measures +30 mv (inside positive) The reloading occurs after the firing, the membrane’s permeability to K+ to the rise and the K+ ions to move outward The permeability to Na+ declines, the Na-K pump swings back into action, and the ionic balance gradually returns to its resting level (-70 mv) K-Na pump keeps the neuron depolarized at resting state -90 mv = hyperpolarization The firing process occurs quickly but recovery takes more time For about .5 mili-seconds after the neuron has fired, it is in a completely insensitive state and cannot be re-excited – is called absolute refractory period, coincides with the inrush of Na+ Then for approximately for 2-4 msec, as the neuron begins to regain its sensitivity, it is capable of being re-excited, but only by intense stimulation – it is called relative refractory period, coincides with the outrush of K+ Speed of a neural impulse: slowest 10 meters/second & fastest 120 meters/second Ion channels are microscopic pores that penetrate the membrane and govern the flow of ions into and out of the neuron during the resting and action potentials There are 2 types of ion channels: o Active (gated) and passive (always open) Each channel, whether active or passive, is selective for a specific ion – such as Na+, K+, and Cl- Passive channels determine membrane permeability during the resting potential There are more passive channels for K+ than for Na+ and hence the membrane is more permeable to K+ than to Na+ (50 – 70 times) Active channels, on the other hand, determine membrane permeability during the action potential The gates of the active channels are controlled by the electrical charge (voltage) across the membrane During the resting potential (-70 mv) – the gates are closed During the action potential (+30 mv) – the gates are open 1. The gates of the Na+ channels open, accounting for the rush of Na+ into the neuron and the subsequent firing phase of the action potential; 2. Then, roughly 1.0 msec later, the gates of the Na+ channels close and the gates of the K+ channels open for the next 2-4 msec The resulting outward flow of K+ repolarize the membrane back to -70 mv There are approximately 500 Na+ channel per square micrometer of membrane surface in the giant axon of the squid, the channels are separated by a distance of roughly 450 angstroms Sequential depolarization – a stimulus initiates a succession of action potentials that propagate down the membrane and produce the neural impulse Even though the Na+ rush occurs only in the section of the neural membrane that receives stimulation, the neighboring areas are affected as well They produce a charge in membrane permeability and allow for additional Na+ flow The domino effect that occurs in unmyelinated axons: Push the first domino and it will knock down the second, which will knock down the third, and so forth Changes in permeability along the membrane take place in a sequential pattern and produce the Na+ movement point by point along the membrane The neural impulse is a wave of depolarization along the neural membrane The neural impulses occur in unmyelinated axons are slow (10 meters per second) Saltatory conduction that occurs in myelinated axons: Myelin sheath covers the axon, and speeds up conduction along the axon The myelin sheath, along with the nodes of Ranvier, plays a crucial function in the conduction of the neural impulse Myelin makes neural impulse jump from node to node, instead of moving sequentially along every part of the neural membrane Depolarization in a myelinated axon gives rise to a phenomenon called saltatory conduction which can speed up to 120 meters per second The neural impulse in the axon follows all or non principle In order to generate electrical activity in the axon, the stimulus must reach a certain level of intensity – it must pass a threshold of .55 mv When the threshold is reached, the resting potential in the neuron goes from -70 mv to -55 mv and axonal firing occurs Once the intensity of the stimulus has passed the threshold, the magnitude of the impulse remains the same (+30 mv) no matter how intense the stimulus becomes This is an all or none effect in the neural firing of the axon The impulse does not decrease its strength as it travels farther from the point of stimulation – is called nondecremental impulse A mild stimulus applied to either the dendrites of the soma of a neuron produces an impulse Nondecremental – no decrease If a neural impulse is to influence behavior, it cannot stay within the neuron; it must travel from neuron to neuron or from neuron to muscle Before information can pass from one neuron to another (or to a muscle) a series of complex chemical events must take place across the microscopic gap separating the neurons The gap is called the synaptic gap, or synapse (Greek word for fasten together) it is about 100 to 200 angstroms (1/10 nanometer) wide The chemical substances that govern the transmission of neural impulses from one neuron to the next are called neurotransmitters Graded – degree on intensity of stimulus on dendrites or soma determines impulse Excitation vs. inhibition ( + vs. - ) Summation – multiple stimuli to crease neural impulse, two types: o Spatial – different spot stimulation o Temporal – same spot over and over stimulation A neurotransmitter is capable of changing their membrane permeability of the neuron of muscle it has reached Thus, it triggers or inhibits the neuron impulse or muscle contraction Up to now, 100 or more neurotransmitters have been discovered in the human nervous system The production of most neurotransmitters takes place in the axonal endings of the neuron The chemical reactions that produce neurotransmitters are usually directed by enzymes The enzymes themselves are produced in the soma of the neuron Then they move down the axon to the axonal endings through structures known as neurotubules (microtubules) The process of the movement of substance from soma to axonal endings is called axonal transport After the neurotransmitters are produced in the axonal endings, they are stored in tiny sacs, called synaptic vesicles Synaptic vesicles, sometimes described as neurotransmitter warehouses, are formed in the soma and transported to the axonal endings by axonal transport Axonal flow involves more than the flow from soma to the axonal endings of synaptic vesicles and of the enzymes needed to produce neurotransmitters (axonal transport) Transport from axonal endings to soma is called retrograde transport, the axonal flow is a two-way traffic The neuron that released the neurotransmitters into the synaptic gap is called the presynaptic neuron The neuron that receives the neurotransmitters is called the postsynaptic neuron Pre-synaptic neuron – neurotransmitters Post-synaptic neuron – receptor sites Permeability to Na+ and Cl Na+, K+ goes up = + Cl-, K+ goes up = - Axonal transport ----> Retrograde transport <---- Modulation neuron releases neuromodulators which control how much calcium ions are released to enter the end of pre-synaptic neuron Nissl substances – mitochondrion, lysosomes, ribosomes, etc. Auto-receptors on membrane of axonal endings monitors how many neurotransmitters are released into the synapse and stops the release when there is enough (regulation mechanism) Exocytosis – when Ca+ comes into axon endings after neural impulse, synaptic vesicles move to the membrane and fuse with inner membrane after neural impulse at (axonal ending), then membrane ruptures so neurotransmitters are released into the synapse, neurotransmitters look to bind to receptor sites o 1. Move to o 2. Fuse o 3. Rupture Sub-sensitivity of post synaptic neuron, opening more receptor sites to increase binding of neurotransmitters, post-synaptic neuron, super sensitivity when there are not enough neurotransmitters (defense mechanism) 100 kinds of neurotransmitters o Excitatory EPSP – excitatory post-synaptic potential Caused by binding to neurotransmitters + Change membrane permeability to allow more Na+ Summation – depolarization 30 mv o Inhibitory IPSP – inhibitory post-synaptic potential Caused by binding to neurotransmitters - Change membrane permeability to allow more Cl- Summation – hyperpolarization -90 mv WEEK 8 NOTES The process by which it translates one form of energy (the environmental stimulus) into another (the neural impulse) is known as transduction A receptor cell is a biological transducer Receptors convert physical stimuli into neural impulses indirectly Receptors convert physical stimuli into electrical signals, and it is these signals – known as generator or receptor potential – that produce the neural impulse The intensity of a stimulus has to exceed the absolute threshold – the minimal intensity of a stimulus that can be detected about 50% of the time – in order to be detected by receptor cells A stimulus that is below the absolute threshold is said to be subliminal and will not be able to influence behaviors The absolute thresholds for the five major senses are: o Vision – a candle light that is 30 miles away in a totally dark night o Sound – a ticking watch that is 20 feet away in a quiet room o Taste – a teaspoon of sugar in 2 gallons of water o Smell – a drop of perfume in a 3-room apartment o Touch – the falling of a bee’s wing that is 1 cm away from the cheek Psychophysicists measured not only absolute thresholds, but also what the called the differential threshold – often referred to as the just noticeable difference (JND) Max Weber and Gustav Fechner were interested in finding out the least amount of change in stimulation that would be noticeable o i.e. the differential threshold or JND JNDs, said Weber, are a constant proportion of a stimulus Fechner labeled this Weber’s Law Brightness -- .08; Taste -- .08; Loudness -- .05; Heaviness -- .02; and Electric Shock -- .01 We possess a variety of receptors; every receptor has a membrane that is surrounded by ions The presence of such a membrane and ions creates a resting potential in the receptor in much the same way that it does in the neuron convert a stimulus into an electric signal (generator potential) - the environmental stimuli change the permeability of the cell membrane and Na+ ions flow into the cell, causing it to depolarize The resulting generator potential travels to the sensory neuron, where it produces the neural impulse (action potential) Every receptor has the capacity to convert a stimulus to a generator potential The membrane of each type of receptor is sensitive to a specific type of environmental stimulus The generator potential must do 2 things to produce the action potential on the sensory neuron o 1. Generator potential spreads to the site of impulse initiation, usually the axon of the sensory neuron o 2. Generator potential produces depolarization sufficient to reach threshold – it must depolarize the axon of the sensory neuron from -70 mv to -55 mv in order to fire the all-or-none action potential that travels to the brain The generator potential is a graded potential (its magnitude depends directly on the intensity of the stimulus) The generator potential is decremental (it decreases as it spreads to the axon of the sensory neuron) A single generator potential is usually insufficient to produce the action potential Multiple generator potentials (all below threshold) must work collectively in order to exceed the threshold Once the generator potential passes the threshold, it elicits not only the action potential but also the rain of action potentials The greater the magnitude of the generator potential, the greater the frequency of the action potential Stimuli of long duration produce decreasing generator potential and progressively fewer action potentials This is called adaptation – a decrease in the resultant sensory experience Sensory coding is the one-to-one correspondence between some aspect of the physical stimulus and some aspect of the nervous system’s activity Where the neural impulse travels and how it travels correlate with certain sensory experiences – sights, sounds, smells, touches, and so forth We refer to these correlates as codes, and to the brain’s capacity to produce them as coding Because the human nervous system is capable of coding an enormous variety of stimuli, we can differentiate among a multitude of colors, sounds, tastes, smells, and tactile feelings We also make discriminations among stimuli on the basis of their intensity There are two types of sensory coding: the anatomical coding and the functional coding The anatomical coding is used to describe the correlation between sensation and brain area It is a theory proposed in 1826 by Johannes Muller, who called it the law of specific nerve energies (1838) The doctrine of specific nerve energies states that sensation depend less on the environmental stimuli that activate them than on the nerves that are stimulated and ultimately on the part of the brain that nerves stimulate Each sensory nerve is ordinarily excited by only one kind of energy, and the brain interprets any stimulation of that nerve as being that kind of energy According to Muller’s theory, sound and light produce different sensations because auditory nerves and optic nerves travel to different parts of the brain The functional coding involves the differences in neural activity (the frequency of neural impulses) triggered by varying amounts of environmental stimuli It states that various sensations do not necessarily correspond only to specific anatomical areas; they are also differentiated in accordance with the degree of neural activity within an area The sensation of brightness or loudness, for instance, varies in accordance with the number of neural impulses arriving per unit of time (per second) at the visual or auditory cortex An intense stimulus, for instance, will increase the level of firing and produce more impulses than a less intense stimulus A stimulus does not initiate the firing of the neural impulse Sensory areas in the brain and the neurons leading to these areas are spontaneously firing even if no stimulus is present What the stimulus does is to modulate this spontaneous activity All stimuli must produce 2 effects in the nervous system in order to be recognized: o 1. They must be received by an aroused brain i.e. the brain must be prime to process the stimuli; o 2. They must be attended to i.e. the brain must receive specific sensory information Humans have a highly sophisticated visual system that enables us to detect shapes, follow movement, differentiate colors, and use vision to judge distance But the physiological basis for this versatility is not yet fully understood To understand vision, you must first understand light Light is a form of electromagnetic energy generated by the movement of elementary particles known as photons This movement takes the form of light waves that vary in 3 respects: o Wavelength o Amplitude o Purity All visual sensations are produced by the relative differences in the wavelength, the amplitude, and the purity of wavelengths, and these sensations fall into 3 general categories: o Hue (wavelength) o Brightness (amplitude) o Saturation (purity) Hue is the sensation of color – is produced by differences in the length of electromagnetic waves Only a small fraction of waves can trigger a visual experience in humans We measure wavelength of light in nanometers (nm), one nanometer is equal to 1/1,000,000 millimeters The light waves visible to the human eye fall within a range from 380 nm to around 760 nm – which are capable of being transduced i.e. of producing the neural impulse The shortest wavelength visible to humans, 380 nm produces violet color The longest wavelength, 760 nm produces red color Wavelengths shorter than 380 nm (including ultraviolet rays, X-rays, and gamma rays) are not visible to the naked eye Wavelengths longer than 760 nm (including infrared waves, radar, FM, AM) are also invisible WEEK 9 NOTES The optical process involves preliminary gathering and bending of light by non-neural cornea and lens in the eye The neural process involves the working of the retina and its relationship to the brain The eyeball is basically round and is sheathed by a fibrous layer that contains 2 parts o About 5/6 of the surface is covered by an opaque white coating called the sclera, or white of the eye o The only area the sclera does not cover 1/6 is the little bulge that is covered by a transparent shield known as the cornea Light enters the eye through the cornea, whose principal function is to initiate the focusing process The cornea has no blood vessels It draws its fuel from a fluid-like substance known as the aqueous humor, which occupies the chamber between the cornea and the lens The next layer within the eye, adjacent to the sclera, is the choroid layer or coat This darkly pigmented layer of tissue has 2 basic functions: o 1. To support the blood vessels that supply fuel to the retina; and o 2. To absorb light waves that have scattered after corneal refraction (cat has tapetum, act as a mirror, reflecting light back to the eye for the nocturnal sight) As light passes through the cornea and the aqueous humor, it encounters the iris, a colored contractile membrane (the color of the iris is the color of the eyes) A small opening in the center of the iris, the pupil, controls the amount of light that reaches the back of the eye th The pupil can dilate to about 5/16 of an incthin diameter at its widest; at its narrowest, it measures about 1/16 of an inch The widening and narrowing of the pupil are governed by two sets of smooth muscles under the control of the autonomic nervous system In periods of stress of intense concentration, the sympathetic nervous system stimulates the pupil to dilate (increases of light) In periods of relaxation or in bright sunshine, the parasympathetic nervous system stimulates the pupil to contract (decreases of light) Myopia results from the overreaction of rays of light The focal point of the lens is located in front of the retina because of the lengthened eyeball, resulting in a blurred image The myopic need to wear a concave lens to adjust the vision Hyperopia results from under-refraction of rays of light; the focal point of the lens is located behind the retina because of the shortened eyeball, resulting in a blurred image The hyperopic need to wear a convex lens to adjust the vision, the retina is the place where the neural processing of visual information begins The retina is a membrane consisting of 3 layers of cells One layer is make up of 2 types of receptor cells: rods and cones The other 2 layers are made up of neurons that take 2 forms: bipolar and ganglion cells There are other neurons in the retina: o 1. Horizontal cells interconnect the receptors; and o 2. Amacrine cells interconnect the ganglion cells The arrangement of 3 layers is unusual The photoreceptor cells (rods and cones) are located behind the 2 neural layers This means that when light hits the retina, it must filter through the neural layers (first the ganglion cell layer, then the bipolar cell layer) before reaching and activating the photoreceptor cells It also means that the neural impulse, once it is triggered by the receptors in the rear portion of the retina, travels toward the front of the retina through the bipolar and ganglion cells Ultimately, the neural impulse is routed to the back of the retina through axons of the ganglion cells that make up the optic nerve Rods and cones are named for the shapes Rods are slender and cylindrical Cones are broad and bulbous Rats only have rods; turtles have only cones, humans have both rods and cones Humans have about 125 million rods and 7 million cones Rods are located primarily in peripheral areas, and cones are more numerous in the interior The center of the retina (smaller than a square mm) only has cones, more than 50,000 of them, packed closely together This area is called fovea centralis Both rods and cones are sensitive to light Rods have a low threshold of excitation That rods operate primarily in conditions of low illumination, such as exist at night Rods do not abstract color from the light Rods function in ways that are achromatic (colorless) and scotopic (related to darkness) The more rods that are stimulated by a particular wavelength, the brighter an object will appear Rods are less adept at visual acuity (the capacity to discern detail) Cones function in ways that are chromatic (colorful) and photopic (related to light) With a much higher threshold of excitation to light than rods, they function mainly under high illumination conditions, such as exist during the day The more cones that are stimulated by a particular wavelength, the brighter and more colorful and object will appear Both cones and rods are not equally sensitive to all wavelengths of light The Purkinje effect is an excellent illustration of the varying sensitivity of rods and comes and their relationship to color sensation At dusk some colors seem vibrant than others Green grass, for instance, seems brighter than red roses The reason is that cones are not equally sensitive to all wavelengths When illumination begins to dim, fewer cones are activated by long wavelengths than by short ones. In the low illumination of evening, the rods come into play, because of their super-sensitivity to light Remember, rods are completely insensitive to long wavelengths, therefore in the evening a rod rose may appear black and green grass may appear a somewhat brighter shade of gray After transduction takes place in the retina, the converted signals are converted from the retina to the brain via axons that issue from ganglion cells and are collected into a bundle called optic nerve At the point where the optic nerve leaves the retina of each eye, there are neither rods nor cones This area is called the blind spot or optic disc Each eye has its own optic nerve, but the two nerves converge at the base of the brain at a place called the optic chiasm There they go through a kind of restoring process, but they do not form synapses there In frogs (a lower animal), the optic nerves from two eyes simply cross to the opposite sides of the brain In humans, only half of the neurons contained in the two optic nerves cross The neurons that cross are those that originate in the nasal half of each retina The neurons that do not cross are those that originate in the temporal half of each retina
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