Study Guides (248,398)
Canada (121,510)
York University (10,209)
Psychology (1,203)
PSYC 2240 (30)
Midterm

Biological Basis Exam 1 Notes.doc

29 Pages
261 Views
Unlock Document

Department
Psychology
Course
PSYC 2240
Professor
Alistair Mapp
Semester
Winter

Description
Biological Basis Exam 1 Notes Chapter 1 – The Major Issues Main ideas 1. biological explanations of behaviour fall into several categories (physiology, development, evolution, and function) 2. Reject the idea that the mind exists independently of the brain 3. The expression of a gene depends on the environment and on interactions with other genes 4. Research with animals is important, but sometimes doing so can be an ethical issue The Mind-Brain Relationship • Biological psychology is the study of the physiological, evolutionary, and developmental mechanisms of behaviour and experience • Relating biology and psychology • Biological psychology point of view: proper way to understand behaviour is in terms of how it evolved and how the functioning of the brain and other organs controls behaviour • Think and act as we do because we have beneficial brain mechanisms that have arrived through evolution. Animals with these mechanisms survived to reproduce better than animals with other mechanisms Biological Explanations of Behaviour • Physiological explanation – relates behaviour to the activity of the brain and other organs. Deals with the machinery of the body. o Ex. Birds who drink with their heads down do so because of certain nerve patterns and throat muscles • Ontogenetic explanation – describes how a structure or behaviour develops, including the influences of genes, nutrition, experiences, and their interactions. o Refers to development, from infancy to maturation • Evolutionary explanation – reconstructs the evolutionary history of a structure or behaviour o Ex. Goose bumps are useless to humans because our shoulder and arms hairs are so short, but in other animals it makes a difference as it makes them seem larger. In humans the behaviour evolved in our remote ancestors and we inherited the mechanism • Functional explanation – describes why a structure or behaviour evolved as it did, its purpose or use o Camouflaged appearance makes the animal inconspicuous to predators o Many behaviours alleged to be part of our evolutionary heritage could have been learned instead, controversial See page 5 for examples of all explanations 1 The Brain and Conscious Experience • Mind-body or mind-brain problem: what is the relationship between the mind and the brain? • Widespread view among non-scientists is dualism, the belief that mind and body are different kinds of substance that exist independently • Current philosophers and neuroscientists reject dualism, conflicts with the law of the conversation of matter and energy • Total amount of energy in the universe is fixed, matter transform into energy, energy into matter, but neither appears out of nothing and nothing disappears into nothing • A mind could not exist on its own as it has to be composed to matter to create something, or an effect, such as moving muscles • Alternative to dualism is monism; the universe consists of only one kind of substance. Various forms of monism are possible • Materialism – everything that exists is material or physical. Mental events don’t exist at all • Mentalism – only the mind really exists and that the physical world could not exist unless some mind were aware of it • Identity position – mental processes and certain kinds of brain processes are the same thing, described in different terms. Every mental experience is a brain activity. Most reasonable hypothesis • Stimulation of any brain area provokes an experience and any experience evokes brain activity, no mental activity without brain activity • Can not observe consciousness, therefore have no idea if other animals are conscious • Solipsism - I alone exist, people believe they are the only conscious organism. • The difficulty of knowing whether other people or animals have conscious experiences is known as the problem of other minds • Easy problems – the difference between wakefulness and sleep, mechanisms that enable us to focus our attention • Hard problems concerns why and how any kind of brain activity is associated with consciousness. Why do minds exist at all? The Genetics of Behaviour Mendelian Genetics • Inhertiance occurs through genes • Genes come in pairs because they are aligned along chromosomes (strands of genes) which also come in pairs • A gene is a portion of a chromosome, which is composed of the double stranded molecule DNA • Genetic outcome depends on parts of two or more chromosomes • Part of a chromosome sometimes does not code for a protein of its own, but alters the expression of genes elsewhere 2 • Some proteins form part of the structure of the body, others serve as enzymes, biological catalysts • Identical pair of genes on the two chromosomes is homozygous for that gene, unmatched pair of genes is heterozygous Sex-Linked and Sex-Limited Genes • Genes located on the sex chromosomes are known as sex-linked genes • All other chromosomes are autosomal chromosomes • A female mammal has two X chromosomes, a male has X and Y • Y chromosome is small, genes for 27 proteins. X for 1500 proteins • Red-green colour deficiency • Sex-limited genes, present in both sexes generally on autosomal chromosomes but active mainly in one sex. Examples include amount of chest hair in men, breast size in women • Both sexes have these genes Heredity and Environment • Variations in behaviour depend on combined influence of many genes and environmental influences. Every behaviour requires both, take away either one and nothing is possible. • However the observed difference among individuals could depend more on differences in heredity or differences in environment • To study nurture vs. nature, study monozygotic (one egg) and dizygotic (two egg) twins • Monozygotic has the same genes, a stronger resemblance between monozygotic and dizygotic represents an impact from genes • A second kind of evidence is studies of adopted children. Any tendency for adopted children to resemble biological parents is hereditary. Variations in some characteristic depend largely on genetic differences, the characteristic has high heritability Possible Complications • Multiplier effect – if genetic or prenatal influences produce even a small increase in some activity, the early tendency will change the environment in a way that magnifies that tendency o Ex. Child born with genes promoting great height, running speed and coordination. Will have many more environmental opportunities to play basketball and magnify that effect Environmental Modification • Traits with high heritability can be modified by environmental interventions • If you are born with a sickness, you can be treated 3 The Evolution of Behaviour • Evolution is a change over generations in the frequencies of various genes in a population. Any change in gene frequencies, whether positive or negative Common Misunderstandings about Evolution • Lamarckian evolution – disuse of a structure causes us to lose it. Not the case at all • Only lose a structure if people with the coding to not have that structure have an advantage • Humans have stopped evolving due to modern medicine and everyone stays alive, however if a gene allows someone to reproduce more, those genes will spread more through a population • Evolution improves the average fitness of the population, the number of copies of ones genes that endure in later generations. Being fit is having more offspring • Evolution does not benefit the individual or the species, it benefits the genes Evolutionary Psychology • How behaviours have evolved, especially social behaviours • Emphasis is on evolutionary and functional explanation • Species evolve functions that help with their behaviour • Altruistic behaviour – action that benefits someone other than the actor. Can help individuals as well as the group • Reciprocal altruism – the idea that individuals help those who will return the favour • Altruistic genes could spread because they facilitate care for one’s kin or because they facilitate exchanges of favours with others. Group selection may also work under some circumstances if the cooperative group has a way to punish or expel an uncooperative individual The Use of Animals in Research Reasons for Animal Research 1. The underlying mechanisms of behaviour are similar across species and sometimes easier to study in a nonhuman species 2. We are interested in animals for their own sake 3. what we learn about animals sheds light on human evolution 4. certain experiments cannot use humans because of legal or ethical reasons • Minimalists – tolerate animal research under certain conditions • Abolitions – maintain that all animals have same rights as humans • Legal standard • The Three Rs – reduction, replacement, and refinement • Reduction – using as few participants as possible • Replacement – using computerized or simulated tests when possible • Refinement – refining study to use minimal discomfort and pain 4 Chapter 2 – Nerve Cells and Nerve Impulses Main Ideas 1. Nervous system is composed of two types of cells: neurons and glia. Only the neurons transmit impules from one location to another. 2. The larger neurons have branches, known as axons and dendrites, which can change their branching pattern as a function of experience, age, and chemical influences 3. Many molecules in the bloodstream that can enter other body organs cannot enter the brain 4. The action potential, an all-or-none change in the electrical potential across the membrane of a neuron, is caused by the sudden flow of sodium ions into the neuron and is followed by a flow of potassium ions out of the neuron 5. Local neurons are small and do not have axons or action potentials. Instead they convey information to nearby neurons by graded potentials Anatomy of Neurons and Glia • Neurons receive information and transmit it to other cells • Human brain contains approximately 100 billion neurons The Structures of an Animal Cell • Neurons similar to other body cells, other than its distinctive features • Membrane, composed of two layers of fat molecules that are free to flow around one another • Most chemicals cannot pass membrane, but specific protein channels permit a controlled flow of water, oxygen, sodium, potassium, calcium, chloride, and other important chemicals • Except for red blood cells, all animal cells have a nucleus, the structure that contains the chromosomes • Mitochondrion performs metabolic activities • Ribosomes synthesize new protein molecules, sometimes attached to endoplasmic reticulum The Structure of a Neuron • Distinguished from other cells by their shape • Larger neurons have dendrites, a soma, an axon, and presynaptic terminals • The tiniest neurons lack axons and lack well-defined dendrites • A motor neuron has its soma in the spinal cord (at the end) • Receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle • A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation, such as light, sound, or touch • Soma is located more towards the middle • Dendrites are branching fibres that get narrower near their ends 5 • The dendrite’s surface is lined with specialized synaptic receptors, where it receives information from other neurons • The greater the surface area of a dendrite, the more information it can receive • Dendritic spines – short outgrowths that increase the surface area available for synapses • The shape of the dendrite has much to do with how the dendrite combines different kinds of input • Cell body (soma) contains the nucleus, ribosomes, mitochondria, and other structures found in most cells • Cell body is also covered with synapse on its surface in many neurons • The axon is a thin fibre of constant diameter, in most cases longer than dendrites • Axon is the information sender • Myelin sheath is insulating material, with interruptions known as nodes of Ranvier • Invertebrate axons are not insulated • An axon has many branches, each of which swells at its tip forming a presynaptic terminal, or an end bulb or bouton • This is the point from which the axon releases chemicals that cross through the junction between one neuron and the next • A neuron can only have one axon, but any number of dendrites • Axons can range to a meter in length (spinal cord to feet) • Afferent axon – brings information into a structure • Efferent axon – carries information away from a structure • Every sensory neuron is an afferent to the rest of the nervous system and every motor neuron is an efferent from the nervous system • Every neuron is efferent to one structure, and afferent to another • Efferent starts with e as in exit, afferent a for admission • If a cells dendrites are entirely within one structure, it is an interneuron or intrinsic neuron of that structure Variations among Neurons • Vary in size, shape, and function • Shape determines its connections with other neurons and thereby determines its contribution to the nervous system • Neurons with wider branching connect with more neurons • Function relates to shape Glia • Do not transmit information over long distances • Exchange chemicals with adjacent neurons • Glia means glue, hold neurons together • Smaller, and also more numerous than neurons. Occupy about the same volue • Astrocytes o Star-shaped 6 o Wrap around presynaptic terminals of a group of functionally related axons o By taking up chemicals released by axons and then releasing them back to axons, an astrocyte helps synchronize the activity of the axons, enabling them to send messages in waves o Also remove waste when neurons die o Control amount of blood flow to each brain area o Dilate blood vessels, allowing more nutrients into that area o Release chemicals that modify the activity of neighbouring neurons • Microglia o Very small cells, also remove waste as well as viruses and fungi o Function like part of the immune system • Oligodendrocytes o In the brain and spinal cord o Build the myelin sheaths that surround and insulate certain vertebrate axons • Schwann cells o In the periphery nervous system o Also build myelin • Radial glia o Guide the migration of neurons and their axons and dendrites during embryonic development o When embryological development finishes, most radial glia differentiate into neurons and some into astrocytes and oligodendrocytes The Blood-Brain Barrier • Many chemicals cannot cross from the blood to the brain due to blood-brain barrier • When a virus invades a cell, mechanisms within the cell extrude the virus through the membrane so the immune system can track it, find it, and destroy it • These body cells are replaced easily when destroyed, nervous cells are not • The blood-brain barrier is a wall that minimizes the risk of invaders getting it • Along the sides of the brains blood vessels • Keeps out most viruses, bacteria, but also keeps out most nutrients • If a virus does enter brain (rabies) could lead to death. Sometimes microglia can attack the virus or slow their reproduction without killing the neurons they occupy • A virus that enters your nervous system is probably with you for life – chicken pox, shingles, herpes How the Blood-Brain Barrier Works • Depends on the arrangement of endothelial cells that form the walls of the capillaries • Joined very tightly that virtually nothing passes between them 7 • Brain has mechanisms to allow certain chemicals to cross through the endothelial cells • Small uncharged molecules, such as oxygen and carbon dioxide, cross freely • Water crosses through special protein channels • Molecules that dissolve in the fats of the membrane can cross passively • Active transport – protein mediated, expends energy to pump chemicals from the blood into the brain (glucose, amino acids, purines, choline, a few vitamins, iron) • In Alzheimer’s the endothelial cells lining the brains blood vessels shrink, and harmful chemicals enter the brain • Barrier is a difficulty as it also keeps out many medications The Nourishment of Vertebrate Neurons • Neurons depend almost entirely on glucose • Metabolic pathway that uses glucose requires oxygen, the neurons consume an enormous amount of oxygen compared with cells of other organs • Neurons have enzymes to metabolize other nutrients, however glucose is best as it can pass blood brain barrier in adults • Glucose shortage is rarely a problem, liver makes glucose from many kinds of carbs and amino acids The Nerve Impulse The Resting Potential of the Neuron • Membrane of a neuron maintains an electrical gradient, a difference in electrical charge between the inside and outside of the cell • All parts of a neuron are covered by a membrane composed of two layers made up of phospholipid • Embedded among the phospholipids are cylindrical protein molecules • In the absence of any outside disturbance, the membrane maintains an electrical polarization (a difference in electrical charge between two locations) • Neuron inside the membrane has a slightly negative electrical potential with respect to the outside • Resting potential = -70mV • Mainly the result of negatively charged proteins inside the cell • Measured by inserting a very thin microelectrode into the cell body Forces Acting on Sodium and Potassium Ions • If charged ions could flow freely across the membrane, the membrane would depolarize at once • Membrane is selectively permeable • Sodium, potassium, chloride cross through membrane channels • Membrane is at rest, the sodium channels are closed, potassium channels are nearly but not entirely closed so potassium flows slowly 8 • Sodium-potassium pump, a protein complex, repeatedly transports three sodium ions out of the cell while drawing two potassium ions into • Active transport requires energy • Results in higher concentration of Na outside cell, higher K inside cell • Pump only effective due to selective permeability, prevents Na from leaking right back in and K leaking right back out • Some K do leak out, carrying a positive charge with them • Leakage increases electrical gradient across the membrane • When at rest, two forces act on sodium both pushing it into the cell • Electrical gradient – Na is positively charge and inside the cell is more negative • Opposite electrical charges attract each other • Concentration gradient – more Na outside the cell than inside the cell, pushes inward • Sodium would move rapidly if it could, but the channels are closed at rest • Potassium subject to competing forces • Electrical gradient – potassium is kept in the cell as it is positive and at rest the cell is negatively charged • Concentration gradient – potassium is more concentrated inside the cell than outside, driving K out of the cell • If channels were wide open, K would have moderate flow out of the cell • Electrical gradient and concentration gradient for potassium are almost in balance, but the pump keeps puling potassium in so they can not be completely balanced • Negatively charged proteins also exist within the cell, which are responsible for the membrane’s polarization • Chloride ions, being negatively charged, are mainly outside the cell • Opening chloride channels produces little effect when the membrane is at rest because the gradients balance Why a Resting Potential? • A lot of energy goes into operating pump and maintain resting potential • Prepares the neuron to respond rapidly • Excitation of the neuron opens channels that let sodium enter the cell explosively The Action Potential • Hyperpolarization – increased polarization, increasing the negative charge • Depolarize – reduce the polarization toward zero • When depolarization hits threshold, action potential fires, a massive depolarization caused by opening of sodium channels and permits a rapid flow of ions across the membrane • Any subthreshold stimulation produces a small response proportional to the amount of current, any stimulation beyond the threshold, regardless of how far beyond, produces the exact same response • The peak of the action potential varies from one axon to another, usually +30mV 9 The Molecular Basis of the Action Potential • The membrane protein gates that control sodium entry are voltage-gated channels • At resting potential, the channels are closed, as the membrane becomes depolarized the sodium channels begin to open and sodium flows more freely • If the depolarization is less than the threshold, sodium crosses the membrane only slightly more than usual • Once potential hits peak, hits reversed polarity • Less than 1% of sodium ions actually cross the membrane during an action potential. Sodium is still much more highly concentrated outside the cell • At the peak of the potential, the sodium gates quickly close and resist reopening for about the next millisecond • Potassium channels then open allowing the action potential to return to resting potential • K flows out of the axon because they are much more concentrated inside than outside and are no longer held in by a positive charge • Membrane temporarily hyperpolarizes • Chloride tends to go inward to make up for hyperpolarization • Pump restores the original distribution of ions • After many action potentials, a pump can not keep up and excessive build up of sodium can be toxic to a cell • Scorpion venom keeps sodium channels open and closing potassium channels The All-or-None Law • Dendrites can depolarize, but they don’t have voltage gated sodium channels • Dendrites don’t have action potentials, if they depolarize enough the axon produces an action potential • For a given neuron, all action potentials are approximately equal in amplitude and velocity. This is the all or none law • Amplitude and velocity of an action potential are independent of the stimulus that caused it • More frequent action potentials signal a greater intensity of stimulus • An axon might show one rhythm of APs for one type of stimulus, and another rhythm for another stimulus The Refractory Period • The cell can not produce another action potential while the current action potential is returning to resting potential, as it is in a refractory period • Absolute refractory period – no matter what, can not produce AP • Relative refractory period – can produce an AP if a stronger than usual stimulus is applied Propagation of the Action Potential • How the AP moves down the axon, must convey impulses without loss of strength • AP begins at the axon hillock, a swelling where the axon exits the soma 10 • Each point along the membrane regenerates the action potential in much the same way that it was generated initially • Na enter a point on the axon, that location is temporarily positive • The positive ions flow down the axon and across the membrane • The positive charges slight depolarize the adjacent areas of the membrane, continuing the AP across • Causes the next area to reach its threshold and open the voltage-gated sodium channels • Travels like a wave along the axon • Propagation of the action potential – the transmission of an action potential down an axon • Gives birth to a new AP at each point along the axon, just as strong at the end as it was at the beginning • Slow than electrical conduction, requires the diffusion of sodium ions at successive points along the axon • Refractory period prevents AP from flowing backwards Summary on page 44 The Myelin Sheath and Saltatory Conduction • Increasing the diamet increases conduction velocity up to about 10 m/s, compared to the thinnest axons which travel at 1 m/s • Sheaths of myelin, the insulating material composed of fats and proteins, increase the diameter • Sodium channels rarely exist between nodes of Ranvier, concentrated at the nodes • After an action potential occurs at a node, Na ions enter the axon and diffuse within the axon, repelling positive ions that were already present and pushing a chain of positive ions along the axon to the next node, where they regenerate the action potential. Summary, APs only occur at nodes • This flow of ions is faster than the regeneration of an action potential at each point • The action potential jumps from node to node • Saltatory conduction • Also conserves energy • MS attacks myelin, causes symptoms Local Neurons • Axons produce action potential, some neurons do not have axons • These neurons are smaller but very important Graded Potentials • Neurons without axons exchange information only with their closest neighbours, local neurons • Receives information from other neurons and produces graded potentials, membrane potentials that vary in magnitude without following the all-or-none law 11 • Depolarizes or hyperpolarizes in proportion to the magnitude of the stimulus • Travels in all directions, decays as it travels Chapter 3 – Synapses (omit hormones section) Main Ideas 1. At a synapse, a neuron releases neurotransmitters that excite or inhibit another cell or alter its response to other input 2. In most cases a single release of neurotransmitter produces only a subthreshold response in the receiving cell. This response summates with other subthreshold responses to determine whether or not the cell produces an action potential 3. transmission at synapses goes through many steps and interference at any of them can alter the outcome 4. nearly all drugs that affect behaviour or experience do so by acting at synapses 5. nearly all abused drugs increase the release of dopamine in certain brain areas 6. addiction changes certain brain areas, increasing the tendency to seek the addictive substance and decreasing the response to other kinds of reinforcement The Concept of the Synapse The Properties of Synapses • Sherrington’s research showed reflexes are slower than conduction along an axon, and several weak stimuli presented at slightly different times or slightly different locations produce a stronger reflex than a single stimulus does • Also when one set of muscles becomes stimulated one set becomes relaxed • Sherrington came up with idea of synapses Temporal Summation • Repeated stimuli within a brief time have a cumulative effect • Postsynaptic neuron – the cell that receives the message • Presynaptic neuron - the cell that delivers the synaptic transmission • Subthreshold excitation decays but can combine with a second excitation that quickly follows it • Pinching twice is temporal summation, pinching in different locations at the same time is spatial summation • Excitatory postsynaptic potential (EPSP) – depolarization graded potential • Inhibitory postsynaptic potential (IPSP) – hyperpolarization graded potential Spatial Summation • Summation over space • Synaptic inputs from separate locations combine their effects on a neuron 12 Inhibitory Synapses • A pinch in the foot sends a message along a sensory neuron to an interneuron in the spinal cord which in turn excites the motor neurons • Interneuron also sends a message to block activity of other muscles • Inhibitory synapses input from an axon hyperpolarizes the postsynaptic cell • Increases the negative charge within the cell, moving it further from the threshold and decreasing the probability of an action potential • Inhibitory postsynaptic potential (IPSP) • Synaptic input selectively opens the gates for potassium ions to leave the cell carrying a positive charge with them, or for chloride ions to enter the cell carrying a negative charge Relationship among EPSP, IPSP, and Action Potentials • Spontaneous firing rate – a periodic production of action potentials even without synaptic input • EPSPs increase this rate, IPSPs decrease it Chemical Events at the Synapse The Discovery of Chemical Transmission at Synapses • T.R. Elliott applied adrenaline to the surface of the heart, stomach, pupils. All produce similar effects as those of the sympathetic nervous system, Sympathetic nerves stimulate muscles by releasing adrenaline or a similar chemical • Loewi stimulated the vagus nerve, decreasing the frog’s heart rate. Collected fluid from the heart, transferred it to a second frog’s heart, had the same effect. Did same thing with stimulation. Concluded nerves send messages by releasing chemical The Sequence of Chemical Events at a Synapse 1. Neuron synthesizes chemicals that serve as neurotransmitters o Synthesizes small neurotransmitters in the axon terminals and neuropeptides in the cell body 2. The neuron transports the neuropeptides that were formed in the cell body to the axon terminals or to the dendrites o Neuropeptides are released from multiple sites in the cell 3. Action potentials travel down the axon. At the presynaptic terminal, an action potential open voltage gated calcium channels, calcium enters the cell and releases neurotransmitter from the terminals and into the synaptic cleft, the space between the presynaptic and postsynaptic neuron 4. The released molecules diffuse across the cleft, attach to receptors, and alter the activity of the postsynaptic neuron 5. The neurotransmitter molecules separate from their receptors. Depending on the neurotransmitter, it may be converted into inactive chemical 13 6. The neurotransmitter molecules may be taken back into the presynaptic neuron for recycling or may diffuse away. In some cases, empty vesicles are returned to the cell body 7. Some postsynaptic cells send reverse messages to control the further release of neurotransmitter by presynaptic cells Types of Neurotransmitters • Most are amino acids, derivatives of amino acids, or chain of amino acids (neuropeptides) • Nitric oxide signals that a brain area has become more active, increasing blood flow to that area Synthesis of Transmitters • Catecholamines – contain a catechol group and an amine group o Epinephrine, norepinephrine, acetylcholine, dopamine, serotonin • Synthesis begins with substances found in diet Transport and Storage of Transmitters • Most abundant neurotransmitters are synthesized in the presynaptic terminal • Neuropeptides are synthesized in the cell body and then transported down the axon or into the dendrites • Presynpatical terminal stores neurotransmitter molecules in vesicle, tiny spheres • Nitric oxide is the exception. The gas is released as soon as it is formed • MAO (monoamine oxidase) breaks down excess transmitters into inactive chemicals Release and Diffusion of Transmitters • Depolarization opens voltage gated calcium channels, which allow calcium to enter and cause exocytosis of the neurotransmitters • Amount of neurotransmitter released varies • Neurotransmitter diffuses across cleft, attaches to receptor on postsynaptic membrane • No single neuron releases all neurotransmitters, usually release a combination of two or more transmitters • Neurons release different transmitters from different branches of its axon • Combination makes the neuron’s message more complex, such as brief excitation followed by a slight but prolonged inhibition • Neuron can have ability to receive and respond to many neurotransmitters at different synapses Activation of Receptors of the Postsynaptic Cell • The meaning of a neurotransmitter depends o
More Less

Related notes for PSYC 2240

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit