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by: Mr. Elbert Greenholt

Neurobiology BIOLOGY 222

Mr. Elbert Greenholt
GPA 3.9

Naomi Nagaya

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Naomi Nagaya
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This 6 page Class Notes was uploaded by Mr. Elbert Greenholt on Thursday October 29, 2015. The Class Notes belongs to BIOLOGY 222 at University of Michigan taught by Naomi Nagaya in Fall. Since its upload, it has received 12 views. For similar materials see /class/231615/biology-222-university-of-michigan in Biology at University of Michigan.


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Date Created: 10/29/15
Chapter 3 A goal of cellular neurophysiology is to understand the biological mechanisms that underlie a nervous system to collect distribute and integrate information Action potential a brief uctuation in membrane potential caused by the rapid opening and closing of voltagegated ion channels also known as spike nerve impulse or discharge39 it sweeps like a wave along axons to transfer information from one place to another in the nervous system Excitable membrane any membrane capable of generation action potentialsaxons and muscle cells Resting membrane potential the membrane potential or membrane voltage maintained by a cell when it is not generating action potentials also called resting potential Neurons have a resting potential of about 65 mV Water is the main ingredient of the cytosol uid inside the neuron and the extracellular uid lon an electrically charged atom or molecule Water has an uneven distribution of electrical charge The electrons covalently bonded between the oxygen and hydrogen atoms tend to be more attracted to the oxygen therefore the oxygen atom acquires a negative charge making the molecule polar Polar molecules dissolve well in water Spheres of hydration water molecules that surround ions when being dissolved Monovalent when the difference between the number of protons and electrons in an ion is l Na K and C139 are important for cellular neurophysiolo y Divalent when the difference between the number of protons and electrons in an ion is 2 Ca2 is important for cellular neurophysiolo Cations a positively charged ion Anion a negatively charged ion Hydrophilic waterloving ions and polar molecules that dissolve well in water Hydrophobic water fearing molecules with nonpolar covalent bonds that do not dissolve in water Phospholipid bilayer the arrangement of phospholipid molecules that forms the basic structure of the cell membrane The core of the bilayer is lipid creating a barrier to water and to watersoluble ions and molecules out of nonpolar chains of carbon atoms bonded to hydrogen atoms as well as a phosphate group P atom bonded to three 0 atoms known as the polar Protein a molecule made from various combinations of 20 difference amino acids Amino acids have a central carbon a hydrogen atom an amino group NH and carboxyl group C00 and an R group The properties of the R group determine the chemical relationships in which each amino acid can participate Peptide bond the covalent bond between the amino group of one amino acid and the carboxyl group of another Polypeptide a string of amino acids held together by peptide bonds Protein structure 9primag chain amino acids are linked together by peptide bonds in a chain 9alpha helix secondary structure where the primary chain coils into a spirallike configuration 9tertiary structure created when R groups interact with one another and change its threedimensional shape bend fold create globular shape 9guaternag structure when different polypeptide chains subunits bond and create a larger molecule Ion channela membranespanning protein that forms a pore that allows the passage of ions from one side of the membrane to the other Ion selectivity a property of ion channels that are selectively permeable to some ions and not to others Gating a property of many ion channels making them open or closed in response to specific signals such as membrane voltage or the presence of neurotransmitters Ion pump a protein that transports ion across a membrane at the expense of metabolic energy ionic movements through channels are in uenced by two factors diffusion and electricit diffusion the temperaturedependent movement of molecules from regions of high concentration to regions of low concentration resulting in a more even distribution the rate of diffusion is proportional to the temperature concentration gradient a difference in concentration from one region to another Tonic concentration gradients across the neuronal membrane help determine the membrane potential ions are driven across the membrane through diffusion when the membrane possesses channels permeable to the ions and there is a concentration gradient across the membrane electrical current the rate of movement of electrical charge represented by the symbol 1 and measured in amperes amp electrical potential voltage and electrical conductance determine how much current will ow electrical potentialvoltage the force exerted on an electrically charged particle represented by the symbol V and measured in volts also called voltage or potential difference more current will flow as the difference in charge between the anode and cathode increases electrical conductance the relative ability of an electrical charge to migrate from one point to another represented by the symbol g and measured in siemens S it depends on the number of particles available to carry electrical charge and the ease with which these particles can travel through space electrical resistance the relative inability of an electrical charge to migrate from one point to another represented by the symbol R and measured in ohms W the inverse of conductance Ohm s law the relationship between electrical current I voltage V and conductance g lgV Because electrical conductance is the inverse of resistance R Ohm39s law may also be written VIR ions are driven across the membrane electrically when the membrane possesses channels permeable to that ion and if there is an electrical potential difference across the membrane membrane potential the voltage across a cell membrane represented by the symbol Vm microelectrode a probe used to measure the electrical activity of cells has a very fine tip and can be fashioned from etched metal or glass pipettes filled with electrically conductive solutions the inside of the neuron is electrically negative with respect to the outside the resting potential of a typical neuron is 65mV millivolts equilibrium potential the electrical potential difference that exactly balances an ionic concentration gradient represented by the symbol Egoquot also known as equilibrium potential Ex The inside of a cell has a high and equal concentration of positively and negatively charged ions K and A39 The outside of the cell has a lower but still equal concentration of the same positively charged ions They are separated by a phospholipid bilayer and therefore there is no net movement of ions If a potassium channel was inserted into the phospholipid bilayer then the K ions would move from the inside to the outside of the cell due to the concentration gradient As the K ions leave the cell they leave the A ions inside the cell making the inside of the cell negative This attracts the K ions back through the potassium channel Eventually an equilibrium state is reached when the diffusional and electrical forces are equal and opposite and the net movement of the K ions is zero again large changes in membrane potential are caused by miniscule changes in ionic concentrations the net difference in electrical charge occurs at the inside and outside surfaces of the membrane the membrane is so thin that the anions on the inside and the cations on the outside tend to be mutually attracted to the cell membrane This property of the membrane storing electrical charge is called capacitance ions are driven across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential ionic driving force the difference between the real membrane potential Vm and the ionic equilibrium potential Eion if the concentration difference across the membrane is known an equilibrium potential can be calculated for that ion IS THIS CORRECT77 if the ion concentration is higher on the inside that ion has a negative equilibrium potential if the membrane was selectively permeable to that ion IS THIS CORRECT77 if the ion concentration is higher on the outside that ion has a positive equilibrium potential if the membrane was selectively permeable to that ion each ion has its own equilibrium potential if the membrane were permeable only to that ion Nernst equation a mathematical relationship used to calculate an ionicequilibrium potential sodiumpotassium pump an ion pump that removes 39 Na and 39 because it pushes the ions across their concentration gradients up to 70 of the brain s ATP calcium pump an ion pump that removes cytosolic Ca2 Goldman equation a 39 39 391 used to predict membrane potential from the concentrations and membrane permeabilities of ionsthe neuronal membrane at rest is mostly permeable to K so the membrane potential is close to ER depolarization a change in membrane potential taking it from the value at rest to a less negative value ex 65 mV to 0m This means that increasing extracellular potassium depolarizes neurons bloodbrain barrier a specialization of the walls of brain capillaries that limits the movement of bloodborne substances like potassium into the extracellular uid of the brain potassium spatial buffering the mechanism of regulating K ions by astrocytes K using ATP for energy pter 2 there are two types of cells in the nervous system neurons and glia in the brain glia outnumber neurons which are about 100 billion tenfold neurons are the most important cells for the unique functions of the brain neuron the informationprocessing cell of the nervous system also called nerve cell Most neurons use action potentials to send signals over a distance and all neurons communicate with one another using synaptic transmission glia glial cell a support cell in the nervous system it insulates supports and nourishes neighboring neurons can be classified into four categories astrocytes oligodendroglia Schwann cells and microglia brain tissue has a jellolike consistence therefore not firm enough to make thin slices in order to harden the brain tissue scientists immersed it in formaldehyde then were able to cut thin slices histology the microscopic study of the structure of tissues Nissl stain a class of basic dyes that stain the somata of neurons named for its discoverer German historologist Franz Nissl It s useful for two reasons 1 it distinguishes neurons and glia from one another and 2 it enables histologists to study the arrangement or Cytoarchitecture of neurons in different parts of the brain c oarchitecture the arrangement of neuronal cell bodies in various parts of the brain Golgi stain a method of staining brain tissue that shows neurons and all of their neurites named for its discoverer Italian histologist Camillo Golgi cell bodysomaperikagon the central region of the neuron containing the nucleus neurite a thin tube extending from a neuronal cell body39 the two types are axons and dendrites w a neurite specialized to conduct nerve impulses or action potentials normally away from the soma dendrite a neurite specialized to receive synaptic inputs from other neurons Golgi proposed that the neurites of different cells are fused together to form a continuous reticulum network similar to the arteries and veins of the circulatory system Santiago Ramon Caj al Golgi s rival argued that the neurites of different neurons are not continuous with one another and must communicated by contact not continuit neuron doctrine the concept that the neuron is the elementary functional unit of the brain and that neurons communicate with each other by contact not continuity c osol the watery uid inside a cell organelle a membrane enclosed structure inside a cell c oplasm cellular material contained by the cell membrane including the organelles but excluding the nucleus nucleus 1 the roughly spherical organelle in a cell body containing the chromosomes 2 a clearly distinguishable mass of neurons usually deep in the brain It is enclosed by a double membrane called the nuclear envelope which is perforated by pores chromosome a structure in the cell nucleus containing a single linear thread of DNA DNA deox ibonucleic acid a doublestranded molecule constructed from four nucleic acids that contain the genetic instructions for a cell g a unit of hereditary39 a sequence of DNA that encodes a single polypeptide or protein gene expression the process of transcribing the information from a gene into messenger RNA protein a polymer of amino acids strung together by peptide bonds protein smthesis the assembly of protein molecules in the cell39s cytoplasm according to genetic instructions mRNA messenger ribonucleic acid a molecule constructed from four nucleic acids that carries the genetic instructions for the assembly of a protein from the nucleus to the cytoplasm transcription the process of synthesizing a messenger RNA molecule according to genetic instructions encoded in DNA promoter a region of DNA that binds RNA polymerase to initiate gene transcription transcription factors a protein that regulates the binding of RNA polymerase to a gene promoter terminator a region of DNA that the RNA polymerase recognizes as the end point for transcription introns regions of the DNA within the gene itself that cannot be used to code for proteins exons regions of the DNA within the gene itself that can be used to code for proteins RNA splicing the process by which intronsare removed and remaining exons are fused together In some cases specific exons are also removed with the introns leaving an alternatively spliced mRNA that encodes a different protein amino acids a chemical building block of protein molecules containing a central carbon atom an amino group a carboxyl group and a variable R group translation the process of synthesizing a protein molecule according to genetic instructions carried by a mRNA molecule molecular biology DNA 917anscriplian 9 mRNA 9Iranslal ian9 protein ribosome a cellular organelle that assembles new proteins from amino acids according to the instructions carried by mRNA rough ER a membraneenclosed cellular organelle with ribosomes attached to its outer surface39 a site of synthesis for proteins destined to be inserted into the membrane or to be enclosed by membrane It abounds in neurons Also called Nissl bodies because they are the organelles that are stained with the dye Nissl introduced pol ibosomes a collection of several ribosomes oating freely in the cytoplasm free ribosomes the difference between ribosomes synthesized on the rough ER and free ribosomes is that the free ribosomes make proteins that reside in the cytosol of the neuron while rough ER ribosomes make membrane cell or organelle proteins smooth ER a membraneenclosed cellular organelle that isheterogeneous and performs different functions in different locations like folds proteins that come from the rough ER or regulates the internal concentration of substances such as calcium Golgi apparatus an organelle that sorts and chemically modifies proteins that are destined for delivery to different parts of the cell posttranslational chemical processing of proteins Mitochondrion an organelle responsible for cellular respiration it generates adenosine triphosphate using the energy produced by the oxidation of food Cristae multiple folds of inner membrane within the enclosure of the mitochondria Between these folds is a space called the matrix Kreb s cycle a cycle that uses pyruvic acid and oxygen to create ATP with the help of the electrontransport chain in the mitochondria adenosine triphosphate ATP cell39s energy source molecule hydrolysis of ATP to produce ADP releases energy that fuels most of the biochemical reactions of the neuron ADP is converted back to ATP in the mitochondria neuronal membrane the barrier about 5 nm thick that separates the inside of a nerve cell from the outside consists of a phospholipid bilayer with proteins embedded in it encloses the intracellular organelles and vesicles the protein composition of the membrane varies depending whether its in the soma the dendrites or the axon c oskeleton the internal scaffolding that gives a cell its characteristic shape consists of microtubules neurofilaments and microfilaments microtubule a polymer of the protein tubulin forming a straight hollow tube 20 nm in diameter running down neurites longitudinally its a component of the cytoskeleton play an important role in axoplasmic transport 39 1 39 quot Droteins MAPS anchor the 39 to one another and to other parts of the neuron microfilament a polymer of the protein actin forming a braided strand 5 nm in diameter its a component of the cytoskeleton particularly in the neurites as well as anchored in the cell membrane in a net shape Actin is critically involved in muscle contraction neurofilam ent a type of intermediate filament found in neurons 10 nm in diameter an important component of the neuronal cytoskeleton mechanically very strong due to their structure single protein chain subunits organized like sausages axon hillock a swelling of the axon where it joins the soma no rough ER extends into the axon and there are few if any ribosomes exist This means that all proteins must originate in the soma the protein composition of the axon membrane is fundamentally different from that of the soma membrane an axon can branch from less than a mm to over a meter ong axon collateral a branch of an axon axon diameter also varies which is important because it effects how quickly the nerve impulse is thicker axon faster axon terminal terminal bouton the end region of an axon usually a site of synaptic contact with another cell also called terminal bouton or presynaptic terminal synapse the region of contact where a neuron transfers information to another cell terminal arbor branches at the end of an axon terminating in the same region of the nervous system innervation when a neuron makes synaptic contact with another cell differences between axons and axon terminals a axon terminals do not have microtubules b axon terminals have synaptic vesicles c the inside of the membrane that faces the synapse of an axon terminal is densely covered in proteins d axon terminals have numerous mitochondria synaptic vesiclesa membraneenclosed structure about 5 nm in diameter which contains neurotransmitter and found at a site of synaptic contact synaptic cleft the region separating the presynaptic and postsynaptic membranes of neurons synaptic transmission the process of transferring information from one cell to another at a synapse neurotransmitter a chemical released by a presynaptic element upon stimulation that activates postsynaptic receptors information in the form of electrical impulses travel down the axon and is translated into neurotransitters chemical signal which are stored and transported across the synapse by synaptic vesicles Once on the postsynaptic membrane they are then converted back to electrical signals the synapse is the location for many toxins and for most psychoactive drugs Wallerian degeneration degeneration of the axons when they are cut away from the soma due to no protein production axoplasmic transport the process of transporting materials down an axon material is enclosed within vesicles that walk down the microtubules with legs made from kinesin anterograde transport axoplasmic transport from a neuron39s soma to the axon terminal The legs are made from kinesin retroggade transport axoplasmic transport from an axon terminal to the soma39 usually occurs when providing signals regarding metabolic needs The legs are provided by dyenein dendritic tree all the dendrites of a single neuron dendrites are covered with thousands of synapsesbecause they function as the antennae of the neuron receptors 1 a specialized protein that detects chemical signals such as neurotransmitters and initiates a cellular response 2 a specialized cell that detects environmental stimuli and generates neural responses dendritic spine a small sac of membrane that protrudes from the dendrites of some cells and receives synaptic input They are believed to isolate various chemical reactions that are triggered by some types of synaptic activation Spine structure is sensitive to the type and amount of synaptic activity Unusual spine structure is linked with cognitive impairments Dendrite structure is very similar to axon structure The major difference is that polyribosomes have been found in dendrites often right underneath the spines Neurons can be classified by the total number of neurites axons and dendrites that extend from the soma unipolar neuron a neuron with a single neurite bipolar neuron a neuron with two neurites multipolar neuron a neuron with three or more neurites most fall into this category in the cerebral cortex there are two broad classes of neurons stellate cells and pyrmidal cells stellate cell a neuron characterized by a radial starlike distribution of dendrites p amidal cell a neuron characterized by a pyramidshaped cell body and elongated dendritic tree39 found in the cerebral cortex spiny neuron a neuron with dendritic spines aspinous neuron a neuron lacking dendritic spines primary sensory neuron a neuron specialized to detect environmental signals at the body39s sensory surfaces motor neuron a neuron that synapses on a muscle cell and causes muscle contraction interneuron any neuron that is not a sensory or motor neuron39 also describes a CNS neuron whose axon doesn39t leave the structure in which it resides it forms connections between other neurons this is the most common neuron in the nervous system Golgi type I neurons projection neurons neurons with axons that extend from one part of the brain to the other pyramidal cells in the cerebral cortex Golgi type II neurons local circuit neurons neurons with axons that don t extend past the soma stellate cells in the cerebral cortex cholinergic term used to describe neurons that can be classified by their use of a particular neurotransmitter astroc e a glial cell in the brain that supports neurons and regulates the extracellular ionic and chemical environment the most numerous glial cell in the brain a envelop synaptic junctions thereby restricting the spread of neurotransmitters b have proteins in their membranes that actively remove many neurotransmitters from the synaptic cleft c have membranes that possess neurotransmitter receptors that can trigger electrical and biochemical events inside the glial ceH d control the extracellular concentration of several substances such as K ions astrocytes probably influence whether a neuron can grow or retract oligodendroglial cell a glial cell that provides myelin in the central nervous system39 can contribute myelin to several axons Schwann cell a glial cell that provides myelin in the peripheral nervous system39 myelinates only a single axon myelin a membranous sheath around axons provided by oligodendroglia in the central nervous system and Schwann cells in the peripheral nervous system node of Ranvier a space between two consecutive myelin sheaths where an axon comes in contact with the extracellular uid ependmal cell a type of glial cell that provides the lining of the brains ventricular system microglial cell a type of cell that functions as a phagocyte in the nervous system to remove debris left by dead or dying neurons and glia Chapter 4 rising phase the first part of an action potential characterized by a rapid depolarization of the membrane overshoot the part of an action potential when the when the membrane potential is more positive than 0 mV falling phasethe part of an action potential characterized by a rapid fall of membrane potential from positive to negative undershootthe part of an action potential when the membrane potential is more negative than at rest39 also called after hyperpolarization afterhypegpolarizationthe hyperpolarization that follows strong depolarization of the membrane the last part of an action potential also called undershoot the action potential lasts about 2 milliseconds msec threshold a level of depolarization sufficient to trigger an action potential absolute refractog periodthe period of time measured from the onset of an action potential during which another action potential cannot be triggered relative refractory periodthe period of time following an action potential during which more depolarization current than usual is required to achieve threshol depolarization of the cell during the action potential is caused by the in ux of Na ions across the membrane and repolarization is caused by the ef ux of K ions the number of open potassium channels is proportional to an electrical conductance gK IK will ow as long as Vmi EK39 the current ow is in the direction that takes Vm toward EK When Vm EK the membrane is at equilibrium and no net current will ow even though there is still a large potassium conductance The rising phase of the action potential could be explained if in response to depolarization of the membrane beyond threshold membrane sodium channels opened allowing Na to enter the neuron causing a massive depolarization until the membrane potential approached EN The sodium current lna is inward across the membrane in this case vm EN 9 80mV 62mV l42mV The in ux of Na depolarizes the neuron until Vm approaches ENE assuming the membrane permeability is now far greater to sodium than it is to potassium Sodium channels quickly close and the potassium channels remain open so the dominant membrane ion permeability switches back from Na to K then K would ow out of the cell until the membrane potential again equals EK The rising phase of the action potential is explained by an inward sodium current and the falling phase is explained by an outward potassium current voltage clampa device that enables an investigator to hold the membrane potential constant while transmembrane currents are measured the rising phase of the action potential was caused by a transient increase in gNa and an in ux of Na ions the falling phase was associated with an increase in gK and an ef ux of K ions voltagegated sodium channela membrane protein forming a pore that is permeable to Na ions and gated by depolarization of the membrane the VG sodium channel is made from a single long polypeptide with four distinct domains each made of six transmembrane alpha helices the four domains form a pore between them39 the pore is closed at the negative resting membrane potential39 when the molecule is depolarized to threshold the molecule twists and allows Na to ow through patch clampa method that enables an investigator to hold constant the membrane potential of a patch of membrane while current through a small number of membrane channels is measured properties of voltagegated sodium channels a they open quickly b they stay open for mlmsec then inactivate c they cant be opened by depolarization again until the Vm returns to a negative value near threshold accounts for the absolute refractory perio channelopathya human genetic disease caused by alterations in the structure and function of ion channels quot J epilepsv with febrile seizures a A with epileptic seizures caused by high fevers This is linked to mutations that slowed the inactivation of the VG sodium channels prolonging the action potential tetrodotoxin TTX a toxin that binds to the outside of the Na channel and clogs the Na permeable pore39found in puffer fish saxitoxin another channel blocking toxin found in dino agellates which when bloom create the red tide batrachotoxin found in poison dart frogs veratridine lillies and aconitine buttercups causes the channels to open at more negative potentials and stay open much longer delayed rectifier when channels take about lmsec to open upon depolarization rather than immediately ex potassium gates voltagegated potassium channela membrane protein forming a pore that is permeable to K ions and gated by depolarization of the membrane the VG potassium channel is made from four individual polypeptide with four distinct domains when the membrane is depolarized the voltage sensors detect a change in electrical field and twist the subunits into a shape that allows the K ions to pass through


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