Exam 1 Notes
Exam 1 Notes BIOH 313-001
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This 11 page Study Guide was uploaded by Rebeka Jones on Monday September 12, 2016. The Study Guide belongs to BIOH 313-001 at Montana State University taught by Noudoost, Behrad in Fall 2016. Since its upload, it has received 36 views. For similar materials see Neurophysiology in Cell Biology and Neuroscience at Montana State University.
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Date Created: 09/12/16
Neurophysiology Even simple reflexes require the nervous system to collect, distribute, and integrate information. Action Potential – a special type of signal – a nerve impulse – that overcomes biological constraints – they do not diminish over distance. The information encoded in the frequency of action potentials of individual neurons as well as the distribution and number of neurons firing actions potentials in a given nerve. The cells that generate and conduct action potentials (nerve and muscle cells) are said to have excitable membranes. In resting neurons, the cystol along the inside surface of the membrane has a negative electrical charge compared to the outside. – The difference in this charge is called the resting membrane potential. Action potentials are just the reverse of the resting membrane potential meaning the inside briefly becomes positively char ged compared to the outside. There are 3 main players in the resting membrane potential - the salty fluid surrounding the membrane - the membrane - the proteins that span the membrane water is the main ingredient in fluid inside the neuron (the int racellular fluid in the cystol) and the fluid that bathes the neuron (the extracellular fluid). Electrically charged atoms-ion are in the water and responsible for action potentials *water is polar *ions are atoms or molecules with a net charge polar substances are hydrophilic nonpolar covalent substances are hydrophobic lipids are also hydrophilic Phospholipids – contain long nonpolar chains of carbon atoms bonded to hydrogen atoms Phospholipids have a polar phosphate group this is why the y are said to have a polar head that is hydrophilic and a nonpolar tail that is hydrophobic The neuronal membrane consists of a sheet of phospholipid two molecules thick. The polar heads face out and the tails face each other -This is called the phospholipid bilayer Enzymes catalyze chemical reactions in the neuron The cytoskeleton gives the neuron its special shape The receptors that are sensitive to neural transmitters – are all made up of protein molecules 2 All amino acids have a central carbon atom that is covalently bonded to four molecule groups: a hydrogen atom, an amino group, a carboxyl group, and a variable R group. The R groups cause the differences in the amino acids. Proteins are synthesized by the ribosomes of the neural cell body. Amino acids are connected into a chain connected by peptide bonds. Primary structure – Chain Secondary Structure – alpha helix Tertiary Structures – bend, folds, globular shape Quaternary – bonds to form large molecule Ion channels are made from proteins suspended in the phospholipid bi- layer, with its hydrophobic portion inside the membrane and its hydrophilic ends exposed to the watery environments on either side. These channels usually require 4-6 similar protein molecules to assemble to form a pore. The subunit composition varies from one type of channel to the next and that’s what gives them their special properties. Ion selectivity – specified by the diameter of the pore and the R-groups lining it. Gating – opening and closing of channels by changes in the local microenvironment of the membrane Ion pumps – other membrane spanning proteins come together to form these. They are enzymes that use ATP to transport certain ions across the membrane. 3 *Ionic movements through channels are influences by two factors: diffusion and electricity diffusion – net movement of ions from regions of high concentration regions to low concentration. Concentration gradient – difference in concentrations. Diffusion only occurs when -the membrane possesses channels permeable to the ions - there is a concentration gradient across the membrane *another way to create movement of ions is electrical fields Electrical current – movement of electrical charge (I) *current is defined as positive in the direction of positive charge movement Two factors determine how much current will flow: Electrical potential (voltage) – force exerted on a charges particle it reflects the difference in charge between the anode and cathode, more current will flow as the difference increases (V) Electrical conductance – the reductive ability of electrical change to migrate from on point to another (g) measured in Siemens (s) it depends on the number of particles available to carry electrical charge and the ease with which these particles travel through space. *can also be expressed as electrical resistance which is the relative inability of an electrical charge to migrate (R) measured in ohms *Resistance is the inverse of conductance R = 1/g 4 Ohms law I =gV Driving ions across the membrane requires -the membrane to have channels permeable to that ion - there is an electrical potential difference across the membrane membrane potential – voltage across neural membrane (Vm) The resting potential for a typical neuron is about -65 millivolts The electrical potential difference that exactly balances an ionic concentration gradient is called an ionic equilibrium potential or equilibrium potential (E ion -Large charges in membrane potential are caused by minuscule changes in ion concentrations -the net difference in electrical change occurs at the inside and outside of the membrane -ion are driven across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential - if the concentration difference across the membrane is known for an ion are equilibrium potential for any ion *using the Nernst equation we can calculate the value of the equilibrium, potential for any ion The neural membrane potential depends on the ionic concentrations of either side of the membrane. 5 *K+ is more concentrated on the inside and Na+ and Ca+ is more concentrated on the outside Ion concentration gradient are made by the action of ion pumps -sodium-potassium pump: enzyme that breaks down ATP in the presence of internal Na+ -Calcium pump – actively transports Ca+ out of the cell Goldman Equation – a mathematical equation that takes into consideration the relative permeability of the membrane to different ions A change in membrane potential from normal resting value to a less negative value is called depolarization of the membrane *increasing extracellular potassium depolarizes neurons Transport Across the membrane Active (requires energy usually against concentration gradients) Ion pumps: e.g. Na/K pump Cotransport (secondary active): a process in which two substances are simultaneously transported across a membrane by a protein which does not have ATPase actively: e.g. Glucose Endocytosis: a form of active transport in which a cell transports molecules (such as proteins) in the cell by engulfing them in an energy using process 6 Passive (does not require energy; uses the concentration gradient) Simple diffusion: Movement of molecules from an area of high concentration to an area of low concentration. Osmosis: passage of water across a membrane from an area of high concentration to an area of low concentration Facilitated diffusion: similar to simple diffusion by a molecule E.g. Ions passing through ion channels When we say channel we mean follows concentration gradient When we say pump we mean goes against concentration gradient Ion channels are proteins that span the cell membrane The flux of ions through the ion channel is passive, But selective (through size and charge) *phospholipids form self-sealing lipid bilayers that are the basis for all cellular membranes through their hydrophilic head and hydrophilic tails *ions in solution are surrounded by a cloud of water molecules that are attracted by the net charge of the ion. This cloud is carried along by the ion as it diffuses through solution, increasing the effective size of the ion. Selective Sodium Channels Sodium, channels have a selectivity filter somewhere along the length of the channel, with a sight that weakly binds to Na+ ions. It is believed that a Na+ ion binds transiently at an active sight as it moves through the filter. 7 In channels where the pore diameters are large enough to accommodate several water molecules an ion isn’t needed to be stripped of its water shell. Several Types of stimuli control the gating of ion channels. The binding of exogenous ligands, such as drugs, can make an ion channel favor an open or closed state through a verity of mechanisms. Summary… -ion channesl are proteins that span the cell membrane - The flux of ions through the ion channel is passive but selective - Several types of stimuli control the opening and closing of channels - their functions can be modulated by other molecules (drugs) Passive Transport Passive Potassium Leakage Channel -potassium comes and goes from the cell as the concentration gradient requires using no energy - involved in maintaining the resting membrane potential Voltage-gated Sodium Channel -ligand must bind to allow sodium to pass -involved in generating local potential due to release of the neurotransmitter Acetylcholine Stretch-gated sodium channel -opening of channel must be stretched open by movement of muscles or by touch 8 -involved in generating local potential in things like somatosensory Active transport Sodium-Potassium pump -requires the use of energy and potassium leaves the cell and sodium enters the cell -involved in maintain membrane potential Cation – positively charged ion Anion – negatively charged ion Hypertonic – a solution with high amount of molecules Hypotonic – a solution with low amount of molecules Current – flow of charge (I) – rate of change Voltage – difference in potentials (V) Potential – energy to act – potential to do something Resistance – inverse of conductance Equilibrium potential – fight between charge and concentrations Diffusion Potential is the potential difference ge nerated across a membrane because of a concentration difference of an ion. -it can be generated only if the membrane is permeable to the ion -the size of the diffusion potential depends on the size of the concentration gradient 9 when the membrane is only permeable to one ion there will be one wrong (charge or concentration gradient) at the expense of the other. They will balance to make the sum of the forces equal. The equilibrium (or reversal) potential is the membrane potential where the net flow through any open channels is 0. In other words, at E rev, the chemical and electrical forces are in balance. E rev n be calculated using the Nernst equation. Ohlms Law Ion flux = (electrical driving force + chemical driving force) x membrane conductance Nernst equation For a given ion, the equilibrium/reversal potential can be calculated by the Nernst equation ???????? [????] ▯ ???????? = ???????? ln [????] ▯ Ex = potential at which equilibrium happens for ion x X = Concentration of the ion X outside the cell o X = Concentration of the ion X inside the cell i R = Gas constant T = Temperature (K) Z = ion charge F = Faraday’s constant 10 The flux of K+ across the membrane is determined by both the K+ concentration gradient and the electrical potential across the membrane -In a cell permeable only to K+ the resting membrane potential is generated by the efflux of K+ sown its concentration gradient -The continued efflux of K+ builds up an excess of positive charge on the outside of the cell and leaves behind on the sided and excess of negative charge. This buildup of charge leads to a potential difference across the membrane that impedes the further efflux of K+ so that eventually an equilibrium is reached: The electrical and chemical driving forces are equal and opposite, and as many K+ ions move in as move out Goldman Equation Vm is relted to E k+ E na E cl- Vm is related to P k k P na na P cl- cl P = permability to ion ???????? ???? [????+] + ???? [????????+] + ???? [????????−] ???????? = ln ▯ ▯ ▯▯▯ ▯ ▯▯▯ ▯ + ???? ????▯[????+] + ▯ ▯▯▯ [????????+] + ▯ ▯▯▯[????????−] ▯ hyperpolarize – more negative hypopolarize – more positive 11
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