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Chapter 3

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University of Toronto Scarborough
Janelle Leboutillier

Chapter 3 NROB60 Chapter 3 The Neuronal Membrane at Rest Introduction  Action Potential o Do not diminish over distance; signals of fixed size and duration o Information is encoded in the frequency of action potentials of individual neurons, as well as in the distribution and number of neurons firing action potentials in a given nerve  Information is encoded in the pattern of electrical impulses  Excitable membrane  When a cell with excitable membrane is not generating impulses, it is said to be at rest. o In the resting neuron, cytosol along the inside surface of the membrane has a negative electrical charge compared to the outside o ^Resting membrane potentials/resting potential:  Action potential is a reversal of this condition The Cast of Chemicals  Three main players in resting membrane potential o Salty fluid on either side of membrane o Membrane o Proteins that span the membrane Cytosol and Extracellular Fluid  Water is the main ingredient of the fluid inside the neuron, intracellular fluid or cytosol  Fluid that bathes the neuron is the extracellular fluid  Electrically charged atoms (ions) are dissolved in this water and they are responsible for the resting and action potentials Water  Water has an uneven distribution of electrical charge o Hydrogen atoms are bonded covalently with oxygen (share electrons) o Oxygen has a greater affinity for electrons that does the hydrogen atom o Therefore, shared electrons will spend more time associated with the oxygen atom than with two hydrogen atoms o Therefore, oxygen acquires a net negative charge and hydrogen acquires a net positive charge  H2O is a polar molecule and other polar molecules tend to dissolve in water Ions  Ions: atoms or molecules that have a net electrical charge  Held together by ionic bonds (held together by electrical attraction of oppositely charged atoms)  Salt dissolved readily in water since the charged portion of water have a stronger attraction for ions (like salt)  Electrical charge of an atom depends on the difference between the number of protons and electrons o When the difference is 1  monovalent o When the difference is 2  divalent  Cations: ions with a net positive charge  Anions: ions with a net negative charge  Ions of particular importance for cellular neurophysiology are monovalent cations (Na+ and K+) Chapter 3 NROB60 o Divalent cations Ca2+ o Monovalent anions Cl- The Phospholipid Membrane  Substances (including ions and polar molecules) that dissolve in water are  hydrophilic  Compounds whose atoms are bonded by nonpolar covalent bonds have no basis for chemical interactions with water  Nonpolar covalent bonds occurs when the shared electrons are distributed evenly in the molecule so that no portion acquires a net electrical charge o Will not dissolve in water  hydrophobic  This includes lipids in the body The Phospholipid Bilayer  Phospholipids are the chemical building blocks of cell membranes  Contain long nonpolar chains of carbon atoms bonded to hydrogen atoms  Phospholipid has a polar phosphate group (phosphorous bonded to 3 O atoms) attached at end molecule o Therefore phospholipids have a polar head (containing phosphate) and is hydrophilic o Has a nonpolar tail (containing hydrocarbon) that is hydrophobic  Neuronal membrane consists of a sheet of phospholipids: two molecules thick  Hydrophilic head faces the outer and inner watery environment  Hydrophobic tails face each other  Phospholipid bilayer: Protein  Type and distribution of protein molecules distinguish neurons from other types of cell  Enzymes: catalyze chemical reactions in neuron  Cytoskeleton: gives a neuron its special shape  Receptors: are sensitive to neurotransmitters  Resting and action potentials depend on special proteins that span the phospholipid bilayer o Provide routes for ions to cross neuronal membrane Protein Structure  Proteins are molecules assembled from various combinations of 20 different amino acids  Basic structure of amino acid – PAGE 56/90  All amino acids have a central carbon atom (alpha carbon) which is covalently bonded to four molecular groups: o A hydrogen atom o An amino group (NH3+) o A carboxyl group (COO-) o And a variable group called R group (R for residue)  Differences in amino acids result from differences in the size and nature of these R groups o Properties of R group determine the chemical relationships in which each amino acid can participate  Peptide bond  Polypeptides  Four levels of protein structure – PAGE 57/91 o Primary structure – is like a chain in which amino acids are linked together by peptide bonds o Secondary structures  Alpha helix: polypeptide chain coils into a spiral-like configuration o Tertiary structure: interactions among R groups can cause the molecule to change its 3-D conformation – bend, fold and assume a globular shape Chapter 3 NROB60 o Quaternary structure: different polypeptide chains can bond together to form a larger molecule  Doesn’t happen to all proteins  Subunit: each different polypeptide contributing to a protein with quaternary structure Channel Proteins  Exposed surface of a protein may be chemically heterogeneous  Regions where nonpolar R groups are exposed will be hydrophobic and will tend to associate readily with lipid  Regions with exposed polar R groups will be hydrophilic and will tend to avoid a lipid environment  Ion channels: o Made from just these sorts of membrane-spanning protein molecules o 4-6 similar protein molecules assemble to form a pore btw them o Subunit composition varies from one type of channel to the next  Ion selectivity: o Important property of most ion channels specified by diameter of the pore and nature of the R groups lining it  Potassium channels are selectively permeable to K+  Sodium channels are permeable almost exclusively to Na+  Calcium channels to Ca2+  Gating o Another important property o Channels with this property can be opened and closed – gated – by changes in the local microenvironment of the membrane Ion Pumps 
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