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Chapter 3

NSC 2201 Chapter 3: CH 3 The Neuronal Membrane at Rest

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Vanderbilt University
NSC 2201

CH 3 THE NEURONAL MEMBRANE AT REST p. 55-79 INTRO • Action potentials can travel along the axons of neurons because they have excitable membranes • Excitable membranes are cell membranes that are capable of capable of generating and conducting action potentials • The resting membrane potential is the difference in electrical charge across the membrane when the cell is at rest • The cytosol is negative relative to the extracellular fluid THE CAST OF CHEMICALS CYTOSOL AND EXTRACELLULAR FLUID Water • Most abundant on both sides as it holds dissociated ions • Most important characteristic is uneven distribution of electrical charge across the covalent bond between oxygen and hydrogen molecules • Oxygen has net negative charge and hydrogens have positive charge • Water turns out to be great solvent for charged or polar molecules because it is held together by polar covalent bonds and is a polar molecule Ions • Spheres of hydration insulate ions from reassociating with each other • Monovalent/divalent/etc. is difference in charge between protons and electrons in ion • Most important ions are monovalent cations of sodium and potassium, monovalent anion of chloride, and divalent cation calcium THE PHOSPHOLIPID MEMBRANE • Phospholipids are main chemical building blocks of cell membranes • Have hydrophilic head and hydrophobic tail • Form phospholipid bilayer with hydrophilic heads facing out and hydrophobic tails facing in PROTEIN Protein Structure • Proteins made from mRNA translation by ribosomes • Amino acids connected by peptide bonds which join amino group of one amino acid to carboxyl group of next • Polypeptide is protein made of a single chain of amino acids • Primary structure: chain in which amino acids are linked by peptide bonds • Secondary structure: forms alpha helix or beta sheets • Tertiary structure: R-groups interact to form further 3D shape • Quaternary structure: different polypeptide chains bond to form larger molecule • Subunit: each polypeptide quaternary structure that makes up a whole protein Channel Proteins • Membrane-spanning protein often has hydrophilic ends and hydrophobic middle • Ion channels are like this with about 4-6 similar protein molecules assembled to form a pore between them • Ion selectivity: important property of most ion channels given by specific diameter of pore and nature of R groups lining it • Gating: channels can be opened and closed by specific changes in local microenvironment Ion Pumps • Use ATP to transport ions against their concentration/charge gradients THE MOVEMENT OF IONS DIFFUSION • Diffusion: movement of ions from areas of high concentration to areas of low concentration • Concentration gradient: difference in concentration between two areas • Diffusion can only happen with: o 1. Ions in membrane that are permeable to the ion o 2. There is concentration gradient across the membrane ELECTRICITY • Electrical current: movement of electrical charge, rep by I and amps, defined as positive in the movement of positively-charged environment • Electrical potential AKA voltage: difference in charge and force exerted on a charged particle • Ohm’s law (I=gV) shows that for current to flow there needs to be: o 1. Membrane must possess channels permeable to that ion (conductance) o 2. There is electrical potential difference across membrane THE IONIC BASIS OF THE RESTING MEMBRANE POTENTIAL • Membrane potential is voltage across the neuronal membrane at any time • Typical resting potential is -65 mV EQUILIBRIUM POTENTIALS • Equilibrium state is reached where diffusional and electrical for
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