Cellular Neurophysiology I Membranes and Ion Channels

Transcription

1 Cellular Neurophysiology I Membranes and Ion Channels Reading: BCP Chapter 3

2 All living cells maintain an electrical potential (voltage) across their membranes (V m ). Resting Potential At "rest", V m is negative (typically V rest -65 mv) relative to extracellular fluid. Neurons are rarely at rest for long. Neurons exploit this separation of charge across membrane (a form of potential energy) to generate electrical currents for encoding and transmitting information. Resting Potential

3 Biological currents are carried mainly by charged ions liberated when inorganic salts dissolve in water. Water molecules are polarized, and bind to ions electrostatically. Hydrated diameter : the water/ion molecular complex has a larger diameter than the ion itself. Hydrated diameter is an important factor limiting the movement of ions through pores (channels) in membrane. Ions in Solution

4 Ions and Membrane Currents Membranes are semi-permeable : Only certain ions and other small molecules move through membranes. Ions flow through complex proteins that form channels or pumps. Ions flowing through open channels generate electrical currents that in turn create a voltage according to Ohm s law: V (voltage) = I (current) R (resistance) or current = voltage conductance where conductance G = 1/R. Thus, the magnitude of current I is directly proportional to voltage across membrane (V m ) and ion channel conductance (G ion ).

5 Phospholipid Bilayer Membranes composed primarily of longchain fatty acids called phospholipids (PL). PLs in aqueous solution naturally aggregate into a bilayer: Polar phosphate heads interface with polar water molecules, form intra- and extracellular faces of membrane. Non-polar lipid tails form core of bilayer. Membrane chemistry regulates molecular traffic across membrane: Non-polar, fat-soluble (lipophilic/hydrophobic) molecules cross by dissolving in the lipid bilayer. Polar, water-soluble (hydrophilic/lipophobic) molecules (e.g. ions) cross only through ion channels or carrier molecule pores in membrane.

6 Ion Channels Channels are complex proteins composed of 4 to 6 polypeptide subunits. Each subunit has a hydrophobic surface region (shaded) that readily associates with the phospholipid bilayer. Pore Ion channels span the lipid bilayer, facing both extracellular and intracellular (cytoplasmic) compartments. Subunits form a aqueous pore through which ions may pass.

7 Ion Channel Structure Ion channels show four levels of protein structure: primary: the chain of amino acids linked together by peptide bonds secondary: the membrane-spanning segments, composed of lipophilic ( fat-loving ; non-polar; non-charged) amino acids, naturally coil into alpha helices and will insert into the lipid bilayer [the hydrophilic ( water-loving ) amino acids are excluded and will associate with the intra- and extracellular fluids] tertiary: folded alpha helices create a subunit quaternary: multiple subunits assemble together Tertiary structure Quaternary structure Primary structure Secondary structure

8 Fundamental Properties of Channels Channels are gated: transition from open to closed states by changing their 3D configuration. Channels are selective: Only certain ions permitted to pass. Selectivity arises from: chemical properties (mainly charge state and distribution) of amino acid sequences lining the pore. diameter of the pore when open. Na + Ion flow is passive: movement through open channels does not directly expend energy. K + biomhs.com

9 Channel Types Ion channels can be gated open in a number of ways: Random: open or close randomly, leakage channels Voltage: open or close depending on membrane voltage. Chemical: open or close by binding with a ligand often referred to as a messenger (extracellular messenger: neurotransmitter ; intracellular messenger: second messenger ) Chemical AND voltage-gated: opened by binding transmitter only when membrane voltage is favorable Mechanical-gated ( stretch channels): opened by membrane deformation.

10 Diffusion Diffusion: The movement of substances down a concentration gradient. Why? Random (Brownian) motion of molecules in solution eventually distributes substances uniformly within a compartment. X Concentration gradient: Established by a difference in (ion) concentration within compartment or across a barrier (e.g., membrane). A lipid bilayer lacking channels resists movement of ions across it, creating a chemical gradient. Ion channels permit certain ions to cross membrane down their concentration gradients.

11 The magnitude of a concentration gradient W c depends on the log-ratio of extracellular to intracellular (i.e. cytosol) concentration: W c = RT (ln[out] - ln[in]) = 2.3 RT log 10 ([out]/[in]) where: R = gas constant (=8.314 joules/(t ( K) / mol)) T = absolute temperature K ( = T C) [out] = extracellular concentration [in] = intracellular concentration Concentration Gradient

12 Unchecked, ion flow through open channels by diffusion would eventually result in equal concentrations on both sides of membrane Equilibrium: ion exchange is equal in magnitude, and opposite in direction. Chemical Equilibrium

13 Electrical Gradients 1 Current flows when a battery is connected to an electric circuit. However, even if a voltage is applied across membrane (symbolized by a battery), ions cannot flow across a lipid bilayer in absence of ion channels: By definition, current (I) = 0. When channels are present and open: Any separation of charge creates a voltage difference across the membrane. Voltage difference creates an electrical gradient that drives ion currents through open channels. Current can flow if an electrical gradient is present, even if there is no concentration gradient.

14 Applying a voltage across membrane (symbolized by battery) causes positivelycharged Na + ions and negatively-charged Cl - ions to move in opposite directions through their respective channels Direction of current flow is from left to right (by convention, current flows in the direction of positive charge movement). Electrical Gradients 2 Magnitude (strength) of electrical gradient W E is determined from the product of three variables: W E = zfe where Valence (z) of the atom (=charge of ion) F, Faraday constant Voltage across the membrane (in volts, E)

15 Equilibrium Potential 1 Initial conditions: K concentration gradient exists across membrane, but no open K ion channels. Intracellular and extracellular compartments are electrically balanced (neutral). Potassium channels Opening K channels allows outflow ( efflux ) of K + down its concentration gradient, creating a (net) outward K current.

16 K efflux immediately results in separation of charge across membrane, creating a voltage difference that acts as an electrical gradient opposing outward K + current (like charges repel). Electrical gradient (i.e. voltage) grows stronger as K efflux continues Net negative charge inside membrane (fewer K + ions to balance A - ) attracts K + back inside (unlike charges attract). Equilibrium Potential 2

17 When efflux of K down its concentration gradient equals influx of K down its electrical gradient, ion currents are equal and opposite. At equilibrium, W C = W E, therefore no net ion flux zero net current. Equilibrium Potential 3 The membrane potential at which there is no net current is the ion s equilibrium potential (E ion ).

18 Review Electrical properties of membranes determined by physical and chemical properties of the lipid bilayer and ion channel proteins. Ion channels are gated, either open or closed, and selective to a greater or lesser degree for particular ions. Ion flow across membrane determined by the magnitude and direction of the Concentration gradient (W C ) Electrical gradient (W E ) Equilibrium for a particular ion channel occurs when ion flow is equal in magnitude and opposite in direction, at which point there is no net current. Membrane voltage at which equilibrium occurs for a particular channel is called the equilibrium potential (V m = E ion ).