Chapter 3 The Neuronal Membrane at Rest

Introduction Action potential in the nervous system Action potential vs. resting potential

The Cast of Chemicals Cytosolic and Extracellular Fluid Water Key ingredient in intracellular and extracellular fluid Key feature – water is a polar solvent

Cytosolic and Extracellular Fluid (Cont’d) Ions: Atoms or molecules with a net electrical charge Cations: positive charge Anions: negative charge Spheres of hydration

The Phospholipid Membrane Hydrophilic Dissolve in water due to uneven electrical charge (e.g., salt) Hydrophobic Does not dissolve in water due to even electrical charge (e.g., oil) Lipids are hydrophobic Contribute to resting and action potentials

The Phospholipid Bilayer

Protein Molecules Enzymes Cytoskeletal elements Receptors Special transmembrane proteins Control resting and action potentials

Protein Structure Amino acids Alpha carbon and R groups

Protein Structure (Cont’d) Peptide bonds and polypeptides

Protein Structure (Cont’d) Four levels of protein structure Primary, Secondary, Tertiary, Quaternary

Protein Channel Proteins Polar R groups and nonpolar R groups Ion selectivity and gating

Protein Ion Pumps Formed by membrane spanning proteins Uses energy from ATP breakdown Neuronal signaling

The Movement of Ions Diffusion Dissolved ions distribute evenly Ions flow down concentration gradient when: Channels permeable to specific ions Concentration gradient across the membrane

Electricity Electrical current influences ion movement Electrical conductance (g) and resistance (R); R = 1/g Electrical potential (voltage)

The Chemicals Involved in the Conduction of Electricity Electrical current flow across a membrane Ohm’s law I = gV

The Ionic Basis of The Resting Membrane Potential Membrane potential: Voltage across the neuronal membrane

Equilibrium Potential (Eion) No net movement of ions when separated by a phospholipid membrane Equilibrium reached when K+ channels inserted into the phospholipid bilayer Electrical potential difference that exactly balances ionic concentration gradient

Equilibrium Potentials Four important points Large changes in Vm Minuscule changes in ionic concentrations Net difference in electrical charge Inside and outside membrane surface Rate of movement of ions across membrane Proportional Vm – Eion Concentration difference known: Equilibrium potential can be calculated

Equilibrium Potential Inside positively charged relative to outside

Voltmeter Plasma membrane Ground electrode outside cell Microelectrode inside cell Axon Neuron

K+ A– Na+ Cl– Cell exterior Na+ 15 mM Na+ Cell interior 150 mM ion Na+ Diffusion us Na+–K+ pump Diff -70 mV Cl– 10 mM Na+ Na+ Na+ 150 mM A– 100 mM K+ Na+ A– 0.2 mM Na+ Plasma membrane K+ 5 mM K+ Cl– 120 mM K+ Cell interior Cell exterior K+ K+

Depolarizing stimulus Hyperpolarizing stimulus +50 +50 Inside positive Inside negative Membrane potential (voltage, mV) Depolarization –50 Membrane potential (voltage, mV) –50 Resting potential –70 –70 Resting potential Hyper- polarization –100 –100 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Time (ms) Time (ms) (a) (b)

Depolarized region Stimulus Plasma membrane (a) Depolarization (b) Spread of depolarization

Active area (site of initial depolarization) Membrane potential (mV) – 70 Resting potential Distance (a few mm)

Outside cell Na+ Outside cell Na+ Inside cell Inside cell K+ K+ 2 Depolarizing phase: Na+ channels open Repolarizing phase: Na+ channels inactivating, K+ channels open Action potential +30 3 Relative membrane permeability 2 Membrane potential (mV) PNa PK Threshold –55 1 1 4 –70 1 2 3 4 Time (ms) Outside cell Sodium channel Potassium channel Outside cell Na+ Na+ Inside cell Activation gates K+ Inside cell K+ Inactivation gate 4 Hyperpolarization: K+ channels remain open; Na+ channels resetting 1 Resting state: All gated Na+ and K+ channels closed (Na+ activation gates closed; inactivation gates open)

Voltage at 2 ms +30 Membrane potential (mV)) Voltage at 0 ms Voltage at 4 ms –70 (a) Time = 0 ms (b) Time = 2 ms (c) Time = 4 ms Resting potential Peak of action potential Hyperpolarization

Action potentials +30 Membrane potential (mV) – 70 Stimulus amplitude Threshold Voltage Time (ms)

Absolute refractory period Relative refractory period Depolarization (Na+ enters) +30 Repolarization (K+ leaves) Membrane potential (mV) After-hyperpolarization –70 Stimulus 1 2 3 4 5 Time (ms)

Equilibrium Potentials (Cont’d) The Nernst Equation Calculates the exact value of the equilibrium potential for each ion in mV Takes into consideration: Charge of the ion Temperature Ratio of the external and internal ion concentrations

The Distribution of Ions Across The Membrane K+ more concentrated on inside, Na+ and Ca2+ more concentrated outside

The sodium-potassium pump Enzyme - breaks down ATP when Na present Calcium pump: Actively transports Ca2+ out of cytosol

Relative Ion Permeabilities of the Membrane at Rest Neurons permeable to more than one type of ion Membrane permeability determines membrane potential Goldman equation Takes into account permeability of membrane to different ions Selective permeability of potassium channels - key determinant in resting membrane potential Many types of Potassium Channel Lily & Yuh Nung Jan—amino acid sequences; Family of K+ channels e.g., Shaker Potassium Channel

Seymour Benzer – research leader in studies of Shaker Flies Seymour Benzer – research leader in studies of Shaker Flies. In the 1970s, his lab was able to associate this mutant’s behavior to a gene that affects potassium channel development.

Relative Ion Permeabilities of the Membrane at Rest K+ channels: 4 subunits Channel selectively permeable to K+ ions MacKinnon—2003 Nobel Prize Mutations of specific K+ channels; Inherited neurological disorders

Relative Ion Permeabilities of the Membrane at Rest Resting membrane potential is close to EK because it is mostly permeable to K+ Membrane potential sensitive to extracellular K+ Increased extracellular K+ depolarizes membrane potential

Relative Ion Permeabilities of the Membrane at Rest Regulating the External Potassium Concentration Blood-Brain barrier Potassium spatial buffering

Concluding Remarks Activity of the sodium-potassium pump Large K+ concentration gradient Electrical potential difference across the membrane Similar to a battery Potassium channels Contribute to resting potential Roles of ion pumps