BSCI440 EXAM 1 STUDY GUIDE Joshua Singer Chapter 8 Neurons Cellular and Network Properties I Electrical Signals in Neurons Nerve and muscle cells are called excitable tissues they have the ability to propagate fast long distance electrical signals in response to a stimulus Nernst equation predicts the membrane potential Vm for a single ion if the membrane was permeable to only one ion Two factors influence membrane potential 1 Uneven distribution of ions across the cell membrane Na Cl and Ca2 are higher in extracellular fluid K is higher in the cytosol 2 Differing membrane permeability to those ions The resting membrane is more permeable to K than to Na or Ca2 so K contributes more to the resting membrane potential For a given ion the membrane potential is called the equilibrium potential Eion Eion mV 61 z log ion out ion in The Eion of K is 90 mV Ek but the resting membrane potential of a neuron is 70 mV Neurons at rest a slightly permeable to Na which makes the membrane potential more positive when it leaks inside the cell GHK equation Goldman Hodgkin Katz predicts membrane potential using multiple ions GHK equation includes permeability values see page 249 for equation remember Cl has a negative charge so it is in out instead of out in In words resting membrane potential is determined by the combined contributions of the concentration gradient X membrane permeability for each ion If the ion is not permeable it drops of out the equation at rest cell is only permeable to K so it reverts back to the Nernst equation GHK equation used to predict what happens to membrane potential when ion concentrations or permeabilities change Permeability increases for Na depolarizes cell membrane AP produced if strong enough Permeability increases for K hyperpolarize cell membrane It takes a very SMALL amount of ions to change the membrane potential Gated channels control the ion permeability of the neuron There are four major types of selective ion channels 1 Na channels 2 K channels 3 Ca2 channels and 4 Cl channels Conductance G the ease with which ions flow through a channel 1 Mechanically gated ion channels respond to physical forces such as pressure or stretch neuromodulaters 2 Chemically gated ion channels respond to ligands such as neurotransmitters and 3 Voltage gated ion channels respond to changes in the cell s membrane potential i e ions moving during an action potential 1 Channel opening to allow ion flow is called channel activation Inactivation occurs when some channels that open with a stimulus close even though the activating stimulus continues There are two types of voltage changes across the membrane graded potentials and action potentials i Graded potentials vary in strength travel shorter distances and lose strength as they travel through the cell if strong enough when it reaches the integrating region it can produce an A P ii Action potentials brief large depolarizations travel long distances without losing strength primary function Ohm s Law current flow The flow of electrical charge carried by an ion is called the ion s current Iion Ohm s Law states that the current flow I is directly proportional to the electrical potential difference V between two points and inversely proportional to the resistance R of the system to current flow As resistance increases current flow decreases and vice versa Resistance comes from two sources 1 The resistance of the cell membrane Rm 2 The internal resistance of the cytoplasm Ri I V R A membrane with no open channels has high resistance and low conductance if ion channels are open it has low resistance and high conductance Action Potentials A P Graded potentials that are strong enough will reach the initial segment and will depolarize the membrane opening up voltage gated Na channels and an A P begins Depolarizing graded potentials excitatory Hyperpolarizing graded potentials inhibitory moves membrane potential further from threshold A P conduction is of uniform strength fast and does not lose strength over distance the way a graded potential does All or none phenomena strength of graded potential that initiates the A P has no influence on amplitude A P Step by step 1 Rising phase of A P temporary increase in cell s permeability to Na when depolarization hits threshold 55 mV more Na channels open and Na flows in membrane becomes more positive and moves toward Na equilibrium potential ENa of 60 mV 2 Falling phase of A P the A P peaks at 30 mV and Na channels close and K channels just finished opening increase in K permeability in response to depolarization but K channel gates are slower to open at 30 mV K flows out of the cell and membrane potential becomes negative and goes toward resting potential K continues to leave cell after 70 mV is reached and membrane undershoots toward EK of 90 mV K channels finally close retention of K and leak of Na into axon bring membrane potential back to 70 mV 2 Na Channels Have Two Gates Voltage gated Na channels have two gates activation and inactivation gates which flip flop back and forth to open and close the Na channel At resting potential the activation gate of Na channel closes and no Na can move through channel The inactivation gate an amino acid sequence behaving like a ball and chain on the cytoplasmic side of the channel is open When the cell membrane near channel depolarizes the activation gate swings open and Na moves into the cell a positive feed back loop starts when more Na rushes in When the slower inactivation gates close Na influx stops and A P peaks K repolarizes the neuron and Na channel gates reset to their original positions Double gating of Na channels contributes to the absolute refractory period the time required for the Na channel gates to reset to their original positions a second A P cannot occur before the first has finished no overlap A relative refractory period follows the absolute refractory period some but not all Na channels have reset and K are still open the Na channels can be reopened by a stronger than normal graded potential but the A P will be smaller than normal Speed of A P Influenced by two things 1 diameter of axon and 2 resistance of the axon membrane to ion leakage out of the cell The larger the diameter of the axon or more leak resistant the membrane the faster the action potential will move Conduction is faster in myelinated axons limits the amount of contact between membrane and extracellular fluid and prevents ion flow out of the cytosplasm A P uses