Physiology Unit 2 MEMBRANE POTENTIALS and SYNAPSES
In Physiology Today
Ohm s Law I = V/R Ohm s law: the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them. I =V/R (I=current, V=voltage, R=resistance) Charges separated across plasma membrane Voltage (V) = Electrical potential measured in mv Current (I) = movement of electrical charge I =V/R (I=current, V=voltage, R=resistance) Plasma membrane (lipids) have high electrical resistance Water has low electrical resistance (cytosol, ECF)
Resting Membrane Potential Cells under resting conditions have an electrical potential across their plasma membranes ECF voltage = 0 Voltage is stated in terms of the sign of the excess charge on the inside of the cell
Resting Membrane Potential Ions collect in a thin shell very close to the faces of the membrane The bulk of the intracellular and extracellular fluids remain the same ** The number of +/- charges that have to be separated across the membrane to account for potential is an infinitesimal fraction of the total number of charges in the two compartments**
Resting Membrane Potential Mg 2+, Ca 2+, H +, HCO 3-, HPO 4 2-, SO 4 2-, amino acids, and proteins are also present in both compartments Determined By Difference in [ion] across the membrane Permeability of the membrane to each ion Na + /K + /ATPase pump rmp Varies from -5 to -100 mv Neurons -40 to -90 mv
Equilibrium Potential Theoretical voltage produced across a cell membrane with the movement of only ONE ion Individual ions have different membrane permeabilities So, each ion contributes its own potential across the membrane
Nernst Equation Calculates the equilibrium potential of a single ion E ion = _61_ log (C o /C i ) E ion = equilibrium potential of a particular ion in mv C i = concentration of ion inside the cell C o = concentration of ion outside the cell Z = valence of ion Z
Understanding Equilibrium Cell cytoplasm has an overall net (-) charge Electrochemical gradients favor: Inward movement of Na + Opposite direction for K + Applying the Nernst Equation Equilibrium potential for Na + = +60mV Equilibrium potential for K + = -90mV Potential
Goldman Equation Resting Membrane Potential (rmp) is determined by the sum of the equilibrium potentials of all the ions that move across the membrane rmp = -70 mv
Establishing RMP Na + /K + /ATPase pump Establishes concentration gradients for Na + and K + Generates a small (-) potential Greater permeability of K + makes membrane potential more (-)
Voltage-Gated Ion Channels K + channels Gated and non-gated Slower to close than Na + channels Leaky K + channels Na + channels All gated open at threshold depolarization = +15mV from rmp Faster to open than K + channels Closed at RMP Contain inactivation gates Intracellular structure Limits ion flux by blocking ion channels after depolarization
Voltage-Gated Na + and K + Channels
Action Potentials All-or-none event in an axon or muscle fiber in which the polarity of the membrane potential is rapidly reversed and reestablished Characteristics: Membrane potential may change 100mV -70mV to +30mV Rapid (1-4m/sec) Occurs in cells with excitable membranes Membrane capable of producing action potentials Nerve cells Muscle cells Some others (some endocrine, immune, reproductive not all!)
Graded Potentials and Action Potentials Transient changes in membrane potential produce electrical signals This is how nerve cells process and transmit information 2 forms Graded potentials Transmit info over a short distance Action potentials Long distance signals of nerve and muscle cell membranes
Changes in Membrane Potential Depolarization less negative/towards 0 voltage Overshoot = cell becomes more positive relative to the outside Repolarization return to rmp Hyperpolarization more negative than resting membrane potential
Graded Potentials Changes in membrane potential that occur in a very localized part of the membrane Variable magnitude Depolarization or hyperpolarization fizzle out Conducted decrementally Examples: Receptor potential Synaptic potential Pacemaker potential EPP
Action Potential Mechanism 1. Steady RMP maintained due to leaky K + channels and Na + /K + /ATPase pumps 2. Local membrane brought to threshold by a stimulus 3. Voltage changes through opening voltage-gated Na + channels rapidly depolarizes the membrane, causing more Na + channels to open 4. Inactivation of Na + channels and delayed opening of voltage-gated K + channels halts membrane depolarization
Steps of AP 1. rmp 2. Threshold stimulus 3. Membrane depolarizes due to opening of vg Na + channels 4. Na + channels deactivated, vg K + channels open
Action Potential Mechanism 5. Outward current through open voltage-gated K + channels repolarizes the membrane back to a negative potential 6. Persistent current through slowly closing voltage-gated K + channels hyperpolarize membrane toward E K+ ; Na + channels return from inactivated state to closed state (without opening) 7. Closure of voltage-gated K + channels and the activity of Na + /K + /ATPase pumps return the membrane to rmp
Steps of AP 5. Membrane repolarizes due to opening of vg K + channels 6. Membrane hyperpolarizes due to slowly closing vg K + channels 7. rmp reestablished as vg K+ channels close PLUS the activity of Na + /K + /ATPase pumps
Action Potentials are All-or-None If threshold is reached, depolarization proceeds with the same amplitude No threshold = no response Subthreshold stimuli = subthreshold potentials How is greater stimulus reflected? Frequency modulation Recruitment
Refractory Period Prevents a subsequent AP from beginning before the first AP is complete Absolute refractory period Relative refractory period Limits number of AP a nerve can produce in a given time period Ensures that AP moves in one direction down the axon Presynaptic neuron to postsynaptic neuron One-Way Conduction
One-Way Conduction Absolute refractory period Axon membrane is incapable of producing another AP Na + channels already open Na + channels inactivated Relative refractory period Follows absolute refractory period Stronger than normal stimulus required to produce an AP Voltage gated K + channels are open
Absolute versus Relative Refractory Period
Action Potential Propagation An AP can only travel the length of an axon if each point along the membrane is depolarized to threshold The current during an AP is sufficient to easily depolarize the adjacent membrane to threshold potential Ions leak to next segment, initiating AP Sequential opening/closing of Na + and K + channels along the membrane AP does not move, it sets off a new AP in the region just ahead of it
Velocity of AP Propagation Velocity of AP propagation depends on 1. Diameter of fiber Small slower Large - faster 2. Myelination Insulation Increases speed of conduction Nodes of Ranvier Saltatory conduction Conduction Velocities Small, unmyelinated 0.5 m/s Large, myelinated 100 m/s
Velocity of AP Propagation Unmyelinated axons Depolarization of each segment Slower Myelinated axons Saltatory conduction Nodes of Ranvier and Na + channel concentration Faster
The Synapse Functional connection between cells Electrical gap junctions act as one cell Chemical synaptic cleft synaptic vesicles neurotransmitter
Impulses travel from cell to cell Gap junctions Adjacent cells electrically coupled through a channel Examples Smooth and cardiac muscles, brain, and glial cells. Electrical Synapse
Terminal end bulb is separated from postsynaptic cell by synaptic cleft NT released from synaptic vesicles Vesicles fuse with axon membrane and NT released by exocytosis Amount of NT released depends upon frequency of AP Chemical Synapse
The Structure of a Synapse