Membrane Potentials Resting Membrane Potential Excitatory Post-synaptic Potential (EPSP) Inhibitory Post-synaptic Potential (IPSP) Action Potential

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Resting Membrane Potential Click on animation website or main website (here) Neurons have a selectively permeable membrane During resting conditions membrane is: permeable to potassium (K+) (channels are open) impermeable to sodium (Na+) (channels are closed) Diffusion force pushes K+ out (concentration gradient) This creates a positively charged extra-cellular space. Electrostatic force pushes K+ in Thus, there is a ‘dynamic equilibrium’ with zero net movement of ions. The resting membrane potential is negative (- 60mv) Diffusion: Movement of molecules from a region of high concentration to regions of low concentration. Electrostatic pressure: The attractive force between atomic particles charged with opposite signs or the repulsive force between two atomic particles charged with the same sign. Intracellular fluid: The fluid contained within cells. Extracellular fluid: Body fluids located outside cells. Sodium-potassium transporter: A protein found in the membrane of all cells that extrudes sodium ions.

Cell Membrane Copyright © Allyn & Bacon 2004

Resting Membrane Potential OUTSIDE Na+ Cl- Force of Diffusion Electrostatic Force K+ Force of Diffusion Electrostatic Force + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 3Na/2K pump open channel open channel Closed channel no channel - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - Charged particles are called ions. Negatively charged ones are ‘anions’, while positive ones are ‘cations’. Proteins are negatively charged particles and they remain inside the cell Pr- INSIDE - 65 mV K+ = Potassium; Na+ = Sodium; Cl- = Chloride; Pr- = proteins

Resting membrane potential: (things you need to know) Concept of ‘Selective membrane’ How permeable the membrane is to proteins, K+, and Na+ Diffusion and electrostatic forces and how they act on K+ and Na+ Concept of ‘Dynamic equilibrium’ Concept of ‘Membrane potential’ ATP Na/K pump and its role in maintaining the membrane potential Membrane potential: The electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell. Resting membrane potential: The membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials; approximately -70 mV in the giant squid axon.

Copyright © Allyn & Bacon 2004

Membrane Potentials 3. Action Potential 2. Excitatory Post-synaptic potential Ion channel: A specialized protein molecule that permits specific ions to enter or leave the cell. Voltage-dependent ion channel: An ion channel that opens or closes according to the value of the membrane potential. Depolarization: Reduction (toward zero) of the membrane potential of a cell from its normal resting potential. Normal depolarizing events are termed EPSPs – excitatory post-synaptic potentials – and result from opening of Na+ channels. Hyperpolarization: An increase in the membrane potential of a cell, relative to the normal resting potential. Hyperpolarizing events are termed IPSPs – inhibitory post-synaptic potentials – and result from opening of Cl- channels (Cl- higher outside than in, so flow in and make cell more negative). Also can result from opening K+ channels. Action potential: The brief electrical impulse that provides the basis for conduction of information along an axon. Threshold of excitation: The value of the membrane potential that must be reached to produce an action potential. Cable properties: The passive conduction of electrical current, in a decremental fashion, down the length of an axon. Saltatory conduction; Conduction of action potentials by myelinated axons. The action potential appears to jump from one node of Ranvier to the next. All-or-none law: The principle that once an action potential is triggered in an axon, it is propagated without decrement to the end of the fiber. Rate law: The principle that variations in the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires. 4. Inhibitory Post-synaptic potential threshold 1. Resting Potential (just described) Copyright © Allyn & Bacon 2004

2. Excitatory Post-synaptic Potential (EPSP) Post-synaptic neuron Pre-synaptic neuron

Excitatory Post-synaptic Potential (EPSP) Post-synaptic neuron Pre-synaptic neuron The pre-synaptic neuron releases a neurotransmitter. Neurotransmitter diffuses across extra-cellular space - synaptic cleft. Neurotransmitter binds to post-synaptic receptor. Binding of neurotransmitter causes Na+ channels in post-synaptic membrane to open. Depolarization occurs (excitatory potential) Presynaptic membrane: The membrane of a terminal button that lies adjacent to the postsynaptic membrane and through which the neurotransmitter is released. Synaptic cleft: The space between the presynaptic membrane and the postsynaptic membrane. Synaptic vesicle: A small, hollow, beadlike structure found in the terminal buttons; contains molecules of a neurotransmitter. Postsynaptic receptor: A receptor molecule in the postsynaptic membrane of a synapse that contains a binding site for a neurotransmitter. Neurotransmitter-dependent ion channel: An ion channel that opens when a molecule of a neurotransmitter binds with a postsynaptic receptor Excitatory postsynaptic potential (EPSP): An excitatory depolarization of the postsynaptic membrane of a synapse caused by the liberation of a neurotransmitter by the terminal button. Inhibitory postsynaptic potential (IPSP): An inhibitory hyperpolarization of the postsynaptic membrane of a synapse caused by the liberation of a neurotransmitter by the terminal button. Neuromodulator: A naturally secreted substance that acts like a neurotransmitter except that it is not restricted to the synaptic cleft but diffuses through the extracellular fluid.

Excitatory Post-Synaptic Potential (EPSP) Resting Membrane Potential OUTSIDE Na+ Cl- Force of Diffusion Electrostatic Force K+ Force of Diffusion Electrostatic Force + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + open channel open channel OPEN Na+ CHANNEL Closed channel no channel - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - Charged particles are called ions. Negatively charged ones are ‘anions’, while positive ones are ‘cations’. Proteins are negatively charged particles and they remain inside the cell + + + Pr- INSIDE - 50 mV - 65 mV K+ = Potassium; Na+ = Sodium; Cl- = Chloride; Pr- = proteins

EPSP EPSP is a “graded” potential Multiple EPSPs are integrated across space and time. Once the threshold is reached, voltage-dependent sodium channels are opened Neural integration: The process by which inhibitory and excitatory postsynaptic potentials summate and control the rate of firing of a neuron. The cell is depolarized (action potential) Excitatory Post-synaptic potential Integration

3. Inbibitory Post-synaptic Potential (IPSP)