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

2. Levels of Investigation
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3. Resting Membrane Potential
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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.

4. Cell Membrane
Copyright © Allyn & Bacon 2004

5. 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

6. 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.

7. Copyright © Allyn & Bacon 2004

8. 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

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

10. 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.

11. 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

12. 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

13. 3. Inbibitory Post-synaptic Potential (IPSP)