Basic Electrophysiology

Valeria Fu

November 3, 2014

Neurons

• Cells in the nervous system • Vary in structure and properties • Their fundamental task is

signaling • Neurons respond to stimuli,

conduct impulses and send signals

• Neurons encode information by a combination of electrical and chemical signals.

Electrical Signaling in Neurons

Electrical Signaling in Neurons

Presynaptic neuron Postsynaptic neuron

• Electrical signals in the presynaptic neuron cause the release of neurotransmitter; the neurotransmitter binds to receptors of the postsynaptic neuron and triggers electrical signal (synaptic potentials)

Plasma Membrane

• The entire neuron is enclosed by a plasma membrane

• The plasma membrane: • a double layer (bilayer) of phospholipid

molecules • provides a resistance to the flow of ions

entering or leaving the cell (resistor) • Allows to store charge inside the cell

(capacitor). • Separating electrical charges between

extracellular and intracelluar spaces

Membrane Potential • Uneven distribution of ions inside and outside of the membrane • Sodium (Na+) and Chloride (Cl-) are more concentrated outside the cell. • Potassium (K+) and organic anions (A-) are more concentrated inside the cell • Inside cell membrane, it is negatively charged. • The electrical potential difference across the membrane at any moment in time

is known as the Membrane Potential (Vm) • Vm = Vin - Vout

Resting Membrane Potential • When the cell/axon is not conducting impulses; it is said to be at rest • Resting potential ranges from about -60 mV to -70 mV • Membrane is selectively permeable to K+ • Permeable to Na+ is low

• To maintain the steady resting membrane potential, the charge separation across the membrane must be constant:

• Influx of positive charge ≈ efflux of positive charge

• Resting state of the cell is achieved by: • 1. Gibbs-Donnan Equilibrium • 2. Equilibrium Potential & Nernst Equation • 3. ATP-dependent 3 Na+-K+ Pump • 4. Ion Channels

1. Gibbs-Donnan Equilibrium • Uneven distribution of charged ions on one side of a semipermeable membrane • Diffusion occurs when ions move from areas of high concentration to low

concentration down a concentration gradient (Chemical driving force). • Concentration of 2 ions of opposite sign on each side (extracellular and intracellular) of

the membrane are affected by the electrostatic repulsion (same sign repulses) and attraction (opposite sign attracts).(Electrical driving force)

• At rest, K+ diffuses down to the concentration out of the cell (Chemical driving force)leaving surplus of anion (A-) inside the cell

• Excess cation outside the cell pushes K+ back to the cell (Electrical driving force)

• Until K+ concentration inside and outside the cell are in equilibrium (-75mV)

• Resting membrane potential (Vr ) settles at K+ equilibrium potential (Ek)

• Vr = Ek

2. Equilibrium Potential & Nernst Equation

• membrane potential where the net flow through any open channels is zero • Depends on the concentration gradient of an ion • Equilibrium potential of an ion can be calculated from the Nernst equation

derived by Walter Nernst (1888): • Ex = RT In [X+]o

ZF [X+]i

• Ex = membrane potential at which ion X is in equilibrium • R = gas constant • T = temperature • Z = charge on the ion • F = Faraday constant • [X+]o & [X+]I = concentration of ion inside and outside of the cell • For monovalent ions at room temperature, the Nernst equation reduces to: • Ex = 58 log10 [X+]o

[X+]i

Goldman-Hongkin Katz Equation • Changing extracellular concentration of each ion has a strong effect on

resting membrane potential and could be calculated by:

• Permeability ability ratios for membrane at rest : • PK:PNa:PCl = 1/0.04/0.45

3. ATP-dependent Na+-K+ pump

• 3 Na+ transport outward for every 2 K+ inward • And contributes to the resting potential

• Consequences: • 1. causes a net transfer of charge across the

membrane. Pump is electrogenic: cause to cell to hyperpolarize

• 2. passive movements of Na+ and K+ are unequal • 3. decreases the osmolarity of the intracellular fluid

and balances the effect of impermeant anion in the cytosol

4. Ion Channels • Neuronal signaling is based on the movements of ions across cell membrane. • Hydrophilic pores through which ions flow from extracellular space to intracellular

space or vice versa down their concentration gradients. • There are gated and non-gated ion channels in membrane. • Voltage-dependent gated ion channels: • 1. Probability of opening of channels is strongly influenced by voltage only • 2. Opening and closing of channels are influenced by changes in membrane

potential close to normal resting potential.

Ohm’s Law • Current flow (I) between extraceullar space and intracellular space depends on the

voltage difference (V) and Resistance to current flow (R)

• I = V R • Voltage difference (Vm – Ex) is called Driving Force • Conductance (G) is the inverse of resistance: • G = 1/R

• Ix = (Vm – Ex) Gx

• X = ion

Equivalent Electrical Circuit

• Membrane is a Capacitor and a Resistor connected in parallel.

• Neuronal signaling is based on the movements of ions across cell membrane.

• We can calculate the number of ions must move through membrane in order to give rise to the membrane potential:

• V = Q/C • Q = charge • C = capacitance • V = electrical potential

Neuronal Signal Transmission

• Neurons use a single type of signal to transmit information over a long distance.

Action Potentials • When the dendrite/axon/nerve is conducting an impulse. • Large, brief, invariant signals propagate along the axons

without decrement. • “all-or-none” • Opening and closing of a sodium channel and a potassium

channel in a precise timing give a transient changes in membrane potential which allows electrical signals travels along axons at speed up to 120 meter per second.

• Four Stages: • 1. Resting Potential • 2. Depolarization (Rising phase) • 3. Repolarization (Falling phase) • 4. Undershoot (After-hyperpolarization)

1. Resting Potential • When the axon is at rest (-70mV) • Neither Na+ or K+ ion channel is open • Steady Na+ influx is balanced by steady efflux of K+

• ATP-dependent Na+-K+ pump is at work

2.Depolarization (Rising phase) • When the nerve fiber is stimulated, synaptic

inputs (Post-synaptic neuron ) to a neuron cause the membrane to depolarize (membrane potentials are less negative)

• A transient depolarizing potential (i.e. excitatory synaptic potential) causes opening of some voltage-gated Na+

channels. • Increase membrane Na+ permeability and

allows influx of Na+ to further depolarize the membrane

• Increase in depolarization allows influx of more positive charge flow inside the cell.

• When depolarization of membrane exceeds threshold, Action Potentials result.

Action Potentials • To transfer information from one part of

the neuron to another • ‘All-or None’ law: • The strength of neuronal response is

independent of stimulus strength; Once the stimulus is above threshold, it produces full size action potentials to minimize the possibility that the information is lost along the way down the axon.

Action Potentials • Latency, the time delay from the onset of

stimulus to the peak of action potentials, is the function of stimulus strength (strength-latency relationship)

• The larger the depolarizing stimulus, the greater the frequency of action potential firing

Frequency coding in axons

2. Depolarization (Rising Phase) • Sodium conductance exceeds Potassium

conductance • Net inward current drives the membrane

potentials toward Na+ equilibrium (+55mV) • Sodium permeability decreases when the

action potential approaches the peak which results from inactivation of sodium channel -> no longer respond to depolarization

• Potassium conductance responds more slowly and starts to increase when the action potential is near to its peak.

3. Repolarization (Falling phase) • Delay opening of Potassium channel • Inactivation of Sodium channel • Efflux of K+ increases carrying their

positive charge with them • Thus lead to hyperpolarization

(membrane potential is more negative) • Bring the membrane potential back to

resting state (-60 mV to -70 mV)

4. After-hyperpolarization

• Potassium conductance is higher than normal

• Sodium conductance is lower than normal

• Membrane potentials is driven closer to the equilibrium of K+ (-90mV) than it is at rest (60 – 70mV)

Refractory Period

• Immediately after an action potential • Refractory period: threshold is higher than

normal to initiate another action potential.

• Absolute refractory period: the threshold is infinite, impossible to evoke another action potential

• Relative refractory period: requires a larger than normal stimulus to evoke an action potential.

Review: Action Potential

Action Potential Propagation • Passive spread (local potential): voltage

change spreads from one point to another, but with attenuation with distance.

• Vd = Voe -d/

• Even voltage changes at distance, local depolarization is large enough to spread to the adjacent region of the axon to generate a full size action potential

Action Potential Propagation • Part of the inward Na+ current flows down to

interior of axon to produce local potential in advance of an action potential

• Local potential depolarizes the membrane • Activated voltage-gated Na+ channels • When reach threshold, inward current

further depolarizes the membrane and acts as a source for local potential change.

• The inward current flows downstream and moves the action potential along the axon.

• Due to refractory period, inward current will not initial another action potential in towards the cell body.

• Therefore, action potentials propagate in ONE direction.

Saltatory Conduction • An axon is myelinated. • In the myelinated area, there is no inward current of Na+ when the Na+

channels open because there is no extracellular Na+

• The only place that the myelinated axon comes to contact with extracellular fluid is at the Node of Ranvier where the axon is unmyelinated.

• Action potential jumps from node to node to propagate down the axon. This is called saltatory conduction.

Synaptic Potentials • Action potential travels along the axon down to

the presynaptic terminal • The depolarization of the presynaptic neuron

triggers the release of neurotransmitter in the cleft.

• When the neurotransmitter binds to the receptor of the post-synaptic neuron, it gives rise to the synaptic potentials.

• In the central nervous system (CNS), glutamate is the major excitatory neurotransmitter which generate excitatory postsynaptic potentials (epsp).

• While GABA or glycine is commonly the inhibitory neurotransmitter which open the channels of K+ and Cl-. In turn, it hyperpolarizes the cell and makes depolarization to threshold more difficult. This synaptic potential is called inhibitory postsynaptic potentials (ipsp).

Passive Membrane Properties • They are constant during neuronal signaling • They affect the electrical signaling process:

• 1. Time course of electrical signals • 2. Efficiency of signal conduction

Time Course of Electrical Signals • Membrane acts as an electrical capacitor(Cm) and resistor (Rm) • The shape of change of a potential (voltage) is determined by the fact that

membrane capacitance and resistance are in parallel. • Charged (discharged) of membrane capacitance does not occur instantaneously. • Time constant . • = Rm Cm

• Voltage changes exponentially with time (t) • Vt = Vo e –t/

• Voltage falls to 1/ e of its initial value Vo in a time equal to one time constant (). • The longer the time constant, the longer duration of synaptic potential -> more

chance for temporal summation (small potentials adding together) -> higher chance to drive membrane potential for an action potential

Efficiency of Signal Conduction • 1. Axoplasmic resistance: • the greater length of the axoplasmic core, the greater the resistance (ion collisions

along the dendrite) , the smaller the current. • Vm = Imrm • 2. Insulation of the membrane: • the better the insulation, the further the current spread along the dendrite/axon,

the faster the velocity of action potential • Length constant () = rm/ra • Where rm = membrane resistance; ra = axial resistance • Myelination of axon affects velocity of action potential

• 3. Axon Diameter: • The larger the axon diameter, the lower resistance of axoplasm to flow of current;

more effective depolarization of membrane, the faster the velocity of action potential.

Review

Review

Review

Review

Review

Presynaptic neuron Postsynaptic neuron

References

• Principles of Neural Science (Kandel, Schwartz & Jessell) • Molecular Neurobiology (Zach Hall) • The Neuron (Levitan & Kaczmarek) • Ionic Channels of Excitable Membranes (Bertil Hille)