ELECTRICAL PROPERTIES OF NEURON

Dr. Atif Mahmood

LEARNING OBJECTIVES At the end of the lecture, student should be able to: • know the basic structure and function of neuron• understand its basic physical and electrical

properties of neuron• to know the factors on which the resting membrane

potential depends • to appreciate the mechanics of action potential

initiation and measurements of action potentials. • Know the basics of patch-clamp technique

Two classes of cells

• Nerve cells (neurons)•is the basic structural and functional unit of nervous tissue

− For electrical signaling − The human brain has 1011 neurons

• Glial cells − Not for electrical signaling − 10 to 50 times more glial cells than neurons

− Play an essential role in brain metabolism − Support neurons − Cover neurons with myelin − Clean up debris

Neuron structure

• cell body – contains nucleus & organelles

• dendrite – conducts signals to cell body

• axon – long fibers – specialized for electrical conduction

Structure of a typical neuron

DendriteAxon Terminal

Myelin Sheath

Node of Ranvier

Axon

Cell Body

Nucleus

Schwann Cell

Differences between neurons and other cells

1. Neurons have specialized extensions calleddendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body

2. Neurons communicate with each other through an electrochemical process

3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters)

MORPHOLOGY OF NERVE CELLS (THE NEURON)

A NEURONNucleus

Dendrites

Myelin

Axon

• Dendrites -- Input• Cell body (soma) -- Integration• Axon -- Output

STRUCTURE OF NEURONS -DENDRITES

At dendrites, the neurons recieve input via axons of other neurons at synapses

dendritic spine

STRUCTURE OF NEURONS -AXON

The axon transmits the information electrically from the soma to the synapses –it is surrounded by myelin that insulate the axon, provided by oligodendrocytes (glial cells)

NEURON: MORPHOLOGICAL CLASSIFICATION

• Unipolar• Bipolar• Multipolar

•Single axon & multiple dendrites•Most common type in men, e.g•Motor cortex, interneurons, …

• Pseudounipolar•Single process arises from body•Branches into an axon and dendrite, e.g•Present in spinal and cranial ganglia

Two processes extending from the cell body. (retinal cells, olfactory epithelium cells)

Galvani frog’s legs experiment

Ions Cannot Diffuse Across the Hydrophobic Barrier of the Lipid Bilayer

Ions and Ion Channels

[K+]

[K+][Na+] [Ca2+] [Cl-]

[Cl-][Ca2+][Na+]

Cations: +

Anions: -

Channels are selective to particular ions. Passive vs. Active channels.

Permeability is very high to K and Na, medium to Cl and very low to big anions.

[K+]in=20 [K+]out

[Na+]out=10 [Na+]in

Main question: How is the membrane potential is related to [charges] in and out?

Outside

Inside

Equilibrium potential for one ion

[K+]

[K+]

Two competing forces:1.Diffusion by concentration gradient.2.Motion by voltage gradient.

Diffusion Voltage difference

Outside

Inside

1.3 Equilibrium potential for one ion

[K+]

[K+]

Diffusion Flux: Jdif(x)= -D d[K+](x) / dx

D, diffusion coefficient-> D=µkT/q

µ, mobilityk, Boltzmann constantq, ion charge

x

Voltage difference Flux: Jelec(x)= -µz [K+](x) dV(x) / dx

µ, mobilityz, ion valence, +/-1, +/-2, etc.

Equilibrium happens when Jdif(x) + Jelec(x) = 0, which leads to the Nernst equation:

E K+ = kT / zq ln [K+]out / [K+]in = -75,-90 mV

E K+ is the potential necessary to maintain the concentration gradient [K+]out / [K+]in

1.3 Equilibrium potential for one ion

[K+]

[K+]

x

E K+ = kT / zq ln [K+]out / [K+]in = -75,-90 mV

E Na+ = +55 mV

E Ca2+ = +150 mV

E Cl- = -60,-65mV

K+ Na+

Vm = -70 mV

E K+ < Vm < E Na+

Compensated by Na-K pump.

K+ Na+

The Bulk Solution Remains Electroneutral

Ion Channel Selectivity and Ionic Concentration Gradient Result in an Electromotive Force

• Cell membrane: 2-3 nm thick and is impermeable to most charged molecules and so acts as a capacitor by separating the charges lying on either side of the membrane.

• NB Capacitors, store charge across an insulating medium. Don’t allow current to flow across, but charge can be redistributed on each side leading to current flow.

The Lipid Bilayer Acts Like a Capacitor

+ + + +

- - - -

Vm = Q/C

∆Vm = ∆Q/C

The Lipid Bilayer Acts Like a Capacitor

∆Q must change before∆Vm can change

The voltage (Vm)across a capacitor is

proportional to the charge (Q) stored on the capacitor:

Capacitance is Proportional to Membrane Area

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Vm = Q/C

Electrical Signaling in the Nervous System isCaused by the

Opening or Closing of Ion Channels

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The Resultant Flow of Charge into the CellDrives the Membrane Potential Away From its Resting Value

Equilibrium and reversal potentials

Equilibrium potentials (The Nernst equation applies when the channels allow only one type of ion to pass through them)

Some channels are not so selective, and in this case the potential E is estimated by the Goldman equation

Reversal potentials takes a value intermediate between the equilibrium potentials of the individual ion types that it conducts

RESTING MEMBRANE POTENTIAL RMP

• All cells (not just excitable cells) have a resting potential (membrane potential) : an electrical charge across the plasma membrane

• The interior of the cell negative• Typically -70mV ( varies -40 to -90 mV depends

upon the type of neuron)

RESTING MEMBRANE POTENTIAL RMP

•Neurons are enclosed by a membrane separating interior from extra cellular space•The concentration of ions inside is different (more –ve) to that in the surrounding liquid

• -ve ions therefore build up on the inside surface of the membrane and an equal amount of +ve ions build up on the outside

• The difference in concentration generates an electrical potential (membrane potential) which plays an important role in neuronal dynamics..

• The ion channels are proteins in the membrane, which lower the effective membrane resistance by a factor of 10,000 (depending on density, type etc)

RESTING MEMBRANE POTENTIAL RMPRESTING MEMBRANE POTENTIAL RMP

• In order for a potential difference to be present across a membrane lipid bilayer, two conditions must be met.

• 1.unequal distribution of ions of one or more species across the membrane (ie, a concentration gradient).

• 2.Two, the membrane must be permeable to one or more of these ion species. The permeability is provided by the existence of channels or pores in the bilayer; these channels are usually permeable to a single species of ions.

• represents an equilibrium situation at which the driving force for the membrane-permeant ions down their concentration gradients across the membrane is equal and opposite to the driving force for these ions down their electrical gradients.

Resting Potential

In a resting neuron (one that is not conducting an impulse), there is a difference in

electrical charges on the outside and inside of the plasma membrane. The outside has a positive charge and the inside has a negative charge.

Contribution of Active Transport – Factor 1

There are different numbers of potassium ions (K+) and sodium ions (Na+) on either side of the membrane. Even when a nerve cell is not conducting an impulse, for each ATP molecule that’s hydrolysed, it is actively transporting 3 molecules Na+ out of the cell and 2 moleculesof K+ into the cell, at the same time by means of the sodium-potassium pump.

Contribution of facilitated diffusion

The sodium-potassium pump creates a concentration and electrical gradient for Na+ and K+, which means that K+ tends to diffuse (‘leak’) out of the cell and Na+ tends

to diffuse in. BUT, the membrane is much more permeable to K+, so K+ diffuses out along its concentration gradient much more slowly.

RESULTS IN:a net positive charge

outside & a net negative charge inside. Such a membrane is

POLARISED

EXCITATION AND CONDUCTION

• Most biological neurons communicate by short electrical pulses called action potentials or spikes or nerve impulses

• These action potentials are generated by means of influx and out flux of ions through the ion channels embedded in membrane

• Suitable electrical probe (electrode) and measurement instrumentation (amplifier and read-out) can measure these tiny potentials on the order of few milli-volts

EXCITATION AND CONDUCTION• Nerve cells have a low threshold for excitation –

may easily be excited by electrical, chemical or mechanical stimuli

• Two types of physiochemical disturbances are produced as a result

• Local or Non-Propagated Potentialssuch as Synaptic, Generator or Electrotonic Potentials• Propagated Disturbances such as Action Potentials

or Nerve Impulses

ELECTRICAL PROPERTIES OF NEURONS

The cell membrane isolates the intracellular from extracellular space

extracellular

intracellular

The membrane potential

In the resting state, the intracellular space contains more negative ions than the extracellular space

extracellular

intracellular

difference of -70 mV

ELECTRICAL PROPERTIES OF NEURONS

ION CHANNELS CONNECT THE INTRA- AND

EXTRACELLULAR SPACE

Opening of ion channels lead to a flux of ions through the membrane and to a change of the membrane potential

THE ACTION POTENTIAL

The action potential is generated by ion flux through voltage gated channels

All or none principle!!

PROPAGATION OF THE ACTION POTENTIAL

SYNAPSE – COMMUNICATION BETWEEN NEURONS

Presynaptic vesicles with

neurotransmitter

Released transmitter

Transmitter binds to receptor

Na+Transmitter-Resorption

from synaptic cleft

SYNAPSE – COMMUNICATION BETWEEN NEURONS

SYNAPSE – COMMUNICATION BETWEEN NEURONS

THE MORPHOLOGY OF AN ACTION POTENTIAL

• Action potential is a rapid, reversible, and conductive change of the membrane potential after the cell is stimulated.

• Nerve signals are transmitted by action potentials.• Reduction in membrane potential (depolarization) to

"threshold" level leads to opening of Na+ channels, allowing Na+ to enter the cell

• Interior becomes positive• The Na+ channels then close automatically followed by a

period of inactivation.• K+ channels open, K+ leaves the cell and the interior again

becomes negative.• Process lasts about 1/1000th of a second.

PROPERTIES OF THE ACTION POTENTIAL

• “All or none” phenomenon• A threshold or suprathreshold stimulus

applied to a single nerve fiber always initiate the same action potential with constant amplitude, time course and propagation velocity.

• Propagation• Transmitted in both direction in a nerve fiber

THE PATCH-CLAMP TECHNIQUE

• • This is a novel technique in which physiological

currents flowing through the cells can be detected without disrupting the cell or its contents

• A micropipette (diameter in microns) filled with a buffer solution and carrying a metal electrode is gently touched to the cell membrane and the membrane contents are sucked in. This is called a patch-clamp.

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THE END