biological electricity and action potentials4

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Copyright 2009 John Wiley & Sons, Inc. 1

Chapter 12Biological electricity and

action potentials




Electrical Signals in Cells

Cells are electrically excitable due to thevoltage difference across their membrane

This voltage difference is created by ions(which are electrical charged particles)flowing through protein channels in the cellmembrane.

Cells can send messages inside themselvesor to another cell by changing the voltagedifference.




Resting Membrane PotentialN

egative ions along inside of cellmembrane & positive ions alongoutside

Voltage (or µpotential¶) difference atrest is -70 milliVolts (mV)cell is said to be ³polarized´

Resting potential exists because thereare more positively charge particlesoutside the cell compared to inside thecell




Graded PotentialsG raded potentials are small changes to the restingpotential of -70mV

hyperpolarization = membrane potential becomes morenegative (e.g. -90mV) because more positively chargedparticles have exited the cell to the extracellular fluidd epolarization = membrane potential becomes morepositive (e.g. -70mV) because more positively chargedparticles have entered the cell to the intracellular fluid/cytosol



Graded Potentials

These changes in membrane potential aregra d e d , meaning they vary in amplitude

(µsize¶), depending on the strength of thestimulus.These changes in membrane potential arelocalise d .





Hyperpolarized/Depolarized Graded

Potential N erve cell or neuron




Stimulus strength and graded potentials




Summation




Generation of Action PotentialsW hen the membrane depolarizes it might reach theµthreshol d¶ potential (usually -55mV). Reaching thethreshold depends on how many positive chargedparticles enter the cell.

If this happens, protein channels open and allowmore positive charged particles to enter the cell.This causes the cell membrane to d epolarizerapidly to +30 mV; (it reverses ± negative outside

and positive inside) and is known as an actionpotential




Generation of Action PotentialsN ote that an action potential is µall-or-none¶ -

meaning it either happens or it doesn¶t! there isno big or small action potential ± it is always thesame size (+30mV).

N ow the cell restores its membrane potential(repolarization to -70mV) by opening channelswhich allow positive charged particles to exit thecell (i.e. making the inside more negative)

A strong stimulus will result in many actionpotentials close together in time




Stimulus strength and Action Potential

generation




Continuous versus Saltatory Conduction

Continuous conduction (unmyelinated fibers)step-by-step depolarization of each portion of thelength of the cell membrane of a nerve cell

Saltatory conductiondepolarization only at nodes of Ranvier wherethere is a high density of protein channels in nerve

cells




Propagation of an Action Potential in a

neuron after it arises at the trigger zone




Factors that affect speed of propagation

A mount of myelination

A xon diameter

Temperature




Signal Transmission at the SynapseA synapse is the small space (or junctionbetween) to cells (usually nerve cells)2 Types of synapses

electricalionic current spreads to next cell through gap junctionsFast; information can travel in both directions

chemical

one-way information transfer See next slide




Signal transmission at the chemical

synapseP resynaptic cell

P ostsynaptic cell




Chemical Synapses

A ction potential reaches the end of the presynapticcell and calcium channels openCalcium flows inward stimulating release of

neurotransmitter N eurotransmitter crosses the synapse binds to achannel on the postsynaptic cell and causes thechannel to open ± this allows charged particles in or

out and changes the membrane potential of thepostsynaptic cell




Signal transmission at a chemical synapse




Excitory and Inhibitory Postsynaptic

PotentialsThe effect of a neurotransmitter can be either excitatory or inhibitory on the postsynaptic cell

a depolarizing postsynaptic potential is called an excitatorypost synaptic potential (EPSP) (e.g. -70mV to -60mV) ±positively charged particles move inside the cell

A hyperpolarising postsynaptic potential is called aninhibitory postsynaptic potential is called an IPSP ( IPSP; -70 mV to -90mV - Positively charge particles move out of thecell




Removal of Neurotransmitter

Enzymatic degradationUptake by nerve cells




Summation

If several presynaptic cells release their neurotransmitter at about the same time, thecombined effect may generate a nerveimpulse due to summationSummation may be spatial or temporal .




Spatial Summation

Summation of effects of neurotransmitters releasedfrom several end bulbs onto one neuron




Temporal Summation

Summation of effect of neurotransmitters releasedfrom 2 or more firings of the same end bulb in rapidsuccession onto a second neuron




Summation




C opyright 2009 John Wiley & Sons, Inc. A ll rights reserved. Reproduction or translation of this work beyondthat permitted in section 117 of the 1976 United States Copyright A

ct without express permission of the copyright owner is unlawful.Request for further information should be addressed to thePermission Department, John W iley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publishers assumes no responsibility for errors, omissions, or damages caused by the use of thesesprograms or from the use of the information herein.

End of Chapter 12

biological electricity and action potentials4

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