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Action Potential vs. Graded PotentialAction Potential vs. Graded Potential

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Resting Membrane PotentialResting Membrane Potential

the cell membrane acts like a battery in that it separates positive and

negative charges and therefore establishes a difference in voltage

across the membrane

resting membrane potential refers to this voltage difference

it is 70 mV in nerve cells and 90 mV in muscle cells

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Ion ChannelsIon Channels

stimulation of ligand- ormechanically gatedchannels alter membranepotential

these channels aregenerally found indendrites

this change in potential then

stimulates the voltage gated channel

these channels aregenerally found in axons

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Maintaining the Resting Membrane PotentialMaintaining the Resting Membrane Potential

Na+outside, K+ inside

protein- , phosphate- , and Cl- inside all contribute as well

Na+

/ K+

pump is key especially following an action potential

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Graded PotentialsGraded Potentials

occur in the dendrites and cell body

change in membrane potential is local anddoes not propogate down the axon

may be hyperpolarizing (inhibitory post

synaptic potential, IPSP)

K+ leaves the cell

Cl-enters the cell

may be depolarizing (excitatory post synaptic potential, EPSP)

Na+enters the cell

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Graded PotentialsGraded Potentials

graded potentials can summatebut if the threshold of 55 mV is

not reached then the

depolarising potential will

simply dissipate and no action

potential will be generated

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Graded Potentials- SummationGraded Potentials- Summation

all IPSPs and EPSPsreceived by the

dendrites and cell

body are summed

if the result is anEPSP of sufficient

strength (> -55 mV)

then an action

potential will be

generated

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Action Potentials- OverviewAction Potentials- Overview

generated in the axon hillock and

propogate down the axon

all or none- APs always look the

same, i.e. they dont have a graded

amplitude

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Action Potentials- MechanismAction Potentials- Mechanism at rest

Na+ activation gate is closed

K+

gate is closed

depolarisation

Na+ activation gate opens

K+ gate is closed

repolarisation

Na+ inactivation gate closes

K+ gate opens

hyperpolarisation and refratory period K+ gate slowly closes

Na+ inactivation gate opens, Na+

activation gate closes

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Action Potential- Repeated StimulationAction Potential- Repeated Stimulation

repeated supra threshold stimulations will trigger repeated APs

due to the absolute and relative refractory periods however, APs do

not summate to larger depolarisations

APs remain distinct from those that were generated before and after

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Action Potentials- PropagationAction Potentials- Propagation

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Stimulus Intensity and SummationStimulus Intensity and Summation

spatial summation recruits more neurons

temporal summation recruits a single neuron several

times

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Synaptic TransmissionSynaptic Transmission

electrical synapse

very fast due to directconnection betweencells

e.g. cardiac and visceralsmooth muscle

chemical synapse

slower due to synapticcleft and NT movementacross

Beyond the Cleft- fate ofNTs

diffusion

degradation

reuptake and recycling

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EPSPs and IPSPsEPSPs and IPSPs

NTs may be excitatory or inhibitory and cause an EPSP or IPSP

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Temporal and Spatial SummationTemporal and Spatial Summation spatial summation recruits more neurons

temporal summation recruits a single neuron several times

if summated EPSPs outweigh IPSPs, there is a greater chance ofreaching threshold and generating and AP

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Classes of NeurotransmittersClasses of Neurotransmitters

Ach- primary NT of NMJ; excitatory

glutamate and aspartate- excitatory in the CNS GABA and glycine- inhibitory in the CNS

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Classes of NeurotransmittersClasses of Neurotransmitters

NE and Epi- generally stimulatory

dopamine- emotion and muscle tone

serotonin- mood, sleep, sensation

e.g. ATP, GTP-

excitatory in CNS

relaxes vasculature

varied class with different actions

e.g. substance P, CCK, endorphins

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Problems with Synaptic TransmissionProblems with Synaptic Transmission

altered excitability

altered activation

threshold

inhibited NT degradationor reuptake

receptor agonists or

antagonists

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Neuronal Circuits and PoolsNeuronal Circuits and Pools

diverging- e.g. corticospinal path innervates 1000s of skeletal muscles converging- think of indirect motor pathways and final common pathway

reverberating- operates like a positive feedback loop until interrupted

parallel after discharge- allows temporal summation

reciprocal inhibition- agonist activity inhibits its antagonist

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Neuronal RegenerationNeuronal Regeneration

plasticityrefers to the ability for neurons

to change throughout its life

this change includes dendritic and

synaptic growth, and new NT

production

following injury however, there is limited

ability to repair especially in the PNS

chromatolysis refers to degeneration of

the cell body

Wallerian degeneration occurs at thedistal axon

both of the above are reversible if the

injury is not severe