Transcript
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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
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