action potential synapse hb 10 sep 2010

8/8/2019 Action Potential Synapse HB 10 Sep 2010

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Dr. Isabel Hwang

Teaching and Learning Unit,School of Biomedical Sciences

Faculty of MedicineThe Chinese University of Hong Kong10 Sep 2010

Action Potential

(synaptic transmission)


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Lecture Outline

Propagation of action potentialsSynaptic transmission


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Propagation (Conduction) of Nerve

Impulses

Defined as the traveling of anerve impulse from one part

of the body to anotherEffects of neurotoxins and local anesthetics


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Axonal conduction

Cable Properties

Refer to how axonsproperties affect itsability to conductcurrent

Includes highresistance ofcytoplasm resistance decreases

as axon diameterincreases

current leaks outthrough ion channels


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Propagation of nerve signals Defined as the traveling of

a nerve impulse from one

part of the body to another


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Conduction in

Un-myelinated() axon After axon hillock reaches

threshold and fires AP, itsNa+ influx depolarizesadjacent regions tothreshold

Generating a new AP

Process repeats allalong axon

So AP amplitude is

always same

Conduction isslow


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Conduction in myelinated() axon

Ions can't flow across myelinated membrane

thus no APs occur under myelin

this increases current spread

alsosaves energy!


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Gaps in myelin are called Nodes of Ranvier APs occur only at nodes

voltage-gated Na+

channels are present only at nodes Current from AP at 1 node can depolarize next node to

threshold

fastbecause APs skip from node to node

is called saltatory conduction

Conduction in myelinated () axon


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Remember: The strength of the stimulus changes the

frequencyof action potentials but not thestrength of action potentials


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Dendrites

Cellbody

Axon

Myelin(speeds signaltransmission)

Synapticterminals

Actionpotentialsignal

Conduction speeddepends on1. Axon diameter and

2. Myelination


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Synaptic transmission


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Neurons communicate at synapses

Synapses arejunctions wheresignals aretransmittedbetween

two neurons

or betweenneurons and

effector cells


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Synaptic transmission

Synapses are interaction pointswithin the nervous system

Dendrites

Myelinsheath

Axon

Receivingcell body


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Electrical synapses Electrical signals pass between cells

Chemical synapses

Sending (presynaptic) cell secretes a chemical signal,a neurotransmitter

The neurotransmitter crosses the synaptic cleft

The neurotransmitter binds to a receptor on thesurface of the receiving (postsynaptic) cell

Types of synapse


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Electrical Synapse Not as common as chemical synapse Gap junctions connect presynaptic membranes

with postsynaptic membranes

Gap junctions have pores that allow ions to flow betweenthe two cells

Very efficient propagation of action potentials

Formed byconnexin proteins

Found in smooth

and cardiacmuscles, brain, andglial cells (to sustainneurons)


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Chemical synapses make complex

information processing possible

Some neurotransmitters

1.1. ExciteExcitethe receiving cell2.2. InhibitInhibitthe receiving cells activity by

decreasing its ability to develop action

potentials


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Sending neuron 1

23

4

65

Axon of

sendingneuron

Vesicles

Synapticterminal

Vesicle fuseswith plasma

membrane

Synapticcleft

Receivingneuron

ReceivingneuronIon channels

Neurotransmittermolecules

Neurotransmitteris released intosynaptic cleft

Neurotransmitterbinds to receptor

Synapse

Actionpotentialarrives

Neurotransmitter

Receptor

Ions

Neurotransmitter brokendown and released

Ion channel closesIon channel opens


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A neuron may receive information

from hundreds of other neuronsvia thousands of synaptic terminals

The summation of excitation and inhibition

determines if a neuron will transmit a nervesignal


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Dendrites

Myelin

sheath

Axon

Receivingcell body

Inhibitory Excitatory

Synaptic terminals

Synapticterminals


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Amount of NT released depends upon frequencyof APs


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Acetylcholine (Ach) Most widely used neurotransmitter

used in brain and autonomic nervous system

used at all neuromuscular junctions

Ligand-GatedChannels

Contain both a Ach

receptor site and an ionchannel Open when ligand (Ach)

binds


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Acetylcholinesterase An enzyme that inactivatesAch, stopping its

action

located in the synaptic cleft


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Summary of events

A single neuron postsynapticto one cell can be presynaptic

to another cell.

ExcitatoryPostSynapticPotential


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The end.

action potential synapse hb 10 sep 2010

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