communication between neurons (chemical). action potential
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Communication between neurons(chemical)
Action potential
Saltatory conductionPassive current is much faster within myelin(speed of conduction can range from 1 to 100 meters/sec
2. Na/K pumps only in the nodes (saves energy)3
Action potential super-threshold depolarization in nearby regionsOpens Na+ channels Saltatory conduction5
Relatively slowConsumes more energy (need more channels along the membrane)Action potentials in a non-mylinated neuronAction potentials characteristics (regardless of myelin):
All or none (always same shape and size)
Does not diminish with distance (or splits along the axon) until it reaches the terminal buttons that form synapses with the target neuron
The Action Potential is All or NoneThe rate lawThe strength of the signal = frequency of Action potentialsNOT magnitude of potentialSo how can we differentiate btw strong and weak stimulations?8
Decreases with distance Rate of decrease depends on properties of axon (like diameter)Passive conductance (graded potentials)Graded Potential vs. Action potentialAction PotentialGraded PotentialAll or none: fixed magnitudeLarger for stronger triggersStrengthActive Does not decay. Saltatory conductionUnidirectional (from soma to axon)PassiveDecays with distance from sourceMulti directionalconduction10
The synapseThe junction between the terminal button of one neuron and the membrane of another neuron (~20 nm) Presynaptic neuron vs. Postsynaptic neuron11
Axo-dendriticAxo-somaticAxo-axonicTypes of synaptic connections
Terminal button (structure)13The action potential has reached the terminal button. Now what?Voltage dependent calcium (Ca2+) channels
Action potential -> depolarization. Opens Ca2+ ion channels Without Ca2+ - there will be no release of neurotransmitter 14
Each action potential release of a fixed # of vesiclesExocytosis - Followed by recycling15
ExocytosisDocking vesicles containing neurotransmitters
An action-potential arrives, and causes voltage-gated Calcium-channels to open (Ca2+ enters the cell)
Ca2+ causes fusion of the vesicles with the cell membrane
Neurotransmitter in vesicle is expelled (exocytosis)
Vesicle membrane either fuses with cell membrane or closes back inside the cellStages in the process of neurotransmitter release:Now that we have neurotransmitters in the synapse..Neurotransmitters bind to postsynaptic receptorsOpen/close neurotransmitter-dependent channels:1) Direct2) Indirect
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Post-synaptic receptors20How does it affect the cell?Post-synaptic Potential Excitatory postsynaptic potential - depolarization Inhibitory postsynaptic potential - hyperpolarization21
Excitatory Post-Synaptic Potential (EPSP)
Inhibitory Post-Synaptic Potential (IPSP)What determines whether the post-synaptic potential is Excitatory or Inhibitory is NOT the neurotransmitter !!!
The same neurotransmitter can have an IPSP or EPSP effect on the target neuron.
The ultimate effect depends on the target RECEPTOR and the channels it open !!!
Post-synaptic integrationNeural integration
Summation by timeSummation by spaceAxon hillock26A single cell can receive synaptic input from up to ~100,000 !!!
150,000!! contacts Purkinje cell in cerebellumNeurotransmitter binds to postsynaptic receptorsand can Influence neurotransmitter-dependent channels:1) Direct2) Indirect
neurotransmitter-dependent channels2. Metabotropic Receptors1. Ionotropic Receptor
29neurotransmitter-dependent channels
2. Metabotropic Receptors1. Ionotropic receptor30Inotropic MetabotropicFast effectOnly opens transmitter dependent channels Slow effectmay open/close transmitter dependent channels31
Sensory neuronMotor neuronReceptor potentialAction potentialSynaptic potentialAction potential32
Removal of neurotransmitter from the synapseReuptakeEnzymatic deactivationDiffusion33Regulation of neurotransmitter releaseAutoreceptors (pre-synaptic)Usually inhibitory effect Presynaptic facilitation/inhibitionAxoaxonic synapse
34Important notes:
IPSP does NOT necessarily mean inhibition of behavior
Auto-receptors are pre-synaptic receptors that respond to the neurotransmitter the same cell released. They do not open ion channels on the post-synaptic cell
Axo-dendritic and Axo-somatic synapses cause either IPSP or EPSP. Axo-Axonic synapses can regulate the amount of neurotransmitter that will be released.
Inihibition => reduction in the amount of neurotransmitter eventually releasedFacilitation=> increase in the amount of neurotransmitter released.