4-1 3. transmission across synapses a. depolarization of presynaptic cell b. increase in inward gca...

4-1

3. Transmission across synapses

a. Depolarization of presynaptic cell

b. Increase in inward gCa++ via voltage gated Ca++ channels

c. Vesicle migration and exocytosis of neurotransmitters

d. NT diffusion across cleft

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e. NT binding and activation of receptors

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Receptors

NT active without entrance

Transmembrane proteins

Specifically bind NT outside cell

Initiate response in postsynaptic cell

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receptor is channel, e.g., nicotinic ACh

receptor is enzyme

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Each NT has at least one receptor, usually more

ACh = “cholinergic”, nicotinic and muscarinic

Ep/Norep = “adrenergic”, and

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f. Reception of NT results in change in ion permeability of subsynaptic membrane

Leads to a change in the EM of the postsynaptic cell

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f. Reception of NT results in change in ion permeability of subsynaptic membrane

Leads to a change in the EM of the postsynaptic cell

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f. Reception of NT results in change in ion permeability of subsynaptic membrane

Leads to a change in the EM of the postsynaptic cell

4-9Postsynaptic potential

f. Reception of NT results in change in ion permeability of subsynaptic membrane

Leads to a change in the EM of the postsynaptic cell

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PSP is type of “passive potential”

low and variable magnitude

local: decreasing magnitude with distance from synapse

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g. 2 possible responses of the postsynaptic cell

Record from postsynaptic cell as you stimulate a.p.s in presynaptic cell

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(1) “excitatory” synapses

Reception of NT leads to passive depolarization of postsynaptic membrane

NT reception makes the postsynaptic cell more likely to reach threshold

“Excitatory PostSynaptic Potential”

Receptor linked to depolarizing ion channels

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(2) “inhibitory” synapses:

Reception of NT leads to passive hyperpolarization

NT reception makes the postsynaptic cell less likely to reach threshold

“Inhibitory PostSynaptic Potential”

Receptor linked to hyperpolarizing ion channels

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Every synapse programmed chemically as inhibitory or excitatory by channels involved

Most neurons receive both types of input

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h. Degradation of NT, recovery of presynaptic cell

PSPs are brief

Therefore NT action must be terminated quickly

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(1) NT can be enzymatically degraded in cleft

e.g., acetylcholinesterase on subsynaptic membrane

cleaves ACh

A and Ch recycled presynaptically

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(2) NT can be reabsorbed by presynaptic cell or glial cell

e.g., norepinephrine is reabsorbed, metabolized, recycled

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(3) Ca++ is pumped out of presynaptic terminal

Synapse restored to resting condition

Synapse is one-way structure

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4. Action potential generation in postsynaptic cell

How are passive potentials converted to action potentials?

a. Postsynaptic integration:

incoming electrical activity is “summated” to depolarize postsynaptic membrane to threshold

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b. Area of cell called “spike initiation zone” or “axon hillock”

has lower threshold than other areas

due to high density of sensitive sodium channels

c. Summation

Additive effects of multiple passive potentials on total membrane voltage

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Multiple epsps summate to depolarize SIZ to threshold

Ipsps cancel out epsps and prevent SIZ from reaching threshold

Threshold for new postsynaptic cell action potential can only be

reached in response to multiple epsps