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synaptic transmission. Basic Neuroscience NBL 120 (2007). how is the signal transferred?. electrical currents in the presynaptic process induce currents in the postsynaptic process not very efficient………. high. low. how do synapses work?.

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synaptic transmission
synaptic transmission

Basic Neuroscience NBL 120 (2007)

how is the signal transferred
how is the signal transferred?
  • electrical currents in the presynaptic process induce currents in the postsynaptic process
    • not very efficient………..

high

low

how do synapses work
how do synapses work?

“I vividly remember visiting him [Eccles] in his pleasant house with its fine tennis court and beautiful view over Sydney harbor.” (Katz, 1985)

pharmacologists versus physiologists

chemical transmission
chemical transmission
  • a synapse is both anatomically and functionally optimized
    • Ca2+
    • vesicles
    • postsynaptic receptors
    • central synapses are just smaller than the nmj
    • integration
nmj structure anatomy overview
nmj structure - anatomy overview

axon

endplate

boutons

mitochondria

vesicles

active zone

10,000 / m2

synaptic cleft

basement

membrane

junctional

fold

acetylcholine receptors

nmj physiology overview
nmj - physiology overview
  • stimulate motorneuron: muscle contraction
  • record potential change in muscle fiber
  • AP (high safety factor)
origin of the epp
origin of the EPP
  • EPP: passive decay
    • length constant
  • AP: regenerative
nmj actetylcholine receptor

PORE

BINDING

SITE

GATE

nmj - actetylcholine receptor

a-bungarotoxin

Bungarus multicinctus (many-banded krait)

Torpedo californica (pacific electric ray)

acetylcholine receptor channel
acetylcholine receptor channel

EPP

single channel

closed

open

non-selective cation channel

efficiency of the epp
efficiency of the EPP
  • hi-fidelity synaptic transmission
  • design of the perfect receptor
  • high transmitter concentration in the cleft
  • rapid binding to many receptors
  • very fast opening (opening rate 100,000 s-1)
  • 99+% receptors that are bound - open
  • channel closes after ≈ 1 ms
  • agonist unbinds quickly (low affinity)
  • degradation and diffusion
  • receptor recovers without desensitization
mepps quantal hypothesis
mEPPs quantal hypothesis

smallest evoked EPP = spontaneous mEPP

“It has been suggested that the end-plate potential (epp) at a single nerve-muscle junction is built up statistically of small all-or-none units [quanta or discrete packets of transmitter] which are identical in size with the spontaneous ‘miniature epp’s’” (Del Castillo & Katz, 1954)

Fatt and Katz (1952)

normal EPP ≈ 200 quanta or vesicles (quantal content)

presynaptic mechanisms
presynaptic mechanisms
  • object:
  • synchronous + fast release of many vesicles
vesicle cycling
vesicle cycling…….
  • synthesis of transmitter
  • storage of transmitter in vesicles
  • docking+priming of vesicles
  • release (fusion) of vesicles
  • action of transmitter on postsynaptic receptors
  • termination of transmitter action
  • recycling of vesicle membrane (endocytosis)
release

delay?

release…..
  • depolarization and Ca2+ are required
synaptic delay
synaptic delay
  • Ca2+ channels open slowly…
presynaptic ca 2 microdomains
presynaptic Ca2+ microdomains
  • Ca2+ is only high while channels are open

presynaptic

terminal

Llinas et al (1995)

ca 2 channels vesicles
Ca2+ channels / vesicles
  • synaptotagmin (on vesicle): Ca2+ sensor
  • low affinity for Ca2+
  • vesicles must be close to Ca2+ channels
clearance of transmitter
clearance of transmitter
  • acetycholinesterase:
    • 10 molecules ACh per ms (one every 100 s)
  • inhibition prolongs synaptic transmission…..
  • diffusion is very fast

Katz and Miledi (1973)

myesthenia gravis
myesthenia gravis

cholinesterase inhibitor

locations of synapses
locations of synapses

axosomatic

axodendritic

axoaxonic

dendrodendritic

(e.g. inhibition)

(e.g. excitation

spines)

(e.g. presynaptic

inhibition)

(e.g. reciprocal

excitation)

coping with multiple synapses
coping with multiple synapses

how do the multiple inputs combine to determine the output firing pattern of the neuron?

dendritic

integration and other mechanisms

central synapses
central synapses
  • smaller (<1 m) synaptic contact
  • fewer active zones:
    • release few vesicles
    • failures
    • don’t reach AP threshold
inhibition versus excitation
inhibition versus excitation
  • GABA
  • glycine
    • chloride
  • hyperpolarizing?

glutamate

ACh

serotonin

depolarizing

combining excitation and inhibition

excitatory input

excitatory input

inhibitory input

EPSP

EPSP

IPSP

inhibitory

input

threshold

action potential

no action potential

combining excitation and inhibition
mechanism of inhibition
mechanism of inhibition
  • “Shunting inhibition”
  • Inhibitory transmitters (e.g. GABA) open Cl- permeable channels. ECl isalways more negative than AP threshold. Thus, opening up a large amount of inhibitory channels will oppose the depolarzation by any excitatory transmitter/receptor and keep the membrane close to Ecl.
slide30

general rule

ENa

+67

membrane

potential (mV)

  • relationship between: membrane potentialion equilibrium potentials
  • if the membrane becomes more permeable to one ion over other ions then the membrane potential will move towards the equilibrium potential for that ion (basis of AP).

RMP

ECl

-90

EK

-98

temporal summation

action potentials closely spaced in time

threshold

postsynaptic action potential

temporal summation

action potentials separated in time

threshold

no postsynaptic action potential

spatial summation

threshold

spatial summation

membrane time constant

dual synaptic components
dual synaptic components…..
  • Wait for lecture on synaptic plasticity……
terminating the synaptic signal
terminating the synaptic signal

just how much glutamate is around?

one role of glia at cns synapses
one role of glia at CNS synapses
  • transmitter transporters
  • re-uptake
  • prevent excitotoxicity
synaptic summary
synaptic summary
  • neuromuscular junction
  • fast synaptic transmission - highly efficient
    • Ca2+-dependent release of vesicles (quanta)
    • postsynaptic ligand-gated ion channels
  • synaptic integration in the CNS
    • synapse location
    • inhibition versus excitation
    • “shunting” inhibition
    • temporal versus spatial summation