Neurobio lecture 6: Electrical vs Synaptic Transmission

Cajal and neuron doctrine

proved that neurons are individual autonomous tissues and not in a reticular by using the Golgi stain

types of neurons

chemical vs electrical

loewi and vagusstoff aka ach

• stimulates vagus nerve caused heart to slow down (vagusstoff)

• if due to signal if you move to box 2 and the heart there also slows down which is does it means that it has to be due to chemical transmission

Eccles and central neurotransmission

showed that what Loewi showed in the CNS. He originally wanted to prove electrical transmission, but turns out he was wrong. If electrical we can still have the same current map. A depolarizing current can’t produce a hyperpolarizing current

electrical transmission

direct

through specialized organelles called GAP junctions

gap junctions are mediated by

connexons

connexons

two hexamers,

protein connexin, one on each of two connected cells

makes a direct pore

pre synaptic voltage and NT release was found through

the giant squid axon

• able to show that neurotransmission is voltage dependent by plugging in electrodes in pre and post voltage clamp. Induced TTX and saw less of an AP present

Chemical transmission always has what feature in comparison to electrical transmission

synaptic delay. this mechanism is not due to diffusion but is dependent on calcium release .

Different Ca experiments and their effect on synaptic transmission

• only nerve stimulation= no response

• calcium before nerve stimulus= induces a response

• nerve stimulation and then calcium= no response

CaV

voltage inactivating channel and has an inward current which will produce an outward post synaptic potential

24 transmembrane domains

4 subdomains

synaptic delay and Ca2+ currents

before the current clamp reaches the Eca, there is a snaptic delay and a huge voltage. As we reach Eca the delay becomes less

testing for calcium

use DM nitrophen, and flash a light and release calcium and we see a big inward current!

calcium affects the signaling of

multiple proteins: PKC, G proteins, SNARES, CAMK II

An example of C terminal facilitation AND inactivation via the calcium binding protein calmodulin

How does the calmoudin channel work

CBD is in the open again, as more Ca2+ binds in it moves to a facilitated position and then it moves to inactivation via carboxyl inactivation

evoked vs spontaneous release

• evoked implies a depolarization which results in a calcium spike and is multiquantual

• spontaneous release of individual quanta is independent of action potentials, depolarization, and calcium

evoked release

triggered by APs

blocked b y TTX

voltage, calcium dependent

spontaneous

not triggered by action potentials

not blocked by TTX

voltage calcium independent shaped like evoked

shaped like evoked

very small

current clamps can result in

EPSP, EPP, IPSP

voltage clamps results in

EPSC, EPC, IPSC

What are minis?

minis could be the effects of multimolecular packets of transmitter binding to many receptors

if minis are due to action of individual ACh molecules

we should see an increase in EPSC frequency but not in amplitude. more ACh molecules activate individual channels if we inhibit AChase

if mini are due to multipocket packets of ACh

we increase the amplitude but not the frequency! which is what we see.

m

number of quanta

q

quantal size

evoked EPSP size

q x m

method of failure

predicts EPP size by doing observed/predicted size

why is release is quantal

vesicular

freeze slammer of Heuser and Reese

• has an NMJ on plunger,

• Drop plunger into liquid nitrogen to flash freeze after stimulating the nerve. Then you are freezing when a vesicle is getting released

picture of electrical and chemical transmission

chemical vs electrical transmission

synaptic delay for chemical, not electrical

bidirectional transmission for most electrical

current injection in cell 1 affects cell 2

injected dye passes from cell 1 to cell 2

“Sign flip” indicates chemical transmission

Electrical NT only does one thing: pass a smaller copy

electrical transmission

to measure the voltage passing through Cell 1 and Cell 2

V2= V1 (R2/ R2 +Rc)

check vesicles

use the dye FM1-43

• lipophilic dye

• labeled synaptic vesicles, so it becomes labeled. inside vesicle maintains the dye

NSF and alpha SNAP were isolated how

dividing cytoplasm and add back fractions

Trick concept

NSF does not provide the energy through ATP!

SNAREs (SNAP receptors)

synaptobrevin

syntaxin

SNAP 25

• SNARE motif that are composed of alpha helices that wrap around each other

Exocytosis

• regulated release of NT is a specialized form of exocytosis, which takes place in numerous cell types and in multiple cell compartments.

• The same 3 families of proteins mediate these different forms of vesicle fusion

• highly conserved across cell types, cellular compartments, and species

Clostridial toxins

bloc neurotranmission by affecting the snare proteins

Reduction of syntaxin or synaptobrevin

they did an experiment where they blocked it with a null and we see less of an inward current but that is rescued by when we dump in the NT.

Ca2+ release is

specialized form of exocytosis.

• Ca2+ of NT is rapid when Ca2+ is elevated and when Ca2+ is low

• synaptotagmin and complexin act as a Ca2+ sensitive brake holding back fusion when calcium is low but allowing it to proceed when high

elimination of synaptotamin

increases rate of spontaneous release and decreases envoked release

steps of vesicle fusion in neurons

• docking occurs where Rab 3 and Rab 27 recruit munc13. syntaxin is kept in closed position by munc18

• munc 13 dissociates from munc18 leavin syntaxin open.

• snare protein form coiled coil helix and bring vesicle and membrane close together

• arrested fusion by synaptotagmin until there are high enough ca to remove it

• SNAP and NSF come in to facilitated the coiled coil complexes to break apart SNARE proteins via ATP

in hair cells what is the calcium brake or sensor

oterfilin

oterfilin (6) and synaptotagmin (2)

also interact with domains II and II

• synaptotagmin binds with CaV2

• oterfilin binds with CaV1

• helps target vesicles to microdomains