Module 3 Lecture 2: The Phenomenon of Chemical Synaptic Transmission Flashcards
what was the knowledge of Ca2+ dependence before Katz and Miledi did their 1965 experiment
they knew that proper synaptic transmission requires Ca2+
- they didn’t know if it’s working acutely or if it’s just something needed to keep neurons alive
how did Katz & Miledi confirm that Ca2+ is necessary for synaptic transmission in 1965 experiment
removing extracellular Ca2+ retains the presynaptic AP and stimulus artifact, but results in no postsynaptic AP
- saw that more amounts of Ca2+ resulted in a more intense postsynaptic response
what experiment did Katz and Miledi do in 1967 to answer when Ca2+ is necessary?
- methods: recorded directly from the muscle with an intracellular electrode & delivered Ca2+ in a dose-dependent manner to the motor neuron. removed extracellular Ca2+
- results: supplying Ca2+ before AP = postsynaptic response; supplying after AP = no postsynaptic response
methods for Kats & Miledi (1967) squid giant synapse preparation
- 2 electrodes in presynaptic neuron (1 delivers current, the other recording presynaptic voltage)
- 1 postsynaptic electrode that measures voltage
- block Na+ and K+ channels w/ TTX and TEA
results of Katz & Miledi 1967 squid giant synapse prep
no AP, but there is inward Ca2+ and some postsynaptic depolarization that allows you to specifically focus on Ca2+ current & prevent APs
what did adding TTX in a dose dependent way to the squid giant synapse show
while APs cause NT release, it’s not true that APs are required; it seems that depolarization causes NT release
Ca2+ hypothesis
depolarization –> Ca2+ influx –> quantal NT release
results from Llinàs (1981) experiment
- depolarization causes Ca2+, but results are not sufficient to prove it
how did they determine that Ca2+ is both necessary and sufficient for NT release
they directly injected Ca2+ into the presynaptic neuron and saw a spike in postsynaptic membrane potential
- shows NT release
how do calcium chelators affect synaptic transmission
prevents increases in presynaptic intracellular Ca2+ concentration, and blocks synaptic transmission
- does not affect presynaptic AP, but stops postsynaptic AP
how did mEPPs play into the story
Fatt and Katz observed them, but didn’t know what they were
- probably not noise bc they were all the same shape & style as synchronous release, just smaller
- assumed to be due to release of small quantities of Ach due to activity of isolated terminal spots
what did Fatt and Katz propose about mEPPs
that there are several small terminals that all have a probability of release, and that Ca2+ is what increases the probability
del Castillo & Katz (1954) experiment on mEPPs
created a histogram based on magnitude of mEPPs
- followed a Pussand distribution
- got 0 or 1 a lot bc probability of release is very low
Boyd & Martin (1956) statistical contribution
estimated the number of release sites, based on the assumption that endplate potential should equal magnitude * probability
general required processes for chemical synaptic transmission
- synthesis of NT
- release of NT
- recognition of NT
- conversion of the chemical messenger back into electrical signaling
- clearing of the NT to stop the message
which steps of chemical synaptic transmission contribute to the synaptic delay?
- release of NT
- recognition of NT
- conversion of chemical messenger back into electrical signaling
Sabatini & Regehr (1996) contribution to synaptic delay
- new technique to study timing & synaptic delay in rat cerebellum: one dye detects Ca2+, another detects voltage
- compared timing between presynaptic AP and presynaptic Ca2+ current, and observed how that leads to the postsynaptic response
what was surprising about Sabatini & Regehr’s 1996 experiment on synaptic delay
they did not expect that Ca2+ would come in as early as the falling phase
- people expected it to come in during the overshoot
- delay increases with colder temps, there is almost no delay for physiological temp
what causes the first third of the synaptic delay
vesicles are getting fused and starting to release –> diffusion across cleft –> binding to receptors –> changing in receptor conformation –> voltage change in postsynaptic cell
