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Chapter 11-12

CSB332H1 Chapter Notes - Chapter 11-12: Chemical Synapse

Cell and Systems Biology
Course Code
Melanie Woodin

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CSB332H1S Ch.11 -12 Notes (related to lecture figures)
Direct chemical transmission via ionotropic recs
Membrane pot changes w/i 1ms
Slower indirect chemical transmission via
metabotropic recs
At excitatory synapses, cation channels open
At inhibitory synapses, anion channels opens
membrane pot stays (-) w.r.t. threshold
Direction of current flow depends on the permeant
ion’s conc & electrochem grads
Electrical transmission at synapses specialized for
very fast reflex responses & in mammalian CNS to
coordinate nerve cell activity
A: Electrical synapse – direct current flow btwn cells
thru intercellular connexons (cluster to form gap
B: Chemical synapse – depolarization of presynaptic
nerve terminal triggers release of chemical
neurotransmitters that open ion channel recs on the
postsynaptic membrane excitation or inhibition
Predominant method in vertebrate CNS & PNS
A: Voltage clamp recording scheme; using 2
microelectrodes to voltage clamp the end-plate
region of a frog muscle fibre (1 records membrane
pot Vm , 1 injects current to clamp at desired pot)
Nerve then stimulated ACh or Ach applied
B: Synaptic currents recorded at membrane pots
btwn -120 & +38mV
When muscle/end-plate membrane pot clamped
below 0mV synaptic current flows into muscle;
would depolarize the muscle if wasn’t voltage
At more (-) pots higher end-plate current
When end-plate pot clamped above 0mV
current flows out of muscle
Depolarization eventually causes current
dir’n to reverse
C: End-plate current as a f’n of membrane pot
Nearly linear
Reversal pot Vr close to 0mV membrane pot
Found that changing extracellular concs of cations
changed reversal pot, no effect from changing
extracellular Cl
concluded ACh increased general cation
In CNS, most excitatory synapses on dendrites; pots
spread down to soma to axon hillock
Pots integrated in dendrites or soma, 1 is too small to
initiate act pot on own
In many neurons (ex. hippocampal & cortical
pyramidals, cerebellar Purkinjes) – excitatory
synapses on dendritic spines
Spines play special role in long-term changes of
synaptic activity & brain f’n
A: Dendrites labeled w red, counterstained for GFP-
labeled, actin-binding prot (concentrated in dendritic
B: Electron micrographs of vesicle-filled synaptic
boutons contacting spines; arrows mark pts of pre- &
postsynaptic membrane adhesion
Excitation opening postsynaptic membrane
channels whose reversal pot is (+) to threshold
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