You seem to be mixing models.
Action potentials are considered an all or none phenomena, but the release
of neurotransmitter is certainly a graded reponse. The only function of
action potentials is to drive axonal currents (Iaxon) which ultimately
depolarize the axon terminals leading to a graded release of
neurotransmitter. There are several models describing graded release on NT
but all share several common features:
1. Neurotransmitters are contained in vesicles and quantal release
vesicles results in miniature post synaptic potentials.
2. A voltage dependent calcium influx (or the release of calcium for
internal stores) is critical to exocytosis of NT.
3. The number of NT vesicles exocytosed is proportional to the magnitude of
the depolarization at the axon terminal.
4. Spike frequency correlates with the degree of depolarization in the
axon terminal and the amount of NT released.
The variations in models attempt to explain presynaptic facilitation and
inhibition, but the essential components are based on GRADED RELEASE OF NT.
What may be confusing the issue is the observation that non-spiking neurons
NT release is frequently described as "graded" as opposed to spike driven
NT release. At the level of the synaptic terminal both are graded
responses regardless of the driving force for Iaxon.
At 12:43 PM +0100 10/19/97, Brian Bj=AFrn wrote:
>Lars Thomsen wrote:
>>> Brian Bj=F8rn wrote in message <344598C0.875E1A20 at mdb.ku.dk>...
>>>> >> If it is electro/chemical what sort of levels. I assume the greater t=
>> >> level the greater the reaction?
>> >No, not really. Action potentials are all-or-none responses: If a certai=
>> >treshold level is reached an action potential with a distinct shape
>> >(amplitude and duration) is fired. The transmitter release from one sing=
>> >nerve terminal is therefore also an all-or-none response.
>>>> A little correction here...
>> Gradually changing the frequency of action potential will give a gradual
>> release of neurotransmitters. So the neurotransmitter realease is not an
>> all-or-none event. Furthermore, the post-synaptic membrane potential is
>> strongly modulated (in many systems e.g. by GABA)
>>I agree on that, but if you look at an axon where there is no change in
>modulation and no change in action potential frequency, I still believe
>makes sense to claim that neurotransmitter release is an all-or-none
>of course there will be an increased (frequency of) transmitter release if
>is an increased AP frequency.
Comparative Animal Physiologist
Division of Sciences and Mathematics
University of the Virgin Islands
St. Thomas, USVI 00802
rhall at uvi.edu