Synaptic augmentation
Encyclopedia
Augmentation is one of four components of short-term synaptic plasticity
that increases the probability of releasing synaptic vesicles during and after repetitive stimulation such that
when all the other components of enhancement and depression are zero, where
is augmentation at time 
and 0 refers to the baseline response to a single stimulus. The increase in the number of synaptic vesicles that release their transmitter leads to enhancement of the post synaptic response. Augmentation can be differentiated from the other components of enhancement by its kinetics of decay and by pharmacology. Augmentation selectively decays with a time constant of about 7 seconds and its magnitude is enhanced in the presence of barium
. All four components are thought to be associated with or triggered by increases in internal calcium ions
that build up and decay during repetitive stimulation.
During a train of impulses the enhancement of synaptic strength due to the underlying component
that gives rise to augmentation can be described by
where
is the unit impulse function at the time of stimulation, 
is the incremental increase in 
with each impulse, and 
is the rate constant for the loss of 
. During a stimulus train the magnitude of augmentation added by each impulse, a*, can increase during the train such that
where
is the increment added by the first impulse of the train, 
is a constant that determines the increase in 
with each impulse, 
is the stimulation rate, and 
is the duration of stimulation.
Augmentation is differentiated from the three other components of enhancement by its time constant of decay. This is shown in Table 1 where the first and second components of facilitation, F1 and F2, decay with time constants of about 50 and 300 ms, and potentiation
, P, decays with a time constant than ranges from tens of seconds to minutes depending on the duration of stimulation. Also included in the table are two components of depression D1 and D2, along with their associated decay time constants of recovery decay back to normal. Depression at some synapses may arise from depletion of synaptic vesicles available for release. Depression of synaptic vesicle release may mask augmentation because of overlapping time courses. Also included in the table is the fraction change in transmitter release arising from one impulse. A magnitude of 0.8 would increase transmitter release 80%.
†The magnitude of augmentation added by each impulse can increase during the train.
‡The time constant of P can increase with repetitive stimulation.
The balance between various components of enhancement and depression at the mammalian synapse is affected by temperature so that maintenance of the components of enhancement is greatly reduced at temperatures lower than physiological. During repetitive stimulation at 23o C components of depression dominate synaptic release, whereas at 33-38o C synaptic strength increases due to a shift towards components of enhancement.
Synaptic plasticity
In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...
that increases the probability of releasing synaptic vesicles during and after repetitive stimulation such that
when all the other components of enhancement and depression are zero, where
is augmentation at time and 0 refers to the baseline response to a single stimulus. The increase in the number of synaptic vesicles that release their transmitter leads to enhancement of the post synaptic response. Augmentation can be differentiated from the other components of enhancement by its kinetics of decay and by pharmacology. Augmentation selectively decays with a time constant of about 7 seconds and its magnitude is enhanced in the presence of bariumBarium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
. All four components are thought to be associated with or triggered by increases in internal calcium ions
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
that build up and decay during repetitive stimulation.
During a train of impulses the enhancement of synaptic strength due to the underlying component
that gives rise to augmentation can be described bywhere
is the unit impulse function at the time of stimulation, is the incremental increase in with each impulse, and is the rate constant for the loss of . During a stimulus train the magnitude of augmentation added by each impulse, a*, can increase during the train such thatwhere
is the increment added by the first impulse of the train, is a constant that determines the increase in with each impulse, is the stimulation rate, and is the duration of stimulation.Augmentation is differentiated from the three other components of enhancement by its time constant of decay. This is shown in Table 1 where the first and second components of facilitation, F1 and F2, decay with time constants of about 50 and 300 ms, and potentiation
Long-term potentiation
In neuroscience, long-term potentiation is a long-lasting enhancement in signal transmission between two neurons that results from stimulating them synchronously. It is one of several phenomena underlying synaptic plasticity, the ability of chemical synapses to change their strength...
, P, decays with a time constant than ranges from tens of seconds to minutes depending on the duration of stimulation. Also included in the table are two components of depression D1 and D2, along with their associated decay time constants of recovery decay back to normal. Depression at some synapses may arise from depletion of synaptic vesicles available for release. Depression of synaptic vesicle release may mask augmentation because of overlapping time courses. Also included in the table is the fraction change in transmitter release arising from one impulse. A magnitude of 0.8 would increase transmitter release 80%.
| decay constant | magnitude/impulse | |
|---|---|---|
| F1 | 50 ms | 0.8 |
| F2 | 300 ms | 0.12 |
| A | 7 s | 0.01† |
| P | 20 s to minutes‡ | 0.01 |
| D1 | 5 to 6 s | -0.15 |
| D2 | minutes | -0.001 |
†The magnitude of augmentation added by each impulse can increase during the train.
‡The time constant of P can increase with repetitive stimulation.
The balance between various components of enhancement and depression at the mammalian synapse is affected by temperature so that maintenance of the components of enhancement is greatly reduced at temperatures lower than physiological. During repetitive stimulation at 23o C components of depression dominate synaptic release, whereas at 33-38o C synaptic strength increases due to a shift towards components of enhancement.