Hebbian synapses and spike-timing-dependent plasticity

Assignment

In a history of neuroscience article, H. Sebastian Seung wrote “In 1949, Donald Hebb predicted a form of synaptic plasticity driven by temporal contiguity of pre- and postsynaptic activity. This prediction was verified decades later with the discovery of long-term potentiation, securing Hebb’s place in the scientific pantheon.” 

Let us assume then that the persistence or repetition of a reverberatory activity (or “trace”) tends to induce lasting cellular changes that add to its stability. The assumption can be precisely stated as follows: When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s effi- ciency, as one of the cells firing B, is increased.

Donald Hebb

This simple principle was paraphrased as “Neurons that fire together wire together” by Carla Shatz (not Hebb himself). 

Later in the article, after the quotation shown above, Seung continues, “I have emphasized the first sentence here, because it is very important, though not so well-known. It hypothesized a specific function for the Hebbian synapse: the conversion of short-term into long-term memory by stabilization of reverberatory activity patterns. Once such an activity pattern was stored in synaptic connections, thereafter it could be recalled repeatedly by excitation from sensory neurons, or from other reverberatory activity patterns.”

When neuroscience majors think of synaptic plasticity, usually the first thing that comes to mind is long-term potentiation and depression (see video below).

But even putting its role learning and memory to the side, synaptic plasticity is of tremendous import to neural computations.

Neurons are often considered to be the computational engines of the brain, with synapses acting solely as conveyers of information.  But the diverse types of synaptic plasticity and the range of timescales over which they operate suggest that synapses have a more active role in information processing.  Long-term changes in the transmission properties of synapses provide a physiological substrate for learning and memory, whereas short-term changes support a variety of computations.  By expressing several forms of synaptic plasticity, a single neuron can convey an array of different signals to the neural circuit in which it operates.

Abbott and Regehr 2004

A backpropagating action potential (AP) could also represent a global signal to the dendritic arbor that reports output activity to sites of synaptic input and induces localised long-term changes in efficacy of excitatory synapses. The induction of such changes depends on correlated pre- and postsynaptic AP activity during a relatively narrow time window on the order of about 100 ms. Depending on the precise order of occurrence of the axonal AP in the presynaptic neurone and the backpropagating AP in the dendrites of a postsynaptic cell, synaptic efficacy may either increase or decrease after repeated epochs of such correlated activity. Changes in synaptic efficacy induced by repeated epochs of near coincident pre- and postsynaptic APs have been termed spike time dependent plasticity (STDP). The long-term changes in synaptic efficacy are often dependent on increased dendritic calcium influx through NMDA receptors, suggesting that backpropagating APs are part of a mechanism that controls the plasticity of synapses through metabolic cascades that are driven by a brief rise in intracellular calcium concentration.

Waters et al. (2005)

Further Reading