Julian Jack, M.D.
University Laboratory of Physiology
Oxford, United Kingdom
January 22-28, 2002
Fifty Years of Quantal Analysis: What Have We Learnt?
The goal of understanding the workings of the brain is still very far from being reached. One area where significant progress has been made is the study of the detailed mechanism by which the connections between two individual brain cells operate. For the great majority of these connections-called synapses-the signaling is achieved by the incoming message causing the release of a chemical, the-"transmitter." Once released, the transmitter diffuses across a very narrow cleft and binds on to specific molecules, the receptors, on the surface of the target cell. Although there are a variety of mechanisms by which the transmitter receptor complex can influence the target cell, the best studied mechanism is one in which membrane channel opens, allowing the flow of ions into or out of the cell. The net ion flow leads to a direct change in the membrane potential of the cell, affecting its excitability, and hence the likelihood of the incoming message being relayed.
Historically, the understanding of the working of the synapse has been influenced by the technical ease with which these connections can be studied. Much of the early work was performed on the junction, in vertebrates, between the motor nerve and muscle. Subsequently, the connections to the large nerve cells in the spinal cord, giving rise to the motor nerve to muscle, became the "model" synapse. It is only in the last two decades that the techniques have developed so that the most numerous type of brain synapse, that between two nerve cells in regions of the brain such as the cerebral cortex, can be studied in adequate detail. It still remains a problem that much of the work on this last type of synapse has, for reasons of technical tractability, been performed on immature, developing brains rather than those of the adult. The importance of drawing attention to these issues is that the functional task performed at these three types of synapse can be quite different, and hence some features of the operation of the synapse may reflect its specific job (or state of development).
In a series of papers starting 50 years ago, Bernard Katz and his colleagues revealed a then startling feature of the way in which the transmitter was released at the nerve-muscle junction. Instead of an expected mechanism in which variations in the amount of transmitter released was continuous, it was found the amount was in units of approximately 5,000 molecules. Taking a term from physics for an "irreducible minimum," Katz called these units quanta. The underlying structural mechanism turned out to be that the transmitter was concentrated inside small intracellular organelles, called vesicles, and release was achieved by enabling these vesicles to empty their content into the intracellular space, adjacent to the receiving cell's specialized receptors. Quantal analysis refers to the methods used to deduce, for a particular synapse, how many quanta of transmitter are released and what the average effect of each quantum is on the target cell, measured in terms of amount of ions flowing (charge) or change in the membrane potential. The importance of such an analysis is partly related to the insight it gives into the detailed mechanism of the signaling process and partly because it provides an effective method to quantify the extent to which the change in the strength of a synapse (for example, as a memory mechanism) reflects primarily a change in the number of quanta released (presynaptic) or a change in the response of the target cell to each quantum (postsynaptic change).
What have we learnt from the last 50 years? At the synapse between nerve and muscle, under normal conditions, a large number of quanta of transmitter are released producing a very large effect on the membrane potential, which is more than sufficient to activate the muscle. At a more detailed level, the released transmitter in a single quantum has ample opportunity to bind to the adjacent postsynaptic receptors, so that the factor which is dominant in setting the size of the postsynaptic effect is the exact number of molecules shared in the particular vesicle which releases its content. By contrast, when brain cells are the postsynaptic target, there is a restricted number of such receptors so that the limitation on postsynaptic affect is not necessarily the number of molecules, but can be the number of receptors. Why this difference? There is good evidence for the motor nerve cell (motoneurone) and for other brain cells that this feature has been used functionally in two ways. In the motoneurone, where the synapses show no long-term plasticity (i.e., no "memory" mechanism), it is used exclusively to ensure a synaptic "democracy." In nerve cells, unlike muscle, the synapses can end at various distances from the zone where the cell makes the decision to fire, sending the message onwards. Everything else being equal, those synapses further away have less of a voice. However, the postsynaptic cell has found a mechanism, still mysterious, by which it adjusts the number of postsynaptic receptors, with larger numbers of receptors the further the synapse is from the decision point. This adjustment is made in such a way that, quantitatively, the efficacy of the synapse in causing firing of the cell are equal, whatever their location. This result was first reported for the motoneurone 20 years ago and it is an indication of how difficult and slow it has been to achieve progress that the same result has only now been found in a looser form for brain cells in the cortex. Apart from technical difficulties, there is an additional factor in these latter cells; that the synapses also show long-term changes in their efficacy. One of the mechanisms by which this occurs is postsynaptic, as an increase in the number of response receptors. Thus, depending on the "memory-state" of the synapse, there are more or less postsynaptic receptors than would be expected if the only factor operating was the mechanism of synaptic "democracy."
One issue, about which there has been considerable controversy, is whether the only memory mechanism at the synapse is a change in the postsynaptic receptors. There is also evidence that there is an additional, presynaptic mechanism viz. a greater likelihood of releasing more quanta. This evidence was reviewed and it was concluded that it is compelling, not just from quantal analysis but from other methods as well. Thus, in principle, the synapse is modifiable, both presynaptically and postsynaptically. Although there is some preliminary evidence about the rules governing which process predominates, further research is required. What the result does emphasize is that the synapse is a unified device, with memory mechanisms being available for changes in both the amount of transmitter released and the postsynaptic response to a fixed amount of transmitter.