Memory as Change of Synaptic Structure and Efficacy

A complete discussion of neuronal function and synaptic transmission of information is beyond the scope of this article. Briefly, a synapse is a functional juxtaposition of two or more neurons (Fig. 4). When a neuron is stimulated to a sufficient degree, chemicals known as neurotransmitters are released from its axon terminal into the microscopic space that separates it from its neighboring neuron. The presence of neuro transmitters within this region produces characteristic changes in the membranes of neighboring neurons.

Each neuron in the brain interconnects with many other neurons in this fashion, forming a series of networks and feedback loops. In the first half of the 20th century, D. O. Hebb proposed that psychologically important events such as memory were manifestations of the flow of activity within a given network of neurons that were acting together as a single unit. He suggested that when an event or experience caused a set of neurons to be excited together, the synapses involved in that pathway became functionally connected.

Although some of the specific neuronal mechanisms that Hebb proposed were not supported by subsequent research, the basic hypothesis of memory as structural

Synaptic Memory
Figure 4 Illustration of the basic structure of neurons and neuronal synapses. Modified from Kandel, E. "Principles of Neural Science,'' 3rd ed. (1992), The McGraw-Hill Companies. Reproduced with permission of The McGraw-Hill Companies.

and functional change involving neuronal activity and synaptic transmission has endured. Research has shown that when animals are trained to perform specific tasks or are exposed to enriched environments, new synapses grow and preexisting synaptic connections become better developed. Synaptic change in response to new learning is observed in both young and adult animals and can occur subsequent to a single learning experience.

B. Neuronal Processes Underlying Short- and Long-Term Memory

A distinction can be made between the neuronal processes underlying short- versus long-term memory. Although temporary changes in the dynamics of neuronal functioning can maintain information for up to several minutes, more permanent structural changes are believed to be necessary to retain information for days or longer.

1. Neuronal Processes Underlying Short-Term Memory

Studies show that a neuron's function can be modified by intense activity. High-frequency stimulation by a presynaptic neuron, for example, tends to increase the responsiveness and efficiency of postsynaptic membranes, a phenomenon known as potentiation. Some neurons can demonstrate increased responsiveness lasting for several minutes to more than 1 hr after active stimulation has ceased. This temporary increase in synaptic effectiveness is known as posttetanic potentiation (PTP) and reflects one possible way that recently learned information can be remembered. PTP may cause neurons activated during learning to remain active for a brief time after learning has ceased, thus allowing that information to remain available.

Another possible mechanism by which neuronal activity associated with recently learned information may be maintained for a brief period of time is by means of a feedback loop. Specifically, excitatory input from exposure to information could theoretically be maintained after active input ceases if neurons within a closed loop excite each other (Fig. 5). Excitatory neuronal feedback systems such as these are known as reverberatory circuits.

The amount of time that elapses before information in short-term memory degrades is believed to be a function of PTP strength and/or the level of neuronal excitement in reverberatory circuits caused by the

Figure 5 Schematic of a reverberatory circuit. Neuronal responses are prolonged by reexcitation of excitatory neurons.

Figure 5 Schematic of a reverberatory circuit. Neuronal responses are prolonged by reexcitation of excitatory neurons.

learning experience. More intense changes in these neuronal dynamics are typically associated with more persistent memory, whereas weaker changes are associated with more rapid forgetting. Are short- and long-term memories, then, different ends of a single graded continuum of neuronal dynamics, with long term-memory reflecting a chronic state of persisting neuronal activity?

If both types of memory relied on dynamic mechanisms such as PTP and reverberatory circuits, then interrupting those dynamics should interfere with both short- and long-term memory. Studies of patients undergoing electroconvulsive therapy (ECT), however, show that this is not the case. ECT is a treatment for medically refractory depression that works by delivering low-voltage electric current to the brain. Neuronal activity in the brain is briefly ''short-circuited'' by this procedure, thus interfering with all electrical activity, including PTP and activity within reverberatory circuits. Following recovery from ECT, patients demonstrate memory loss for information immediately preceding treatment; however, memories in long-term storage remain intact.

2. Neuronal Processes Underlying Long-Term Memory

The ability to encode information into a more permanent long-term storage system is believed to be a function of long-term potentiation (LTP). LTP is similar to PTP in that high-intensity stimulation increases the effectiveness of the neuronal synapse. LTP is much more powerful and longer lasting than PTP, however, and it cannot be produced by activation of only a single presynaptic neuron in a single pathway. Instead, a minimum number of inputs must be present to produce an effect.

Another difference between LTP and PTP is that LTP is associated with the activation of genes that direct the growth and structure of synapses. These neuronal changes are believed to underlie long-term memory formation and are not observed in short-term memory.

It is unlikely that every neuronal synapse is modified by each learned experience, nor is it likely that each memory corresponds to changes in a single neuron in a one-to-one fashion. So where in the brain are long-term memories formed and stored? The first neurons to be discovered that were capable of LTP were found in the hippocampus and surrounding structures, although it has since been recognized that neurons in other brain areas also demonstrate LTP. Similarly, behavioral studies of memory function and dysfunction in humans and animals indicate that a number of different brain regions are important for memory.

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