Evoked responses

Each hippocampal (or cortical) area receives 'afferent' synaptic inputs from other areas. Most afferents are excitatory; stimulating them provides a convenient tool to study the operation of the neuronal circuits involved. The responses evoked in hippocampal neurones typically start with an excitatory postsynaptic potential. If the excitatory postsynaptic potential is strong enough, it will result in an action potential triggered at a low-threshold zone near the cell body, probably a short distance down the axon. The excitatory postsynaptic potential is followed by a fast and a slow inhibitory postsynaptic potential. Both the fast inhibitory and the excitatory postsynaptic potentials are due to ligand-gated channels where the transmitter receptor (GABA A and glutamate a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainic acid or W-methyl-D-aspartate respectively) is part of the same molecular structure as the ion channel (chloride and partially selective cationic respectively). The slow inhibitory postsynaptic potential is due to GABA B receptors which are G protein coupled and use second messengers to open separate potassium channels.

Inhibitory neurones can be triggered both by activity in the principal cells (pyramidal or granule), resulting in recurrent or feedback inhibition, and directly by the incoming afferents, resulting in feed-forward inhibition. The synchrony of the stimulation of the afferent input imposes synchrony on the response with the useful consequence that the extracellular currents generated by the activity of individual pyramidal or granule cells can summate (because the cells are located in tight layers) and produce large 'field potentials' comprising a population excitatory postsynaptic potential, followed by a population spike.

Evoked field potentials are over in 10 to 20 ms and the slowest intracellular components end within a few hundred milliseconds to a second. However, stimulation can elicit much more prolonged responses in the hippocampus. The best known of these is long-term potentiation, in which a brief train of stimuli can result in an increase, lasting hours or days, in the response to a fixed test stimulus. The modest conditioning event and the enduring consequence make long-term potentiation an attractive model of learning and memory, although the evidence that it really is the direct cellular substrate for learning remains circumstantial. (3) It is perhaps more likely that long-term potentiation provides an artificial experimental tool that depends on cellular and molecular mechanisms that may also be involved in learning and/or other plastic changes in synaptic strength.

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