SLIDE 6 6
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Properties of Place Cells
Position of place cells is influenced by visual cues, and by vestibular cues. According to O’Keefe and Nadel (1978). The hippocampus, with its place cells, is the site for the cognitive map (although there is no evidence that the place cells are mapped in any way).
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Place cells can fire in anticipation of a turn in a given maze.
prior to a left turn trial, cell fires very little Prior to right turn trial, vigorous firing. Rat is trained to turn left, and then turn right
runs through maze. From experiments of Howard Eichenbaum.
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Head direction cells
Navigation requires both a ‘map’
- f space and a compass to tell
direction. Head direction specific cells discovered James Ranck (1984) in postsubiciculum. Cells fire when head is in fixed direction, both in standard and in novel environments.
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Synaptic plasticity in the Hippocampus
Stimulation at any one of the three excitatory connections shows plasticity (Bliss and Lømo, 1973) Brief, high frequency burst causes increased EPSP. Can last for hours. Long Term Potentiation (LTP).
Stimulation of Schaffer collaterals, record from CA1 neuron in the hippocampus.
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LTP Characteristics
Stimulation of just a few fibers does not generate an
- LTP. Instead, different inputs
must cooperate to get the effect. Here, just a weak input is given shocks. No LTP. Similar stimulation of strong input gives LTP. Specificity: Stong stimulation at one site produces LTP there, but there is no LTP at a different site. The effect is plasticity at the post synaptic site. If a weak input arrives at the same time as a strong input, there will be association between the two. LTP will be produced at the weak input.
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LTP mediated by NMDA type Glutamate Receptor
The NMDA receptor binds to glutamate,
for Calcium ions. In the absence
depolarization, Mg++ ions plug up the NMDA receptor, not much of a response
