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Synaptic Plasticity. Synaptic efficacy (strength) is changing with time. Many of these changes are activity-dependent , i.e. the magnitude and direction of change depend on the activity of pre- and post-synaptic neuron. Some of the mechanisms involved:.

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slide1

Synaptic Plasticity

Synaptic efficacy (strength) is changing with time.

Many of these changes are activity-dependent, i.e. the magnitude and direction of change depend on the activity of pre- and post-synaptic neuron.

Some of the mechanisms involved:

- Changes in the amount of neurotransmitter released.

- Biophysical changes in ion channels.

- Morphological alterations of spines or dendritic branches.

- Modulatory action of other transmitters.

- Changes in gene transcription.

- Synaptic loss or sprouting.

slide2

Hebb’s Postulate

“When an axon of cell A is near enough to excite a cell B and repeatedly and persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased.”

Donald Hebb, “Organization of Behavior”, 1949

slide3

Animal Models of Plasticity

Long-Term Potentiation (LTP)

Cross-section of the hippocampus:

Cajal’s drawing

slide4

Animal Models of Plasticity

Brain slice preparation of the hippocampus:

slide5

LTP

Typical LTP experiment: record from cell in hippocampus area CA1 (receives Schaffer collaterals from area CA3). In addition, stimulate two sets of input fibers.

slide6

LTP

Typical LTP experiment: record EPSP’s in CA1 cells (magnitude)

Step 1: weakly stimulate input 1 to establish baseline

Step 2: give strong stimulus (tetanus) in same fibers (arrow)

Step 3: continue weak stimulation to record increased responses

Step 4: throughout, check for responses in control fibers (input 2)

slide7

LTP

LTP is input specific.

LTP is long-lasting (hours, days, weeks).

LTP results when synaptic stimulation coincides with postsynaptic depolarization (achieved by cooperativity of many coactive synapses during tetanus).

The timing of the postsynaptic response relative to the synaptic inputs is critical.

LTP has Hebbian characteristics (“what fires together wires together”, or, in this case, connects together more strongly).

LTP may produce synaptic “sprouting”.

slide8

The NMDA Receptor

  • At the resting potential (postsynaptic neuron), glutamate binds to the NMDA channel, the channel opens, but is “plugged” by a magnesium ion (Mg2+).
  • Depolarization of the postsynaptic membrane relieves the magnesium block and the channel open to allow passage of sodium, potassium and calcium.
slide9

The Associative Nature of LTP

Old(er) view: Associative requirement is mediated by the voltage-dependent characteristics of the NMDA receptor.

New discovery (1994): Active conductances in dendrites mediate back-propagation of AP’s into the dendritic tree.

slide10

Spike-Timing Dependent Plasticity

Basic Idea: Change in synaptic strength depends on the precise temporal difference between pre- and post-synaptic neuronal firing (causality!).

slide11

The Neuron:

Integrator or Coincidence Detector?

Synchronous inputs really matter!

slide12

Data Analysis in Neurophysiology

Spike train

data sets:

Neuron in MT

Colby and

Duhamel, 1991

slide13

Data Analysis in Neurophysiology

Neuron in IT (object selective)

Desimone et al., 1984

slide14

Data Analysis in Neurophysiology

Neurons in V1 (orientation selective)

PSTH (firing rate)

Auto-Correlation

Shift Predictor

Cross-Correlation

Engel et al., 1991

slide15

Neural Coding

Rate coding versus temporal coding

One major mechanism of how neurons encode information is through their firing rate (number of AP’s per second). – Example: orientation selectivity.

Another major mechanism is synchronization (AP’s occurring together in time). – Example: perceptual grouping.

Synchrony could affect other neurons (e.g. through spatial summation – see unit 1).

slide16

Computational Neuroscience

Components of (most) neural models:

- Units and connections

- Inputs and outputs

- Activation function

- Learning rule

slide19

“Why the Mind is in the Head”

“Why is the mind in the head? Because there,

and only there, are hosts of possible

connections to be formed as time and

circumstance demand. Each new connection,

serves to set the stage for others yet to

come and better fitted to adapt us to the

world, for through the cortex pass the

greatest inverse feedbacks whose function

is the purposive life of the human intellect.”

Warren S. McCullogh, Hixon Symposium 1951.