Question

Explain the concept of Hebbian modification of synapses and why it is important both to brain development and memory formation.

Answer 1

Ans : Hebbian modification of synapses was described with Hebian theory or Hebb's rule by Donald Olding Hebb in 1949 in his book The Organization of Behavior. The theory has 3 postulates. The first postulate describes the process of synaptic modification by stating that "when an axon of a cell A near enough to excite a cell B and repeatedly or persistently take part firing it, some growth process or metabolic change takes place in one or both of the cells such that, A's efficasy, as one of the cells firing B, is increased".

Hebb stated that learning and memory are  neurophysiologival processes that involve 3 stages :

1. Synaptic changes : synatic potentiation
2. Formation of a cell assembly
3. Formation of a phase sequence

He describes these processes using 3 postulates.

First postulate :

• Repeated and persistent stimulation of synapses increases the synaptic efficacy in the post synaptic cell. This results in a phenomenon called Long Term Potentiation (LTP) by which even a weak signal input can cause postsynaptic response as the synapse is potentiated.
• However, to generate LTP, an initial stimulation by a strong signal is required. This is called Cooperativity.
• A weak signal combined with a strong signal also can potentiate a synapse with LTP. This ia called Associativity.
• Once generated, LTP can last for a long time, a long as several days. This is an example of synaptic plasticity as described by Hebb.

Second postulate :

• Such repeated signalling leads to a slow development of a cell assembly containing a group of neurons. This assembly is called the Hebbian Cell Assembly and can function as a unified system in response to subsequent stimulation.

Third postulate :

• A phase cycle is defined as a unique perceptive stimulus that activates the fring of an assembly of cells that had been potentiated previously. When multiple stimuli are perceived, a series of phase cycles occus, called a phase sequence. The temporal overlap due to the transient nature of phase cycles involved in a group of stimuli that has a similar perceptive element, would establish facilitation between them to form a phase sequence. This is how a train of thought is triggered.

These findings from Hebb theory can be used to describe learning and memory mechanisms in hyppocampus. They demostarate how repeated stimulus and synaptic plasticity improves the neurological response action involved in learining and memroy formation. Hebb also proposed that burning of a memory is facilitated by a change in cell to cell connection.