Kenzo Le Vaillant

Adult neurogenesis produces new neurons into the dentate gyrus. However, these neurons cannot function like an older neuron and also doesn’t have any connections with other neurons.

They have to develop into the neural circuit of the dentate gyrus from scratch, “do novo”.

There survival isn’t an assured fact either. There are chances that they don’t survive until maturation.

However, it was found that activities that stimulate the hippocampus such as exercise, new environments and learning can increase the chance of survival.

Stage 1 : up to 2 weeks

From their creation to 2 weeks after, the GABA neurotransmitter is extremely important for their survival and growth. The new cells receive mainly GABA inputs that help survival, migration and neurite outgrowth.

Neurite outgrowth

A neurite is any kind of projection from a young neuron. They will then develop to become axons and dendrites.

Neurite outgrowth is the process in which new extensions of the neuron are created.

As a result, mossy fiber axons start to extend.

One thing to note is that in the young cell, the GABA input is actually depolarising because of the high intracellular concentration of chloride ions.

Stage 2 : 2 to 3 week old

The NMDA receptor also becomes important in survival.

During this stage, several growth processes occur such as the formation of dendritic spines and rapid spine growth.

This means that dendrites start to appear and excitatory synapses from the mossy fiber axons to downstream CA3 neurons are created.

Downstream

A signalling pathway in which a response is caused after the activation of the pathway.

These links to other neurons mean the neuron starts to hold a function in the network.

Stage 3 : 19 days to 4 weeks old

The ionic concentration inside the cell changes cause the transition of GABA inputs from being excitatory to inhibitory. Although it is still more excitable and less inhibited than fully mature cells.

At 4 weeks, the density and composition of inward spines starts to develop and finally at 8 weeks, they are approximately mature.

Spine

The part of the axon that is involved in the synaptic transmission. The name comes from the shape of this part.

The number of their synapses becomes stable. But, they still have one difference in that they have a high plasticity.

Emotional states on neurogenesis

Emotional states are found in our everyday lives and have a big influence on memories, behaviours and our brains.

It is logical to think, considering the closeness of the amygdala to the hippocampus, they would have an impact on the growth of these new neurons.

For example, certain anti-depressants can increase the survival of new neurons.

They increase monoamine levels whose signalling regulates the effects on neurogenesis.

Monoamine

The group of neurotransmitters that holds one amino group.

Serotonin, dopamine and noradrenaline are a part of this group.

A pathway called the BDNF-TrkB signalling pathway is involved in the regulation of mood but not only.

BDNF TrkB signaling pathway

The response system of BDNF and its receptor TrkB.

It is involved in activity dependent neuroplasticity and also protects neurons from reactive oxygen species.

It was also found to be involved in survival, dendritic morphogenesis and LTP induction of new neurons.

Morphogenesis

The development of the shape of an organ, tissue, cell.

Part 1 : The dentate gyrus and pattern separation

Part 2 : How neurogenesis is involved in pattern separation

How neurogenesis is involved in pattern separation

Accessible Science

Introduction

This page is the second part of a study of the article

Adult neurogenesis: integrating theories and separating functions by James B. Aimone, Wei Deng, and Fred H. Gage

Computational theories

Theories in neuroscience aren’t always easy to understand as they hold a large amount of variants and combine ideas from many fields. Such as mathematics.

This is one of the reasons why most brain models are made in a computer. The computer also has an advantage of being able to replicate some characteristics of the brain and neurons.

These include, the amount of cells in the brain ( around 86 billion neurons and much more glia cells ) and their digital way of response.

Computational theories of pattern separation and neurogenesis

Computational theories of pattern separation show that only a small amount of neurons in the dentate gyrus are activated for any event.

This decreases the probability that the same neural pattern won’t be assigned to another event.

New neurons, from neurogenesis, may be important in this aspect by increasing the number of patterns the dentate gyrus can create and therefore increase its capacity.

They also aren’t linked to any existing memories, meaning they possibly help create memories of new events.

These new, immature neurons are easily excited and more plastic meaning they are more likely to create a response and be involved in it.

This enables it from integrating the neural network of the dentate gyrus whilst also creating a similarity in certain dentate gyrus outputs called pattern integration.

Although, with the number of mature neurons, this will hardly make a difference in the distinction of events, this will help in finding the relations of different events.

Part 1 : The dentate gyrus and pattern separation

Next Part : The maturation after neurogenesis