Introduction
This page is the first part of a study of the article
Adult neurogenesis: integrating theories and separating functions by James B. Aimone, Wei Deng, and Fred H. Gage
Context
During pregnancy and childhood, the brain develops by creating new neurons. However, this stops once it has reached adulthood.
This is except for two regions.
The sub ventricular zone, which creates new neurons that are then moved to the olfactory bulb.
Ventricle
Areas in the brain, mostly in the midbrain and brainstem areas where Cerebrospinal fluids are stored.
Olfactory bulb
A brain region near the eyes that sends olfactory information ( sense of smell ) to the amygdala and the hippocampus.
This strong connection is what makes the smell of sense one of the most powerful senses regarding of memory.
The Dentate gyrus of the hippocampus.
Here, local progenitor cells produce new neurons which develop to become excitatory granule cells.
Progenitor cell
Cells that differentiate to become a specialised cell.
Granule cell
The most abundant type of neuron in the brain found in the cerebellum, the dentate gyrus and more areas.
In the dentate gyrus, they are the principal projection neurons.
This, in the hippocampus, is called neurogenesis.
Neurogenesis
Neurogenesis is the creation of new neurons. In this context, we talk of adult neurogenesis, which is the creation of new neurons in an adult brain.
It is still something that we don’t understand much of. However, it is believed to be involved in learning and memory as it occurs in the hippocampus.
Several studies suggest this as they have found rates of neurogenesis to correlate with performance of hippocampus dependent behaviour and activities that improve learning and memory have been found to increase neurogenesis.
The opposite has also been found. Stress, aging and other diseases which all impair memory also decrease neurogenesis rates.
The Dentate Gyrus
This area where the neurogenesis occurs is responsible for pattern separation of information coming from the cortices to the hippocampus.
Pattern separation
The process done by the dentate gyrus. When the dentate gyrus represents an event, it uses several neurons.
Pattern separation makes sure that similar events aren’t represented with the same neurons and that the are as different as possible.
The distinction is more between events than features.
Pattern separation prevents interference of two different memories on each other.
Information of a new event is sent from the entorhinal cortex, through the perforant path to the different areas of the hippocampus.
Each granule cell in the dentate gyrus receives information from thousands of entorhinal cortex neurons.
However, they need a strong signal to be activated meaning only the specific ones receiving the strong signals will send it on and induce an action potential in CA3 neurons.
Action potential
A rapid increase in membrane voltage at which the neuron will be activated, pass on the signal and then get back to its normal voltage.
The increase in voltage is called depolarisation and the act of passing on the signal is called to fire.
CA3
A region of the cornu Ammoni of the hippocampus.
It is involved in memories by forming associations during encoding and reassembling during recollection.
The signals from the granule cells are strengthened by afferent mossy fiber projections. The signal is strengthened to the point that input from one granule cell is enough activate the CA3 neuron.
Mossy fiber projections
The unmyelinated axons of the dentate gyrus that output to mossy hilus cells and pyramidal CA3 cells.
They form powerful excitatory glutamatergic synapses, sometimes called “detonator” synapses.
They are important for memory and learning
The other granule cells that were only weakly activated have an effect on CA3 interneurons and increase the inhibition of the CA3 region.
This makes sure that only the really strong signals, which are supposed to be the correct ones, will be activated.
This will usually activate the bi-directional synapses connecting the CA3 and the entorhinal cortex where the original information came through creating a neural network of that memory.
Next Part : How neurogenesis is involved in pattern separation
Part 3 : The maturation after neurogenesis
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