A memory does not reside in a small brain area. Rather, it is represented by a large population of neurons distributed over the brain
(Herry and Johansen, 2014). The neurons involved in the storage of long-lasting memories are referred to as
"memory engram cells". Several lines of evidence suggest that the engram cells could be spiny neurons (Figure 7-1)
located in the cerebral cortex, hippocampus, basolateral amygdala, and striatum.
- Spiny neurons are characterized by a large number of dendritic branches and spines, well-suited for the storage of memory.
- During deep sleep (also known as "slow wave sleep"), the above mentioned brain areas exhibit slow oscillations between UP and DOWN states
(Figure 7-2) which have been demonstrated to orchestrate memory consolidation (Mölle and Born, 2011).
- The spiny neurons may produce NMDA spikes which play a pivotal role in the activation of engram cells.
- Synaptogenesis is crucial for the formation of enduring memory. Learning-induced synaptogenesis has been reported in these memory storage areas
(Restivo et al., 2009;
Xu et al., 2009;
Fu et al., 2012;
Mahmmoud et al., 2015;
Bello-Medina et al., 2016).
Figure 7-1. Examples of spiny neurons. The upper image is a camera lucida drawing, while the lower image is a conceptual representation
of the dendritic tree. The granule cells of the dentate gyrus (not shown) may also be considered as spiny neurons.
[Source: Oikonomou et al., 2014]
Not all spiny neurons are involved in memory storage. For instance, the stellate cells of entorhinal cortex are unlikely to be the memory engram cells because they are highly excitable
(Alonso and Klink, 1993). The engram cells should be prone to silence, encoding
Author: Frank Lee
First Published: January, 2018
Last updated: April, 2018