Novel connection between newborn granule neurons and the hippocampal CA2 field

Beginning less than 1 week after their generation, new granule cells extend mossy fiber axons through the hilus toward the CA3 and CA2 subregions.

“Beginning less than 1 week after their generation, new granule cells extend axons, known as mossy fibers, through the hilus toward the CA3 and CA2 subregions of the hippocampus (Llorens-Martín et al., 2015; Zhao et al., 2006).”

Mossy fibers from new granule cells begin to form synapses with excitatory pyramidal cells in CA3 and CA2 and with hilar mossy cells by approximately 2-3 weeks post-mitosis.

“Mossy fibers from new granule cells begin to form synapses with excitatory pyramidal cells in the CA3 and CA2 subregions (Gu et al., 2012; Llorens-Martín et al., 2015; Toni et al., 2008) as well as with hilar mossy cells (Toni et al., 2008) by approximately 2–3 weeks post-mitosis (Figure 1B).”

Physical exercise enhances the survival of new cells, increases the development of their dendrites, and increases the complexity of both their afferent and efferent connections.

“After these new cells are produced and start down the path toward neuronal differentiation, physical exercise enhances their survival, as it increases the development of their dendrites, and the complexity of both their afferent and efferent connections (Ambrogini et al., 2013; Gonçalves et al., 2016b; Llorens-Martín et al., 2015; Snyder et al., 2009b; Zhao et al., 2015) (Table 1).”

Running increases the ratio of inhibitory interneuron versus excitatory mossy cell innervation onto new granule neurons and enhances the number of new axon fibers projecting to the CA3 and CA2 subregions.

“Using viral-based methods, running has been found to increase the ratio of inhibitory interneuron versus excitatory mossy cell innervation onto new granule neurons (Sah et al., 2017; Vivar et al., 2016) as well as to enhance the number of new axon fibers projecting to the CA3 and CA2 subregions (Llorens-Martín et al., 2015).”