Mesopontine median raphe regulates hippocampal ripple oscillation and memory consolidation :: Dong V Wang et al, National Institutes of Health, Baltimore :: www.nature.com/neuro :: Nature Neuroscience, Vol. 18, No. 5, May 2015
Summary and review of the above paper
The consolidation of memories depends on the hippocampus. The hippocamus generates ripple activity, which is a sharp wave oscillation at 100-300 Hz, mainly within the CA1 region of the hippocampus. Sharp-wave associated field oscillations of the hippocampus, referred to as ripples, are thought to be involved in the consolidation of memories. The median raphe region (MnR) is involved in regulating hippocampal ripple activity and memory consolidation. Ripple activity is suppressed when these neurons are active, and is enhanced when these neurons are inhibited.
Studies have shown that place-cells firing in the hippocampus are later replayed in the same sequence, but in a compressed form, and this correlates with the ripple activity. Disruption of ripple activity is seen to impair the ability to learn a task. Ripple activity is associated with inattentive states, notably slow-wave sleep.
Ripple activity is also associated with inhibition of subcortical regions. The subcortical MnR projects to the whole of the hippocampal formation, and regulates theta activity there, thus possibly also regulating learning and memory. Many of the MnR neurons are inactive during sleep. It is thought that the MnR may also regulate memory consolidation through projections to the thalamus and cortex. Memory is thought to depend on interactions between the thalamus, hippocampus and the cortex, notably the medial prefrontal during slow-wave sleep. This involves spindles which are low-frequency oscillations generated by the reticular thalamic neurons, and detected by the medial prefrontal cortex which generate spindles timed to the hippocampal ripples. It is known that MnR neurons project directly to the medial prefrontal cortex.