Sharp waves and ripples

Sharp waves and ripples (SPW-R), also called sharp wave ripples (SWR), are oscillatory patterns produced by extremely synchronized activity of neurons in the mammalian hippocampus and neighboring regions which occur spontaneously in idle waking states or during NREM sleep.

They are composed of large amplitude sharp waves in local field potential and produced by thousands of neurons firing together within a 30–100 ms window.

[3] György Buzsáki and his collaborators studied and characterized SWRs in further detail and described their physiological functions and role in different states of the animal.

[3][5] These patterns are large amplitude, aperiodic recurrent oscillations occurring in the apical dendritic layer of the CA1 regions of the hippocampus.

Studies have shown that elimination of SWRs by electrical stimulation interfered with the ability of rats to recall spatial memories.

Emergence of these self-organized hippocampal events are dependent on interactions between pyramidal cells and different types of the interneurons in this circuit.

Pyramidal cells of CA3 and CA1 are important in generating these waves, and they affect the subiculum, parasubiculum, entorhinal cortex, and ultimately neurons of the neocortex.

[4] Sparse firing of CA1 pyramidal cells and in-phase inhibition from the activated interneurons, give rise to high frequency (200 Hz) network oscillations, which are the ripples.

Sharp waves and associated ripples are present in the mammalian brains of the species that have been investigated for this purpose, including mice, rats, rabbits, monkeys and humans.

Based on the research findings about SWRs, in 1989 an influential two-stage model of memory proposed by Buzsáki, that subsequent evidences supported it.

Acquisition occurs by theta and gamma waves activating a neuronal pathway for initial formation of the memory.

Stronger excitation from sharp waves results in ripple oscillations, whereas weaker stimulations generate fast gamma patterns.

These fast ripples are field potentials of hypersynchronous bursting of excitatory neurons pyramidal cells at frequencies between 250 and 600 Hz.