In this post, I want to discuss the reticular nucleus of the brain and its potential relation to meditation as outlined in James Austin’s seminal work Zen and the Brain.
The reticular nucleus is best described as a thin sheet of nerve cells that surrounds the thalamus, like a cap. Here is a picture of the thalamus to give you an idea of what exactly it is encapsulating:
The reticular nucleus contains large GABA nerve cells, which are inhibitory in nature. It receives input from a network of long dendrites that cover the outer surface of the thalamus. This network monitors the information flow between the thalamus and the cortex. When too many impulses flow in, the GABA cells fire and serve to inhibit the information that would otherwise be going to the “sensory relay cells” of the thalamus.
Most of the nerve cells of the reticular nucleus fire have average firing rate between 5 and thirty-four times a second. Some, however, can fire much faster, at rates between 200 to 350 times a second. After these bursts of inhibition shut down the thamalic gate, they can pause for as long as three to four seconds, which is a relatively long time in brain terms. Because of variability of firing rates and the potential for long pauses , it is believed that these GABA cells of the reticular nucleus don’t just “shut down” the thalamus in a crude fashion, but rather, regulate its activity through complex, rhythmic oscillations of hyperpolarization and depolarization.
Okay, so what is the relation between all of this and meditation? Well, in his book, James Austin proposes two hypothesis:
The first is that the brain could shut off sensation during internal absorption by prolonging its burst phase of GABA activity. The second is that prolonging the pause phase could help sponsor the entry into consciousness of several kinds of “quickenings.” At the sensate end of the spectrum, these events could include visual episodes perceived as blinding white lights. At the mental end, brief “illuminations” of other kinds might be added to.
Cortical excitatory states descend to excite the reticular nucleus and block sensation and brainstem excitatory states ascend to inhibit the nucleus and allow more sensate messages to flow through the thalamus into consciousness
…what this thin GABA nucleus seems to be preparing us for is a relatively novel concept: a high-level blockade caused by strong afferent inhibition. Even so, outside of it,-creating it in fact- are other lays of extra excitation, still going on elsewhere.
What this means for meditation is that the prefrontal areas associated with executive control and attention can send down messages to activate the reticular nucleus, blocking sensation. This would result in various sensory modalities dropping out to the point of losing your “reference frame” and subsequently your sense of self would dissipate. Furthermore, when the lower-centres of the brain send up information to the block the inhibitory functions of the nucleus, this would result in an increase of “novel, secondary fluctuations of cognitive functions as high up as in the frontal lobe.”