for in that sleep, what dreams may come

During sleep, the body rests, and does not move much, particularly during a dream. If we were able to move during a dream, during a dream of a swimming pool we may just jump out of the window. The nervous system has a mechanism in place to make sure that we do not enact our dreams.

The striatum, the structure in charge of initiating movement, has two types of output neurones: these are the gabaergic medium-spiny neurones. The resting membrane potential of these neurones has the interesting ability to alternate between two states; one close to threshold where the cell can be excited (“up”), and another hypepolarised state (“down”). Wilson (2005), in a work done on cholinergic neurones, proposed that this biestable state occurs by the activity of a hyperpolarising activating current (HCN), and an inward-rectifier potassium current (Kir).

The transition between “up” and “down” states has also been found in vivo. But it is difficult to compare the result in vivo with those done in brain slices because the experiment in vivo are done under anaesthesia. It is know that certain anaesthetics can modify the HCN channels (Chen et al., 2005).

Mahon and coworkers (2006) managed to train animals to stay quiet in the stereotactic apparatus to perform intracellular recording. Brain surgery is not painful, just remember the Penfield experiments where he stimulated brain regions of an awake woman and asked her how she felt. The animals of Mahon could even sleep in the stereotactic apparatus. There is when things became interesting. He found that during wakefulness the cells almost never go to the down state. During the slow-wave sleep, the cells go frequently to the “down” state. And, during REM sleep, the cells stay always in the “down” state. Unfortunately, Mahon did not fill the cells and reconstructed the morphology to make sure that those were indeed medium-spiny neurones and not other cells. But, in spite of these technicalities, these results mean that during REM sleep the medium-spiny neurons seldom can be excited. In this way, during REM sleep, the globus palidus is kept active and the thalamus inhibited. During REM sleep thus no movement can be initiated, for you never know, in that sleep, what dream may come.