As we discussed during my lecture on sleep, slow wave sleep consists of up and down states with cortical neurons silent during the down (hyperpolarized) states. A research article in Science magazine demonstrated that a relatively small number of neurons fire during these down states. Furthermore, these “delta spikes” (so-called because the neurons fire or ‘spike’ during the down state signaled by a very low frequency delta wave) occur in neurons that were most related to a memory when the animal (rat) was in an awake state. Thus, the down state during slow wave sleep were associated with the selective activation of those neurons most involved in the memory that is presumably being consolidated.

There was an interesting paper in Cell Development that shows how a single neuron the C. elegans worm can provide proprioceptive information that coordinates wiggling behavior, but also can respond to harsh touch and evoke a different behavioral response from the worm. How can a neuron with just a single axon code for these different behaviors?

It appears that the proprioceptive signaling is handled completely by the dendrites using calcium signaling without involving the neurons’s cell body or generating an action potential. The nociceptive response, however, does involve the cell body and the generation of an action potential in the axon.

This is quite a departure from the cell doctrine concepts we discussed in class and adds to the growing list of deviations from the canonical “dendrite – axon hillock – axon” sequence.

One possible explanation for the need to sleep is the clearance of metabolic waste products from the brain that have built up over the course of the waking day. The journal Science had a perspective article on this topic in today’s issue, which reinforces many of the points I raised in lecture.