Head direction cells efference copy") preparing the network for an impending head turn. HD cells continue to fire in an organized manner during sleep, as if animals were awake. However, instead of always pointing toward the same direction—the animals are asleep and thus immobile—the neuronal "compass needle" moves constantly. In particular, during rapid eye movement sleep, a brain state rich in dreaming activity in humans and whose electrical activity is virtually indistinguishable from the waking brain, this directional signal moves as if the animal is awake: that is, HD neurons are sequentially activated, and the individual neurons representing a common direction during wake
Nanoparticles for drug delivery to the brain stabilizing the nanoparticles and preventing the nanoparticles from aggregation. Metallic nanoparticles typically cross the BBB via transcytosis. Nanoparticle delivery through the BBB can be increased by introducing peptide conjugates to improve permeability to the central nervous system. For instance, recent studies have shown an improvement in gold nanoparticle delivery efficiency by conjugating a peptide that binds to the transferrin receptors expressed in brain endothelial cells. Also, solid lipid nanoparticles (SLNs) are lipid nanoparticles with a solid interior as shown in the diagram on the right. SLNs can be made by replacing the liquid lipid oil used in the emulsion process