Effect of the glial envelope on extracellular K(+) diffusion in olfactory glomeruli

In many species, including vertebrates and invertebrates, first-order olfactory neuropils are organized into spherical glomeruli, partially enveloped by glial borders. The effect of this characteristic organization on olfactory information processing is poorly understood. The extracellular concentration of potassium ions ([K(+)]) must rise around olfactory receptor axons in specific glomeruli following odor-induced activation. To explore the time course and magnitude of K(+) accumulation and possible effects of such accumulation on neural activity within and among glomeruli, we developed a theoretical model to simulate the diffusion of K(+) in extracellular spaces of the glomeruli of the moth Manduca sexta. K(+) released by activated axons was assumed to diffuse through the extracellular spaces in glomeruli and the glial borders that surround them. The time-dependent diffusion equations were solved in spherical coordinates using a finite-difference method. The results indicate that the glial envelope forms a significant barrier to the spread of K(+) between neighboring glomeruli, thus reducing the likelihood of cross-talk between glomeruli, and may cause elevation of extracellular [K(+)] to levels that influence neural activity within the activated glomerulus for many seconds. Such effects could enhance olfactory discrimination and sensitivity, respectively.