Neurons, the basic building block of the nervous system, are integral to the working of our human brains, as well as the brains of all other animals.
If that is the case, why do animals have such varying levels and types of behavior? Why do humans seem to have greater "intellectual" abilities than squirrels? Is it at all related to amounts of neurons in the nervous system??
There are two methods for thinking about neuron density within the brain. The first, most basic method is to take the ratio of volume of gray matter and volume of body of nerve cells. According to Kuhlenbeck, the average value of this ratio for the human cerebral cortex is 27, meaning that for each volume of nerve cells there are 27 volumes of other substance. The latter may include glial cells, intercellular space, vascular components, etcetera.
A second method as food for thought is looking at nerve activity. Macphail suggests that in mammalian cortex in general, the number of neurons per unit volume (i.e. neuron density) declines with increases in brain size. However, as neuron density declines with increasing brain size, so too does neural connectivity increase. In other words, the activity of a neuron may be proportional to the length of its dendritic tree, and thus activity per unit volume of brain is independent of a species' brain size.
How do we make sense of all this? There seem to be two main limitations to this discussion. First, the independence of neuronal activity and brain size is assumed from the linear relationship between dendritic tree length and activity. However, I could not find asolute evidence to support or refute this assumption. Second, the data for both density and connectivity were obtained from mammalian cortex, so we don't know if it would hold true for other types of vertebrates.
In other words, it is probable that the volume of nerve cells may be relatively irrelevant in a discussion of intelligence.