In any square millimeter of cortex there are roughly 100,000 neurons, for a total of about 15 billion neurons in the entire cortex.
Neurons are excitable cells that are the functional unit of the nervous system.
These processes include multiple dendrites, and usually a single, longer axon. In a large neuron, the total volume of these processes may be from 100 to 1000 times that of the cell body. The cell bodies are concentrated in areas of gray matter, whereas axons compose the white matter or nerves. Neurons are specialized to react to stimuli received on their dendrites or cell body, to transmit the resulting excitation rapidly to other portions of the cell, including their long axon, and then to pass this signal via their axon to other neurons, muscle cells, and glandular cells. The points of contact between cells, through which this information passes, are called synapses .