Excitable Tissue: Nerve

The human central nervous system (CNS) contains about 10¹¹ (100 billion) neurons. It also contains 10-50 times this number of glial cells. It is a complex organ; it has been calculated that 40% of the human genes participate, at least to a degree, in its formation. The neurons, the basic building blocks of the nervous system, have evolved from primitive neuroeffector cells that respond to various stimuli by contracting. In more complex animals, contraction has become the specialized function of muscle cells, whereas integration and transmission of nerve impulses have become the specialized functions of neurons. This chapter is concerned with the ways these neurons are excited and the way they integrate and transmit impulses.