6. How do nerve signals flow from neuron to neuron?

⁶ The key brain cells​—the neurons—​do not actually touch one another. They are separated by synapses, tiny spaces less than one millionth of an inch across. These gaps are bridged by chemicals called neurotransmitters, 30 of which are known, but the brain may possess many more. These chemical signals are received at one end of the neuron by a maze of tiny filaments called dendrites. The signals are then transmitted at the other end of the neuron by a nerve fiber called an axon. In the neurons the signals are electrical, but across the gaps they are chemical. Thus the transmission of nerve signals is electrochemical in nature. Each impulse is of the same strength, but the intensity of the signal depends upon the frequency of the impulses, which may be as high as one thousand a second.

8. What is one of the great unresolved issues concerning the brain?

⁸ The vast numbers of microscopic nerve fibers making these connections within the brain are often referred to as its “wiring.” They are precisely placed within a maze of staggering complexity. But how they are placed in the exact spots called for by the “wiring diagrams” is a mystery. “Undoubtedly the most important unresolved issue in the development of the brain,” one scientist said, “is the question of how neurons make specific patterns of connections. . . . Most of the connections seem to be precisely established at an early stage of development.”⁠⁴ Another researcher adds that these specifically mapped-out areas of the brain “are common throughout the nervous system, and how this precise wiring is laid down remains one of the great unsolved problems.”⁠⁵

9. How many connections do scientists estimate exist within the brain, and what does one authority say as to its capacity?

⁹ The number of these connections is astronomical! Each neuron may have thousands of connections with other neurons. Not only are there connections between neurons, but there are also microcircuits that are set up directly between the dendrites themselves. “These ‘microcircuits,’” says one neurologist, “add a totally new dimension to our already mind-boggling conception of how the brain works.”⁠⁶ Some researchers believe that the “billions upon billions of nerve cells in the human brain make perhaps as many as a quadrillion connections.”⁠⁷ With what capacity? Carl Sagan states that the brain could hold information that “would fill some twenty million volumes, as many as in the world’s largest libraries.”⁠⁸