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Our brain controls almost everything we do. It is responsible for our perception, consciousness, memory, language, intelligence, and personality—everything that makes us human. This would seem to be a daunting job for an organ that only weighs about three pounds. The brain, however, has typically been estimated to consist of about 100 billion nerve cells, called neurons. A recent study of the cellular composition of the brain, for example, estimated 86 billion (Azevedo et al., 2009). Each neuron may receive information from thousands of other neurons; therefore, the number of possible communication connections between these billions of neurons is in the trillions! One estimate goes as high as one thousand trillion, the number 1 followed by 15 zeroes (Sweeney, 2009). Just as your genome is the entire sequence of nucleotides in your DNA, the totality of the connections between neurons in your nervous system is referred to as your connectome (Seung, 2012). Unlike your genome, which is fixed at conception, your connectome changes throughout your life. With this vast collection of changing neuronal connections, the human brain is the most complex device in the known universe (Buonomano, 2011). Hence, this complexity may never be completely understood, but it’s an intriguing puzzle—human brains trying to understand how human brains work.

In this chapter on neuroscience (the scientific study of the brain and nervous system), we will first examine neurons, the cellular building blocks of the nervous system. We will look at how neurons transmit and integrate information, and how drugs and poisons interrupt these processes (and change our behavior and mental processes). We will also consider how some diseases and disorders are related to transmission problems.

Once we understand how neurons work, we will consider the hierarchical structure of the nervous system, discussing its various subsystems—the central nervous system and the peripheral nervous system—along with the body’s other major communication system, the endocrine glandular system. We will also consider emotions and the role of the autonomic nervous system, a division of the peripheral nervous system, in explaining how our emotional experiences are generated.

Next, the major parts of the brain (vast collections of neurons) and their functions will be detailed. We will focus mainly on the cerebral cortex, the seat of higher mental functioning in humans. Last, we will consider what consciousness is and what brain activity during sleep (a natural break from consciousness) tells us about the five stages of sleep and the nature of dreaming.