How the Body Works: The Facts Simply Explained

Book Preface

What the
Brain Does
The brain is the body’s control center. It coordinates the basic functions required for survival, controls body movements, and processes sensory data. However, it also encodes a lifetime of memories and creates consciousness, imagination, and our sense of self.

The physical brain
At the largest scale, the human brain appears as a firm, pink-gray solid. It is made mostly from fats (about 60 percent) and has a density just a little greater than that of water. However, neuroscientists, the people who study the form and function of the brain, see the organ as being constituted from more than 300 separate, although highly interconnected, regions. On a much smaller scale, the brain is made from approximately 160 billion cells, half of which are neurons, or nerve cells, and about half are glia, or support cells of one kind or another (see pp.20–21).

LEFT BRAIN VS. RIGHT BRAIN

It is often claimed that one side, or hemisphere, of the brain dominates the other—and that this has an impact on someone’s personality. For example, it is sometimes said that logical people use their left brain hemisphere, while artistic (and less logical) people rely on the right side. However, this is an extreme oversimplification. While it is true that the hemispheres are not identical in function—for example, the speech centers are normally on the left—most healthy mental tasks deploy regions on both sides of the brain at the same time.

The Brain in the Body
The brain is the primary component of the human body’s nervous system, which coordinates the actions of the body with the sensory information it receives.

The nervous system
The two main parts of the nervous system are the central nervous system (CNS) and the peripheral nervous system. The CNS is made up of the brain and the spinal cord, a thick bundle of nerve fibers that runs from the brain in the head to the pelvis. Branching out from this is the peripheral system, a network of nerves that permeates the rest of the body. It is divided according to function: the somatic nervous system handles voluntary movements of the body, while the autonomic nervous system (see opposite) handles involuntary functions.

KEY The Brain in the Body
The brain is the primary component of the human body’s nervous system, which coordinates the actions of the body with the sensory information it receives. Spinal nerves Most peripheral nerves connect to the CNS at the spinal cord and split as they connect. The rear branch carries sensory data to the brain; the forward branch carries motor signals back to the body

CRANIAL NERVES

Within the peripheral system, 12 cranial nerves connect directly to the brain rather than the spinal cord. Most link
to the eyes, ears, nose, and tongue and are also involved in facial movements, chewing, and swallowing, but the vagus nerve links directly to the heart, lungs, and digestive organs

The autonomic nervous system
The involuntary, or autonomic, system maintains the internal conditions of the body by controlling the involuntary muscles in the digestive system and elsewhere, as well as heart and breathing rates, body temperature, and metabolic processes. The autonomic system is divided into two parts. The sympathetic system generally acts to elevate body activity and is involved in the so-called “fight-or-flight” response. The parasympathetic system works in opposition to this, reducing activity to return the body to a “rest-and-digest” state.