Neuroscience For Dummies

Frank Amthor

• Publisher: For Dummies
• Year: 2016
• ISBN:
  9781119224914
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Neuroscience
• Index: Biological Sciences

Get on the fast track to understanding neuroscience

Investigating how your senses work, how you move, and how you think and feel, Neuroscience For Dummies, 2 nd Edition is your straight-forward guide to the most complicated structure known in the universe: the brain. Covering the most recent scientific discoveries and complemented with helpful diagrams and engaging anecdotes that help bring the information to life, this updated edition offers a compelling and plain-English look at how the brain and nervous system function.

Simply put, the human brain is an endlessly fascinating subject: it holds the secrets to your personality, use of language, memories, and the way your body operates. In just the past few years alone, exciting new technologies and an explosion of knowledge have transformed the field of neuroscience—and this friendly guide is here to serve as your roadmap to the latest findings and research. Packed with new content on genetics and epigenetics and increased coverage of hippocampus and depression, this new edition of Neuroscience For Dummies is an eye-opening and fascinating read for readers of all walks of life.

• Covers how gender affects brain function
• Illustrates why some people are more sensitive to pain than others
• Explains what constitutes intelligence and its different levels
• Offers guidance on improving your learning

What is the biological basis of consciousness? How are mental illnesses related to changes in brain function? Find the answers to these and countless other questions in Neuroscience For Dummies, 2 nd Edition

Part 1

Introducing the Nervous System

IN THIS PART …

Discover what neurons are and what they do that allows 100 billion of them to make up a human brain.

See the overall structure of the central nervous system from the cortex to the brainstem and spinal cord.

Look at the details of neurons as electrical signaling devices that process inputs and secrete messenger molecules far away as their outputs.

Chapter 1

A Quick Trip through the Nervous System

IN THIS CHAPTER

Following the evolution of the nervous system

Understanding how the nervous system works

Listing the basic functions of the nervous system

Looking at types of neural dysfunction

Peeking into neuroscience’s future contributions

My brain: it’s my second favorite organ.

— WOODY ALLEN (SLEEPER, 1973)

The brain you are carrying around in your head is by far the most complicated structure known in the universe, and everything you are, have been, and will be arises from the activity of this three-pound collection of 100 billion neurons.

Although this book is about neuroscience, the study of the nervous system, it’s mainly about the brain, where most of the nervous system action takes place, neurally speaking. (The central nervous system consists of the brain, retina, and spinal cord.) If your brain functions well, you can live a long, happy, and productive life (barring some unfortunate circumstances, of course). If you have a brain disorder, you may struggle to overcome every detail of life, a battle that will take place within your brain. So read on for an introduction to the nervous system, how it works, what it does, and what can go wrong.

Understanding the Evolution of the Nervous System

The earth formed 4.5 billion years ago. Evolutionary biologists believe that single-celled prokaryotic life (cells without a cell nucleus) appeared on earth less than one billion years after that. What’s remarkable about this date is that geophysicists believe this was the earliest point at which the planet had cooled enough to sustain life. In other words, life appeared almost the instant (in geological time) that it was possible.

For unknown reasons, it took more than another billion years for eukaryotic life (cells with nuclei) to appear, another billion years for multicellular life to evolve from eukaryotic cells, and another billion years for humans to appear — which we did less than a million years ago. The processes that lead to multicellular life all took place in the earth’s oceans.

Specializing and communicating

In multicellular organisms, the environment of cells on the inside of the cell group is different from the environment of the cells on the outside of the group. These different environments required the cells in these multicellular life forms to develop a way to specialize and communicate. Understanding this specialization is one of the keys to understanding how the nervous system works.

Imagine a ball of a few dozen cells in a primitive ocean billions of years ago. Because the cells on the inside of the ball aren’t exposed to the seawater, they might be able to carry out some digestive or other function more efficiently, but they don’t have any way to get the nutrients they need from the seawater, and they don’t have a way of ridding themselves of waste. To perform these tasks, they need the cooperation of the cells around them.

For this reason, multicellular life allowed — in fact, mandated — that cells specialize and communicate. Eukaryotic cells specialized by regulating DNA expression differently for cells inside the ball of cells versus those on the outside. Meanwhile, some of the substances secreted by cells became signals to which other cells responded. Cells in multicellular species began specializing and communicating.

Moving hither, thither, and yon — in a coordinated way

Currents, tides, and waves in Earth’s ancient oceans moved organisms around whether they wanted to be moved or not. Even organisms specialized for photosynthesis developed buoyancy mechanisms to keep themselves in the upper layer of the ocean where the sunlight is. Other Darwinian “survival of the fittest” mechanisms caused other changes.

Some multicellular organisms found an advantage in moving more actively, using flagella. But having different cells on different sides of a multicellular organism move flagella without coordination is not the best way to direct movement (picture a sculling team that has every member rowing in a different direction). Without some form of communication to synchronize their activity, the boat — the organism in this case — would go nowhere fast. The result? Networks of specialized cells with gap junctions between them evolved. These networks allowed rapid electrical signaling around ringlike neural nets that became specialized for synchronizing flagella on the outside of the organism.

Evolving into complex animals

Balls of cells with nervous systems that had become capable of moving in a coordinated fashion in the oceans evolved into complex animals with sensory and other specialized neurons.

About half a billion years ago, invertebrates such as insects crawled onto the land to feast on the plants that had been growing there for millions of years. Later, some vertebrate lung fish ventured onto the land for brief periods when tidal pools and other shallow bodies of water dried up, forcing them to wriggle over to a larger pool. Some liked it so much they ended up staying on the land almost all the time and became amphibians, some of which later evolved into reptiles. Some of the reptiles gave rise to mammals, whose descendants are us.

Enter the neocortex

When you look at a human brain from the top or sides, almost everything you see is neocortex. It’s called “neo” because it is a relatively recent invention of mammals. Prior to mammals, animals like reptiles and birds had relatively small brains with very specialized areas for processing sensory information and controlling behavior.

What happened with the evolution of mammals is that a particular brain circuit expanded enormously as an additional processing layer laid over the top of all the older brain areas for both sensory processing and motor control.

Neuroscientists are not exactly sure how and why the neocortex evolved. Birds and reptiles (and dinosaurs, for that matter) did pretty well with their small, specialized brains before the massive expansion of the neocortex that occurred in mammals. However it happened, once mammals arrived, the neocortex enlarged tremendously, dwarfing the rest of the brain that had evolved earlier. This occurred despite the fact that large brains are expensive, metabolically. The human brain consumes about 20 percent of the body’s metabolism despite being only about 5 percent of body weight.

Looking at How the Nervous System Works

Look at just about any picture of the brain, and you see immediately that it consists of a number of different regions. The brain does not appear to be an amorphous mass of neural tissue that simply fills up the inside of the skull.

Given the appearance of the brain, you can ask two very important and related questions:

• Do the different regions of the brain that look different really do different things?
• Do the regions that look the same do the same thing?

The answer to both questions? Sort of. The next sections explain.

FIELDS AND BUMPS: EARLY THEORIES ABOUT HOW THE BRAIN WORKS

The early history of neuroscience saw a number of brain function theories. Two of the more interesting are phrenology and the aggregate field theories.

The aggregate field theories supposed that, for the most part, the brain is a single, large neuronal circuit whose capabilities are related mostly to its total size. These theories assumed that the brain’s internal structure is of little consequence in understanding its function.

At the other extreme were the phrenologists, who believed that almost every human characteristic, including attributes such as cautiousness, courage, and hope, are located in specific parts of the brain. These folks believed that the development of these attributes can be determined by measuring the height of the skull over those areas (bumps), the presumption being that the underlying brain grows and pushes the skull upward for traits that are highly developed. You can read more about phrenology in Chapter 12.

The important role of neurons

The nervous system, explained in detail in Chapter 2, consists of the central nervous system (the brain, retina, and spinal cord), the peripheral nervous system (the sensory and motor nerve axons that connect the central nervous system to the limbs and organs). The peripheral nervous system also includes the autonomic nervous system (which regulates body processes such as digestion and heart rate), and the enteric nervous system, which controls the gastrointestinal system.

All the divisions of the nervous system are based universally on the functions of neurons. Neurons are specialized cells that process information. Like all cells, they are unbelievably complicated in their own right. All nervous systems in all animal species have four basic types of functional cells:

• Sensory neurons: These neurons tell the rest of the brain about the external and internal environment.
• Motor (and other output) neurons: Motor neurons contract muscles and mediate behavior, and other output neurons stimulate glands and organs.
• Projection neurons: Communication neurons transmit signals from one brain area to another.
• Interneurons: The vast majority of neurons in vertebrates are interneurons involved in local computations. Computational interneurons extract and process information coming in from the senses, compare that information to what’s in memory, and use the information to plan and execute behavior. Each of the several hundred distinguishable brain regions contains several dozen distinct types or classes of computational interneurons that mediate the function of that brain area.

What really distinguishes the nervous system from any other functioning group of cells is the complexity of the neuronal interconnections. The human brain has on the order of 100 billion neurons, each with a unique set of about 10,000 synaptic inputs from other neurons, yielding about a quadrillion synapses — a number even larger than the U.S. national debt in pennies! The number of possible distinct states of this system is virtually uncountable.

You can read a detailed discussion on neurons and how they work in Chapter 3.

Computing in circuits, segments, and modules

The largest ...