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Part 4: Intelligence: The Thinking Brain
Part 5: The Part of Tens
Dividing and conquering: Sympathetic and parasympathetic subsystems
lobes
Universal versus small-world connectivity
Minicolumns and the six degrees of separation
Defining the six-layered structure of the cortex
Hail to the neocortex!
Controlling the Content of Thought: Sensory Pathways and Hierarchies
Sensory relays from the thalamus to the cortex
The hippocampus: Specializing for memory
Dividing and Conquering: Language, Vision, and the Brain Hemispheres
Specialized brain systems for language
Seeing the whole and the parts: Visual processing asymmetries
Where Consciousness
Language and left- or right-hemisphere damage
Understanding the “left side interpreter”
Getting the Brain You Have Today: The Neocortex versus Your Reptilian Brain
My neocortex is bigger than yours: Looking at relative sizes
The relationship between prefrontal cortex size and the ability to pursue goals
Working Memory, Problem-Solving, and the Lateral Prefrontal Cortex
Brain processes managing working memory
The limits of working memory
Perseveration: Sticking with the old, even when it doesn’t work anymore
Making Up and Changing Your Mind: The Orbitofrontal Cortex
Feeling it in your gut: Learned emotional reactions
Gambling on getting it right: Risk taking, aversion, and pleasure
Case-based reasoning: Thinking about social consequences .
Are We There Yet? The Anterior Cingulate Cortex
Logging errors and changing tactics
Acting without thinking
Who’s minding the store? Problems in the anterior cingulate cortex
Developing from Conception
Arising from the ectoderm: The embryonic
The development of the cerebral cortex
Wiring it all together: How axons connect various areas of the brain to each other
Learning from Experience: Plasticity and the Development of Cortical Maps
Mapping it out: Placing yourself in a visual, auditory, and touching world
Firing and wiring together: Looking at Hebb’s law
Genetics: Specifying the
Introduction
The central mystery about the brain is simply this: How can a bunch of interconnected cells make each of us what we are — not only our thoughts, memories, and feelings, but our identity. At present, no one can answer this question. Some philosophers think it is not answerable in principle.
I believe we can understand how the brain makes us what we are. This book, while surely not containing the complete answer, points the way to what the answer looks like: In short, the brain is made of neurons, each of which is a complex little computer. Parts of the nervous system make suggestions to the rest of it about what you should do next. Other parts process the sensory inputs you receive and tell the system how things are going so far. Still other parts, particularly those associated with language, make up a running dialog about all of this as it is going on; this is your consciousness.
Those concepts aren’t too difficult to grasp, but people think of neuroscience as hard. And why? Because in order for your nervous system to perform these functions, it takes 86 billion neurons and a quadrillion synaptic connections. All this was structured over billions of years of evolution and over all the human years of development and learning.
You need to know three main things to understand how the nervous system works. The first is how the neurons themselves work. The second is how neurons talk to each other in neural circuits. The third is how neural circuits form a particular set of functional modules in the brain. The particular set of modules that you have make you a human. The content of your specific modules make you unique.
Our nearest animal relative, the chimpanzee, has pretty much the same neurons and neural circuits that you and I do. They even have most of the same modules. We humans have a few extra modules that permit consciousness. Understanding this is what this book is about.
About This Book
Let’s face it. Neuroscience is a complex topic. How could it not be since it deals with the brain, the most complex structure in the known universe. In this book, I explain some very complex ideas in a way that both students enrolled in introductory neuroscience courses and those who are just interested in the topic for fun can understand.
To use and understand this book, you don’t have to know anything about the brain except that you have one. In this book, I cover as much of the basics as possible with simple language and easy-to-understand diagrams, and when you encounter technical terms like anterior cingulate cortex or vestibulospinal reflex, I explain what they mean in plain English. This edition also adds a glossary for technical terms.
This book is designed to be modular for the simple reason that I want you to be able to find the information you need. Each chapter is divided into sections, and each section contains information about some topic relevant to neuroscience, such as
» The key components of the nervous system: neurons and glia
» How neurons work and what the different kinds of neurons are
» What systems are involved in planning and executing complex actions
» The role of the neocortex in processing thoughts
The great thing about this book is that you decide where to start and what to read. It’s a reference you can jump into and out of at will. Just head to the table of contents or the index to find the information you want.
Note: You can use this book as a supplemental text in many undergraduate courses because I discuss the neuron and brain function as a system. Typical undergraduate perception courses, for example, give short (and usually unsatisfactory) introductions to neurons and neural processing and little if any coverage of cognition. Cognitive psychology and neuroscience courses typically cover cognition well but often don’t ground cognition at the level of neurons. Behavioral neuroscience courses sometimes ignore cognition and neurophysiology almost altogether while doing a decent job explaining heuristics and phenomenology of behavior and learning. You can also use this book as an adjunct to graduate or health profession courses where the nervous system or mental illnesses or disorders are mentioned but little explicit coverage is given of the nervous system and the brain.
Within this book, you may note that some web addresses break across two lines of text. If you’re reading this book in print and want to visit one of these web pages, simply key in the web address exactly as it’s noted in the text, pretending as though the line break doesn’t exist. If you’re reading this as an e-book, you’ve got it easy — just click the web address to be taken directly to the web page.
Foolish Assumptions
In writing this book, I made some assumptions about you. To wit:
» You’re not a professional neuroscientist or neurosurgeon but may be a beginning student in this field. (If you notice your neurosurgeon thumbing through a copy of this book before removing parts of your brain, you may want to get a second opinion.)
» You’re taking a course that relates to brain function, cognition, or behavior and feel that you would do better if you had a firm grasp of how the nervous system and its components work.
» You want information in easy-to-access and easy-to-understand chunks, and if a little humor can be thrown in, all the better!
If you see yourself in the preceding points, then you have the right book in your hands.
Icons Used in This Book
The icons in this book help you find particular kinds of information. They include the following:
Looking at things a little differently or thinking of them in a new way can make potentially confusing concepts easier to understand. Look for this icon to find these “think of it this way” types of discussions.
This icon appears next to key concepts and general principles that you’ll want to remember.
In a subject as complicated as neuroscience, it’s inevitable that some discussions will be very technical. Fortunately for you, you don’t need to know the detailed whys and wherefores, but I include this info anyway for those who are voraciously curious about the details. Read or skip paragraphs beside this icon at will.
Beyond This Book
In addition to the material in the print or e-book you’re reading right now, this product also comes with some access-anywhere material on the web. The Cheat Sheet fills you in on types and function of cells in the central nervous system, the role of the neocortex, the left and right hemispheres of the brain, the brain’s four lobes, and more. To get this Cheat Sheet, simply go to www.dummies.com and type Neuroscience For Dummies Cheat Sheet in the Search box.
Where to Go from Here
Finally, the purpose of this book is to get you up to speed fast in understanding neurons and the nervous system, particularly the brain, but there are many important neuroscience topics that fall well beyond the scope of this book. Here’s just a sampling: intra-neuronal metabolism and second messenger cascades, association of neurological deficits with lesions in specific tracts and nuclei, traditional learning theory, and modern genetics. You can find detailed discussion of most of these subjects in Kandel et al, Principles of Neural Science, 6th Edition (McGraw-Hill), the bible of neuroscience books.
1 Introducing the Nervous System
IN THIS PART . . .
Discover what neurons are and what they do that allows 86 billion of them to make up a human brain.
See the overall structure of the central nervous system from the cortex to the brain stem 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.
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
Chapter 1
A Quick Trip through the Nervous System
My brain: it’s my second favorite organ.
TWOODY
ALLEN (SLEEPER, 1973 )
he 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. (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 singlecelled 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 and another billion years for multicellular life to evolve from eukaryotic cells. The processes that lead to multicellular life all took place in the earth’s oceans. Finally, it took another billion years for humans to appear after plants and animals migrated to the land. Humans finally appeared less than a million years ago.
Specializing and communicating
In multicellular organisms, the environment of cells on the interior is different from the environment of the cells on the exterior. 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. The cells on the inside of the ball might be able to carry out some digestive or other function more efficiently, but because they aren’t exposed to the seawater 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. 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.
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. But organisms developed several
