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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
mechanisms for having some control over their location. For example, organisms specialized for photosynthesis developed buoyancy mechanisms to keep themselves in the upper layer of the ocean where the sunlight is. Some multicellular organisms evolved flagella for moving. But, in multicellular organisms, flagella must move in a coordinated way for efficient movement (picture a sculling team that has every member rowing at their own pace). 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 to become 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 that have been shown to be incorrect. 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 86 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 part of the brain, which is what you actually see when you look at a brain from above or the side, is the neocortex. The neocortex is really a 1.5 square
foot sheet of cells wadded up a bit to fit inside the head. The neurons in the neocortex are organized in complex neural circuits that are repeated millions of times across the cortical surface. The repeated neural circuits are called minicolumns.
The brain contains many specialized areas associated with particular senses (vision versus audition, for example) and other areas mediating particular motor outputs (like moving the leg versus the tongue). The function of different brain areas depends not on any particular structure of the minicolumns within it, but its inputs and outputs.
So, even though the cell types and circuits in the auditory cortex are similar to those in the visual and motor cortices, the auditory cortex is the auditory cortex because it receives inputs from the cochlea (a part of the ear) and because it sends output to areas associated with processing auditory information, using it to guide behavior.
Many other parts of the nervous system also are made up of repeated circuits or circuit modules, although these are different in different parts of the brain:
» The spinal cord consists of very similar segments (cervical, thoracic, lumbar, and so on), whose structure is repeated from the border of the medulla at the top of the spinal cord to the coccygeal segments at the bottom.
» The cerebellum, a prominent brain structure at the back of the brain below the neocortex, is involved in fine-tuning motor sequences and motor learning. Within the cerebellum are repeated neural circuits forming modules that deal with motor planning, motor execution, and balance.
All the modules that make up the central nervous system are extensively interconnected. If you were to take a section through about any part of the brain, you’d see that the brain has more white matter, or pale-appearing axon tracts (the neural “wires” that connect neurons to each other) than darker gray matter (neural cell bodies and dendrites, which receive inputs from other neurons and do the neural computations). Here’s why: The brain uses local interconnections between neurons to do computations in neural circuits. However, any single neuron contacts only a fraction of the other neurons in the brain. To get to other brain modules for other computations, the results of these computations must be sent over long distance projections via axon tracts of communication neurons.
What a charge: The role of electricity
Most neurons are cells specialized for computation and communication. They have two kinds of branches: dendrites (which normally receive inputs from other neurons) and axons (which are the neuron’s output to other neurons or other targets, like the muscles) emanating from their cell bodies.
