CHAPTER 2
Brain and Behavior
Theme: Brain activity is the source of human consciousness, intelligence, and behavior.
Preview
Finding Music in Tofu
When I watch the movements of a gifted musician, I often think about the brain. I recently saw Yo-Yo Ma, a master cellist, play a Bach suite with such skill that I was utterly amazed. If Ma had been an athlete, you would say he was “in the zone.” His performance was unforgettable. Of course, in everything from rock to rap, musicians regularly make music that no machine could duplicate. A virtual Carlos Santana? A mechanical Eric Clapton? A synthetic Bono or Bon Jovi? I don’t think so. That’s why music is a good example of the central role the brain plays in all that is human.
Your brain is the size of a grapefruit. It weighs about 3 pounds and looks a lot like tofu. The next time you are in a market that sells beef brains, stop and have a look.What you will see is similar to your own brain, only smaller.
How could such a squishy little blob of tissue allow us to make music of exquisite beauty? To seek a cure for cancer?
To fall in love? Or read a book like this one?
Each nerve cell in your brain is linked to as many as 15,000 others. This network makes it possible to process immense amounts of information. In fact, there may be more possible pathways between the neurons in your brain than there are atoms in the entire universe! Undeniably, the human brain is the most amazing of all computers.
Scientists use the power of the brain to study the brain.
Yet, even now we must wonder if the brain will ever completely understand itself. Nevertheless, it is clear that answers to many age-old questions about the mind, consciousness, and knowledge lie buried within the brain.
Let’s visit this fascinating realm.
¡ Biopsychologists study how processes in the body, brain, and nervous system relate to behavior.
¡ Ultimately, all behavior can be traced to the activity of nerve cells.
¡ To map the brain, researchers activate or disable specific areas and observe changes in behavior.
¡ Bioelectrical recordings and computer-generated images of brain activity provide additional insights into how the brain works.
¡ Sensations, thoughts, feelings, motives, actions, memories, and all other human capacities are associated with brain activities and structures.
¡ Endocrine glands serve as a chemical communication system within the body. Behavior is greatly influenced by the ebb and flow of hormones in the bloodstream.
¡ Brain dominance and brain activity determine if you are right-handed, left-handed, or ambidextrous.
¡ The brain’s circuitry is not static.The brain grows new nerve cells, and it can “rewire” itself in response to changing environmental conditions.
Gateways to Brain and Behavior
How do nerve cells operate and communicate?
What are the functions of major parts of the nervous system?
How do we know how the brain works?
How is the brain organized, and what do its higher structures do?
Why are the brain’s association areas important? What happens when they are injured?
What kinds of behaviors are controlled by the subcortex?
Does the glandular system affect behavior?
How do right- and left-handed individuals differ?
Is brain damage always permanent?
Key Questions
Neurons—Building a “Biocomputer”
All of your thoughts, feelings, and actions can be traced back to electrical impulses flashing through spidery nerve cells within the brain. Although they may seem far removed from daily life, everything you do begins with these tiny cells. Let’s see how nerve cells operate, how the nervous system is “wired,” and how scientists study the brain.
The brain consists of some 100 billion neurons (NEW-rons: individual nerve cells). Neurons carry and process information.
They also activate muscles and glands. A single neuron is not very smart—it would take at least several just to make you blink.
Yet, when neurons form vast networks, they produce intelligence and consciousness. Neurons are linked to one another in tight clumps and long “chains.” Each neuron receives messages from many others and sends its own message on.Millions of neurons must send messages at the same time to produce even the most fleeting thought (Carter, 1998). When Carlos Santana plays a guitar riff, literally billions of neurons may be involved.
Parts of a Neuron
What does a neuron look like? What are its main parts? No two neurons are exactly alike, but most have four basic parts (œFig.
2.1). The dendrites (DEN-drytes), which look like tree roots, receive messages from other neurons. The soma (SOH-mah: cell body) does the same. In addition, the soma sends messages of its own (nerve impulses) down a thin fiber called the axon
(AK-sahn).
Most axons end in axon terminals. These “branches” link up with the dendrites and somas of other neurons. This allows information to pass from neuron to neuron. Some axons are only 0.1 millimeter long. (That’s about the width of a pencil line.)
Others stretch up to a meter through the nervous system.
(From the base of your spine to your big toe, for instance.) Like miniature cables, axons carry messages through the brain and nervous system.Altogether, your brain contains about 3 million miles of axons (Hyman, 1999b).
Now let’s summarize with a metaphor. Imagine that you are standing in a long line of people who are holding hands. A person on the far right end of the line wants to silently send a message to the person on the left end. She does this by pressing the
Neuron An individual nerve cell.
Dendrites Neuron fibers that receive incoming messages.
Soma The main body of a neuron or other cell.
Axon Fiber that carries information away from the cell body of a neuron.
Axon terminals Branching fibers at the ends of axons.
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hand of the person to her left, who presses the hand of the person to his left, and so on. The message arrives at your right hand (your dendrites). You decide whether to pass it on (you are the soma). The message goes out through your left arm (the axon).
With your left hand (the axon terminals), you squeeze the hand of the person to your left, and the message moves on.
The Nerve Impulse
Electrically charged molecules called ions (EYE-ons) are found inside each neuron (œFig. 2.2). Other ions lie outside the cell. Some ions have a positive electrical charge, and some are negative. Different numbers of these “plus” and “minus” charges exist inside and outside of nerve cells.As a result, the inside of each neuron in your brain has an electrical charge of about minus 70 millivolts.
(A millivolt is one thousandth of a volt.) This charge allows each neuron in your brain to act like a tiny biological battery.
The electrical charge of an inactive neuron is called its resting potential. But neurons seldom get much rest: Messages arriving from other neurons raise and lower the resting potential.
If the electrical charge rises to about minus 50 millivolts, the neuron will reach its threshold, or trigger point for firing (see Fig. 2.2). It’s as if the neuron says, “Ah ha! It’s time to send a message to my neighbors.”When a neuron reaches _50 millivolts, an action potential, or nerve impulse, sweeps down the axon at up to 200 miles per hour (œFig. 2.3). That may seem fast, but it still takes at least a split second to react. That’s one reason why hitting a 95-mile-per-hour major league fastball is among the most difficult feats in all of sports.
What happens during an action potential? The axon membrane is pierced by tiny tunnels, or “holes,” called ion channels.
Normally, these tiny openings are blocked by molecules that act like “gates” or “doors.” During an action potential, the gates pop open. This allows sodium ions (Na_) to rush into the axon (Carlson, 2001). The channels first open near the soma. Then, gate after gate opens down the length of the axon as the action potential zips along (œFig. 2.4).
Each action potential is an all-or-nothing event (a nerve impulse occurs completely or not at all). You might find it helpful to picture the axon as a row of dominoes set on end. Tipping over the dominoes is an all-or-nothing act. Once the first domino drops, a wave of falling blocks will zip rapidly to the
Soma (cell body) Dendrites Axon Axon collateral (branch) Nerve cell fiber Axon Myelin sheath
Nerve impulse
Neurilemma Myelin
Axon terminals Other neuron Synapse (see Fig. 2.5 for an enlarged view)
œ Figure 2.1 An example of a neuron, or nerve cell, showing several of its important features.The right foreground shows a nerve cell fiber in cross section, and the upper left inset gives a more realistic picture of the shape of neurons.The nerve impulse usually travels from the dendrites and soma to the branching ends of the axon.The neuron shown here is a motor neuron.Motor neurons originate in the brain or spinal cord and send their axons to the muscles or glands of the body.
BRAIN AND BEHAVIOR 59
end of the line. Similarly, when a nerve impulse is triggered near the soma, a wave of activity (the action potential) travels down the axon. This is what happens in long chains of neurons as Yo- Yo Ma’s brain tells his hands what to do next, note after note.
After each nerve impulse, the cell briefly dips below its resting level and it becomes less willing to fire. This negative after-
Resting potential The electrical charge of a neuron at rest.
Threshold The point at which a nerve impulse is triggered.
Action potential The nerve impulse.
Ion channels Tiny openings through the axon membrane.
Negative after-potential A drop in electrical charge below the resting potential.
–50 –70 +30 0 Resting potential Action potential Negative after-potential Membrane potential (in millivolts) Time Threshold – – – – – – – – – – – – – – Axon + + + + + + + + + + + + + +
œ Figure 2.2 Activity in an axon can be measured by placing electrical probes inside and outside the axon. (The scale is exaggerated here. Such measurements require ultra-small electrodes, as described later in this chapter.) At rest, the inside of an axon is about _60 to _70 millivolts, compared with the outside. Electrochemical changes in a nerve cell generate an action potential.When positively charged sodium ions (Na_) rush into the cell, its interior briefly becomes positive.This is the action potential.After the action potential, an outward flow of positive potassium ions (K_) restores the negative charge inside the axon. (See Fig. 2.3 for further explanation.)
Action potential
+ + + – – – – – – – – – + + + + + + + + – – – – + – – – + + + + – – – – – + + + + – – – – + + + + – – – – – – – – + + + + – + – – – – + + + + + + + –
Axon In its resting state, the axon has a negatively charged interior.
1.
During an action potential, positively charged atoms (ions) rush into the axon.
This briefly changes the electrical charge inside the axon from negative to positive.
Simultaneously, the charge outside the axon becomes negative.
2.
The action potential advances as positive and negative charges reverse in a moving zone of electrical activity that sweeps down the axon.
3.
After an action potential passes, positive ions rapidly flow out of the axon to quickly restore its negative charge. An outward flow of additional positive ions returns the axon to its resting state.
4.
– – – – – – – – – – – – + + + + + + + + + + + + –
Action potential Action potential
œ Figure 2.3 The inside of an axon normally has a negative electrical charge.The fluid surrounding an axon is normally positive. As an action potential passes along the axon, these charges reverse so that the interior of the axon briefly becomes positive.This process is described in more detail in Figure 2.4.
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potential occurs because potassium ions (K_) flow out of the neuron while the membrane gates are open (Fig. 2.4). After a nerve impulse, ions flow both into and out of the axon, recharging it for more action. In our model, the row of dominoes is quickly set up again. Soon, the axon is ready for another wave of activity.
Synapses and Neurotransmitters
How does information move from one neuron to another? The nerve impulse is primarily electrical. That’s why electrically stimulating the brain affects behavior. To prove the point, researcher José Delgado once entered a bullring with a cape and a radio transmitter. The bull charged. Delgado retreated. At the last instant the speeding bull stopped short.Why? Because Delgado’s radio activated electrodes (metal wires) placed deep within the bull’s brain. These, in turn, stimulated “control centers” that brought the bull to a halt.
In contrast to the nerve impulse, communication between neurons is chemical. The microscopic space between two neurons, over which messages pass, is called a synapse (SIN-aps) (œFig. 2.5). When an action potential reaches the tips of the axon terminals, neurotransmitters (NUE-roh-TRANS-mit-ers) are released into the synaptic gap. Neurotransmitters are chemicals that alter activity in neurons.
Let’s return to the people standing in a line. To be more accurate, you and the others shouldn’t be holding hands. Instead, each person should have a toy squirt gun in his or her left hand.
To pass along a message, you would squirt the right hand of the person to your left.When that person notices this “message,” he or she would squirt the right hand of the person to the left, and so on.
When chemical molecules cross over a synapse, they attach to special receiving areas on the next neuron (Fig. 2.5). These tiny
receptor sites on the cell membrane are sensitive to neurotransmitters.
The sites are found in large numbers on nerve cell bodies and dendrites.Muscles and glands have receptor sites, too.
Do neurotransmitters always trigger an action potential in the next neuron? No. Some transmitters excite the next neuron (move it closer to firing). Others inhibit it (make firing less likely). At any instant, a single neuron may receive hundreds or thousands of messages. Does it fire an impulse? It depends: If
Resting potential + + + + + – – – – – – – – – – + + + + + + + + + +
Axon Ion channels
– – – – – + + + + + + + + + + – – – – – – + + + + + Axon repolarizes
K+ K+
Na+ Na+
Na+
œ Figure 2.4 Cross-sectional views of an axon.The right end of the top axon is at rest, with a negatively charged interior. An action potential begins when the ion channels open and sodium ions (Na_) enter the axon. In this drawing the action potential would travel rapidly along the axon, from left to right. In the lower axon the action potential has moved to the right.After it passes, potassium ions (K_) flow out of the axon.This quickly renews the negative charge inside the axon so that it can fire again.
Sodium ions that enter the axon during an action potential are pumped back out more slowly.Their removal restores the original resting potential.
Presynaptic axon terminal Postsynaptic dendrite Synaptic vesicle Receptor site Neurotransmitter Synaptic gap
œ Figure 2.5 A highly magnified view of the synapse shown in Figure 2.1. Neurotransmitters are stored in tiny sacs called synaptic vesicles.When a nerve impulse arrives at an axon terminal, the vesicles move to the surface and release neurotransmitters.
These transmitter molecules cross the synaptic gap to affect the next neuron.The size of the gap is exaggerated here; it is actually only about one millionth of an inch.
Transmitter molecules vary in their effects: Some excite the next neuron and some inhibit its activity.
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several “exciting” messages arrive close in time, the neuron will fire—but only if it doesn’t get too many “inhibiting” messages that push it away from its trigger point. In this way, messages are combined before a neuron “decides” to fire its all-or-nothing action potential. Multiply these events by 100 billion neurons and 100 trillion synapses and you have an amazing computer— one that could easily fit inside a shoe box.
More than 100 transmitter chemicals are found in the brain. Some examples are acetylcholine, epinephrine, norepinephrine, serotonin, dopamine, histamine, and various amino acids. Disturbances of any of these substances can have serious consequences. For example, too little dopamine can cause the muscle tremors of Parkinson’s disease. Too much dopamine may cause schizophrenia.
Many drugs imitate, duplicate, or block these transmitters. For example, acetylcholine
(ah-SEET-ul-KOH-leen) normally activates muscles. Without acetylcholine, our musical friend Yo-Yo Ma couldn’t even move, much less play Bach. That’s exactly why the drug curare (cue-RAHree) causes paralysis. By attaching to receptor sites on muscles, curare competes with acetylcholine. This prevents acetylcholine from activating muscle cells. As a result, a person or animal given curare cannot move—a fact known to South American Indians of the Amazon River Basin, who use curare as an arrow poison for hunting.
Neural Regulators
More subtle brain activities are affected by chemicals called neuropeptides
(NUE-row-PEP-tides). Neuropeptides do not carry messages directly. Instead, they regulate the activity of other neurons.
By doing so, they affect memory, pain, emotions, pleasure, moods, hunger, sexual behavior, and other basic processes. For example, when you touch something hot, your hand jerks away. The messages for this action are carried by neurotransmitters.
At the same time, pain may cause the brain to release enkephalins (en-KEF-ah-lins).
These opiate-like neural regulators relieve pain and stress. Related chemicals called endorphins
(en-DORF-ins) are released by the pituitary gland. Together, these chemicals reduce the pain so that it is not too disabling (Drolet et al., 2001).
Ultimately, brain regulators may help explain depression, schizophrenia, drug addiction, and other puzzling topics. For example, women who suffer from severe premenstrual pain and distress have unusually low endorphin levels (Straneva et al., 2002).
The Nervous System—Wired for Action
Jamal and Vicki are playing catch with a Frisbee. This may look fairly simple. However, to merely toss the Frisbee or catch it, a huge amount of information must be sensed, interpreted, and directed to countless muscle fibers. As they play, Jamal and Vicki’s neural circuits are ablaze with activity. Let’s explore the “wiring diagram” that makes their Frisbee game possible.
Neurons and Nerves
Are neurons the same as nerves? No. Neurons are tiny cells. You would need a microscope to see one. Nerves are large bundles of axons and dendrites. You can easily see nerves without magnification.
Synapse The microscopic space between two neurons, over which messages pass.
Neurotransmitter Any chemical released by a neuron that alters activity in other neurons.
Receptor sites Areas on the surface of neurons and other cells that are sensitive to neurotransmitters or hormones.
Acetylcholine The neurotransmitter released by neurons to activate muscles.
Neuropeptides Brain chemicals that regulate the activity of neurons.
Enkephalins Opiate-like brain chemicals that regulate reactions to pain and stress.
Endorphins Chemicals that are similar in structure and painkilling effect to opiate drugs such as morphine.
Nerve A bundle of neuron fibers.
Under some circumstances, pain can produce feelings of relaxation or euphoria.
Endorphins underlie this effect, as explained in Chapter 5, pages 203–204.
B R I D G E S
© Spencer Grant/PhotoEdit
Endorphins protect us at times of stress. A “real-life” example of this effect can be found among sport parachutists.Right after novices make their first jump, they have elevated endorphin levels and they are less sensitive to pain (Janssen & Arntz, 2001).
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