Please proceed to the next section.

2-1 Why are psychologists concerned with human biology?

Your every idea, every mood, every urge is a biological happening. You love, laugh, and cry with your body. Without your body—your genes, your brain, your appearance—you would, indeed, be nobody. Although we find it convenient to talk separately of biological and psychological influences on behavior, we need to remember: To think, feel, or act without a body would be like running without legs.

Our understanding of how the brain gives birth to the mind has come a long way. The ancient Greek philosopher Plato correctly located the mind in the spherical head—his idea of the perfect form. His student, Aristotle, believed the mind was in the heart, which pumps warmth and vitality to the body. The heart remains our symbol for love, but science has long since overtaken philosophy on this issue: It’s your brain, not your heart, that falls in love.

In the early 1800s, German physician Franz Gall proposed that phrenology, studying bumps on the skull, could reveal a person’s mental abilities and character traits (FIGURE 2.1). At one point, Britain had 29 phrenological societies, and phrenologists traveled North America giving skull readings (Dean, 2012; Hunt, 1993). Using a false name, humorist Mark Twain put one famous phrenologist to the test. “He found a cavity [and] startled me by saying that that cavity represented the total absence of the sense of humor!” Three months later, Twain sat for a second reading, this time identifying himself. Now “the cavity was gone, and in its place was … the loftiest bump of humor he had ever encountered in his life-long experience!” (Lopez, 2002). The “science” of phrenology remains known today as a reminder of our need for critical thinking and scientific analysis. Although its initial popularity faded, phrenology succeeded in focusing attention on the localization of function—the idea that various brain regions have particular functions.

You and I are living in a time Gall could only dream about. By studying the links between biological activity and psychological events, those working from the biological perspective are announcing discoveries about the interplay of our biology and our behavior and mind at an exhilarating pace. Within little more than the past century, researchers seeking to understand the biology of the mind have discovered that

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We have also realized that we are each a system composed of subsystems that are in turn composed of even smaller subsystems. Tiny cells organize to form body organs. These organs form larger systems for digestion, circulation, and information processing. And those systems are part of an even larger system—the individual, who in turn is a part of a family, culture, and community. Thus, we are biopsychosocial systems. To understand our behavior, we need to study how these biological, psychological, and social systems work and interact.

For scientists, it is a happy fact of nature that the information systems of humans and other animals operate similarly—so similarly that you could not distinguish between small samples of brain tissue from a human and a monkey. This similarity allows researchers to study relatively simple animals, such as squids and sea slugs, to discover how our neural systems operate. It allows them to study other mammals’ brains to understand the organization of our own. Cars differ, but all have engines, accelerators, steering wheels, and brakes. A space alien could study any one of them and grasp the operating principles. Likewise, animals differ, yet their nervous systems operate similarly. Though the human brain is more complex than a rat’s, both follow the same principles.

Neurons

2-2 What are neurons, and how do they transmit information?

Our body’s neural information system is complexity built from simplicity. Its building blocks are neurons, or nerve cells. To fathom our thoughts and actions, our memories and moods, we must first understand how neurons work and communicate.

Neurons differ, but all are variations on the same theme (FIGURE 2.2 below). Each consists of a cell body and its branching fibers. The bushy dendrite fibers receive information and conduct it toward the cell body. From there, the cell’s single lengthy axon fiber passes the message through its terminal branches to other neurons or to muscles or glands. Dendrites listen. Axons speak.

Unlike the short dendrites, axons may be very long, projecting several feet through the body. A human neuron carrying orders to a leg muscle, for example, has a cell body and axon roughly on the scale of a basketball attached to a 4-mile-long rope. Much as home electrical wire is insulated, some axons are encased in a myelin sheath, a layer of fatty tissue that insulates them and speeds their impulses. As myelin is laid down up to about age 25, neural efficiency, judgment, and self-control grow (Fields, 2008). If the myelin sheath degenerates, multiple sclerosis results: Communication to muscles slows, with eventual loss of muscle control.

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Supporting these billions of nerve cells are spidery glial cells (“glue cells”). Neurons are like queen bees; on their own they cannot feed or sheathe themselves. Glial cells are worker bees. They provide nutrients and insulating myelin, guide neural connections, and mop up ions and neurotransmitters. Glia also play a role in learning and thinking. By “chatting” with neurons they participate in information transmission and memory (Fields, 2011, 2013; Miller, 2005).

In more complex animal brains, the proportion of glia to neurons increases. A postmortem analysis of Einstein’s brain did not find more or larger-than-usual neurons, but it did reveal a much greater concentration of glial cells than found in an average Albert’s head (Fields, 2004).

The Neural Impulse

Neurons transmit messages when stimulated by signals from our senses or when triggered by chemical signals from neighboring neurons. In response, a neuron fires an impulse, called the action potential—a brief electrical charge that travels down its axon.

Depending on the type of fiber, a neural impulse travels at speeds ranging from a sluggish 2 miles per hour to more than 200 miles per hour. But even its top speed is 3 million times slower than that of electricity through a wire. We measure brain activity in milliseconds (thousandths of a second) and computer activity in nanoseconds (billionths of a second). Thus, unlike the nearly instantaneous reactions of a computer, your reaction to a sudden event, such as a child darting in front of your car, may take a quarter-second or more. Your brain is vastly more complex than a computer, but slower at executing simple responses. And if you were an elephant—whose round-trip message travel time from a yank on the tail to the brain and back to the tail is 100 times longer than that of a tiny shrew—your reflexes would be slower yet (More et al., 2010).

Like batteries, neurons generate electricity from chemical events. In the neuron’s chemistry-to-electricity process, ions (electrically charged atoms) are exchanged. The fluid outside an axon’s membrane has mostly positively charged sodium ions; a resting axon’s fluid interior has mostly negatively charged ions. This positive-outside/negative-inside state is called the resting potential. Like a tightly guarded facility, the axon’s surface is very selective about what it allows through its gates. We say the axon’s surface is selectively permeable.

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When a neuron fires, however, the security parameters change: The first section of the axon opens its gates, rather like sewer covers flipping open, and positively charged sodium ions flood in (FIGURE 2.3). The loss of the inside/outside charge difference, called depolarization, causes the next axon channel to open, and then the next, like falling dominos, each tripping the next. This temporary inflow of positive ions is the neural impulse—the action potential.

During a resting pause called the refractory period, the neuron pumps the positively charged sodium ions back outside. Then it can fire again. (In myelinated neurons, as in Figure 2.2, the action potential speeds up by hopping from the end of one myelin “sausage” to the next.) The mind boggles when imagining this electrochemical process repeating up to 100 or even 1000 times a second. But this is just the first of many astonishments.

Each neuron is itself a miniature decision-making device performing complex calculations as it receives signals from hundreds, even thousands, of other neurons. Most signals are excitatory, somewhat like pushing a neuron’s accelerator. Some are inhibitory, more like pushing its brake. If excitatory signals exceed the inhibitory signals by a minimum intensity, or threshold, the combined signals trigger an action potential. (Think of it this way: If the excitatory party animals outvote the inhibitory party poopers, the party’s on.) The action potential then travels down the axon, which branches into junctions with hundreds or thousands of other neurons or with the body’s muscles and glands.

Increasing the level of stimulation above the threshold will not increase the neural impulse’s intensity. The neuron’s reaction is an all-or-none response: Like guns, neurons either fire or they don’t. How, then, do we detect the intensity of a stimulus? How do we distinguish a gentle touch from a big hug? A strong stimulus can trigger more neurons to fire, and to fire more often. But it does not affect the action potential’s strength or speed. Squeezing a trigger harder won’t make a bullet go faster.

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How Neurons Communicate

2-3 How do nerve cells communicate with other nerve cells?

Neurons interweave so intricately that even with a microscope you would have trouble seeing where one neuron ends and another begins. Scientists once believed that the axon of one cell fused with the dendrites of another in an uninterrupted fabric. Then British physiologist Sir Charles Sherrington (1857–1952) noticed that neural impulses were taking an unexpectedly long time to travel a neural pathway. Inferring that there must be a brief interruption in the transmission, Sherrington called the meeting point between neurons a synapse.

We now know that the axon terminal of one neuron is in fact separated from the receiving neuron by a synaptic gap (or synaptic cleft) less than a millionth of an inch wide. Spanish anatomist Santiago Ramón y Cajal (1852–1934) marveled at these near-unions of neurons, calling them “protoplasmic kisses.” “Like elegant ladies air-kissing so as not to muss their makeup, dendrites and axons don’t quite touch,” noted poet Diane Ackerman (2004, p. 37). How do the neurons execute this protoplasmic kiss, sending information across the tiny synaptic gap? The answer is one of the important scientific discoveries of our age.

When an action potential reaches the knob-like terminals at an axon’s end, it triggers the release of chemical messengers, called neurotransmitters (FIGURE 2.4). Within 1/10,000th of a second, the neurotransmitter molecules cross the synaptic gap and bind to receptor sites on the receiving neuron—as precisely as a key fits a lock. For an instant, the neurotransmitter unlocks tiny channels at the receiving site, and electrically charged atoms flow in, exciting or inhibiting the receiving neuron’s readiness to fire. The excess neurotransmitters then drift away, are broken down by enzymes, or are reabsorbed by the sending neuron—a process called reuptake.

How Neurotransmitters Influence Us

2-4 How do neurotransmitters influence behavior, and how do drugs and other chemicals affect neurotransmission?

In their quest to understand neural communication, researchers have discovered several dozen neurotransmitters and as many new questions: Are certain neurotransmitters found only in specific places? How do they affect our moods, memories, and mental abilities? Can we boost or diminish these effects through drugs or diet?

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Later chapters explore neurotransmitter influences on hunger and thinking, depression and euphoria, addictions and therapy. For now, let’s glimpse how neurotransmitters influence our motions and our emotions. A particular brain pathway may use only one or two neurotransmitters (FIGURE 2.5), and particular neurotransmitters may affect specific behaviors and emotions (TABLE 2.1 below). But neurotransmitter systems don’t operate in isolation; they interact, and their effects vary with the receptors they stimulate. Acetylcholine (ACh), which is one of the best-understood neurotransmitters, plays a role in learning and memory. In addition, it is the messenger at every junction between motor neurons (which carry information from the brain and spinal cord to the body’s tissues) and skeletal muscles. When ACh is released to our muscle cell receptors, the muscle contracts. If ACh transmission is blocked, as happens during some kinds of anesthesia and with some poisons, the muscles cannot contract and we are paralyzed.

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Candace Pert and Solomon Snyder (1973) made an exciting discovery about neurotransmitters when they attached a radioactive tracer to morphine, showing where it was taken up in an animal’s brain. The morphine, an opiate drug that elevates mood and eases pain, bound to receptors in areas linked with mood and pain sensations. But why would the brain have these “opiate receptors”? Why would it have a chemical lock, unless it also had a natural key to open it?

Researchers soon confirmed that the brain does indeed produce its own naturally occurring opiates. Our body releases several types of neurotransmitter molecules similar to morphine in response to pain and vigorous exercise. These endorphins (short for endogenous [produced within] morphine) help explain good feelings such as the “runner’s high,” the painkilling effects of acupuncture, and the indifference to pain in some severely injured people. But once again, new knowledge led to new questions.

How Drugs and Other Chemicals Alter Neurotransmission If indeed the endorphins lessen pain and boost mood, why not flood the brain with artificial opiates, thereby intensifying the brain’s own “feel-good” chemistry? But there is a problem: When flooded with opiate drugs such as heroin and morphine, the brain, to maintain its chemical balance, may stop producing its own natural opiates. When the drug is withdrawn, the brain may then be deprived of any form of opiate, causing intense discomfort. For suppressing the body’s own neurotransmitter production, nature charges a price.

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Drugs and other chemicals affect brain chemistry, often by either exciting or inhibiting neurons’ firing. Agonist molecules increase a neurotransmitter’s action. Agonists may increase the production or release of neurotransmitters, or block reuptake in the synapse. Other agonists may be similar enough to a neurotransmitter to bind to its receptor and mimic its excitatory or inhibitory effects. Some opiate drugs are agonists and produce a temporary “high” by amplifying normal sensations of arousal or pleasure.

Antagonists decrease a neurotransmitter’s action by blocking production or release. Botulin, a poison that can form in improperly canned food, causes paralysis by blocking ACh release. (Small injections of botulin—Botox—smooth wrinkles by paralyzing the underlying facial muscles.) These antagonists are enough like the natural neurotransmitter to occupy its receptor site and block its effect, as in FIGURE 2.6, but are not similar enough to stimulate the receptor (rather like foreign coins that fit into, but won’t operate, a candy machine). Curare, a poison some South American Indians have applied to hunting-dart tips, occupies and blocks ACh receptor sites on muscles, producing paralysis in their prey.

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2-5 What are the functions of the nervous system’s main divisions, and what are the three main types of neurons?

To live is to take in information from the world and the body’s tissues, to make decisions, and to send back information and orders to the body’s tissues. All this happens thanks to our body’s nervous system (FIGURE 2.7). A quick overview: The brain and spinal cord form the central nervous system (CNS), the body’s decision maker. The peripheral nervous system (PNS) is responsible for gathering information and for transmitting CNS decisions to other body parts. Nerves, electrical cables formed of bundles of axons, link the CNS with the body’s sensory receptors, muscles, and glands. The optic nerve, for example, bundles a million axons into a single cable carrying the messages each eye sends to the brain (Mason & Kandel, 1991).

Information travels in the nervous system through three types of neurons. Sensory neurons carry messages from the body’s tissues and sensory receptors inward (thus, they are afferent) to the brain and spinal cord for processing. Motor neurons (which are efferent) carry instructions from the central nervous system out to the body’s muscles and glands. Between the sensory input and motor output, information is processed via the brain’s interneurons. Our complexity resides mostly in these interneurons. Our nervous system has a few million sensory neurons, a few million motor neurons, and billions and billions of interneurons.

Our peripheral nervous system has two components—somatic and autonomic. Our somatic nervous system enables voluntary control of our skeletal muscles. As you reach the end of this page, your somatic nervous system will report to your brain the current state of your skeletal muscles and carry instructions back, triggering a response from your hand so you can read on.

Our autonomic nervous system (ANS) controls our glands and our internal organ muscles, thus influencing glandular activity, heartbeat, and digestion. (Autonomic means “self-regulating.”) Like an automatic pilot, this system may be consciously overridden, but usually operates on its own (autonomously).

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The autonomic nervous system serves two important functions (FIGURE 2.8). The sympathetic nervous system arouses and expends energy. If something alarms or challenges you (such as a longed-for job interview), your sympathetic nervous system will accelerate your heartbeat, raise your blood pressure, slow your digestion, raise your blood sugar, and cool you with perspiration, making you alert and ready for action. When the stress subsides (the interview is over), your parasympathetic nervous system will produce the opposite effects, conserving energy as it calms you. The sympathetic and parasympathetic nervous systems work together to keep us in a steady internal state called homeostasis (more on this in Chapter 11).

I [DM] recently experienced my ANS in action. Before sending me into an MRI machine for a routine shoulder scan, the technician asked if I had issues with claustrophobia. “No, I’m fine,” I assured her, with perhaps a hint of macho swagger. Moments later, as I found myself on my back, stuck deep inside a coffin-sized box and unable to move, my sympathetic nervous system had a different idea. As claustrophobia overtook me, my heart began pounding and I felt a desperate urge to escape. Just as I was about to cry out for release, I felt my calming parasympathetic nervous system kick in. My heart rate slowed and my body relaxed, though my arousal surged again before the 20-minute confinement ended. “You did well!” the technician said, unaware of my ANS roller-coaster ride.

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From neurons “talking” to other neurons arises the complexity of the central nervous system’s brain and spinal cord.

It is the brain that enables our humanity—our thinking, feeling, and acting. Tens of billions of neurons, each communicating with thousands of other neurons, yield an ever-changing wiring diagram. By one estimate—projecting from neuron counts in small brain samples—our brains have some 40 billion neurons, each connecting with roughly 10,000 other neurons (de Courten-Myers, 2005). This implies some 400 trillion synapses—places where neurons meet and greet their neighbors. (A different research team projected 86 billion brain neurons [Azevedo et al., 2009]. The moral: Distrust big round numbers, such as the familiar but undocumented claim that the human brain has 100 billion neurons.)

The brain’s neurons cluster into work groups called neural networks. To understand why, Stephen Kosslyn and Olivier Koenig (1992, p. 12) have invited us to “think about why cities exist; why don’t people distribute themselves more evenly across the countryside?” Like people networking with people, neurons network with nearby neurons with which they can have short, fast connections. As in FIGURE 2.9, each layer’s cells connect with various cells in the neural network’s next layer. Learning—to play the violin, speak a foreign language, or solve a math problem—occurs as experience strengthens connections. Neurons that fire together wire together.

The other part of the CNS, the spinal cord, is a two-way information highway connecting the peripheral nervous system and the brain. Ascending neural fibers send up sensory information, and descending fibers send back motor-control information. The neural pathways governing our reflexes, our automatic responses to stimuli, illustrate the spinal cord’s work. A simple spinal reflex pathway is composed of a single sensory neuron and a single motor neuron. These often communicate through an interneuron. The knee-jerk response, for example, involves one such simple pathway. A headless warm body could do it.

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Another neural circuit enables the pain reflex (FIGURE 2.10). When your finger touches a flame, neural activity (excited by the heat) travels via sensory neurons to interneurons in your spinal cord. These interneurons respond by activating motor neurons leading to the muscles in your arm. Because the simple pain-reflex pathway runs through the spinal cord and right back out, your hand jerks away from the candle’s flame before your brain receives and responds to the information that causes you to feel pain. That’s why it feels as if your hand jerks away not by your choice, but on its own.

Information travels to and from the brain by way of the spinal cord. Were the top of your spinal cord severed, you would not feel pain from your paralyzed body below. Nor would you feel pleasure. With your brain literally out of touch with your body, you would lose all sensation and voluntary movement in body regions with sensory and motor connections to the spinal cord below its point of injury. You would exhibit the knee-jerk response without feeling the tap. Men paralyzed below the waist may be capable of an erection (a simple reflex) if their genitals are stimulated (Goldstein, 2000). Women similarly paralyzed may respond with vaginal lubrication. But, depending on where and how completely the spinal cord is severed, both may be genitally unresponsive to erotic images and have no genital feeling (Kennedy & Over, 1990; Sipski & Alexander, 1999). To produce bodily pain or pleasure, the sensory information must reach the brain.

2-6 How does the endocrine system transmit information and interact with the nervous system?

So far we have focused on the body’s speedy electrochemical information system. Interconnected with your nervous system is a second communication system, the endocrine system (FIGURE 2.11 below). The endocrine system’s glands secrete another form of chemical messengers, hormones, which travel through the bloodstream and affect other tissues, including the brain. When hormones act on the brain, they influence our interest in sex, food, and aggression.

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Some hormones are chemically identical to neurotransmitters (the chemical messengers that diffuse across a synapse and excite or inhibit an adjacent neuron). The endocrine system and nervous system are therefore close relatives: Both produce molecules that act on receptors elsewhere. Like many relatives, they also differ. The speedy nervous system zips messages from eyes to brain to hand in a fraction of a second. Endocrine messages trudge along in the bloodstream, taking several seconds or more to travel from the gland to the target tissue. If the nervous system transmits information with text-message speed, the endocrine system delivers an old-fashioned letter.

But slow and steady sometimes wins the race. Endocrine messages tend to outlast the effects of neural messages. Upset feelings may linger beyond our awareness of the reason, and it takes time for us to “simmer down.” The persistence of emotions—without conscious awareness of what caused them—was dramatically evident in one ingenious experiment. Brain-damaged patients unable to form new conscious memories watched a sad film and later a happy film. After each viewing, they did not consciously recall the films, but the sad or happy emotion persisted (Feinstein et al., 2010).

In a moment of danger, the ANS orders the adrenal glands on top of the kidneys to release epinephrine and norepinephrine (also called adrenaline and noradrenaline). These hormones increase heart rate, blood pressure, and blood sugar, providing a surge of energy. When the emergency passes, the hormones—and the feelings—linger a while.

The most influential endocrine gland is the pituitary gland, a pea-sized structure located in the core of the brain, where it is controlled by an adjacent brain area, the hypothalamus (more on that shortly). Among the hormones released by the pituitary is a growth hormone that stimulates physical development. Another is oxytocin, which enables contractions associated with birthing, milk flow during nursing, and orgasm. Oxytocin also promotes pair bonding, group cohesion, and social trust (De Dreu et al., 2010; Zak, 2012). During a laboratory game, those given a nasal squirt of oxytocin rather than a placebo were more likely to trust strangers with their money (Kosfeld et al., 2005).

Pituitary secretions also direct other endocrine glands to release their hormones. The pituitary, then, is a master gland (whose own master is the hypothalamus). For example, under the brain’s influence, the pituitary triggers your sex glands to release sex hormones. These in turn influence your brain and behavior. So, too, with stress. A stressful event triggers your hypothalamus to instruct your pituitary to release a hormone that causes your adrenal glands to flood your body with cortisol, a stress hormone that increases blood sugar.

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This feedback system (brain → pituitary → other glands → hormones → body and brain) reveals the intimate connection of the nervous and endocrine systems. The nervous system directs endocrine secretions, which then affect the nervous system. Conducting and coordinating this whole electrochemical orchestra is that maestro we call the brain.