2.4 The Supporting Systems

Like any complex system, the brain needs a supporting infrastructure to carry out its directives and relay essential information from the outside world. Running up and down your spine and branching throughout your body are neurons that provide the connections between brain and body. The central nervous system (CNS) is made up of the brain and spinal cord. The peripheral nervous system (PNS) includes all the neurons that are not in the central nervous system and is divided into two branches: the somatic nervous system and the autonomic nervous system. The peripheral nervous system provides the communication pathway between the central nervous system and the rest of the body. Thanks to your spinal cord and peripheral nervous system, you can flex your facial muscles into a smirk, feel a soft breeze, and flip through the pages of this book. Figure 2.2 provides an overview of the entire nervous system.

FIGURE 2.2Overview of the Nervous SystemThe nervous system is made up of the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system.

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The Spinal Cord and Simple Reflexes

Brandon suffered a devastating brain injury that temporarily immobilized half of his body. The paralysis would have affected his entire body if the bullet had pierced his spinal cord, the bundle of neurons that allows communication between the brain and the peripheral nervous system, connecting with the body’s muscles, glands, and organs. The spinal cord has two major responsibilities: (1) receiving information from the body and sending it to the brain; and (2) taking information from the brain and sending it throughout the body. If this pathway is blocked, commands from the brain cannot reach the muscles responsible for making you walk, dance, and wash dishes, and sensory information from the skin and elsewhere has no way of communicating crucial information to the brain, like “Ooh, that burner is hot,” or “Oh, this massage feels good.”

How do the brain and spinal cord, which make up the central nervous system, communicate with the rest of the body through the peripheral nervous system? In essence, there are three types of neurons participating in this back-and-forth communication. Sensory neurons receive information about the environment from the sensory systems and convey this information to the brain for processing. Motor neurons carry information from the central nervous system to produce movement in various parts of the body, such as muscles and glands. Motor neurons provide a mechanism for performing activities determined or commanded by the spinal cord and brain. Interneurons, which reside exclusively in the brain and spinal cord, act as bridges connecting sensory and motor neurons. By assembling and processing sensory input from multiple neurons, interneurons facilitate the nervous system’s most complex operations, from solving tough problems to forming life-long memories. They are also involved in a relatively simple operation, the reflex.

Some activities don’t involve the interneurons in the brain (at least at the start). Consider the withdrawal reaction to painful stimuli (Figure 2.3). If you accidentally touch a hot pan, you activate a pathway of communication that goes from your sensory neurons through interneurons in your spinal cord and right back out through motor neurons, without initially involving the brain. In the following example, you can see how the pain reflex includes a number of steps: (1) Your hand touches the hot pan, activating sensory receptors, which cause the sensory neurons to carry a signal from your hand to the spinal cord. (2) In the spinal cord, the signal from the sensory neurons is received by interneurons. (3) The interneurons quickly activate motor neurons and instruct them to respond. (4) The motor neurons then instruct your muscles to contract, causing your hand to withdraw quickly. A sensory neuron has a rendezvous with an interneuron, which then commands a motor neuron in the spinal cord to react—no brain required. We refer to this process, in which a stimulus causes an involuntary response, as a reflex arc.

FIGURE 2.3The Spinal Cord and Reflex ArcWithout any input from the brain, the spinal cord neurons are capable of creating some simple reflexive behavior. While the spinal reflex occurs, sensory neurons also send messages to the brain, letting it know what has happened.

Eventually, your brain does process the event; otherwise, you would have no clue it ever happened. However, you become consciously aware of your reaction after it has occurred (my hand just pulled back; that pan was hot!). Although many sensory and motor neurons are involved in this reaction, it happens very quickly, hopefully in time to reduce damage or injury in cases when the reflex arc involves pain. Think about touching a flame or something sharp. You want to be able to respond, without waiting for information to get to the brain or for the brain to send a message to the motor neurons instructing the muscles to react.

What Lies Beyond: The Peripheral Nervous System

It is now time to look more specifically at how the peripheral nervous system is organized and how it allows for communication between the central nervous system and the rest of the body.

The peripheral nervous system includes all the neurons that are not in the central nervous system. These neurons are bundled together and act like electrical cables carrying signals from place to place. The collections of neurons are called nerves. Nerves are the primary mechanism for communication of information by the peripheral nervous system, supplying the central nervous system with information about the body’s environment—both the exterior (for example, sights, sounds, and tastes) and the interior (for example, heart rate, blood pressure, and temperature). The central nervous system, in turn, makes sense of all this information and then responds by dispatching orders to the muscles, glands, and other tissues through the nerves of the peripheral nervous system. Information flows from the peripheral nervous system (via sensory nerves) into the central nervous system, and then new information is sent back out through the peripheral nervous system (via motor nerves). The PNS has two functional branches: the somatic nervous system and the autonomic nervous system.

The somatic nervous system includes sensory nerves and motor nerves. (Somatic means “related to the body.”) The sensory nerves gather information from sensory receptors, sending it to the central nervous system. The motor nerves receive information from the central nervous system and send this information to the muscles, instructing them to initiate voluntary muscle activity (which results in movement). The somatic nervous system controls the skeletal muscles that give rise to voluntary movements, like using your arms and legs. It also receives sensory information from the skin and other tissues, providing the brain with constant feedback about temperature, pressure, pain, and other stimuli.

Meanwhile, the autonomic nervous system is working behind the scenes, regulating involuntary activity, such as the pumping of the heart, the expansion and contraction of blood vessels, and digestion. Most of the activities supervised by the somatic nervous system are voluntary (within your conscious control and awareness), whereas processes under the control of the autonomic nervous system tend to be involuntary (automatic) and outside of your awareness. Just remember: Autonomic controls the automatic. But this is not a hard-and-fast rule. The knee-jerk reflex, for instance, is managed by the somatic, or “voluntary,” system even though it is an involuntary response.

The autonomic nervous system has two divisions involved in our physiological responses to stressful or crisis situations (Figure 2.4). The sympathetic nervous system initiates what is often referred to as the “fight-or-flight” response, which is how the body prepares to deal with a crisis. When faced with a stressful situation, the sympathetic nervous system prepares the body for action, by increasing heart rate and respiration, slowing digestion and other bodily functions. Earlier, we mentioned that caffeine makes you feel physically energized. This is because it activates the fight-or-flight system (Corti et al., 2002).

FIGURE 2.4The Sympathetic and Parasympathetic Nervous SystemsThe autonomic nervous system has two divisions, the sympathetic and parasympathetic nervous systems. In a stressful situation, the sympathetic nervous system initiates the “fight-or-flight” response. The parasympathetic nervous system calms the body when the stressful situation has passed.

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The parasympathetic nervous system, on the other hand, oversees the “rest-and-digest” process, which basically works to bring the body back to a noncrisis mode. The parasympathetic nervous system takes over when the crisis has ended by reversing the activity of these processes (for example, lowering heart rate and respiration, increasing digestion and other maintenance activities). The two systems work together, balancing the activities of these primarily involuntary processes. Sometimes they even work toward a common goal. For example, parasympathetic stimulation increases blood flow to the penis to create an erection, but it is the sympathetic system that causes ejaculation (Goldstein, 2000). Working together, these two systems allow us to fight if we need to, flee when necessary, and calm down when danger has passed.

The fight-or-flight response would certainly come in handy if fleeing predators was part of your day-to-day life (as it may have been for our primitive ancestors), but you probably are not chased by wild animals very often. You may, however, notice your heart racing and your breathing rate increase during other types of anxiety-producing situations—going on a first date, taking a test, or speaking in front of an audience. You have your sympathetic nervous system to thank for these effects (Chapter 12).

Fighting and running like mad are not the only ways we respond to stress. Many women have an inclination to “tend and befriend” in response to a threat, or direct energy toward nurturing offspring and forging social bonds (Taylor et al., 2000). The tend-and-befriend response is also evident in men, especially in high-pressure situations. In one small study, men placed in a stressful situation were more likely to show increased trust of others; those others, in turn, were more likely to feel that these men were trustworthy. The trusting men were also more willing to share resources than were those who were not subjected to stress (von Dawans, Fischbacher, Kirschbaum, Fehr, & Heinrichs, 2012). Brandon likely experienced this increase of trust while serving in Iraq with his fellow soldiers.

Women are generally more likely to “tend and befriend,” but are there other gender disparities related to the nervous system? Many of us believe males and females are “hardwired” differently or socially conditioned to develop certain tendencies. Let’s take a look at some of the evidence.

THINK again

Male Brain, Female Brain

It is a well-known fact that the numbers of women employed in certain fields, such as science, math, engineering, and technology, continue to be low (Blickenstaff, 2005; Figure 2.5). Does society encourage boys to pursue science and technology interests while pushing girls into the social sciences and humanities? There is no denying that social and cultural factors influence female achievement in math and science. Studies suggest, for example, that gender stereotypes, which are commonly held beliefs about the nature of men and women, can influence performance in math and science. When exposed to statements such as “women possess poor math ability” just before taking a test, some women will actually perform at a lower level (Josephs, Newman, Brown, & Beer, 2003; Chapter 15).

FIGURE 2.5Bachelor’s Degrees Awarded in the United StatesWomen earn the majority (57.3%) of all bachelor’s degrees awarded in the United States. But when it comes to degrees awarded in science, math, and engineering (STEM), the numbers look very different (National Science Foundation, 2011). Why do so many fewer women receive degrees in STEM fields?

SCIENCE AND THE SEXES

But there might also be something biological at play. Research shows that male and female brains are far more alike than they are different, but some intriguing differences exist, both in terms of anatomy and function. An fMRI study by Goldstein and colleagues (2001) found that certain regions of the limbic cortex and the frontal lobes were larger in women, while areas of the parietal cortex, the amygdala, and hypothalamus were larger in men. How these differences translate into behavior is not totally clear, but we know, for example, these regions are involved in spatial reasoning, memory, and emotion (Cahill, 2012). It appears that both nature and nurture are responsible for the gender imbalance in math and the sciences. While women are statistically underrepresented in math and science, their presence in these fields has grown dramatically over the past 50 years, a clear sign of positive change. For more on gender differences, see Chapter 10.

The Endocrine System and Its Slowpoke Messengers

Imagine that you are 19-year-old Brandon Burns fighting in the battle of Fallujah. How would it feel to be immersed in one of the bloodiest battles of the Iraq War? The sound of gunfire rings through the air. Bullets zip past your helmet. People are dying around you. Your life could end at any moment. Unless you have been in a similar situation, it would be difficult to fathom how it feels. But one thing seems certain: You would feel extremely stressed.

When faced with imminent danger, the sympathetic nervous system responds almost instantaneously. Activity in the brain triggers the release of neurotransmitters that cause increases in heart rate, breathing rate, and metabolism—changes that will come in handy if you need to defend yourself or flee the situation. But the nervous system does not act alone. The endocrine system is also hard at work, releasing stress hormones, such as cortisol, which prompt similar physiological changes.

The endocrine system (en-də-krən) is a communication system that uses glands, rather than neurons, to convey messages (Figure 2.6). These messages are delivered by hormones, chemicals released into the bloodstream that can cause aggression and mood swings, as well as influence growth, alertness, cognition, and appetite. Like neurotransmitters, hormones are chemical messengers that can influence many processes and behaviors. In fact, some chemicals, such as norepinephrine, can act as both neurotransmitters and hormones depending on where they are released. Neurotransmitters are unloaded into the synapse, whereas hormones are secreted into the bloodstream by glands stationed around the body. These glands collectively belong to the endocrine system.

FIGURE 2.6The Endocrine SystemThis system of glands communicates within the body by secreting hormones directly into the bloodstream.

left: (face) Hemera/Thinkstock, (body) Yuri Arcurs; right: Asiaselects/Getty Images

Following an action potential, neurotransmitters are released into the synaptic gap and their effects can be almost instant. Hormones usually make long voyages to faraway targets by way of the bloodstream, creating a relatively delayed but usually longer-lasting impact. A neural impulse can travel over 250 mph, which means signals arrive at their destinations within fractions of a second, much faster than messages sent via hormones, which take minutes (if not longer) to arrive where they are going. Although not as fast as neurotransmitters, the messages sent via hormones are more widely spread because they are disseminated through the bloodstream.

If the endocrine system had a chief executive officer, it would be the pituitary gland, a gland about the size of a pencil eraser located in the center of the brain, just under the hypothalamus (a structure of the brain we will return to later). Controlled by the hypothalamus, the pituitary gland influences all the other glands, as well as promoting growth through the secretion of hormones. The thyroid gland regulates the rate of metabolism by secreting thyroxin, and the adrenal glands (ə-drē-nəl) are involved in responses to stress as well as the regulation of salt balance.

Other endocrine glands and organs directed by the pituitary include the pineal gland, which secretes melatonin (controls sleep–wake cycles); the pancreas, which secretes insulin (regulates blood sugar); and the ovaries and testes, whose secretion of sex hormones is one reason that men and women are different. Together, these glands and organs influence a variety of processes and behaviors: (1) growth and sex characteristics, (2) regulation of some of the basic body processes, and (3) responses to emergencies. Just as our behaviors are influenced by neurotransmitters we can’t see and action potentials we can’t feel, the hormones secreted by the endocrine system are also hard at work behind the scenes.

Now that we have discovered how information moves through the body via electrical and chemical signals, let’s turn our attention toward the part of the nervous system that integrates this activity, creating a unified and meaningful experience. Let’s explore the brain.