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 connections between your brain and the rest of you. 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. Figure 2.2 gives an overview of the entire nervous system.

Brandon suffered a devastating brain injury that temporarily immobilized half of his body. The paralysis could have affected his entire body if the bullet had pierced his spinal cord. This bundle of neurons allows communication between the brain and the peripheral nervous system, which connects 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. Likewise, the skin and other parts of the body have no pathway for communicating sensory information to the brain, like “Ooh, that burner is hot,” or “Oh, this massage feels good.”

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LO 6 Explain how the central and peripheral nervous systems connect.

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. 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 lifelong memories. They are also involved in a relatively simple operation, the reflex.

THE REFLEX ARC 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 below). 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. This type of 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.

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Eventually, your brain does process the event; otherwise, you would have no clue it ever happened. 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 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.

LO 7 Describe the organization and function of the peripheral nervous system.

The peripheral nervous system (PNS) includes all the neurons that are not in the central nervous system. These neurons are bundled together in collections called nerves, which act like electrical cables carrying signals from place to place. Nerves are the primary mechanism for communication by the peripheral nervous system, informing the central nervous system 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. The PNS has two functional branches: the somatic nervous system and the autonomic nervous system.

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THE SOMATIC 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 and send 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.

THE AUTONOMIC NERVOUS SYSTEM Meanwhile, the autonomic nervous system (ȯ-tə-ˈnä-mik) 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 directed by 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 below). 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 preps the body for action by increasing heart rate and respiration, and by 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 response (Corti et al., 2002).

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 initiated by the sympathetic system (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 have 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 11).

TEND AND BEFRIEND 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—that is, they direct energy toward nurturing offspring and forging social bonds (Taylor et al., 2000). Men, too, exhibit this response, especially in high-pressure scenarios. 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). While serving in Iraq, Brandon likely experienced this increase of trust with his fellow soldiers.

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

Imagine that you are 19-year-old Brandon Burns fighting in the battle of Fallujah, 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 you would feel. 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 flee or defend yourself. 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.

LO 8 Summarize the role of the endocrine system and how it influences behavior.

The endocrine system (en-də-krən) is a communication system that uses glands, rather than neurons, to send 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 affect 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.

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When neurotransmitters are released into a synaptic gap, their effects can be almost instantaneous. Hormones usually make long voyages to far-away targets by way of the bloodstream, creating a relatively delayed but usually longer-lasting impact. A neural impulse can travel over 250 mph, much faster than messages sent via hormones, which take minutes (if not longer) to arrive where they are going. However, 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 explore 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 and 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 can impact: (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.

Question 1

1. Interneurons

2. Dendrites

3. Sensory neurons

4. Motor neurons

Question 2

1. brain

2. spinal cord

3. axon hillock

4. nodes of Ranvier

Question 3

Question 4