2.8 Digging Below the Cortex

Drama Central: The Limbic System

LO 15     Distinguish the structures and functions of the limbic system.

Buried beneath the cortex is the limbic system, a group of interconnected structures that play an important role in our experiences of emotion, motivation, and memory. It also fuels our most basic drives, such as hunger, sex, and aggression. The limbic system includes the hippocampus, amygdala, thalamus, and hypothalamus (Figure 2.10).

FIGURE 2.10The Limbic SystemThe limbic system fuels basic drives and processes emotions and memories.
Stockbyte/Getty Images

Hippocampus

The largest structure in the limbic system is the hippocampus, a pair of seahorse-shaped structures. The hippocampus is primarily responsible for processing and forming new memories from experiences, but is not where memories are permanently stored (Eichenbaum, 2004). Given its key role in memory, it may come as no surprise that the hippocampus is one of the brain areas affected by Alzheimer’s disease (Henneman et al., 2009; Wang et al., 2003). On the brighter side of things, the hippocampus is also one of the few places in the brain known to give birth to new neurons throughout life (Eriksson et al., 1998).

Amygdala

Another structure of the limbic system is the amygdala (ə-mig-də-lə), which processes basic emotions like fear and aggression and the memories associated with them (Kalin, Shelton, & Davidson, 2004; Kluver & Bucy, 1939; LeDoux, 2000). Having spent many months in a war zone, Brandon encountered more than his fair share of fear-provoking near-death experiences. On one occasion, he was riding at nearly 60 mph in a Humvee that spun out of control and almost flipped over. “My heart was beating faster than ever before,” Brandon recalls. In dangerous situations like this, activity in the nervous system increases drastically, including in the amygdala, triggering an emotional reaction (for example, terror) and orchestrating a whole-body response to the threat (racing heart, sweaty palms, and the like).

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Thalamus

Seated at the center of the limbic system is the thalamus (tha-lə-məs), whose job is to process and relay sensory information to the appropriate parts of the cortex (visual information to the visual cortex, and so on). The great majority of the data picked up by all the sensory systems, except olfaction (sense of smell), pass through the thalamus before moving on to the cortex for processing (Kay & Sherman, 2007). You might think of the thalamus as an air traffic control tower guiding incoming aircraft; when pilots communicate with the tower, the controllers direct the route to take or the runway to use.

Hypothalamus

Just below the thalamus is the hypothalamus (hī-pō-tha-lə-məs; hypo means “under” in Greek), which keeps the body’s systems in a steady state, making sure functions like blood pressure, body temperature, and fluid/electrolyte balance remain within a healthy range. The hypothalamus is also involved in regulating sleep–wake cycles (Saper, Scammell, & Lu, 2005), sexual arousal (Laan & Janssen, 2007), and appetite (Ahima & Antwi, 2008). For example, neurons from the digestive system send signals to the hypothalamus (such as “stomach is empty”), which then sends signals to higher regions of the brain (such as “it’s time to eat”). But deciding what and when to eat does not always come down to being hungry or full. Other brain areas are involved in eating decisions and can override the hypothalamus, driving you to polish off the French fries or scarf down that chocolate bar even when you are not that hungry.

Deeper Yet: The Brainstem and Cerebellum

The brain is made up of structures responsible for processes as complex as being able to rebuild a car’s engine to selecting the right classes for a degree program. Yet delving deeper in the brain, we find structures that control more primitive functions.

LO 16     Distinguish the structures and functions of the brainstem and cerebellum.

The brain’s ancient core consists of a stalklike trio of structures called the brainstem (Figure 2.11). The brainstem extends from the spinal cord to the forebrain, which is the largest part of the brain and includes the cerebral cortex and the limbic system. Located at the top of the brain stem is the midbrain, and although there is some disagreement about which brain structures belong to the midbrain, most agree it plays a role in levels of arousal. The midbrain is also home to neurons that help generate movement patterns in response to sensory input (Stein, Stanford, & Rowland, 2009). An example would be flinching when someone shouts, “Look out!”

FIGURE 2.11The Brainstem and CerebellumLocated beneath the structures of the limbic system, the brainstem includes the midbrain, pons and medulla. These structures are involved in arousal, movement, and life-sustaining processes. The cerebellum is important for muscle coordination and balance and, when paired with the pons and medulla, makes up the hindbrain.
© Fabrice Lerouge/Onoky/Corbis

Reticular Formation

Part of the reticular formation is located in the midbrain. The reticular formation is an intricate web of neurons that is responsible for levels of arousal—whether you are awake, dozing off, or somewhere in between. It is also involved in your ability to attend selectively to important incoming information by sifting through sensory data on its way to the cortex, picking out what’s relevant and ignoring the rest. Imagine how overwhelmed you would feel by all the sights, sounds, tastes, smells, and physical sensations in your environment if you didn’t have a reticular formation to help you discriminate between the important (the sound of a honking car horn) and trivial (the sound of a dog barking in the distance) information.

The hindbrain includes areas of the brain responsible for fundamental life-sustaining processes. The pons, which helps regulate sleep–wake cycles and coordinates movement between the right and left sides of the body, is an important structure of the hindbrain. The pons sits atop the medulla , a structure that oversees some of the body’s most vital functions, including breathing and heart rate maintenance (Broadbelt, Paterson, Rivera, Trachtenberg, & Kinney, 2010).

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Cerebellum

Behind the brainstem, just above the nape of the neck, sits the orange-sized cerebellum (ser-ə-be-ləm). (Latin for “little brain,” the cerebellum looks like a mini-version of the whole brain.) Centuries ago, scientists found that removing parts of the cerebellum from animals caused them to stagger, fall, and act clumsy. Although the cerebellum is best known for its importance in muscle coordination and balance, scientists are beginning to understand just how much this “little brain” influences higher cognitive processes in the “big brain,” such as abstract reasoning and language production (Fine, Ionita, & Lohr, 2002). People with damaged cerebellums struggle with certain fine distinctions, such as telling the difference between words that sound somewhat alike (for example, “pause” versus “paws”) or for producing emotional reactions that are appropriate to a given situation (Bower & Parsons, 2003).

Wrapping It Up

We’ve come a long way in our journey. Let’s step back and review some of the key points we have learned. The body has two main systems of communication: the fast-acting nervous system and the slower endocrine system. Commanding and coordinating the activity of the nervous system is the brain, whose various regions excel in performing certain tasks. While it may be tempting to view the brain as a collection of parts doing various jobs independent of each other, the reality is that most everything the brain does is the result of many components working in sync. Violins alone won’t make a symphony; you need the flutes, trumpets, and all the instruments in the orchestra to create a unified sound. If one violin in the orchestra breaks, there are other violinists to play the same part, compensating for the loss. Likewise, the brain houses many networks of neurons that have the ability to carry out the same job. When one stops working, another can take over (Doidge, 2007).

This allows us to dispel an urban myth: we do use more than 10% of our brains! In fact, people use 100% of their brains, but they don’t always use them effectively, efficiently, or wisely. Brandon Burns and Christina Santhouse have made the most of their brains, and they are living rich and meaningful lives because of it.

show what you know

Question 2.24

1. The __________ represents a group of interconnected structures that process emotions, memories, and basic drives.

  1. left hemisphere
  2. limbic system
  3. corpus callosum
  4. superior temporal sulcus

Question 2.25

2. The specific part of the brain that processes basic emotions, such as fear and aggression and the memories associated with them, is the __________.

Question 2.26

3. How might the specific structures of the limbic system, the brainstem, and the cerebellum come into play if you were out on a first date?

Question 2.27

4. The primary role of the thalamus is to:

  1. relay sensory information.
  2. keep the body’s systems in a steady state.
  3. generate movement patterns in response to sensory input.
  4. regulate sleep–wake cycles.

CHECK YOUR ANSWERS IN APPENDIX C.

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WHERE ARE THEY NOW?

You may be wondering what became of Brandon Burns and Christina Santhouse. Three years after returning from Iraq, Brandon married a young woman named Laura who has witnessed the dramatic unfolding of his recovery. When Laura first met Brandon, he had a lot of trouble communicating his thoughts. His sentences were choppy; he often omitted words and spoke in a flat and emotionless tone. “His speech was very delayed, very slow,” Laura recalls. Over the years, however, Brandon’s vocabulary has expanded considerably, and he can now express himself with greater feeling and fluidity. “He is able to use more humor and emotion when he’s speaking,” Laura says. This young veteran, who was once unable to talk, read, or write, can now articulate his thoughts in lengthy, complex sentences; read a book; and write for his Web site.

Life Is Good Three years after his traumatic brain injury, Brandon celebrated his marriage to Laura. The couple recently had their third child, a daughter named MacCrea Iona.
Laura Burns

These days, Brandon is quite a busy fellow. Much of his time is spent at home in Bartlett, Tennessee, caring for his sons, 4-year-old Porter and 2-year-old Morgan, and his newborn daughter MacCrea Iona. He also works in a church ministry providing much-needed help to people in the inner city of Memphis, and he has traveled to numerous countries, including Haiti, Kenya, and Honduras, to assist pastors in developing new strategies for reaching out to their communities.

As for Christina, she continues to reach for the stars—and grab them. After studying speech–language pathology at Misericordia University in Dallas, Pennsylvania for 5 years (and making the dean’s list nearly every semester), Christina graduated with both a bachelor’s and master’s degree. Those years in academia were not smooth sailing. When Christina first embarked upon her major, she remembers the department chairman telling her that she wouldn’t be able handle the rigors of the program. According to Christina, on graduation day, that same chairman presented her with the department’s Outstanding Achievement Award. “This is the man who said I would never be able to make it, that I should try a new major,” Christina says. “People often don’t expect too much from people with disabilities.”

Hard at Work With her master’s degree in speech–language pathology, Christina now works full time in Pennsylvania’s public school system.
Courtesy, Bucks County Courier Times

Today, Christina works as a full-time speech–language pathologist in Pennsylvania’s public school system. She spends her days helping elementary schoolchildren overcome their difficulties with stuttering, articulation, and other speech problems. She is also a member of the local school district’s Brain STEPS team, which supports students who are transitioning back into school following brain injuries. “Hopefully, I have opened some doors for other people with disabilities,” Christina offers. “There were never doors open for me; I’ve had to bang them down.”

Brandon and Christina provide breathtaking illustrations of neuroplasticity—the brain’s ability to heal, grow new connections, and make do with what is available. And although neuroplasticity played a role in these two people’s recoveries, we would be remiss not to recognize the vast number of medical professionals, such as neuropsychologists, physical therapists, occupational therapists, speech pathologists, nurses, and doctors, who assisted them in their rehabilitation. The recoveries of Brandon and Christina bear testimony to the awesome tenacity of the human spirit.

Christina: Which medical professional had the biggest impact on your recovery?

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