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Now that we have surveyed the brain’s outer terrain, identifying some of the hotspots for language and other higher cognitive functions, let’s dig deeper and examine some of its older structures.

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).

HIPPOCAMPUS The largest structure in the limbic system is the hippocampus, a pair of curved structures. The hippocampus is primarily responsible for processing and forming new memories from experiences, but it 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 generate new neurons throughout life (Eriksson et al., 1998; Tate, Herbet, Moritz-Gasser, Tate, & Duffau, 2014).

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, the amygdala is activated and the nervous system orchestrates a whole-body response (racing heart, sweaty palms, and the like), as well as an emotional reaction (fear).

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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 variables 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.

The brain is made up of structures responsible for processes as complex as rebuilding a car’s engine or 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.

COMPONENTS OF THE BRAINSTEM The brain’s ancient core consists of a stalklike trio of structures called the brainstem (Figure 2.11). The brainstem, which includes the midbrain, pons, and medulla, 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.

The top portion of the brainstem is known as 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). For example, if someone shouted “Look out!” neurons in your midbrain would play a role when you flinch. Also located in the midbrain is part of the reticular formation, an intricate web of neurons that is responsible for levels of arousal—whether you are awake, dozing off, or somewhere in between. The reticular formation 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 information that is important (the sound of a honking car horn) and that which is trivial (the sound of a dog barking in the distance).

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 (mə-´du̇l-ə), 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, researchers are exploring how 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 producing emotional reactions that are appropriate to a given situation (Bower & Parsons, 2003).

Hooray! We have finally completed our tour of the nervous system. We started on the micro level, exploring electrical and chemical signaling between neurons, and then worked our way through the macro structures of the brain, spinal cord, and peripheral nervous system. As you delve into topics covered in other chapters, remember that communication between neurons underlies every psychological phenomenon. Do not hesitate to revisit this chapter throughout the course; this is your biological foundation.

Question 1

1. left hemisphere

2. limbic system

3. corpus callosum

4. superior temporal sulcus

Question 2

Question 3

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.