6-2 What structures make up the brainstem, and what are the functions of the brainstem, thalamus, reticular formation, and cerebellum?

An animal’s capacities come from its brain structures. In primitive animals, such as sharks, a not-so-complex brain primarily regulates basic survival functions: breathing, resting, and feeding. In lower mammals, such as rodents, a more complex brain enables emotion and greater memory. In advanced mammals, such as humans, a brain that processes more information enables increased foresight as well.

This increasing complexity arises from new brain systems built on top of the old, much as the Earth’s landscape covers the old with the new. Digging down, one discovers the fossil remnants of the past—brainstem components performing for us much as they did for our distant ancestors. Let’s start with the brain’s basement.

The brain’s oldest and innermost region is the brainstem. It begins where the spinal cord swells slightly after entering the skull. This slight swelling is the medulla (FIGURE 6.4). Here lie the controls for your heartbeat and breathing. As some brain-damaged patients in a vegetative state illustrate, we need no higher brain or conscious mind to orchestrate our heart’s pumping and lungs’ breathing. The brainstem handles those tasks. Just above the medulla sits the pons, which helps coordinate movements and control sleep.

If a cat’s brainstem is severed from the rest of the brain above it, the animal will still breathe and live—and even run, climb, and groom (Klemm, 1990). But cut off from the brain’s higher regions, it won’t purposefully run or climb to get food.

The brainstem is a crossover point, where most nerves to and from each side of the brain connect with the body’s opposite side (FIGURE 6.5). This peculiar cross-wiring is but one of the brain’s many surprises.

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Sitting atop the brainstem is the thalamus, a pair of egg-shaped structures that act as the brain’s sensory control center (Figure 6.4). The thalamus receives information from all the senses except smell and routes it to the higher brain regions that deal with seeing, hearing, tasting, and touching. The thalamus also receives some of the higher brain’s replies, which it then directs to the medulla and to the cerebellum (see below). Think of the thalamus as being to sensory information what London is to England’s trains: a hub through which traffic passes en route to various destinations.

Inside the brainstem, between your ears, lies the reticular (“net-like”) formation, a neuron network extending from the spinal cord right up through the thalamus. As the spinal cord’s sensory input flows up to the thalamus, some of it travels through the reticular formation, which filters incoming stimuli, relays important information to other brain areas, and controls arousal.

In 1949, Giuseppe Moruzzi and Horace Magoun discovered that electrically stimulating a sleeping cat’s reticular formation almost instantly produced an awake, alert animal. When Magoun severed a cat’s reticular formation without damaging nearby sensory pathways, the effect was equally dramatic: The cat lapsed into a coma from which it never awakened. The conclusion? The reticular formation enables arousal.

Extending from the rear of the brainstem is the baseball-sized cerebellum, meaning “little brain,” which is what its two wrinkled halves resemble (FIGURE 6.6). The cerebellum (along with the basal ganglia, deep brain structures involved in motor movement) enables nonverbal learning and skill memory. It also helps us judge time, modulate our emotions, and discriminate sounds and textures (Bower & Parsons, 2003). And (with assistance from the pons) it coordinates voluntary movement. When a soccer player masterfully controls the ball, give his cerebellum some credit. Under alcohol’s influence, coordination suffers. And if you injured your cerebellum, you would have difficulty walking, keeping your balance, or shaking hands. Your movements would be jerky and exaggerated. Gone would be any dreams of being a dancer or guitarist.

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Note: These older brain functions all occur without any conscious effort. This illustrates another of our recurring themes: Our brain processes most information outside of our awareness. We are aware of the results of our brain’s labor (say, our current visual experience) but not of the how. Likewise, whether we are asleep or awake, our brainstem manages its life-sustaining functions, freeing our newer brain regions to think, talk, dream, or savor a memory.

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6-3 What are the limbic system’s structures and functions?

We’ve now considered the brain’s oldest parts. Its newest and highest regions are the cerebral hemispheres (the two halves of the brain). Between the oldest and newest brain areas lies the limbic system (limbus means “border”). This system contains the amygdala, the hypothalamus, and the hippocampus (FIGURE 6.7). The hippocampus process conscious, explicit memories. Animals or humans who lose their hippocampus to surgery or injury also lose their ability to form new memories of facts and events. Other modules explain how our two-track mind uses the hippocampus to process our memories. For now, let’s look at the limbic system’s links to emotions such as fear and anger, and to basic motives such as those for food and sex.

The AmygdalaResearch has linked the amygdala, two lima-bean-sized neural clusters, to aggression and fear. In 1939, psychologist Heinrich Klüver and neurosurgeon Paul Bucy surgically removed a rhesus monkey’s amygdala, turning the normally ill-tempered animal into the most mellow of creatures. In studies with other wild animals, including the lynx, wolverine, and wild rat, researchers noted the same effect. So, too, with human patients. Those with amygdala lesions often display reduced arousal to fear-and anger-arousing stimuli (Berntson et al., 2011). One such woman, patient S. M., has been called “the woman with no fear,” even of being threatened with a gun (Feinstein et al., 2013).

What then might happen if we electrically stimulated the amygdala of a placid domestic animal, such as a cat? Do so in one spot and the cat prepares to attack, hissing with its back arched, its pupils dilated, its hair on end. Move the electrode only slightly within the amygdala, cage the cat with a small mouse, and now it cowers in terror.

These and other experiments have confirmed the amygdala’s role in fear and rage. One study found math anxiety associated with hyperactivity in the right amygdala (Young et al., 2012). Other studies have shown people angry and happy faces: The amygdala activates in response to the angry ones (Mende-Siedlecki et al., 2013). But we must be careful. The brain is not neatly organized into structures that correspond to our behavior categories. When we feel or act in aggressive or fearful ways, there is neural activity in many areas of our brain. If you destroy a car’s dead battery, you can’t start the engine. Yet the battery is merely one link in an integrated system.

The HypothalamusJust below (hypo) the thalamus is the hypothalamus (FIGURE 6.8), an important link in the command chain governing bodily maintenance. Some neural clusters in the hypothalamus influence hunger; others regulate thirst, body temperature, and sexual behavior. Together, they help maintain a steady (homeostatic) internal state.

As the hypothalamus monitors the state of your body, it tunes into your blood chemistry and any incoming orders from other brain parts. For example, picking up signals from your brain’s cerebral cortex that you are thinking about sex, your hypothalamus will secrete hormones. These hormones will in turn trigger the adjacent “master gland” of the endocrine system, your pituitary (see Figure 6.7), to influence your sex glands to release their hormones. These will intensify the thoughts of sex in your cerebral cortex. (Note the interplay between the nervous and endocrine systems: The brain influences the endocrine system, which in turn influences the brain.)

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A remarkable discovery about the hypothalamus illustrates how progress in science often occurs—when curious, open-minded investigators make an unexpected observation. Two young McGill University neuropsychologists, James Olds and Peter Milner (1954), were trying to implant an electrode in a rat’s reticular formation when they made a magnificent mistake: They placed the electrode incorrectly (Olds, 1975). Curiously, as if seeking more stimulation, the rat kept returning to the location where it had been stimulated by this misplaced electrode. On discovering that they had actually placed the device in a region of the hypothalamus, Olds and Milner realized they had stumbled upon a brain center that provides pleasurable rewards (Olds, 1975).

In a meticulous series of experiments, Olds (1958) went on to locate other “pleasure centers,” as he called them. (What the rats actually experience only they know, and they aren’t telling. Rather than attribute human feelings to rats, today’s scientists refer to reward centers, not “pleasure centers.”) When allowed to press pedals to trigger their own stimulation, rats would sometimes do so more than 1000 times per hour. Moreover, they would even cross an electrified floor that a starving rat would not cross to reach food (FIGURE 6.9).

In other species, including dolphins and monkeys, researchers later discovered other limbic system reward centers, such as the nucleus accumbens in front of the hypothalamus. Animal research has also revealed both a general dopamine-related reward system and specific centers associated with the pleasures of eating, drinking, and sex. Animals, it seems, come equipped with built-in systems that reward activities essential to survival.

Researchers are experimenting with new ways of using brain stimulation to control animals’ actions in search-and-rescue operations. By rewarding rats for turning left or right, one research team trained previously caged rats to navigate natural environments (Talwar et al., 2002; FIGURE 6.10). By pressing buttons on a laptop, the researchers were then able to direct the rat—which carried a receiver, power source, and video camera on a backpack—to turn on cue, climb trees, scurry along branches, and return.

Do humans have limbic centers for pleasure? To calm violent patients, one neurosurgeon implanted electrodes in such areas. Stimulated patients reported mild pleasure; unlike Olds’ rats, however, they were not driven to a frenzy (Deutsch, 1972; Hooper & Teresi, 1986). Moreover, newer research reveals that stimulating the brain’s “hedonic hotspots” (its reward circuits) produces more desire than pure enjoyment (Kringelbach & Berridge, 2012). Experiments have also revealed the effects of a dopamine-related reward system in people. For example, dopamine release produces our pleasurable “chills” response to a favorite piece of music (Zatorre & Salimpoor, 2013).

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Some researchers believe that addictive disorders, such as substance use disorders and binge eating, may stem from malfunctions in natural brain systems for pleasure and well-being. People genetically predisposed to this reward deficiency syndrome may crave whatever provides that missing pleasure or relieves negative feelings (Blum et al., 1996).

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FIGURE 6.11 locates the brain areas we’ve discussed, as well as the next module’s focus, the cerebral cortex.