4-2 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 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 base and work up to the newer systems.

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 4.4). Here lie the controls for your heartbeat and breathing. As 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 4.5). This peculiar cross-wiring is but one of the brain’s many surprises.

Sitting atop the brainstem is the thalamus, a pair of egg-shaped structures that act as the brain’s sensory control center (FIGURE 4.4). The thalamus receives information from all the senses except smell, and routes that information to 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. 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 (“netlike”) 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.

Extending from the rear of the brainstem is the baseball-sized cerebellum, meaning “little brain,” which is what its two wrinkled halves resemble (FIGURE 4.6). The cerebellum 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 executes a perfect bicycle kick, give the player’s 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.

* * *

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 how we construct the visual image. 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.

4-3 What are the limbic system’s structures and functions?

We’ve considered the brain’s oldest parts, but we’ve not yet reached its newest and highest regions, 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 4.7).

THE AMYGDALA Research 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. 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 normally 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 link criminal behavior with amygdala dysfunction (Boccardi et al., 2011; Ermer et al., 2012). 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—not just the amygdala. 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 HYPOTHALAMUS Just below (hypo) the thalamus is the hypothalamus (FIGURE 4.8), 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 4.7), to influence your sex glands to release their hormones. These will intensify the thoughts of sex in your cerebral cortex. (Once again, we see the interplay between the nervous and endocrine systems: The brain influences the endocrine system, which in turn influences the brain.)

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).

Later experiments located other “pleasure centers” (Olds, 1958). (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.”) Just how rewarding are these reward centers? Enough to cause rats to self-stimulate these brain regions more than 1000 times per hour. 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.

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).

Some researchers believe that substance use disorders may stem from malfunctions in natural brain systems for pleasure and well-being (Balodis & Potenza, 2015). People genetically predisposed to this reward deficiency syndrome may crave whatever provides that missing pleasure or relieves negative feelings (Blum et al., 1996).

THE HIPPOCAMPUS The hippocampus processes conscious, explicit memories and decreases in size and function as we grow older. Animals or humans who lose their hippocampus to surgery or injury lose their ability to form new memories of facts and events. Those who survive a hippocampal brain tumor in childhood struggle to remember new information in adulthood (Jayakar et al., 2015). Other modules discuss how hippocampus size and function decrease as we grow older and how our two-track mind uses the hippocampus to process our memories.

* * *

FIGURE 4.9 locates the brain areas we’ve discussed, as well as the cerebral cortex, our next topic.