Older Brain Structures

• 11-2 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.

AP^® EXAM TIP

Your authors, David Myers and Nathan DeWall, are about to take you on a journey through your brain. Focus on the name of each part, its location within the brain, and what it does. Then it’s time to practice, practice, practice. The brain structures have been on previous AP^® exams.

The brain’s increasing complexity arises from new 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 Brainstem

The brainstem is the brain’s oldest and innermost region. Its base is the medulla, the slight swelling in the spinal cord just after it enters the skull (Figure 11.4). 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.

brainstem

the oldest part and central core of the brain, beginning where the spinal cord swells as it enters the skull; the brainstem is responsible for automatic survival functions.

medulla [muh-DUL-uh]

the base of the brainstem; controls heartbeat and breathing.

Figure 11.4 The brainstem and thalamus

The brainstem, including the pons and medulla, is an extension of the spinal cord. The thalamus is attached to the top of the brainstem. The reticular formation passes through both structures.

The brainstem, including the pons and medulla, is an extension of the spinal cord. The thalamus is attached to the top of the brainstem. The reticular formation passes through both structures. The reticular formation is located in the region of the brain behind the ear and the thalamus is in the center of the brain.

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 11.5). This peculiar cross-wiring is but one of the brain’s many surprises.

Figure 11.5 The body’s wiring

Nerves from the left side of the brain are mostly linked to the right side of the body, and vice versa.

The Thalamus

Sitting atop the brainstem is the thalamus, a pair of egg-shaped structures that act as the brain’s sensory control center (Figure 11.5). The thalamus receives information from all the senses except smell, and routes that information 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). For sensory information, your thalamus is something like London’s Heathrow Airport, a hub through which traffic flows in and out on its way to various locations.

thalamus [THAL-uh-muss]

the brain’s sensory control center, located on top of the brainstem; it directs messages to the sensory receiving areas in the cortex and transmits replies to the cerebellum and medulla.

The Reticular Formation

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 and relays important information to other brain areas. Have you multitasked today? You can thank your reticular formation (Wimmer et al., 2015).

reticular formation

a nerve network that travels through the brainstem into the thalamus and plays an important role in controlling arousal.

The reticular formation also controls arousal, as Giuseppe Moruzzi and Horace Magoun discovered in 1949. 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 Cerebellum

Extending from the rear of the brainstem is the baseball-sized cerebellum, meaning “little brain,” which is what its two wrinkled halves resemble (Figure 11.6). As you will see in Module 32, 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 masterfully controls the ball, give the player’s cerebellum some credit. Under alcohol’s influence on the cerebellum, 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.

cerebellum [sehr-uh-BELL-um]

the “little brain” at the rear of the brainstem; functions include processing sensory input, coordinating movement output and balance, and enabling nonverbal learning and memory.

Figure 11.6 The brain’s organ of agility

Hanging at the back of the brain, the cerebellum coordinates our voluntary movements, as when soccer star Cristiano Ronaldo controls the ball.

The diagram shows the brain from below, as if seen from the spinal cord. The cerebellum is located at the very back of the brain next to the spinal cord.

* * *

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.

Check Your Understanding

Ask Yourself

• Are you surprised to learn about all the information-processing that happens automatically, without your knowledge? Why or why not?

Test Yourself

• The ______________ is a crossover point where nerves from the left side of the brain are mostly linked to the right side of the body, and vice versa.

Answers to the Test Yourself questions can be found in Appendix E at the end of the book.