The Basal Region: Your Reptilian Brain

At the basal region of your brain, your spinal cord enlarges to form the medulla blongata, and above it a bulbous structure called the pons, two structures that regulate and control the most primitive aspects of life: breathing, heartbeat, arousal, and primary motor control.This portion of the system is sometimes called the brainstem, considered by scientists to be the most ancient part of the brain, evolutionarily speaking.We share this primary type of structure with reptiles, birds, and probably with the dinosaurs. Your spinal cord itself is a miniature computer of sorts, where some primitive processes are controlled by in-built spinal reflexes.These include the familiar patellar knee jerk, which the physician tests with a little hammer, and automatic recoil responses to sharp pain, heat, or cold.When you put your weight on your feet as you get out of bed or rise from a chair, your spinal reflexes automatically activate the muscles that raise the arches of your feet so that they will support you properly. This stretch reflex is a in built spinal feature that serves most of the muscles in your body. Sexual orgasm also qualifies as a spinal reflex, although it is mediated in complex ways by cortical activity and a dozen or more hormones and neurotransmitters. At this basal level, various other specialized structures control your autonomic, or involuntary, functions, such as hunger, thirst, sleep and wakefulness, sexual drives, various organ processes, blood pressure, and the general level of activity of your entire nervous system. The pupillary reflex—the automatic dilation and constriction of the pupils of your eyes in response to light—is a fairly reliable indicator of these autonomic functions, which emergency medical workers test to assess for brain injury. Curiously, the processes of falling asleep and waking up are not controlled by the main regions of the brain, but rather by a small patch of cells in the brainstem known as the reticular activating system (RAS). By means not yet well understood, the RAS apparently “turns on” your cortex as you wake up and figuratively turns it off to put you to sleep. Although we can resist falling asleep, it’s well proven that a human being cannot voluntarily stay awake indefinitely. General anesthetics typically work their effects on the RAS.While the RAS doesn’t “cause” consciousness, it seems to be necessary for conscious mental activity to occur. It may also be implicated in attention deficit disorder, and possibly hyperactivity disorder. The brainstem contains specialized cells that secrete neurotransmitters, the chemical messengers that enable the neurons to communicate with one another.These include serotonin, dopamine, acetylcholine, and a number of others.The relative concentrations of these messenger molecules in the brain tends to reflect the current state of brain activity. Some researchers claim that romantic infatuation, for example, is signaled by an increased concentration of dopamine (hence the name). This same basal region has another special structure that easily qualifies as a computer—or at least a sub-computer—in its own right. This is the cerebellum, a plum-sized blob of special nerve tissue that handles your habitual motor functions, such as balance and coordination, walking, routine hand and arm movements, speech, eye movements, and other well-learned motor processes such as a golf swing or tennis serve, typing into your computer, or dancing. The cerebellum (Latin for “little brain”) is also divided into left and right hemispheres. Its neurons, known as granule cells, are very tiny, and while it occupies only about 10 percent of your brain’s volume, your cerebellum has nearly 50 percent of all the neurons in your brain. It receives about two hundred million input fibers, compared to, say, the optic nerve, which contains about one million fibers. It’s the job of the cerebellum to reduce the information-processing load on the cerebral cortex, freeing it for more abstract mental activity. Although the motor control region of the cortex can send commands to various muscles throughout the body, it typically delegates responsibility to the cerebellum for “over-learned” activities that become “second nature.” As you learn any new motor activity, such as writing, singing a song, or reciting multiplication tables, your cerebellum tunes in to the neural activity in your cortex and begins to mimic the patterns in its own neurons. After a number of repetitions, the cerebellum has recorded a script of sorts, which it can call on to control the activity itself. Once the function has been fully learned, the cerebellum takes over control and it actually becomes difficult for the cortex to over ride it. As an experiment, try to take over conscious control of the process of walking across the room or up a flight of stairs. Note how the cerebellar auto-pilot seems to operate almost independently of your effort, making it very difficult to control it by conscious intention.These learned scripts actually account for a large proportion of your brain’s activity.