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Brain Stem

The brain is a large expansion of the top end of the spinal cord, filling the skull cavity. Figure 1 shows the left side of the brain. The top of the spinal cord (lower right) expands as it enters the skull to form the brain stem. In humans (and many other mammals), the brain stem is largely hidden by the cerebral hemispheres, which bulge off its front end to form the highest level of the brain.

Figure 2 shows a slice down the middle of the brain. The inner surface of the left hemisphere is shown above and around the brain stem. It shows how the brain stem extends up between the hemispheres.

The brain stem carries out for the face the functions that the spinal cord carries out for the body. It receives sensory information from all senses (except olfaction [smell]) from the face, controls the muscles for the face, and organizes reflexes involving the face. Sensory pathways from the spinal cord to the cerebral hemispheres and motor pathways from the cerebral hemispheres to the spinal cord pass through it.

The brain stem also organizes many of the vital reflexes that manage the internal organs of the body, including heart beat, blood pressure, swallowing, stomach activity, etc. Because these reflex centers are located in the lower brain stem just inside the base of the skull, a sharp blow to the base of the skull can kill by disrupting these reflexes.

In addition, the brain stem has extra, specialized functions. It organizes complex reactions and co-ordinates many different reflexes, so that they work together smoothly. Examples of complex reflex reactions organized in the brain stem are the startle reflex and various balance reflexes. Many things, such as an loud sound, an unexpected touch or sound, etc., can elicit it. A startle reflex involves most of the body: the body, arms, and legs partially flex, neck jerks back, and the face shows a startled expression (partially opened mouth, eyes widened, etc.).

One of many different kinds of balance reflexes is triggered by tripping. Normally, you usually don't notice the action of balance reflexes, yet they work all the time keeping your body erect. If they quit working, the body collapses, as has happened in a couple of rare cases (J. Cole, Pride and the Daily Marathon, MIT Press, 1995. Click HERE for an article describing this case.) You very much notice how strong these reflexes are when your balance is disturbed, as when you trip. When your foot catches on something, the other leg goes way out in front of you to catch your body's weight, your body flexes, and your arms go out to anticipate the fall.

The core of the brain stem contains a structure called the reticular formation or reticular activating system. This structure is critical for "waking up" the rest of the brain (Morruzzi & Magoun, 1949; Adrian, E. D., et al., 1954). Damage to it can produce long-lasting coma, in which patients are unresponsive to anything. Even strong stimulation, like pain, produces only brief arousal (Lindsley et al., 1949); Scheibel, 1980). It also contains areas that block signals in the pathways leading to pain areas of the brain (Reynolds, 1969; Liebsekind et al., 1973; Gebhart et al., 1984).