By Sheila Kun RN, BSN, MS, CPN, FCCP

Last edition we reviewed the role of the hypothalamus ; the suprachiasmatic nucleus (SCN) turns off alerting signals, allowing sleep to occur. Similarly, the ventrolateral preoptic nucleus (VLPO) also can inhibit their activity and turn off the arousal centers, promoting sleep to occur. Today we will go over the function of the brain stem in regulating our sleep pattern. The brain stem, at the base of the brain, communicates with the hypothalamus to control the transitions between wake and sleep. (The brain stem includes structures called the pons, medulla, and midbrain.) Sleep-promoting cells within the hypothalamus and the brain stem produce a brain chemical called GABA, which acts to reduce the activity of arousal centers in the hypothalamus and the brain stem. The brain stem (especially the pons and medulla) also plays a special role in REM (rapid eye movement) sleep; it sends signals to relax muscles essential for body. One area of the brain that promotes arousal is the tuberomammillary nucleus (TMN). It is at the posterior base of the hypothalamus. Here, neurons release histamine as one of their neurotransmitters. Interestingly, many “anti-histamine” medicines block this arousing signal and cause sleepiness.

A “Flip-Flop Switch” Between Sleep and Wakefulness

The ability to remain in a stable period of sleep or wakefulness is a result of what scientists call “mutual inhibition” between the wake-promoting neurons and the sleep-promoting neurons. So, for example, the areas of the brain that maintain wakefulness by activating the cortex also inhibit VLPO neurons. Conversely, when VLPO neurons fire rapidly and induce sleep, they also inhibit activity in the arousal centers such as the TMN. Transitions between these stable states of wakefulness and sleep occur relatively quickly, often in just seconds. Some researchers have compared the neurological mechanism that controls these rapid transitions to the “flip-flop switch” in an electrical circuit. In the brain, the mechanism that maintains stability through mutual inhibition is triggered by changes in factors such as the body’s drive for sleep or the circadian alerting signal. When one of these forces becomes strong enough, it drives the transition to the opposite state. Sheila’s note: now that you understanding the tuberomammillary nucleus (TMN) at the base of the posterior hypothalamus is the site for the release of histamine, one of the neurotransmitters for arousal, be aware of anti-histamine medication for allergy. Anti-histamine medication blocks the arousal signal and therefore causing sleepiness.

Your homework from the Care Ministry this week: check your medications at home and see which one might cause drowsiness?

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