Neuronal Controls of Sleeping, Dreaming, and Waking
We know that sleep is organized as an alternating cycle. The next question for the Western scientist is: How is the cycle organized within the brain? We know that it is controlled by brain structures localized in part of the brain stem. The brain stem looks somewhat like the base of a flower. It connects the stem of the flower to the blossom just as in a lotus plant. The bulb of the flower, by this analogy, is the brain stem. This small but very important part of the brain, between the spinal cord and the rest of the forebrain, supports our conscious activities. Neuronal machinery which controls the alternation among sleep stages and wakefulness is located in a small region of the brain stem called the pons or bridge.
This location is obviously strategic. It can control inputs and outputs for the whole body. It can control activity throughout the whole upper brain, the forebrain, which we consider to be the organ of consciousness. Our third point, then, is that this regular alternation in sleep is controlled by the brain stem.
The obvious next question is: How does the brain stem do this? Within this pontine region are two populations of nerve cells that have distinctive chemical signatures. One is the neuronal population that supports the waking state, and we suppose it to be responsible for arousal and even anxiety. Its chemical signaling involves the release of amino acids, hence it is known as an aminergic system. When this system is very active, we are very alert, but we may also become too alert. We may become anxiously alert.
It is the proper regulation of this system that I think constitutes one of the goals of Buddhist meditative practices. And this same system is obviously of great significance also to Western medicine. This is so not only because this population of aminergic neurons controls arousal, wakefulness, alertness, and anxiety, but also because outputs of this system affect vital functions like breathing, blood pressure, and other visceral as well as cerebral contributions to our experiences.
This aminergic system in the pontine part of the brain stem is also involved in energy regulation and energy flow, and probably also with aggressive behavior. I think it is a key to understanding a number of very important aspects of human life. Moreover, it seems to play a significant role in many functions of prime importance in Buddhist thought and training.
Dalai Lama: Are such emotions as aggression, love, and attachment also associated with that part of the brain?
Allan Hobson: Not specifically, no. But, as part of the general continuum of activation, other forebrain structures will be engaged, which will then, according to external inputs, govern the emotional state of the individual. The emotional system is rather farther forward in the limbic part of the forebrain, including the hippocampus, which was discussed yesterday. This brain stem activation site is not a specific system for the control of emotions. It is a specific system for controlling the level of arousal of the individual as a whole, which thereby affects other systems, including emotional controls.
The aminergic system is one group of neurons in this critical region of the brain stem. The other group of nerve cells in this same region is called a cholinergic system because it’s chemical signature—its neurotransmitter—is acetylcholine. We can identify these two neuronal populations in the pontine brain stem, localize their cells precisely, determine their major connections, their chemical neurotransmitters, and record their patterns of electrical activity.
The cholinergic system is apparently held in restraint by the aminergic system. Thus, when the aminergic system is functioning at a high level, the cholinergic system is functioning at a reciprocally relatively low level. That is the situation in the waking state. As we go to sleep, the aminergic system decreases in its activity, and the cholinergic system becomes relatively more active. The cholinergic system becomes progressively more active throughout the period of deep sleep without dreams. Ultimately these two neuron populations become radically differentiated: the adrenergic system shuts off completely, and the cholinergic system reaches its highest level of activity just when you enter the dream state. Activation of the cholinergic cells generates signals that contribute to eye movements, to inhibition of muscle tone, and to activation of the forebrain.
These reciprocal shifts of functional states can probably also be influenced through meditative practice.
Dalai Lama: Are you indicating that in the dreaming state you are even more relaxed than in the nondreaming state?
Allan Hobson: It is a paradox, because the muscles are completely paralyzed. To speak of relaxation in this case is misleading. The muscles are actively suppressed, or inhibited. But the upper brain, the forebrain, is very active electrically. In contrast to the waking state, this electrically active brain in the dreaming state is chemically distinctly different because of the shift in the neurotransmitter ratios. The dreaming brain is very highly cholinergic, the waking brain is very highly aminergic, while in each of these states the forebrain is highly electrically activated. We believe that this is very important for understanding the differences between the waking state and the dreaming state.
So we now know that sleep is organized into a succession of states. We can identify and measure distinctive sleep states. We know that the brain stem controls the succession of waking/sleeping/dreaming states. And we know that the brain stem controls that succession of states by altering the production of specific neurotransmitters which are represented in two reciprocal systems of neuronal control.
The fifth and final point I want to make about the science of sleep is that we have tested this theory by making microinjections of very small amounts of chemicals into specific, localized regions of the brain stem of experimental animals. By this means, we can control the overall brain states of wakefulness and sleep. In other words, by imitating the activation by acetylcholine in very specific, localized parts of the brain stem, we can convert the whole brain from the waking state to REM sleep almost immediately and keep it there for many hours. If we put the same chemical, acetylcholine, into another part of the brain of experimental animals, we can produce waking. The differentiation of these control systems is specific and precise.
So we have obtained experimental control of the state of sleep in animals. To some extent, similar experiments have been replicated in humans. Obviously, we don’t inject chemicals directly into the brain stem in humans. We use human subjects to measure states of sleep and wakefulness and to obtain reports relating to conscious experiences. We use animal studies to investigate what’s going on neuronally within the brain stem during different sleep and wakefulness states. All mammals share identical organization of alternating states of wakefulness, deep sleep, and REM sleep (presumably dreaming) behavior. They all have obvious waking states complete with apparent awareness and interactive behavior with the environment. Such waking states alternate with slow-wave sleep, which lacks rapid eye movements and is associated with high-amplitude, low-frequency electrical activity throughout the brain. These slow-wave sleep states cycle regularly into the kind of sleep state associated with globally inhibited body movements except for rapid eye movements, specifically accompanied by low-amplitude, high-frequency electrical activity throughout the brain. In this latter state, all of the objective phenomena are equivalent to the state that in humans is identified by subjective testimony of dreaming.
In humans and other mammals we see a complete suppression of muscle tone during REM sleep, so the motor output is actively inhibited. Otherwise our dream states might be accompanied by our getting up and running around—still asleep—acting out our dreams. Our dreams are typically characterized by the hallucination of movements by ourselves and among other animate and dynamic things. That’s because the upper brain, the forebrain, is actually generating elaborate visual and motor patterns which are not allowed to be acted out by our muscles, perforce the general inhibitory control exerted by brain stem mechanisms. Only the eye muscles are permitted to express this internal sensorimotor dreaming state.
Meanwhile, during REM sleep, the brain is electrically activated, even more so than in quiet waking. The brain is intensely internally activated: hence we imagine that the dream arises because the manifestly activated brain is actively processing signals that would ordinarily be associated subjectively with direct, vivid experiences and outgoing behavior. We hallucinate the experiences and the inhibited behavior as if it were not inhibited. And that is our dream!