Vascular theory of sleep
Italian physiologist A. Mosso put forward historically the first concept of sleep mechanisms. Its main postulate states that falling asleep is a consequence of brain anemia (blood deprivation).
To prove this, the scientist used a «balance bed» he designed. The test subject was placed on a carefully balanced platform. As the person drifted off to sleep, the foot end of the bed went down, and the head end went up. This demonstrated the redistribution of blood flow and the outflow of blood to the lower extremities.
Despite the reliability of the hemodynamic change itself, the scientific community found the theory unconvincing. The main argument of critics was the impossibility of establishing a true cause-and-effect relationship: whether blood redistribution is the physiological cause of sleep or merely its autonomic consequence.
Humoral theory: the concept of hypnotoxins
In the 1920s, Belgian researchers Legendre and Pieron proposed a humoral model. In their view, during wakefulness, special substances accumulate in the blood — hypnotoxins (or kenotoxins) — which provoke sleep onset.
As proof, an experiment was conducted: a dog was forcibly sleep-deprived by being forced to run continuously on a treadmill for 24 hours. Then, the blood of the fatigued animal was transfused into a rested, awake dog, after which the recipient immediately fell asleep.
This concept also faced severe criticism:
- Stress factor. The animal on the treadmill experienced colossal physical overexertion. The transfused substances could have been products of muscle fatigue or stress rather than specific sleep regulators.
- The Siamese twins phenomenon. In T.T. Alekseeva's observations of conjoined twins with shared blood circulation, it was found that the children could exhibit independent sleep-wake cycles (one girl slept while the other was awake). Consequently, peripheral blood humoral factors cannot be considered the sole cause of falling asleep.
Neurochemical factors of the brain and CSF
Modern physiology has transformed the humoral theory, shifting the focus to neurochemical processes within the central nervous system (CNS).
American researcher R. Pappenheimer proved the localization of humoral factors in the brain. He deprived goats of sleep for two days, after which he collected their cerebrospinal fluid (CSF). Administering this CSF to awake cats induced immediate sleep in them. This confirmed that the sleep factor (referred to in literature as «factor S») is produced and acts primarily at the level of the cerebrospinal fluid and brain structures.
Delta-sleep peptide and other regulators
Researcher M. Monnier experimentally induced sleep in rabbits using electrical stimulation of the anterior hypothalamus. From the tissues of the thalamic and hypothalamic zones of these animals, he managed to isolate a specific oligopeptide consisting of 9 amino acids (Tyr-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu).
When this compound was administered to awake animals, slow high-amplitude delta waves appeared on the electroencephalogram. The substance was named delta-sleep-inducing peptide (DSIP). This discovery stimulated the search for endogenous factors produced within the brain itself.
In addition, the influence of other biologically active substances on the sleep cycle has been revealed:
- Increase sleep duration: interleukin-1, prostaglandin $D_2$.
- Inhibit sleep development: prostaglandin $E_2$, $\alpha$-melanocyte-stimulating hormone of the pituitary gland.
Briefly: answers to topical questions
What is the physiological role of melatonin in the humoral regulation of sleep?
The physiological role of melatonin is to participate in the circadian regulation of the wake-sleep cycle and to promote the transition from wakefulness to sleep. Melatonin is a pineal gland hormone; its synthesis depends on illumination, changes in which are perceived by the retina.
- During the dark hours melatonin synthesis increases, and it is secreted during the night. The increase in melatonin levels causes a sedative effect and facilitates the transition from wakefulness to sleep.
- In the light / during daytime the conversion of serotonin to melatonin is inhibited; sunlight suppresses melatonin production, which maintains wakefulness.
The criticism of the humoral theory is easily remembered through the image of Siamese twins: «Blood is shared, but dreams are different.» This proves that peripheral blood is not the main conductor of sleep.
Class and exam questions
What is the weakness of Angelo Mosso's vascular theory?
The inability to distinguish cause from effect. The outflow of blood from the head to the legs does occur when falling asleep, but it is an accompanying change in vascular tone, not the root cause of cerebral cortex inhibition.
What are hypnotoxins?
These are hypothetical substances that, according to the theory of Legendre and Pieron, accumulate in the blood during prolonged wakefulness and initiate the process of falling asleep. It later turned out that in their experiments, these were rather products of stress and fatigue.
How does the Siamese twins experiment refute the classical humoral theory?
The twins share a circulatory system. If blood-borne substances were the sole cause of sleep, the children would fall asleep at exactly the same time. In practice, however, their CNS states function independently.
What else to study on this topic
- Changes in hormone secretion during various phases of sleep
- Electroencephalographic picture of slow-wave delta sleep
- The role of the anterior hypothalamus in the induction of inhibitory processes
- Synthesis and functions of neuropeptides in the central nervous system
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