Subcortical theory of sleep and clinical experience
The subcortical theory is based on extensive clinical observations of patients with central nervous system lesions. It has been noted that various pathological processes—vascular catastrophes, tumors, or severe infections affecting the brainstem—lead to pronounced disorders of the wake-sleep cycle. Patients suffered either from debilitating insomnia or deep lethargic sleep. A striking historical example was the epidemics of lethargic encephalitis in the late 1920s, which were accompanied by a specific clinical presentation of "sleeping sickness".
A crucial practical observation was made by the Soviet neurologist N.I. Grashchenkov during the Great Patriotic War. During a surgical operation performed under local anesthesia on a wounded soldier with a shrapnel fragment in the diencephalic region (diencephalon), a surprising phenomenon was discovered. The slightest mechanical touch to the tissues of this zone provoked the immediate induction of sleep: the patient instantly stopped talking and fell asleep, which directly indicated the localization of sleep centers in the subcortical structures.
Experimental evidence (W.R. Hess's experiments)
Clinical data required rigorous experimental confirmation. The Swiss physiologist W.R. Hess developed a unique technique for implanting chronic electrodes into deep brain structures of animals (cats). The scientist purposefully targeted the posterior structures of the subthalamus and hypothalamus.
During the experiments, it was found that low-frequency electrical stimulation of these zones provoked the animal to fall asleep immediately. As soon as the electrical stimulation ceased, the cat woke up right away. These experiments became undeniable proof that the crucial sleep regulation centers are localized precisely in the subthalamus and hypothalamus.
I.P. Pavlov's cortical theory
Despite the persuasiveness of the subcortical concept, it had a significant flaw—the inability to explain the phenomenon of hypnotic sleep (falling asleep on command). The hypnotist's verbal command is perceived by the higher divisions of the CNS, i.e., the cerebral cortex, and cannot directly affect the brainstem or hypothalamus. To resolve this contradiction, I.P. Pavlov substantiated the involvement of the cerebral cortex in the mechanisms of falling asleep.
In experiments with conditioned reflexes, it was established that animals consistently fall asleep upon prolonged and persistent presentation of an unreinforced conditioned stimulus, as well as when there is a need for excessively fine differentiation of signals. In both cases, falling asleep developed against the background of pronounced inhibition of conditioned reflex activity.
Mechanisms of "sleepy" inhibition
According to Pavlov's teaching, sleep is a direct consequence of internal inhibition processes. This is a profound, generalized inhibition that irradiates (spreads) to both hemispheres and captures the nearest subcortical formations. The term "sleepy" inhibition was introduced to describe this state.
Three main pathways for the emergence of such inhibition are distinguished:
- Conditioned-reflex pathway: based on processes of internal inhibition (extinction of a reflex, complex differentiation).
- Action of monotonous, weak stimuli: monotonous sounds or rocking gradually depress cortical activity.
- Action of super-strong stimuli: triggers protective inhibition, which acts as a biological defense for nerve cells against critical exhaustion.
Despite its logic, the cortical theory encountered contradictions that were unsolvable at the time. It could not explain why organisms lacking cortical function sleep fully. These included decorticated (hemisphereless) animals in experiments, as well as newborn infants whose cerebral cortex is in a state of pronounced morphological immaturity.
Briefly: answers to topical questions
Which specific hypothalamic nuclei serve as sleep centers (somnogenic zones)?
The sleep centers are structures of the posterior hypothalamus. Specific somnogenic nuclei are not detailed in the text, but it is indicated that the posterior group of nuclei includes:
- Posterior nucleus
Electrical stimulation of posterior hypothalamic neurons causes immediate sleep. Normally, these hypothalamic centers exert an inhibitory, blocking effect on the neurons of the brainstem reticular formation, reducing its ascending activating influences.
What is the role of the ascending reticular activating system in the regulation of the sleep-wake cycle?
Ascending activating influences of the reticular formation on the cerebral cortex maintain wakefulness.
- Wakefulness: active afferentation from the sense organs activates the structures of the reticular formation; the reticular formation exerts ascending activating influences on the cortex.
- Sleep: occurs when the ascending activating influences of the reticular formation on the cortex are eliminated. This may be associated with the inhibitory action of hypothalamic sleep centers on brainstem reticular formation neurons or with a decrease in the tone of the reticular formation during sensory deafferentation.
Which neurotransmitters are released by subcortical structures to induce sleep?
The provided sources do not specify exactly which neurotransmitters are released by subcortical structures to induce sleep.
The neurochemical factors of sleep directly indicated include:
- Serotonin — a key factor inducing sleep.
- GABA, enkephalins, and endorphins — inhibitory and opioid systems involved in sleep regulation.
- Melatonin — ensures the synchronization of sleep phases and circadian rhythms.
- Delta-sleep peptide — when administered to waking animals, it causes slow, high-amplitude delta waves to appear on the EEG.
- Acetylcholine — during REM sleep, it is released in the brainstem reticular formation and is associated with the activation of giant reticular formation neurons.
What is the cellular mechanism for the development of protective inhibition under the action of super-strong stimuli?
Protective inhibition arises under the influence of a stimulus of excessive strength.
Physiological mechanism: the strength of the stimulus exceeds the optimum of the functional lability of nerve cells, and neurons transition into a state of pessimum — transmessional inhibition.
Biological significance: protection of nerve cells from exhaustion and the damaging action of super-strong stimuli.
Pavlov "inhibits" the cortex with monotony, while Hess "turns off the lights" in the hypothalamus.
Class and exam questions
Why couldn't the subcortical theory explain hypnosis?
The hypnotist's verbal command is perceived exclusively by the cerebral cortex. It does not directly affect subcortical structures, which necessitated the creation of a separate, cortical theory of sleep.
What is Pavlov's «sleep» inhibition?
This is a process of profound, generalized internal inhibition that widely spreads (irradiates) to both cerebral hemispheres and the nearest subcortical nodes.
What is the main weakness of the cortical theory of sleep?
It is incapable of explaining the presence of full sleep in organisms with absent or immature cortex, for example, in animals after decortication or in newborn infants.
What else to study on this topic
- Mechanisms of conditioned-reflex sleep and fine differentiation
- The effect of monotonous stimuli on cortical inhibition processes
- Protective inhibition of nerve cells during super-strong stimuli
- Lethargic sleep symptom complex in epidemic encephalitis
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