Phase changes in higher nervous activity
The transition process from active wakefulness to sleep (sleep onset) is accompanied by specific shifts in higher nervous activity. These changes were studied in detail in I.P. Pavlov's laboratory, particularly in the classical works of I.P. Razenkov. The method of classical conditioned reflexes was used for this study.
The main sign of sleep onset is the disruption of the fundamental law of physical strength of stimuli. Normally, a strong signal causes a strong response and a weak signal causes a weak one. However, when slipping into sleep, this rule ceases to work.
Three main phases of higher nervous activity changes during sleep onset are distinguished:
- Equalizing phase. At this stage, animals or humans begin to produce absolutely identical conditioned-reflex responses in magnitude to signals of completely different physical strength.
- Paradoxical phase. Characterized by a distorted reaction: the organism's responses to weak conditioned stimuli turn out to be greater in magnitude and intensity than reactions to objectively strong signals.
- Narcotic phase. The final stage of sleep onset, characterized by the complete cessation of any responses to all conditioned stimuli, regardless of their intensity.
Behavioral changes and research methods
With the onset of sleep, behavior changes dramatically. First and foremost, there is a complete loss of active and, especially, goal-directed activity. Both for humans and animals, this period is characterized by assuming a strictly defined posture that promotes maximum relaxation and rest.
To objectively characterize the state of a sleeping organism in modern physiology, polymetry is used. This is a comprehensive method that includes the simultaneous recording of the electroencephalogram (EEG) and a number of autonomic parameters using a multichannel polygraph.
Conditions for performing polymetry:
- The study is carried out in a special sound-attenuated chamber to exclude the influence of external noise.
- The subject is comfortably positioned on a bed.
- A special helmet with sensors for EEG recording is placed on the head, and sensors for autonomic parameters are simultaneously attached.
- Parameters are recorded continuously: both during the sleep onset process itself and throughout the entire night's rest period.
During the polygraph study, the following must be recorded: EEG in several leads, electrocardiogram (ECG), respiratory parameters, blood pressure (BP) levels, body temperature, and gas exchange indicators.
Somatovegetative and motor manifestations
Changes in the electrical activity of the brain (EEG) are always matched by completely specific somatovegetative indicators.
Motor reactions: A sleeping person does not remain completely motionless throughout the night. Changes in posture and body position occur predominantly during periods of sleep phase transitions (for example, during the transition from slow-wave sleep to REM sleep and vice versa). Directly during REM sleep, subjects generally lie quietly. It is important to note that such a phenomenon as somnambulism (sleepwalking) is exclusively related to the slow-wave phase.
Autonomic reactions: The dynamics of internal organ function are strictly tied to the current phase.
In the slow-wave phase, autonomic inhibition processes predominate: blood pressure regularly decreases, and the pulse and respiratory rate slow down.
In the REM sleep phase, the picture changes dramatically:
- Increased: blood pressure, pulse rate, and local brain temperature.
- Decreased: muscle tone (reaches a minimum).
- Specific manifestations are observed: minor twitching of facial muscles and fingertips, respiration becomes irregular, snoring decreases (if present before this phase), and erections may occur in men.
- Oxygen consumption increases sharply, which reliably indicates an increase in the general metabolic rate in the body.
Briefly: answers to topical questions
What EEG rhythms are recorded during various stages of slow-wave sleep?
During various stages of slow-wave sleep, alpha, theta, and delta rhythms are sequentially recorded.
- Drowsiness stage — alpha waves are recorded on the EEG.
- "Sleep spindles" stage — alpha waves form into characteristic spindles.
- Theta wave stage — high-amplitude slow theta waves appear.
- Delta wave stage — an increase in high-amplitude ultra-slow delta waves occurs.
Which brain structures and neurotransmitters are responsible for generating the REM sleep phase?
The generation of the REM sleep phase is associated with the concerted activity of neuronal groups in the brainstem reticular formation and the locus coeruleus, as well as with cholinergic activation.
- Locus coeruleus — during wakefulness, its neurons are active and exert an inhibitory influence on the giant neurons of the reticular formation; upon transitioning to sleep, their activity decreases.
- Giant neurons of the brainstem reticular formation — are sharply activated during the REM sleep phase.
- Acetylcholine — the excitation of giant neurons is caused by the release of acetylcholine.
- Central region of the reticular formation — the activity of its neurons determines rapid eye movements.
In REM sleep, predominantly cholinergic activation of the cortex from the pons/brainstem and the limbic system is noted.
What mechanisms ensure the sharp decrease in muscle tone during REM sleep?
The decrease in muscle tone during REM sleep is associated with the inhibitory influence of neurons in the caudal part of the brainstem reticular formation.
- Neurons of the caudal reticular formation — inhibit muscle tone.
- Projections to the spinal cord — the axons of these neurons project to the spinal cord.
In the REM sleep phase, despite cortical activation, the motor output is blocked, which manifests as muscle atonia.
To memorize the phases of disruption of the law of strength during sleep onset, use the acronym EPN: Equalizing, Paradoxical, Narcotic.
Class and exam questions
In which phase of sleep does somnambulism (sleepwalking) most frequently occur?
Somnambulism is a motor reaction characteristic exclusively of the slow-wave phase of sleep.
When does a sleeping person most frequently change posture?
Changes in body position (posture shifts) occur predominantly during phase transitions — upon moving from slow-wave sleep to REM sleep and vice versa.
What happens to muscle tone during rapid eye movement (REM) sleep?
In the REM sleep phase, muscle tone decreases significantly, despite the fact that blood pressure, pulse, and brain temperature increase at this moment.
What is the essence of the paradoxical phase of higher nervous activity during falling asleep?
In this phase, the law of strength is violated: the organism's conditioned reflex responses to weak stimuli become greater in magnitude than responses to strong signals.
What else to study on this topic
- Features of the classical conditioned reflex methodology in I.P. Pavlov's laboratory
- The relationship between EEG changes and specific somatovegetative shifts
- Physiological mechanisms of muscle tone reduction in the REM sleep phase
- Dynamics of gas exchange and metabolic indicators at different stages of sleep
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