Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Nervous System (General Outline of the Structure and Development of the Nervous System)
Central Nervous System
Higher Nervous Activity - Sleep

Terrestrial organisms are characterized by a strict daily periodicity (the alternation of day and night, i.e., the circadian rhythm). Among the external factors that drive circadian rhythmicity, illumination and its associated stimuli take precedence; these factors influence The activity of neural centers, regulate the hormonal sphere, and govern metabolic rates. Most mammals are not born with a fully established circadian rhythm; rather, they gradually adapt to it, developing their own internal rhythm over time. The alternation of Sleep and wakefulness is one of the primary manifestations of an Organism's internal circadian rhythm.

Natural sleep can be monophasic—occurring once a day (typically as nocturnal sleep)—or polyphasic, meaning interrupted sleep (both daytime and nighttime), which is characteristic of children in the first 7 years of life. Electroencephalographic analysis of sleep has made it possible to study its distinct stages. Nocturnal sleep lasts 7— 8 hours and consists of 4—5 cycles. Each cycle begins with a phase of "slow" (non-REM) sleep and concludes with "fast" (REM) sleep. The duration of a single cycle in adults is relatively constant, ranging from 60 to 100 minutes. The first two cycles are dominated by "slow sleep" (δ-sleep), whereas the later cycles are dominated by "fast sleep" (ß-sleep). In adults, slow sleep accounts for approximately 6.5 hours, while REM sleep takes up about 1.5 hours. In newborns, REM sleep accounts for 50—60% of total sleep duration.

Physiological Functions of sleep. As an essential expression of the body's internal circadian rhythm, sleep is nowadays viewed as an active state. The Functional Significance of individual sleep stages varies.

During slow-wave sleep (δ-sleep), restorative processes take place across various Tissues, Organs, and body systems. Organ functions, physical and mental performance are restored, and growth processes are carried out. In the Cerebral Cortex, information gathered during wakefulness is organized—transferring data from short-term to long-term memory banks. At the same time, biologically irrelevant information is purged from the Central Nervous system, helping to reduce informational and emotional overload.

REM sleep (ß-sleep, or paradoxical sleep) is prominent in newborns. Only after several days do the signs of slow-wave sleep emerge, establishing the alternating pattern of these two states throughout the night. One of the hallmark features of REM sleep is the occurrence of rapid Eye Movements. Simultaneously, changes occur in the skeletal and muscular systems, associated with a drop in Muscle tone. The differences between REM and non-REM sleep are clearly evident in autonomic functions. For instance, during slow-wave sleep, Respiration and Heart rate slow down, and Blood pressure drops. In contrast, the REM phase triggers an "autonomic storm"—breathing becomes rapid and irregular, the pulse becomes frequent and irregular, blood pressure rises, and hormonal activity increases. REM sleep is a profoundly unique state of the brain compared to slow-wave sleep; it is during REM sleep that the functions of brain Neurons and synapses are restored. REM sleep plays a major role in relieving unproductive, anxious tension. It is essential for periodically assessing situations—thus performing a sentinel function—and prepares the body for the transition to wakefulness.

It is believed that total sleep duration fluctuates primarily due to changes in ß-sleep, specifically through its shortening or lengthening.

Mechanisms of sleep. Current theories regarding The Mechanism of sleep converge on the idea that sleep is an active process driven by the excitation of inhibitory (hypnogenic) structures and the suppression of the brain's activating structures. It is hypothesized that the orbitofrontal cortex and the preoptic nuclei of the Hypothalamus activate the raphe nuclei, which in turn exert inhibitory influences on the Brainstem reticular formation. As the reticular formation is inhibited, its suppressive effect on the nonspecific nuclei of the thalamus weakens, leading to the inhibition of the CEREBRAL CORTEX AND the onset of slow-wave sleep. Conversely, the inhibition of the brainstem reticular formation completely lifts its activating influence on the cerebral cortex, a period that corresponds to the onset of REM sleep. The transition from slow-wave sleep to REM sleep is mediated by Two Types of neurons in the pontine reticular formation: REM-sleep neurons (cholinergic neurons) and slow-wave sleep neurons (noradrenergic). The activation of REM-sleep neurons against the Background of slow-wave sleep triggers the transition into REM sleep, whereas the activation of slow-wave sleep neurons during REM sleep switches the brain back to slow-wave sleep.

According to several authors, the alternation between sleep and wakefulness depends on the functional state of the brain's cholinergic neurons. When excited, these neurons inhibit the secretion of norepinephrine by the locus coeruleus while simultaneously increasing serotonin production in the raphe nuclei, processes that induce sleep. Conversely, when cholinergic neurons are inhibited, norepinephrine secretion rises and serotonin production drops, leading to wakefulness. Serotonin, produced by the raphe nuclei neurons, is known to be involved in The Development of slow-wave sleep. Norepinephrine, produced by locus coeruleus neurons, participates in the development of paradoxical sleep and possibly in the cycling between slow-wave and REM sleep. The pineal hormone melatonin is also implicated in the development of REM sleep. Consequently, sleep represents a state in which the brain's hypnogenic (inhibitory) structures are active, producing hypnogenic BIOLOGICALLY ACTIVE SUBSTANCES and Neurotransmitters. During wakefulness, the activity of hypnogenic structures is suppressed, while the centers that activate the cerebral cortex are enhanced; these latter centers produce antihypnogenic biological substances and neurotransmitters.

REVIEW AND SELF-Control Questions:

1. Discuss the Methods used to study Higher Nervous Activity.

2. Discuss motivations and emotions, along with the mechanisms of their formation.

3. Explain the difference between unconditioned and conditioned Reflexes. How are these reflexes formed?

4. List and explain the characteristics of conditioned reflexes and the stages of their formation.

5. Explain your understanding of The Role of sleep in human life, name its stages, and describe the mechanisms of sleep.

The mechanism of conditioned reflex formation. The foundation of a conditioned reflex is the reflex arc, which develops on The basis of an unconditioned reflex arc. An obligatory component of the conditioned reflex is inhibitory neurons, which ensure precise and dynamic Differentiation of the conditioned response.

According to I.P. Pavlov, when an organism is exposed to both a conditioned and an unconditioned stimulus, two foci of excitation are formed in the cerebral cortex. The excitation focus generated by the unconditioned stimulus is stronger and draws toward itself the excitation from the focus produced by the conditioned stimulus. After several repeated presentations of both stimuli, a stable pathway for signal transmission is established between the two cortical areas. As a result, the isolated presentation of the conditioned stimulus alone triggers the reaction typically caused by the unconditioned stimulus. The consolidation of this temporary connection is, essentially, a matter of memory mechanisms.