Sleep spindles are bursts of neural oscillatory activity that are generated by interplay of the

thalamic reticular nucleus The thalamic reticular nucleus is part of the ventral thalamus that forms a capsule around the thalamus laterally. However, recent evidence from mice and fish question this statement and define it as a dorsal thalamic structure. It is separated fro ...

(TRN) and other thalamic nuclei during stage 2 NREM sleep in a frequency range of ~11 to 16 Hz (usually 12–14 Hz) with a duration of 0.5 seconds or greater (usually 0.5–1.5 seconds). After generation as an interaction of the TRN neurons and thalamocortical cells, spindles are sustained and relayed to the cortex by thalamo-thalamic and thalamo-cortical feedback loops regulated by both

GABAergic In molecular biology and physiology, something is GABAergic or GABAnergic if it pertains to or affects the neurotransmitter GABA. For example, a synapse is GABAergic if it uses GABA as its neurotransmitter, and a GABAergic neuron produces GABA. A ...

and

NMDA ''N''-methyl--aspartic acid or ''N''-methyl--aspartate (NMDA) is an amino acid derivative that acts as a specific agonist at the NMDA receptor mimicking the action of glutamate, the neurotransmitter which normally acts at that receptor. Unlike ...

-receptor mediated glutamatergic neurotransmission. Sleep spindles have been reported (at face value) for all tested mammalian species. Considering animals in which sleep-spindles were studied extensively (and thus excluding results mislead by pseudo-spindlesGottesmann, C. (1996). The transition from slow-wave sleep to paradoxical sleep: evolving facts and concepts of the neurophysiological processes underlying the intermediate stage of sleep. Neuroscience and Biobehavioral Reviews 20, 367–387.), they appear to have a conserved (across species) main frequency of roughly 9-16 Hz. Only in humans, rats and dogs is a difference in the intrinsic frequency of frontal and posterior spindles confirmed, however (spindles recorded over the posterior part of the scalp are of higher frequency, on average above 13 Hz).Iotchev, I. B., & Kubinyi, E. (2021). Shared and unique features of mammalian sleep spindles–insights from new and old animal models. Biological Reviews, 96(3), 1021-1034. Research supports that spindles (sometimes referred to as "sigma bands" or "sigma waves") play an essential role in both sensory processing and long term memory consolidation. Until recently, it was believed that each sleep spindle oscillation peaked at the same time throughout the neocortex. It was determined that oscillations sweep across the neocortex in circular patterns around the neocortex, peaking in one area, and then a few milliseconds later in an adjacent area. It has been suggested that this spindle organization allows for neurons to communicate across cortices. The time scale at which the waves travel at is the same speed it takes for neurons to communicate with each other. Although the function of sleep spindles is unclear, it is believed that they actively participate in the consolidation of overnight declarative memory through the reconsolidation process. The density of spindles has been shown to increase after extensive learning of declarative memory tasks and the degree of increase in stage 2 spindle activity correlates with memory performance. Among other functions, spindles facilitate somatosensory development, thalamocortical

sensory gating Sensory gating describes neural processes of filtering out redundant or irrelevant stimuli from all possible environmental stimuli reaching the brain. Also referred to as gating or filtering, sensory gating prevents an overload of information in the ...

synaptic plasticity In neuroscience, synaptic plasticity is the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. Since memories are postulated to be represented by vastly interconnected neural circuits ...

, and offline memory consolidation. Sleep spindles closely modulate interactions between the brain and its external environment; they essentially moderate responsiveness to sensory stimuli during sleep. Recent research has revealed that spindles distort the transmission of auditory information to the cortex; spindles isolate the brain from external disturbances during

sleep Sleep is a sedentary state of mind and body. It is characterized by altered consciousness, relatively inhibited Perception, sensory activity, reduced muscle activity and reduced interactions with surroundings. It is distinguished from wakefuln ...

. Another study found that re-exposure to olfactory cues during sleep initiate reactivation, an essential part of long term memory consolidation that improves later recall performance. Spindles generated in the thalamus have been shown to aid sleeping in the presence of disruptive external sounds. A correlation has been found between the amount of brainwave activity in the thalamus and a sleeper's ability to maintain tranquility. Spindles play an essential role in both sensory processing and long term memory consolidation because they are generated in the TRN. During sleep, these spindles are seen in the brain as a burst of activity immediately following muscle twitching. Researchers think the brain, particularly in the young, is learning about what nerves control what specific muscles when asleep. Sleep spindle activity has furthermore been found to be associated with the integration of new information into existing knowledge as well as directed remembering and forgetting (fast sleep spindles). During

NREM sleep Non-rapid eye movement sleep (NREM), also known as quiescent sleep, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other cha ...

, the brain waves produced by people with

schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social w ...

lack the normal pattern of slow and fast spindles. Loss of sleep spindles are also a feature of

familial fatal insomnia Fatal insomnia is an extremely rare genetic (and even more rarely, sporadic) disorder that results in trouble sleeping as its hallmark symptom. The problems with sleeping typically start out gradually and worsen over time. Eventually, the patien ...

, a

prion Prions are misfolded proteins that have the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It i ...

disease. Changes in spindle density are observed in disorders. There are some studies that show an increase in sleep spindles i
autistic children
Also some studies suggest a lack of sleep spindles in

epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrica ...

./ Research is currently underway to develop a web-based automatic sleep spindle detection system by using machine learning techniques. The results of the present study show that the automatic sleep spindle detection system has great potential in practical application.

Evolution

No sleep spindles have been observed in reptiles and birds so far. In most mammals sleep spindles were described at face value, but the existence of pseudo-spindlesGottesmann, C. (1996). The transition from slow-wave sleep to paradoxical sleep: evolving facts and concepts of the neurophysiological processes underlying the intermediate stage of sleep. Neuroscience and Biobehavioral Reviews 20, 367–387. invites uncertainty around these observations. Spindle-like oscillations, that show additional analogies (e.g. apparent involvement in learning or thalamic dependence) are currently only known from humans, rats, mice, cats, and dogs.Iotchev, I. B., & Kubinyi, E. (2021). Shared and unique features of mammalian sleep spindles–insights from new and old animal models. Biological Reviews, 96(3), 1021-1034. In these species spindles invariantly oscillate between 9 and 16 Hz, with minor variations (e.g. 7-14 Hz in the cat). Clearly distinct frontal and posterior sleep spindles (i.e. "slow" and "fast") were only confirmed outside humans in the rat and dog.Iotchev, I. B., Kis, A., Turcsán, B., Tejeda Fernández de Lara, D. R., Reicher, V., & Kubinyi, E. (2019). Age-related differences and sexual dimorphism in canine sleep spindles. Scientific reports, 9(1), 1-11.

Sex differences

Sleep spindles play a crucial role in declarative memory consolidation, however, most studies neglect to control for sex , although both sex and

menstruation Menstruation (also known as a period, among other colloquial terms) is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The menstrual cycle is characterized by the rise and fall of ...

affect sleep and online learning periods. Studies have shown that the influence of sleep spindles during the declarative memory process may be affected by modulatory menstrual cycle effects in females. Humanlike sex differences in sleep spindle activity and sleep-dependent learning were observed in the dog.Iotchev, I. B., Kis, A., Turcsán, B., Tejeda Fernández de Lara, D. R., Reicher, V., & Kubinyi, E. (2019). Age-related differences and sexual dimorphism in canine sleep spindles. Scientific reports, 9(1), 1-11. A direct link to specific sexual hormones could not be established, but the effects were stronger in intact animals.Iotchev, I. B., Kis, A., Turcsán, B., Tejeda Fernández de Lara, D. R., Reicher, V., & Kubinyi, E. (2019). Age-related differences and sexual dimorphism in canine sleep spindles. Scientific reports, 9(1), 1-11. Women tend to have 0.16 more sleep spindles per minute than men ( ⁠i.e. roughly 9–⁠10 more over an hour's time). A female advantage has been found for episodic, emotional, and spatial memories as well as recognition of odours, faces, and pictures. These differences are believed to be due to hormonal influence, especially that of

estrogen Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ac ...

. The female sex hormone estrogen primarily influences sexual maturation and reproduction, but has also been found to facilitate other brain functions, including cognition and memory. On verbal tasks where women scored higher than men, women scored higher during the mid-luteal phase, when women have higher estrogen levels, when compared to the menstrual phase. A recent study found that local brain estrogen production within cognitive circuits may be important for the acquisition and consolidation of memories. Recent experiments concerning the relationship between estrogen and the process of offline memory consolidation have also focused on sleep spindles. Genzel and colleagues determined that there was a menstrual effect on declarative and motor performance, meaning that women in the mid-luteal phase (high estrogen) performed higher than the other female participants. Women in the luteal phase were also the only participants to experience an increase in spindles after learning, which led to the conclusion that the effect of the menstrual cycle may be mediated by spindles and female hormones.

References

{{EEG Sleep physiology Electroencephalography Sleep Neural circuits