The word dream is derived from the Latin «somnum» and its original root is preserved in the words somnífero, somnoliento and sonámbulo. According to the Dictionary of the Royal Spanish Academy, dream, in short, means the act of sleeping as the desire to do so and reverie is equivalent to the act of dreaming, it is the dreamlike representation of the one who sleeps. The adjective oniric comes from the Greek «onar» which means dream. Dreaming is the mental representation of images, sounds, thoughts, and sensations during sleep, generally involuntarily.
Sleep, as we conceive it on a daily basis, can be defined as a normal, recurrent and reversible state of diminished perception and responsiveness to the environment. Motor activity ceases and a specific posture is adopted. Sleep is also present in mammals, birds, probably reptiles, amphibians and fish. Contrary to what is commonly believed, sleep is a dynamic state where groups of neurons remain active, playing a role different from that of wakefulness, and it is also necessary for the overall health of the organism, due to its properties of consolidating different forms of memory,1 regulating temperature and the function of certain neurotransmitters, as well as storing energy and maintaining immunocompetence.
The generation of sleep spindles in NREM sleep has certain peculiarities that are worth mentioning. During wakefulness and REM sleep thalamic reticular neurons have «tonic» activity, but during NREM sleep they alter their electrical activity to produce «burst» discharge patterns. The deactivation of the cholinergic nuclei of Meynert and of the pedunculopontine and laterodorsal tegmental nuclei causes the reticular nuclei of the thalamus to become active and through their GABAergic activity to block the passage of sensory information from the outside, thus consolidating sleep.
The EEG translation of the activation of the reticular nuclei of the thalamus is the appearance of sleep spindles (Stage II of NREM sleep). In deep sleep, after progressive hyperpolarization of these reticular nuclei of the thalamus, the sleep spindles disappear, the dorsal nuclei of the thalamus are activated and project to the cortex, and the delta activity characteristic of deep NREM sleep appears in the EEG.
REM sleep is characterized unlike NREM sleep precisely by this particularity of presenting the sleeping person with episodes of rapid eye movements, muscle atonia, in addition to a cortical activity of «awakening» like the activity recorded in the routine EEG by opening the eyelids and the classic «sawtooth» waves.
I invite everyone to read Matias’ post with the topic of the day.