Dept. Biologia F. i C.S.
Universitat de les Illes Balears
07071 Palma de Mallorca (Spain)
Telephone: +34 71 173147, Fax: +34 71 173184
his paper proposes that each one of the main evolutive neuroanatomic developments of the chordate brain gave birth to new and improved kinds of waking. Three evolutive steps are considered: 1) prochordates with only a spinal chord plus a single, tiny brain vesicle and reactive behavior with continuous activity; 2) poikiloterms, with three brain vesicles and differentiation between activity and behavioral sleep and 3) mammals, with a highly developed cortex and three activity states: REM, Slow Wave Sleep and waking. Each one of these three groups have active states supported by different neuroanatomical structures. These three waking types are thus analogous states but not homologous.
In the first step, the evolution from prochordate continuous activity to alternative active and resting periods was the result of improved nervous controls procured by the rostral brain of fishes, amphibians and reptiles, able to suppress the continuous activity of prochordates controlled in lower CNS regions. When suppressed, this activity was transformed into behavioral sleep. The division between rest and activity time allowed a better use of energy and environmental resources.
The three brain vesicles of poikiloterms provides a distributed sensory and motor processing of the environmental signals: olfaction corresponds to telencephalon, vision to mesencephalon and contact sensitivity to rhombencephalon. Motor regions are only placed along the brainstem. This brain evolved to the mammalian one when the cortex developed multisensory and motor primary centers. Additional mechanisms were needed to switch the main control, from the brainstem to the cortex. As a result, the cortical activity caused a new state, the cortical waking. The old poikiloterm waking, supported by the brainstem, was converted into SW sleep and its inactivity remained as REM sleep.
Several proofs are proposed to support the homology between the first poikiloterm inactivity and the mammalian REM: 1) the parallelism between primitive phylogenic homeostasis level and REM sleep, 2) the ontogenic development of the REM sleep and 3) neuroanatomical arguments. The relationship between SW sleep and pokiloterm activity is also supported by 1) their neuroanatomy, 2) the existence of SW EEG in active reptiles, 3) the opposite relationships between mammals and reptiles in EEG amplitude and arousal state, 4) the synchronizing arousal reaction of poikiloterms and 5) the presence of high frequency spindles and high voltage spikes and evoked potentials in the EEG of active reptiles.
