A. Intact sleeping cats (Fig.1)
Two different stages can be distinguished on the records:
(1) Falling asleep and " slow " sleep. This stage is marked by the appearance of spindles followed by slow waves which invade the cortex, the diencephalon and then the mesencephalic reticular formation (R.F.). During this stage, the animal prepares for sleep. It bends its head while the EMG of the neck muscles discreetly falls. Breathing is regular; so is the heart beat which slows down in comparison with that in the waking state. As the invasion of reticular formation by the slow waves proceeds, the threshold of arousal by direct excitation of the R.F. increases from 20 percent to 50 per cent.
(2) "Rapid" sleep phase or "paradoxical phase" (p.p.) (Jouvet, Michel and Courjon, 1959C) (Figs. 1 - 3). This always follows a " slow" sleep phase and never appears immediately after wakeful ness. It starts suddenly and is distinguished by a rapid and low voltage corticomesodiencephalic activity, identical to that of wakefulness, while the dorsal rhinencephalic formations show a slow rhythmic activity, identical to that which has been described during arousal at this level (Green and Arduini, 1953). At the same time, a 6 to 8 per second spindle activity appears at the level of the pontile R.F. The auditory cortical, and particularly reticular evoked responses undergo a marked reduction in amplitude compared with that in the waking state or during "slow sleep." This phase, 10 to 15 minutes in duration, is accompanied immediately and constantly by total disappearance of EMG activity in the recorded muscles. It is periodically repeated during behavioural sleep, with intervals of 20 to 30 minutes (Fig. 2).
During that phase, the posture of the animal is the same as that found in profound sleep. Postural muscles completely lack tone, nictitating membranes almost entirely cover the pupils which are myotic, while the eyeballs are frequently shaken by short rapid jerks. Movements of the vibrissae and, more rarely, brief jerks of the jaws and the tail can also be observed. Cardiorespiratory variations are constantly observed: breathing becomes irregular, more superficial and quicker than during the slow wave phase, while the heart beat is slowed down, or, more rarely, quickened (Fig. 3).
That this phase of sleep is more profound than the slow wave ase is supported by the following arguments :
(I) An auditory stimulation, insufficient to produce arousal, causes the spindle and the slow wave stage to reappear.
(II) The arousal threshold, measured in decibels for an auditory stimulation, is higher during this phase than during the "slow" phase.
(III) Last but not least, the behavioural threshold of arousal through direct stimulation of the mesencephalic R.F. in creases by 200 to 300 per cent compared with that of the slow stage of sleep (Jouvet, Michel and Courjon, 1959C; Benoit and Bloch, 1960).
Identical EEG and behavioural aspects during these two phases of sleep are found in partially decorticate animals or totally cerebellectomized cats (Jouvet and Michel, 1960a). In these animals, the alpha-type rigidity which distinguishes the post-operative period is abolished during p.p.
B. Cats which have undergone total removal of the neocortex (Fig. 4)A surprising alteration in the EEG recorded at the level of the subcortical structures is produced by this operation (Jouvet and Michel, 1958). There is, in effect, a permanent lack of spindles and slow waves during the survival period of the animal (up to three months). Mesodiencephalic structures continuously exhibit a fast low voltage activity. On the other hand, the p.p. is shown by phenomena identical with those of the intact animal: appearance of spindles (6/sec.) at the level of the pontile R.F., rapid rhinencephalic activity, disappearance of EMG activity, cardiorespiratory alterations, and appearance of eye movements (Fig. 3).
The electrical activity of the cortical and diencephalic formations situated in front of the section continually exhibit the classic appearance described at the level of the cerveau isolé (Bremer, 1935), i.e. a continuous mixing of spindles and slow waves, whatever the state of wakefulness.
Behind the section, however, mesencephalic activity constantly remains rapid during waking. The periods of behavioural sleep are marked by the appearance of spindles at the level of the pontile R.F. and a complete disappearance of EMG activity in the neck, which is remarkable since muscular activity is greatly increased during wakefulness (decerebration hypertony) (Fig. 2) (Jouvet and Michel, 1959). During these periods, the eye movements are slower and less frequent than in decorticate or normal animals, while the nictitating membrane is relaxed. Finally, important cardiorespiratory variations are observed (Fig. 3).
The duration of these phases is similar to that observed in intact animals, i.e. 10 to 15 minutes. The interval between phases is longer : 40 to 60 minutes (Fig. 2). The behavioural arousal threshold produced by reticular stimulation (shown by an increase in hypertony and mydriasis) is also increased by 200 per cent during p.p., compared with the state of wakefulness.
D. Posterior pontile animal
In animals sectioned along a plane inclined at 60° in relation to the frontal plane, at the posterior part of the pons (between the pons and the trapezoid bodies), no periodic variation of muscular tone, respiration or heart rhythm, such as appears in mesencephalic or pontile animals during p.p., can be observed. EMG activity consistently persists during the survival of the animal (up to ten days). In front of the section cortical activity is often rapid and resembles that described by Batini and co-workers (1958) in cat sectioned in the middle of the pons. Periods of slow waves and spindles can, however, be observed for approximately 40 per cent of the time.