The Human Brain : From Neurone to Nervous System

THE SLEEP-WAKING CYCLE


  1. When the brain is asleep, cortical neurones show synchronous rhythmic activity. Arousal occurs when this synchronous activity is disrupted, as a result of non-specific sensory activity - pain, bright lights, loud sounds, imposed movements, etc. The ascending reticular activating system (ARAS) of the brainstem initiates arousal.

  2. The activity of cortical neurones can be monitored in humans using the electroencephalogram (EEG). During sleep, cortical neurones show synchronised activity, but arousal is associated with desynchronisation, i.e., independent activity of cortical columns. Synchronised activity during sleep is generated by reciprocal connections between the thalamus and cortex, and desynchronisation occurs as a result of strong non-specific sensory inputs. The latter is integrated by the ascending reticular activating system, and distributed to the hypothalamus, thalamus and cortex by the projections of cholinergic neurones of the basal forebrain (the Nucleus Basalis and the nucleus of Meynert), the noradrenergic locus coeruleus, and the serotoninergic raphe nuclei.

  3. The Sleep-Waking Cycle is controlled by the hypothalamus, with two centres alternating in activity; the anterior hypothalamus actively promotes sleeping, whereas the lateral hypothalamus promotes arousal. The'Flip-Flop' hypothesis is outlined below.

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