Mechanism of Sleep
Mechanism of sleep is not clear and has been explained by several theories:
1. Passive theory of sleep.
It was thought earlier that excitatory areas of reticular
activating system send signals to cortex and are responsible for waking state. Fatigue of reticular activating system makes it inactive and causes sleep. This is the passive theory of sleep.
2. Active theory of sleep.
Some centres located in mid pons actively cause sleep by inhibiting other parts of the brain.
3. Stimulation of raphe nuclei in the lower half of the pons and in the medulla.
This causes sleep. Nerve fibres from these nuclei are spread widely in the reticular formation, upward into the thalamus, neocortex, hypothalamus, and most of the areas of limbic system. Fibres also run downwards in the spinal cord terminating on posterior horn cells to inhibit incoming pain signals. Most of these endings secrete serotonin which is the neurotransmitter associated with production of sleep. Excitation of raphe nuclei stimulates some areas in nucleus tractus solitarius for promoting sleep through serotonergic system (stimulation of certain areas of tractus solitarius also produces sleep). Stimulation of several regions in the diencephalon, e.g. suprachiasmatic nucleus of e.g., hypothalamus, diffuse nuclei of thalamus also produce sleep. Discrete lesions of raphe nucleus produces a state of wakefulness.
Other possible transmitters related to sleep are muramyl peptide and non-apeptide which which accumulate in blood and CSF when animal is kept awake for several days. When they are injected in the third ventricle, they cause sleep. Therefore, it is possible that prolonged wakefulness causes progressive accumulation of a sleep factor in the brain.