[The neural substrate of biological rhythms]

The various bodily functions vary rhythmically along the 24 hour cycle. These circadian rhythms, display common properties and are generated by common cellular and molecular mechanisms. The basic rhythm is endogenously produced in cellular pacemakers in various regions; the most conspicuous are located in the central nervous system. They are composed by cells endowed with the molecular substrate necessary to generate rhythmicity, and to send the circadian time signal to the effectors of the overt rhythms. The rhythmicity in the pacemaker cells is generated by specific genes integrated in a negative feedback double loop. The structure of the genes responsible for the rhythmicity is phylogenetically well preserved. The circadian pacemakers are in turn synchronized by environmental stimuli, being light the best characterized, but other external agents, such as food and some homeostatic factors and products of metabolic activity are also effective. These synchronizing signals are conveyed to the pacemakers via specific receptors and pathways. The role of some neurotransmitters in the synchronizing action has been demonstrated.

The functional integrity of this complex biochronometrical system is necessary for the maintenance of health. Its alteration results in specific ailments, and enhances the vulnerability to certain diseases