The article presents the most important mechanisms related to the functioning of the retina, the suprachiasmatic nucleus (SCN) and the pineal gland as components of the mammalian biological clock. Environmental light influences the biological clock in mammals via light-sensitive, retinal ganglion cells containing a photo pigment - melanopsin. The axons of these neurons form the retionohypothalamic tract, which terminates in SCN. Neurons located in SCN generate cyclic, circadian changes in their activity due to a system of clock genes, the transcription of which is mutually controlled by an auto regulatory feedback loop. Glutamate and pituitary adenylate cyclase activating peptide (PACAP) - neurotransmitters released from terminals of the retionohypothalamic tract, synchronize the activity of the SCN neurons with environmental light conditions. The neuronal cells located in SCN influence the pineal activity via a paired, mulisynaptic pathway, composed of neurons of the paraventricular nucleus, the intermediolateral nuclei of the spinal cord and the cranial cervical ganglion, which supply the pineal gland with sympathetic nerve fibers. Norepinephrine, released from these fibers at night, stimulates melatonin secretion. The intracellular mechanisms controlling melatonin synthesis in the pinealocytes have significant variations between species, which accounts for differences in the diurnal patterns of pineal hormone secretion.
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