How is the communication among the parts of an organism accomplished?
The endocrine system functions in a hierarchical manner. The hypothalamus produces releasing or inhibiting hormones, which travel in neurosecretory cells to the anterior pituitary, where they control the production of its hormones. Hypothalamus also produces two hormones which are stored in and released from the posterior pituitary level. Many hormones are regulated by negative feedback controls. In such a Control, an excess of the hormone or the product of hormone-induced response suppresses the synthesis or release of that hormone until the level returns to normal. In positive feedback control, the hormone causes an event or process that increases the production of this hormone. Thyroid and steroid are lipid-soluble hormones and readily pass through the plasma membrane of a target cell into the cytoplasm. There they bind to the specific intracellular receptors and binds to specific regulatory sites on the chromosomes. The binding alters proteins, forming a complex that enters the nucleus the pattern of gene expression, initiating the transcription of some genes (DNA) while repressing the transcription of others. This results in the production of specific RNA, translation products, proteins, and enzymes. In this way communication among the parts of an organism accomplished.
