Xenoestrogens may be a factor in declining sperm rates in males of many species, including men. Gonadotropin releasing hormones and testosterone levels remain within homeostatic levels through a negative feed-back loop.
Plant cells are totipotent.
Action potential causes release of neurotransmitter. Action potential causes reuptake of neurotransmitter. Action potential causes degradation of neurotransmitter. Hormone receptors are activated. Action potential causes receptors to initiate neurotransmitter reuptake.