Neuroendocrine cells are cells that receive neuronal input (
neurotransmitters released by nerve cells or neurosecretory cells) and, as a consequence of this input, release message molecules (
hormones) to the blood. In this way they bring about an integration between the
nervous system and the
endocrine system, a process known as
neuroendocrine integration. An example of a neuroendocrine cell is a cell of the
adrenal medulla (innermost part of the
adrenal gland), which releases
adrenaline to the blood. The adrenal medullary cells are controlled by the
sympathetic division of the
autonomic nervous system. These cells are modified
postganglionic neurons. Autonomic nerve fibers lead directly to them from the central nervous system. The adrenal medullary hormones are kept in vesicles much in the same way neurotransmitters are kept in neuronal vesicles. Hormonal effects can last up to ten times longer than those of neurotransmitters. Sympathetic
nerve fiber impulses stimulate the release of adrenal medullary hormones. In this way the sympathetic division of the autonomic nervous system and the medullary secretions function together.