Neural regulation of hormone release is when neuronal input to an endocrine cell increases or decreases hormonal secretion. We will consider three different examples: the autonomic innervation of the pancreas, the adrenal medulla, and neurosecretory cells of the hypothalamus.
The pancreatic islets receive inputs from the autonomic nervous system. The figure diagrams the typical autonomic efferent pattern: targets are innervated via two neurons, a preganglionic neuron that has its cell body in the central nervous system, and a postganglionic neuron that has its cell body in an autonomic ganglion.
Parasympathetic input to the pancreatic beta cells stimulates insulin secretion. Parasympathetic stimulation of insulin secretion is part of what is known as cephalic phase stimulation of insulin secretion. Cephalic phase refers to sensory stimuli and neural inputs that are activated when food is first eaten. There is activation of parasympathetic preganglionic neurons whose axons travel in the vagus nerve. These activate postganglionic neurons that stimulate insulin secretion even before there is an increase in blood glucose. This is an example of feedforward regulation: insulin secretion is stimulated in anticipation of the rise in blood glucose.
Sympathetic input to the pancreatic beta cells inhibits insulin secretion. Sympathetic inhibition of insulin secretion is important during exercise. Muscle cells are utilizing glucose at much higher rates, and so the body needs to activate fuel-producing mechanisms, just as it does during fasting. At the same time, the body needs to prevent glucose uptake by non-muscle cells (which insulin stimulates), so insulin secretion is inhibited.
The adrenal gland is a composite endocrine gland: the outer layer is called the adrenal cortex and the inner layer is called the adrenal medulla. The adrenal medulla is a special case of autonomic innervation. The adrenal medulla is considered a modified sympathetic ganglion. Cells in the adrenal medulla are innervated by sympathetic preganglionic neurons. Like the postganglionic neurons of the sympathetic ganglia, cells in the adrenal medulla release norepinephrine (but they also release epinephrine). Epinephrine and norepinephrine are considered hormones because they are released to the circulation; however they both bind to adrenergic receptors, and thus have much the same physiological effects as sympathetic neural stimulation.
The third type of neural regulation concerns endocrine cells that are found in the brain. These cells are called neurosecretory cells. As shown in the figure, they have dendrites, axons, and terminals just like typical neurons. The difference is that the terminals of neurosecretory cells are adjacent to capillaries. The regulatory molecules they secrete enter the circulation and act as hormones. Hormone secretion is regulated by neurons that form synapses with the dendrites of the neurosecretory cell.
Neurosecretory cells are found in the hypothalamus, a part of the brain that is mainly concerned with regulatory processes necessary for survival. For instance, the hypothalamus regulates feeding, drinking, sleep, stress responses, thermoregulation, and reproduction. In addition to hormonal output, the hypothalamus also controls the autonomic nervous system.
There are two types of neurosecretory cells: magnocellular cells
and parvocellular cells. Magnocellular cells are larger,
having longer axons that terminate in the neurohypophysis
(also known as the posterior pituitary). The parvocellular
cells are smaller, with shorter axons that terminate at a
capillary-rich bulge at the base of the hypothalamus known as the
median eminence.
The
figure at right is a schematic view of the communication between
the hypothalamus and adenohypophysis. The parvocellular cells
release tiny amounts of various hormones whose sole function is to
regulate hormone release by endocrine cells in the adenohypophysis
(also known as the anterior pituitary). These hormones are
conveyed to the adenophypophysis via a restricted vascular link
known as the hypophyseal portal vessels. A portal vessel
is a blood vessel that links two capillary beds, in this case, the
capillary bed of the median eminence with the capillary bed in the
adenophypophysis.