The hypothalamus

The hypothalamus produces secretions-and here, as some endocrinologists put it, is what might be considered a two-way chemical bridge between "body" and "mind." Certainly, it is now clear that the endocrine system and the autonomic nervous system work together closely. Consider, for example, what happens when, for any reason, you experience an alarm reaction. 

From the nervous system, a message is transmitted to the cores of the adrenals which then secrete into the blood a hormone that increases heart action and narrows blood vessels so blood is pushed through them with more force; the hormone also relaxes and enlarges airways so more air can reach the lungs more quickly; in addition, the hormone reaches the pituitary which then secretes hormones to cause adrenal cortex, thyroid, parathyroid, even the gonads (which are not exclusively sexual in function) to secrete hormones. 

All these hormones complete the almost instantaneous mustering of body and mind to deal with stress-and account for some of the superhuman feats of thinking, action, and muscular accomplishment which humans often exhibit under stress.

There is no simple, neat categorization for the endocrine system, its functioning, and its relationships with the nervous system. The pituitary, if not the master, is a critical link in a delicate check- and-balance system, and all endocrine glands are involved in the checking and balancing. If one becomes overactive or underactive, it influences all the rest. Some may speed up activities; others may slow them down. A new balance is set up, but it may not always be a healthy balance. For this reason, glandular disorders can be very complex. 

The relationship between endocrine system and nervous system is two-way. Nerve impulses influence glands; and glands influence nerves. Emotions affect the autonomic nervous system, which in turns affects gland activity. And glands work through the nervous system and influence emotions. One example of the latter is evident in the hyperthyroid person whose thyroid is excessively active; he or she is tense, easily exited, stirred up by the slightest disturbance, nervous, irritable.