Nervous System-Endocrine System Interactions

The link between the nervous and endocrine systems lies in two glands located
between the cerebrum and the brain stem, the hypothalamus and the
hypophysis (the pituitary gland). Electrical impulses from neurons in the
cerebral cortex may activate the hypothalamus to release hormones that activate
the hypophysis to release its hormones, which in turn activate or inactivate
other endocrine glands throughout the body. These glands include
the thyroid, parathyroids, thymus, pancreas, adrenals, and reproductive organs.
This entire system operates by negative feedback homeostasis so that,
once information is transferred and specific bodily functions are achieved,
nervous or hormonal signals travel back to the hypothalamus to terminate
any further action.

Animal behavior occurs as a result of the actions of the nervous and endocrine
systems. There is a complex interplay among these two body systems,
the environment, and an individual’s genetic makeup in terms of the causeand-
effect, stimulus-response events that constitute behavior. An animal receives
external information via its special senses (eyes, ears, nose, mouth)
and somatic sense organs (touch, pain, temperature, pressure). This external
information travels along sensory neurons toward the brain and spinal
cord, where the information is analyzed and a motor response to the external
stimulus is initiated. Some of these motor responses will be directed toward
the sense organs, locomotory muscles, and organs such as the heart
and intestines. Other impulses will be directed toward the hypothalamus,
which controls body cycles such as all endocrine system hormones, heart
rate, sleep-wake cycles, and hunger.

When the hypothalamus releases the hormone corticoliberin, the pituitary
gland (the hypophysis) releases the hormones thyrotropin (which activates
the thyroid gland), prolactin (which stimulates milk production in the female breast), and growth hormone (which triggers growth in children
and metabolic changes in adults). When the thyroid gland is activated, hormones
such as thyroxine and triiodothyronine are released to accelerate cellular
metabolism, an event which may occur in certain situations such as
stress or fight-or-flight encounters.

If the pituitary gland releases adrenocorticotropic hormone (ACTH),
the adrenal glands will be activated to release their hormones. The adrenal
cortex produces and secretes a variety of hormones, such as aldosterone,
which regulates the blood-salt balance directly and blood pressure indirectly;
cortisol, which accelerates body metabolism; and androgens, or sex
hormones. All of these are steroid hormones, which are involved in rapidly
preparing the body for strenuous performance. Even more pronounced are
the effects of the adrenal medulla, which produces and secretes the hormone
neurotransmitters epinephrine and norepinephrine; these two hormones
accelerate heart, muscle, and nerve action as well as stimulate the release
of fat and sugar into the bloodstream for quick energy, all of which are
extremely important for spontaneous activity such as fighting with or fleeing
from enemies. The control of sugar storage and release from the liver by the
pancreatic hormones insulin and glucagon also are important in this process.