The Endocrine System

Summarized from works by Lee Weller
Medline Plus


  The endocrine system shares with the nervous system a role for control and coordination throughout the body. The nervous system is mostly thought of as dealing with very short term control (on a time scale of milliseconds to seconds), while the endocrine system, through hormones, may exert relatively short term (seconds to minutes) or long term (days to weeks) control.
  The nervous system and endocrine system should not be considered as entirely separate entities. Some endocrine organs respond to nervous stimuli. The endocrine system is important in the development and maintenance of the nervous system. Some reflexes involve both systems.
The endocrine and nervous system share areas of the brain. For example, within the forebrain lie the thalamus and hypothalamus. The thalamus is a relay station that receives, analyzes and sends on information from all the senses. Located below the thalamus is the hypothalamus. This regulates hunger, thirst, the sex drive, and other motivated behavior (Neitzel, et al; 1998).

The hypothalamus is also part of the limbic system, which regulates emotion and memory. Furthermore, the hypothalamus receives information from the autonomic nervous system (ANS) about the functioning of internal organs. The autonomic nervous system is our involuntary part of the nervous system. It is that part of the nervous system that allows us to do things without thinking (e.g. control over our respiratory and circulatory system), and influences our endocrine system. Thus, it is especially important to the understanding of emotional behavior.

  The ANS is divided into two parts, the sympathetic and the parasympathetic nervous systems. The sympathetic system mobilizes the energies of the body for an emergency. It kicks in when you are in a state of fear excitement, or anxiety. Your heart beats faster and more strongly, blood pressure rises, salivary secretion is inhibited, perspiration increases, blood sugar is released from the liver, and the adrenalin is released from the adrenal gland of the endocrine system. This can be both beneficial or a problem: In a game or emergency, it can give you a boost of energy, but it is not beneficial if it becomes a distraction, or causes the person to over-react.
  The parasympathetic nervous system operates in a way opposite to that of the sympathetic division. It slows the heart beat and reduces blood pressure. In short, the parasympathetic division usually conserves the resources of the body. It is what is operating when you are relaxed.
Not only is the hypothalamus influenced by the nervous system, it also responds to hormones from the endocrine system. The hypothalamus is connected to the pituitary gland, which in turn serves as the director of the endocrine system (Neitzel, et al; 1998).
  Structurally, the endocrine system comprises a number of separate organs and tissues scattered throughout the body. These include:
  * The pituitary (hypophysis) attached to the underside of the brain
* The thyroid, situated in the neck close to the larynx
* The parathyroids, associated with the thyroid but comprising separate structures
* The adrenals, situated on top of each kidney
* The endocrine portion of the pancreas
* The ovaries or testes..
  All the structures have in common the characteristic that they produce one or more hormones. Other structures not listed above also produce hormones, but are not included within the endocrine system. Examples of these other structures which produce hormones include the placenta in the pregnant female; glands within the gastro-intestinal tract; structures within the heart and blood vessels, and structures within the kidney.


  A hormone is defined as "A chemical substance, synthesized by a specific organ or tissue and secreted into the blood stream, in which it is carried to other specific sites in the body, where it exerts its action(s)".

Mechanisms of Hormone Action

  As hormones are transported in the blood stream, their molecules are able to reach where ever the blood circulates. Yet hormones are only able to act on cells which possess receptors specific to that particular hormone. Cells which possess receptors are termed target cells, which in combination may be referred to as target tissues, or target organs.

Control of Hormone Release

  There are three ways in which the release of a hormone from an endocrine gland may be controlled.
  Substrate control. This is the simplest form of control mechanism, where the hormone is directly influenced by the circulating blood levels of the substrate that the hormone itself controls. This sets up a simple control loop in which the substrate is controlling release of the hormone, which by its action(s) is altering the level of the substrate. Examples of this type of control are afforded by calcitonin (substrate is calcium), aldosterone (substrate is potassium) and insulin (substrate is glucose). In effect, the insulin controls the glucose which, in turn, controls the insulin.
  Nervous control. Some hormones are released directly in response to nervous stimuli. Endocrine glands which are most influenced by nerve inputs include the pituitary, where the nervous control arises in the hypothalamus, and the adrenal medulla, where the nervous control arises from the sympathetic nervous system, which is part of the autonomic nervous system.
  Trophic hormones. In some cases, the endocrine gland is itself a target organ for another hormone. Hormones of this type are termed trophic hormones, and they are all released from the anterior pituitary gland (adenohypophysis). Endocrine glands controlled principally by trophic hormones include the thyroid gland and the adrenal cortex.

Feedback Mechanisms

  Except when there is a definite stimulus present to alter the blood levels of a particular hormone, hormone release is largely controlled by the circulating levels of the hormone itself. Exceeding a certain level inhibits further hormone release. This type of control mechanism is termed negative feedback control.
For negative feedback to occur, cells producing hormones must themselves have receptors to that particular hormone, or be influenced by cells which possess such receptors. Feedback operates at many different levels within the endocrine system, but most involve hormones associated with the pituitary gland.
Feedback loops are referred to as direct if the feedback involves the structure immediately preceding it in the control sequence, or indirect if at least one step removed. Terms also used are short and long feedback loops, and in the case of a hormone directly affecting its own release from the same cell in which it is synthesized, the term ultra-short loop may be used.


Endocrine Glands


Endocrine System

  As previously stated, the endocrine system consists of glands that produce and secrete hormones into the blood or lymph systems. These glands include the thyroid, parathyroid, hypothalamus, pineal, pituitary, adrenal, islands of Langerhans in the pancreas, and the gonads (testes and ovaries). The effects of these hormones may affect one organ or tissue, or the entire body.

Disorders may result when too few hormones are secreted (hyposecretion) or too many are secreted (hypersecretion). Because particular endocrine states may indirectly be associated with negative educational outcomes through the influence of psychosocial stress and inappropriate child-rearing styles, a list of glands and disorders are listed below that may result when the produce too many or too few hormones:


  * Cretinism
* myxedema
* goiter
* thyrotoxicosis


  * tetany
* renal calculi
* excessive loss of minerals from bone


  * Addison's disease
* adrenogenital syndrome
* Cushing's syndrome
* pheochromocytoma


  * dwarfism
* acromegaly
* diabetes
* gigantism
* diabetes insipidus

 Testes and ovaries:

  * lack of sex development (ambiguous genitalia)


  * diabetes mellitus
* hypoglycemia



  As stated above, the endocrine system is influenced by the limbic system. You learned earlier, that the limbic system, in turn, is affected by the cortex. Thus, any action which would impact the cortex will effect the limbic system or visa versa (that which effects the limbic system can influence the functioning of the cortex).

As an example, take the case of when you are frightened. What happens is known as the hypothalamic-pituitary-adrenocortical (HPAC) axis. The hypothalamus secretes corticotropin releasing hormone (CRH). The CRH starts a chain of integrated biochemical and physiological defenses against the stressor, being frightened, by telling the pituitary gland to secrete another hormone (adrenocorticotrophic- ACTH). This hormone, in turn, directs the adrenal glands to release adrenal corticosteroids, which are referred to as stress hormones. The stress hormones informs the parasympathetic system to shut down, directing energy away from the digestive system and immune system towards those that activate the body (heart rate, respiratory, etc.).

Next, the hypothalamus influences the brain stem and spinal cord to stimulate the adrenal glands to release the catecholamines (epinephrine and norepinephrine). These hormones stimulate the central nervous system, increasing attention and concentration. This makes the person more vigilant to danger, but feelings of heightened anxiety, or panic, may accompany these changes.

These physical reactions to stressors are temporarily adaptive, however, they can become harmful if they are over used. For example, concern about a stressor may cause constant anxiety, depression, or feelings of helplessness. In addition, because this action effects the immune system, a person becomes more susceptible to illness.

Because of the connection between the nervous and endocrine systems, people can consciously remain calm in situations that could cause them to be frightened. For example, as you learned in the section on neurology, the cortex (e.g., frontal lobe) is connected to the limbic system, which, in turn, connects to the endocrine system. This suggests that a person can consciously control, or influence, the endocrine system. In other words, people can learn to overcome their fears. This is what Cognitive Behavioral Therapy attempts to do, and leads us into looking at the second area; Psychological influences of behavior.