Unit A Conclusion

In this unit, you studied the nervous system and how it communicates to maintain homeostasis in the body. You investigated how various sense cells and organs help a person to see people, notice their smiles, and hear their laughter from across the room. You explored how these sensations were transmitted electrochemically from neurons to specific areas of the brain to be interpreted. You discovered how the brain is able to send nerve impulses to muscles. These communications are conscious. Other parts of the brain, such as the medulla oblongata, send unconscious messages by way of the sympathetic nervous system to increase heart rate. The parasympathetic nervous system slows the heart and returns the body to homeostasis. You investigated how several disorders, such as multiple sclerosis, Alzheimer's disease, and Parkinson's disorder disrupt communication in the nervous system, cause a loss of homeostasis, and require corrective technologies to attempt to bring the system back to homeostasis.

Human life is possible only if the internal environment of the body is kept within a very narrow range of parameters. If these parameters are upset, the result can be sickness or even death. The endocrine system uses chemical messengers called hormones to communicate with target glands, body cells, and organs to regulate the internal environment so that it remains within this narrow range of normal values. This maintenance of a steady state is homeostasis.

Homeostasis is achieved through closed-loop systems called feedback; information about the response of the system is "fed back" to change the response. Negative feedback counteracts or neutralizes the original stimulus and acts to stabilize the system leading to the reestablishment of homeostasis. Positive feedback reinforces the disruptive stimulus and acts to destabilize the system, which can lead to illness or the development of a disorder.

The principal glands of the endocrine system, including the hypothalamus/pituitary complex, the thyroid gland, the parathyroid gland, the adrenal glands, and the islet cells of the pancreas, produce hormones that maintain homeostasis through negative feedback. Thyroid stimulating hormone, thyroxine, calcitonin, parathyroid hormone, adrenocorticotropic hormone, cortisol, glucagon, insulin, human growth hormone, antidiuretic hormone, epinephrine, and aldosterone are hormones with important physiological functions in the body.

For example:

• Thyroxine has an important role in metabolism.
  
• Insulin, glucagon, and cortisol help to regulate blood sugar.
  
• Calcitonin and parathyroid hormone regulate calcium levels in the blood.
  
• Human growth hormone controls growth.
  
• Antidiuretic hormone adjusts water levels in the blood and extracellular fluid.
  
• Aldosterone maintains a constant sodium concentration in the blood.
  
• Epinephrine prepares the body for the fight-or-flight response - a reaction to short-term stress.

Various medical interventions and technologies, such as synthetic insulin, thyroxine, and human growth hormone, have been developed to deal successfully with hormonal imbalances.

Tropic hormones, such as releasing hormone, TSH, and ACTH produced by the hypothalamus, and anterior pituitary control hormonal secretions from the other endocrine glands exert control over the endocrine system much as the brain exerts control over the nervous system.

Homeostasis within the body is brought about by both the nervous system and the endocrine system. The two systems are linked by the hypothalamus. Nervous control is faster, of shorter duration, and targets a muscle or gland precisely; hormonal control is slower, lasts longer, and is more widespread in its effect.