Sensitivity and coordination
By Joanna George-Johnson, Contributor

A very important aspect of living things is the ability to respond to stimuli.

What is a stimulus? This is a factor external or internal to the organism that elicits a response.

Why is this important?

- To protect the organism, or parts thereof, from dangerous situations.

- For reproduction (to continue the species).

- To allow for proper development.

How does it work?

- Humans respond to stimuli in two ways: nervous system and the hormonal system, also known as the endocrine system.

- The nervous system sends its message via electrical impulses.

- And the endocrine system sends chemical messengers.

(See Tables 1 and 2)

It is important that we do not mix up the glands as there are many glands to be found in the body. Not all glands secrete hormones. Therefore, it is important to be able to distinguish between the glands that secrete hormones (endocrine) and those that do not (exocrine).

(See Endocrine diagram at below)

The endocrine system is made up of the endocrine glands that secrete hormones. Although there are eight major endocrine glands scattered throughout the body, they are still considered to be one system because they have similar functions, similar mechanisms of influence, and many important interrelationships. Some glands also have non-endocrine regions that have functions other than hormone secretion. For example, the pancreas has a major exocrine portion that secretes digestive enzymes and an endocrine portion that secretes hormones. The ovaries and testes secrete hormones and also produce ova and sperms.

Pituitary gland: Small marble-sized gland located in the brain directly below the hypothalamus. The gland has two parts, the anterior and posterior.

Anterior pituitary: Produces hormones that stimulate adrenal glands, gonads and thyroid gland to produce hormones or directly affect specific organs.

Posterior pituitary: Produces different hormones that help with contractions of the uterus during childbirth, milk production and ejection, and for regulation of water in the blood (ADH).

Adrenal glands: Small structures located on top of the kidneys that produce numerous hormones.

Gonads (ovaries and testes): Produce gametes and sex hormones that are necessary for the development and function of both female and male reproductive organs.

Thyroid gland: Plays a role in the de-velopment of the central nervous system, normal bone growth; in normal development of teeth, skin and hair follicles; and in the functioning of the nervous, cardiovascular and gastrointestinal systems.

Parathyroid gland: Increases calcium levels in the blood helping to maintain bone quality and enough supply of calcium, which is needed for muscle movement.

Pancreas: Located in the abdomen behind the stomach. Acts as exocrine organ. Produces various enzymes, essential for the digestion of food, that are secreted into the gut. Contains islets of Langerhans, which are cells that produce two hormones: insulin and glucagon. These two hormones are released into the blood and are important in blood glucose regulation.

Questions:

1. Describe how glucagon and insulin work together to help regulate blood glucose levels.

2. Give the names of the sex hormones that are made in the a) testes and b) ovaries.

Joanna George-Johnson teaches at Ardenne High School masterbio@gmail.com.