The sympathetic section has nerve centres located on each side of the spinal cord. Stimulation sets the body up for activity, directing the blood flow to peripheral muscles; the parasympathetic section has nerve centres in the Medulla oblongata and at the base of the spinal cord. Stimulation sets the body for relaxation and digestion, directing the blood flow inwards towards the digestive system (see Fig 5 for diagram of the neuronal connections of the ANS).

The endocrine glands are small bodies which secrete hormones into the bloodstream. The hormones are complex chemicals carried by the blood stream and have either a specific effect on one organ or a general effect on all cells. For instance, thyrotrophic hormone secreted by the pituitary gland only affects the thyroid gland (known as the target organ), stimulating it to secrete a hormone of its own Thyroxine, thus having a specific effect - but thyroxine has a general effect on all cells by regulating their respiratory activity. Hormones have a slower effect on the body organs than the nerves and the position of the main endocrine glands can be seen in Fig 6

Both the ANS and the endocrine glands work towards keeping the internal environment of an organism stable, a state described by Canon as homeostasis. Canon referred to the autonomic nervous system as 'interofective' since it affects the internal environment and the central nervous system as exterofective' since through it there is a direct relationship with the external environment.

It is conceivable that a person could survive without an ANS, but to make up for the lack of internal, homeostatic, mechanisms a constant and very favourable external environment would be required - free from threat.

The stress reaction is caused by a perceived threat to comfort which initiates a series of autonomic and humoral (glandular) responses within the organism. The organs most directly concerned with the response are the hypothalamus, pituitary gland and adrenal gland, each being linked to one or both of the others by nervous and hormonal pathways.

Fig 7 is a diagram of the brain to show the relationship between the hypothalamus and pituitary gland.

The hypothalamus lies above the pituitary gland and below the thalamus. It is the centre of several homeostatic systems as well as being the monitor and controller of many hormone levels in the blood. The circadian clock is located in or near it, hunger and thirst responses are directly mediated by it and it also plays a critical role in the relation between hormones and sexual behaviour.

The pituitary gland (hypophysis) is located on the inferior (lower) surface of the hypothalamus to which it is connected by the hypophyseal portal system of blood vessels. The portal system carries two forms of hormone from the hypothalamus:

a Those which are secreted directly from the pituitary in the form in which they were produced by the hypothalamus.

b Those which stimulate secretion of hormones produced in the pituitary gland. Thus it is entirely under the control of the hypothalamus.

The adrenal glands are located above the kidneys and are really two glands in one. The inner part, the medulla, secretes two hormones in response to impulses in autonomic pathways from the hypothalamus. The hormones, adrenaline and noradrenaline, act to release energy stores, increase metabolism and provide an efficient mechanism for response to a crisis by increasing sympathetic activity. The outer part, the cortex, secretes a variety of hormones, known collectively as corticoids, in response to adreno-cortico-trophic hormone (ACTH) from the pituitary gland. The adrenal cortex secretes over 30 hormones and, depending on their action are designated glucocorticoids, mineralocorticoids and sexcorticoids - only the first two are important in the stress response. Glucocorticoids help to increase blood sugar through liver action and protein breakdown. Excess secretion results in growth retardation, muscle wastage, thinning of the skin and suppression of the immune system. Mineralocorticoids help to control the salt balance in the body and through this the water balance. Excess secretion leads to oedema and high blood pressure.

During the normal response to a short lived threat such as a mild electric shock, the hypothalamus is stimulated by the high level of pain and nervous disruption. Impulses pass to the adrenal medulla. Large amounts of adrenaline are secreted which stimulate the hypothalamus to secrete cortico releasing factor (CRF). CRF in turn stimulates the flow of ACTH which causes the release of corticoids into the blood stream. Corticoids in their turn inhibit the action of the hypothalamus and pituitary gland with the result that all systems return to normal. The effects of this response can be felt as a rapid heart beat, butterflies in the stomach, a dry mouth, and a feeling of anxiety. Fig 8 shows the reaction in diagrammatic form.

Link to The General Adaptation Syndrome of Selye | previous page | index