The other means by which the brain instructs the body organs to alter their activity is through the action of hormones released from endocrine glands. Hormones travel in the bloodstream around the body and therefore have access to all the body organs. Each hormone carries a specific message instructing the organs to alter their activity.
Of particular interest to us in considering the stress response are the adrenal and pituitary glands. The two adrenal glands, one lying on top of each kidney, are the source of several hormones which are released during the stress response. Each gland consists of two parts, an outer part called the adrenal cortex and an inner part called the adrenal medulla. The medulla produces large quantities of two very similar hormones called adrenaline.
The effect of stimulating parasympathetic and sympathetic nerves organ/tissue parasympathetic effect sympathetic effect.
Heart Decreased rate Increased rate and force of beat.
Blood vessels Generally little or no effect but can cause dilation (widening) of blood vessels to heart muscle, lung, brain and sex organs Constriction, except those supplying heart muscles, leg and arm muscles which dilate.
Brain, mental activity Little or none Mental activity increased.
Blood Little or none Increases ability to clot and noradrenaline, technically known as catecholamines (pronounced cat-e-coal-a-means). The hormone noradrenaline is exactly the same as the neurotransmitter noradrenaline produced by the sympathetic nerve endings. In fact, the adrenal medulla can be regarded as an extension of the sympathetic nervous system. The sympathetic nervous system supplies nerves to the adrenal medulla and controls the release of adrenaline and noradrenaline from it. Although noradrenaline and adrenaline are similar in structure their effects on the body organs are different. This is important in instructing the body organs to prepare for either fighting or fleeing, as we shall see later. The other part of the adrenal gland, the adrenal cortex, produces different hormones that prepare the body to deal with long-term demands. One of these is cortisol which is of particular importance as far as the resistance response is concerned.
The release of most adrenal cortex hormones, including cortisol, is controlled by other hormones secreted by the pituitary gland, which lies close to the base of the brain. The pituitary gland is known as the ‹“master endocrine gland' of the body because it controls, by releasing its own hormones, the production and release of hormones from a number of other endocrine glands. The release of hormones from the pituitary is influenced by messages from the hypothalamus.
Cortisol's main role is to ensure a supply of fuel to active body muscles. It helps to convert stores of fat into energy. Cortisol also makes it easier for the catecholmines to carry out their roles. At normal levels, cortisol aids the body's defence mechanisms to deal with infection or injury.
The stress response is largely brought about by the action of the hormones noradrenaline, adrenaline and cortisol.
The brain sends out instructions to the body organs mainly via the sympathetic nervous system, ultimately leading to the release of noradrenaline from nerve endings directly onto the organ, and by hormones (noradrenaline, adrenaline, cortisol) from the adrenal glands via the bloodstream. In the case of noradrenaline this chemical messenger is both a neurotransmitter and a hormone. The reason for such an arrangement is to ensure that the body can react immediately when necessary by using the fast sympathetic nervous system communication line direct to the body organs. However, the effect of noradrenaline at the nerve endings lasts only a few seconds. The additional communication route to the adrenal medulla, releasing both adrenaline and noradrenaline, can prolong and intensify the response initiated by the sympathetic neurotransmitter. This ensures that the body response, once initiated, can be maintained for as long as necessary. In this way physical activity can be sustained to enable the body to deal with and overcome the stressful situation.
When the stress response is no longer required, the brain sends instructions to reduce sympathetic nervous system activity and to increase parasympathetic nervous system activity. The parasympathetic nervous system then produces acetylcholine which acts on the body organs and this leads to a state of rest and relaxation.
With this array of different neurotransmitters and hormones, it is possible for the brain to initiate the most appropriate organic action by selecting a suitable cocktail of chemical messengers.