The endocrine or hormonal system (i.e. the use of body fluid-borne chemical messengers) together with the nervous system make up the control and coordinating systems of animals. However, there are major differences in the way in which control is achieved within the two systems. Firstly, the endocrine system works by transmitting chemical rather than electrical signals, although the nervous system utilizes chemical messengers at synapses. Secondly, the endocrine system has a much slower response time than the nervous system. An action potential is completed in 2-3 ms, but the action of hormones may take minutes or hours to be completed. Finally, endocrine action has a much longer duration of response. For example, reflexes in animals ― fast pre-programmed responses of the nervous system ― take a few milliseconds to be performed. Compare that with growth processes that are achieved utilizing the hormonal system that may take years to be completed. However, having stated that there are major differences between the two systems, it is becoming increasingly recognized that rather than working as two ‘independent’ systems, the nervous and endocrine systems work cooperatively to achieve a common goal. Indeed, some neurons will release neurotransmitters at their synapses that are then used to serve an endocrine function. Most animals have an endocrine system, and it controls many diverse physiological functions, e.g. metabolism, growth, reproduction, osmotic and ionic regulation, and so on.
Definition of endocrine systems
The classical idea of the endocrine system is that of cells, usually of a nonneural origin (although some neural tissue is considered to have an endocrine function), which secrete specific chemical messengers called hormones. The hormones are carried to their target organs (i.e. the organs where they exert their biological effect), usually some distance from their site of release, in the body fluids of the animal concerned. However, this classical view of endocrine organs and function has recently changed. For example, it is now known that some hormones do not need to enter the general circulatory system of animals in order for them to exert an effect, A good example of this is the role of histamine in controlling acid secretion in the vertebrate stomach, whereby various stimulatory factors converge on mast cells in the stomach (as well as parietal cells) leading to the release of histamine which, in turn, stimulates acid production. This type of ‘local’ hormone action is called paracrine control. In general terms, though, endocrine systems may be classified as one of two types. The first is the neuroendocrine system, also called the neurosecretory system or neurosecretory cells. In this case, neurons are specialized for the synthesis, storage and release of neurohonnones ― in reality, this is the neurotransmitter of the neuron concerned. The neurohormone, instead of being released into a synapse, is released into the general circulation from where it travels to its target organ. The neuroendocrine system is found in all invertebrates and vertebrates. In mammals, for example, renal excretion of water is controlled by the secretion of antidiuretic hormone (ADH) released from neurons whose cell bodies lie in the hypothalamic region of the brain and whose axons extend down to the posterior pituitary gland. In some cases, the release of neurohormones into the general circulation may influence other endocrine organs which then exert some biological effect. For example, in crabs, moulting is controlled by the neurohormone moult inhibiting hormone (MIH), which in turn inhibits the activity of a second endocrine gland which produces a hormone that promotes moulting. The widespread presence of neuroendocrine control systems in both invertebrates and vertebrates suggests that they evolved earlier in evolution than the second type of endocrine system, the classical endocrine system. In this case, hormones are released from specialized, nonneural tissue directly into the body fluids. The absence of ducts to transport the hormone from the gland to the circulating body fluids (e.g. plasma, haemolymph) gives rise to the term ductless gland, an alternative term by which endocrine glands are sometimes described. This contrasts with ducted glands (e.g. salivary glands), where an anatomical duct leading from the gland to the body fluids is present. Classical endocrine glands are only found in the higher invertebrates (e.g. some molluscs) and the vertebrates. This suggests that the appearance of this system occurred after the development of the neuroendocrine system. The presence of neuroendocrine control systems and paracrine control blurs the typical definition and concept of endocrine control, and it may be more appropriate not to consider the two control systems of neural and endocrine control as being so clearly distinct from each other.
Identification of endocrine organs
It can be difficult to determine whether a particular structure in an animal serves an endocrine function. The fact that there are no unique anatomical markers that serve to identify endocrine from nonendocrine tissue is just one reason. In order to overcome this problem, criteria have been established by which candidate tissues and their secretions may be classified as true endocrine organs.
(i) Removal of the candidate tissue or organ should produce deficiency symptoms. For example, if a tissue was suspected of producing a substance that maintained Na+ levels in body fluids, then removal would result in disruption of Na+ levels.
(ii) Reimplantation of the candidate tissue or organ should result in the reversal or prevention of the associated deficiency symptoms. In the case described above, Na” levels would return to their correct levels once the candidate tissue had been reimplanted in the animal concerned.
(iii) Administration of an extract of the tissue or organ should also result in reversal or prevention of the associated deficiency symptoms.
(iv) Finally, the suspected hormone must be purified, its structure determined and tested for biological activity. It must exert the same biological effect as that seen previously with the intact organ or tissue.
The chemical nature of hormones
Virtually all hormones from both invertebrate and vertebrate animals, fall into one of three major classes ― peptides or proteins, amino acid derivatives and steroids. There are exceptions to this, such as the range of C20 compounds known as the prostaglandins. The compounds serve many functions in animals and are beyond the scope of the present text. The chemical nature of the hormone is important because ultimately it decides how the hormone exerts its biological effect.