Hormones are organic compounds produced in one part of the body, form which they are transported to other parts where they produce a response. A minute quantity may exert a profound effect on the organism’s development, structure or behavior. Here we shall only be concerned with the general principles involved in their activities especially in what we shall henceforth refer to as hormonal and nervous communication.
Hormones are secreted by endocrine organs directly into the blood stream. The word endocrine means internal secretion and the endocrine organs are therefore glands of internal secretion. Since they shed their secretion into the bloodstream, they have no ducts and are hence known as ductless glands. Once in the bloodstream, the hormones are carried round the body, bringing about responses in various places. Structures that respond to them are called target organs.
Though they may be widely separated from one another spatially, endocrine organs do not exist in functional isolation. They influence one another and through their integrations, are integrated into a highly coordinated system, the endocrine system.
Hormonal Compared With Nervous Communication
A basic similarity between hormonal and nervous communication, i.e the endocrine and nervous systems, is that both provide means of communication within the body of an organism. Both involve transmission of a message which is triggered by a stimulus and produces a response. The target organs of a hormone are equivalent to a nerve’s effectors. The main difference between the two systems concerns the nature of the message. In the endocrine system the message takes the form of a chemical substance conveyed through the blood system. In the nervous system the message is a discrete, all – or – nothing action potential transmitted along a nerve fibre. All other differences spring from this fundamental one. They can be listed as follows:
- Because of the comparatively high speed at which impulses are transmitted along nerves, nervous responses are generally evoked more rapidly than hormonal ones.
- Since it is shed into the bloodstream, there is nothing to stop a hormone being carried to every part of the body. Nervous impulse, however, are transmitted by particular neurons to specific destinations.
- As a result of (2), hormonal responses are often widespread, sometimes involving the participation of numerous target organs far removed from one another. In contrast, nervous responses may be very localized, involving perhaps, the contraction of only one muscle
- Hormonal responses frequently continue over a long period of time: obvious examples of such long – term responses are growth and metabolism. Nervous responses, on the other hand, are usually rapid ad short – lived, such as the contraction of a muscle.
Despite these obvious differences, there is one fundamental similarity between the two systems: both involve chemical transmission. We saw earlier that in the nervous system remission of the message across the neuro-muscular junctions is achieved by a chemical substance. The latter is equivalent to a hormone in the endocrine system. The principal difference between them is that the neuromuscular transmitter has to travel a mere fraction of a micrometer, whereas a hormone may have to travel the full length of the body to achieve its full effect.
This may seem a rather academic point of comparison, but in fact it provides a basis for linking the two systems. This is best illustrated by the adrenal glands. The middle part of these glands, the adrenal medulla, secretes the hormone adrenaline which is chemically almost identical to the transmitter substance noradrenaline produced at the ends of the sympathetic nerves. It is interesting that adrenaline evokes the ames responses as impulses in the sympathetic nerves: acceleration of the heart, constriction of arterioles, dilatation of the pupils, etc. in addition adrenaline induces a marked increase in the metabolic rate, so that the combined effect of the endocrine and nervous systems is to prepare the body for emergency.
We see, then, that there is a close connection between the endocrine and nervous system(hormonal and nervous communication system). In the case of the adrenal medulla and sympathetic nerves, the connection is so close that one suspects that the two share a common evolutionary origin. Innervated by the sympathetic nervous system, the adrenal medulla can be looked upon as an enormous conglomeration of modified nerve cells which being far removed from any effectors shed their transmitter substance into the bloodstream.
