AUTONOMIC NERVOUS SYSTEM 

Despite the quite marked differences in the structural arrangement and neurochemistry of the two divisions of the autonomic nervous system, most visceral organs are innervated by both the sympathetic AND parasympathetic divisions.

Even more weird is the fact that the two divisions usually exert opposite (antagonistic) effects on the tissues they innervate. In most instances, whatever the sympathetic division does to a tissue, the parasympathetic does the opposite. You will see examples of the consequences of this dual innervation as we move through the other systems of the body. However, just to give you a taste of the rich smorgasbord of autonomic nervous system effects, the table below summarises some of the major effects of the two divisions on a few tissues.

One feature of tissues that receive such dual innervation is that the autonomic neurones that innervate them are continuously active (i.e. there are always action potentials travelling along them). This gives rise to the concept that these tissues are subjected to a constant autonomic tone. Activity in these tissues can therefore be controlled by increasing or decreasing the level of sympathetic or parasympathetic tone.

Although this two level control system might seem a little over the top, it does permit very fine control over the activities of what include some of our most vital organs. This arrangement means in effect that the level of activity of any tissue subject to dual innervation is a balance between the opposite effects of the sympathetic and parasympathetic divisions. In fact this makes a lot of sense if you want to exert precise control over important physiological functions. If you only had an excitatory effect it would be a little like riding a bike without any brakes. You'd probably still be able to get around but weaving your way along George Street at 6 pm on a Friday evening might be a little exciting.

Whilst in most tissues that receive dual innervation the sympathetic and parasympathetic divisions exert antagonistic effects, there are some tissues where they act cooperatively to produce the same effect. Probably the best example of this is where the sympathetic and parasympathetic divisions work together to produce an erection. We will revisit this issue when we deal with the reproductive system in Medical Physiology 2 (LQB488).

Despite the predominance of dual innervation (whether it be antagonistic or cooperative) there are tissues which receive innervation from only the sympathetic OR parasympathetic divisions of the nervous system. Some of these are highlighted in the table below.