So far in this consideration of the autonomic nervous system we have only examined the peripheral aspects. We should have by now a fairly good idea of what happens when the activity in symapathetic or parasympathetic preganglionic neurones is altered. The question that we need to address if our knowledge of this topic is to be complete is - what is it that regulates the activity of the preganglionic neurones?
The neuronal circuits responsible for this control are collectively known as the autonomic control systems and, perhaps not surprisingly, these are fairly complex and in many instances remain poorly understood. However what is clear is that there exist fairly simple reflex circuits which act at the level of the brainstem and spinal cord to carryout the more mundane roles of the autonomic nervous system as well as higher order circuits responsible for the regulation and coordination of more complex functions. We'll consider each of these briefly in the subsequent sections.
A. Autonomic Reflexes
Like most reflexes, autonomic reflexes require a population of sensory neurones to monitor the state of play in the peripheral tissues and a population of efferent neurones to mediate the effects. In the case of autonomic reflexes of course the efferent pathway is mediated by the sympathetic or parasympathetic divisions. Two fairly simple autonomic reflexes will serve to illustrate this point:
Pupillary Reflex
The pupillary reflex is the fairly well known change in the diameter of the pupil of the eye in response to varying environmental lighting conditions. If someone shines a bright light into your eye the pupil constricts reflexly. In this case the sensory neurones are in your retina. The axons of these sensory neurones project out of the eye along the optic nerve and synapse in the brainstem. These sensory neurones excite a population of efferent neurones which in turn synapse on preganglionic parasympathetic neurones whose axons exit the brainstem via the IIIrd cranial nerve and synapse with the parasympathetic postganglionic neurones very close to the eye. The parasympathetic postganglionic neurones in turn cause contraction of the constrictor muscles of the iris which constricts the pupil.
Voiding Reflex
Another familiar autonomic reflex controls the contractions of the bladder as it begins to approach full capacity. The distension of the bladder by urine is detected by sensory neurones in its walls. These sensory neurones synapse with parasympathetic preganglionic neurones in the sacral segments of the spinal cord which cause reflex contraction of the smooth muscle in the wall of the bladder and the bladder empties. Fortunately in most healthy adults the reflex can be consciously inhibited until it is socially acceptable for bladder emptying to take place.
B. Autonomic Control Centres
Whilst autonomic reflexes do a very good job of producing rapid and appropriate responses in specific peripheral tissues (the eye and bladder in the examples above) clearly homeostasis requires the involvement of more complex neural systems to integrate the diverse functions that the autonomic nervous system can regulate. One important fact that has come to light in the last decade is that in both the sympathetic and parasympathetic divisions there are very specific pools of preganglionic neurones that make highly ordered synaptic contacts with postganglionic neurones innervating particular peripheral tissues. This idea is illustrated in the figure opposite.
In this example, separate populations of sympathetic neurones innervate the blood vessels of the heart (red) and the skin (blue).
As a result of this highly ordered arrangement, regulation of activity of the preganglionic neurones going to the heart or those going to the skin will permit independent control of blood flow to these particular tissues.
Clearly then all the central control systems need to do is modify the activity of the appropriate population of preganglionic neurones (in the appropriate direction) to produce the desired result.
It is becoming increasingly evident that throughout the brainstem and forebrain there are neural circuits that are able to selectively modify activity in specific populations of preganglionic neurones and thereby coordinate some of the more complex interactions in peripheral tissues.
These so-called higher order autonomic control centres will be considered when the specific system they regulate are covered. The important thing to realise at this juncture is that they mediate their effects by switching on or off the appropriate population of preganglionic neurones.