The digestive system is a large and complicated system that involves a diverse range of functions that are mediated by a number of different organs and has to cope with the variable nature of our feeding habits.
Given its size it is perhaps not surprising that the digestive system makes very significant energy demands on our body. For this reason it makes little sense to have the system constantly primed and ready just in case we decide to pop into our local Indian restaurant and order a chicken chilli masala, large rice and a naan bread.
During fasting the digestive system slows down to a fairly low level of activity but has to wind-up in preparation for a meal. It then has to constantly adjust its response depending on the size of the meal and finally attenuate digestive functions as the meal is processed.
This level of control is enabled by five different control systems that act cooperatively to integrate the activities of many different organs, tissues and cells. These control systems are outlined below.
Enteric Nervous System
The enteric nervous system is a collection of neurones that are intrinsic to the digestive tract. There are around 100 million neurones in the enteric nervous system and in the digestive tract these are collected together in the submucosal nerve plexus and myenteric nerve plexus.
As is shown in the figure on the right, these neurones constitute a true nervous system with afferent (sensory) neurones that detect stimuli, interneurones that integrate this information and efferent (motor) neurones that produce effects such as contraction of smooth muscle or secretion of substances by glands.
The neuronal circuits of the enteric nervous system are why many of the functions of the digestive tract can still be observed in isolated organ preparations where all other external influences have been removed.
Autonomic Nervous System
Like other systems of the body the digestive system is controlled by the autonomic nervous system (see diagram on the left). We have already covered the organisation of the autonomic nervous system and should be aware of the dynamically opposing effects of its two divisions.
In relation to the digestive system the parasympathetic division typically increases motility and exocrine secretions as part of its global function of ‘rest and digest’. The sympathetic division on the other hand is primarily involved in the ‘fight or flight’ reaction so typically attenuates activity in the digestive system.
We will see some more specific examples of the effects of these two divisions of the autonomic nervous system as we move through the digestive tract.
Somatosensory System
As well as the local reflexes mediated by the enteric nervous system there are also long reflexes that involve sensory information from the digestive tract being relayed to autonomic control centres in the brainstem and then effects being mediated by the parasympathetic and sympathetic divisions of the autonomic nervous system.
The afferent (sensory) component of these long reflexes is mediated by neurones of the somatosensory system (also shown in the diagram on the left). These neurones are also responsible for the sensations that arise from our digestive tract (most noticeably nociception).
Endocrine System
Given the size of the digestive system and the diversity of organs involved in its action, it is probably of no surprise to hear that the endocrine system is also intimately involved in its control. There are a large number of hormones that are released by cells in the digestive system that mediate effects of other cells in other organs after travelling through the blood in classic endocrine fashion.
Paracrine System
The digestive system is also uses another form of chemical communication that involves the release of chemicals from cells.
In this case however these chemical messengers don’t reach the blood stream but have their effects on surrounding cells.
As shown in the animation on the right, the chemical simply diffuses through the extracellular space and binds to the receptors associated with its neighbouring target cells.
This form of communication is quite important in some aspects of digestive system physiology and is referred to as paracrine control.
So the regulation of the digestive system is under the control of five discrete systems. Whilst it is sometimes convenient to consider these different systems in isolation, it is probably worth reflecting on the fact that these work cooperatively. As we have seen before this semester the endocrine system is often used to reinforce an effect mediated by the nervous system (and vice versa). Through these types of cooperative interactions the very important functions of the digestive system are very precisely controlled.