The Autonomic Nervous System

The Autonomic Nervous System is a widespread system of nerves that innervates, with the exception of skeletal muscle, nearly every process in the body. This Autonomic Nervous System (ANS) is a completely unconscious process which is responsible for maintaining homeostasis, as well as having many other functions such as control of digestion.

The Autonomic Nervous System was first discovered, and studied by Walter Gaskell (Purves, Augustine, Fitzpatrick and Hall, 2001) who had his work on the ANS first published in 1916, two years after his death. He discovered that certain physiological effects, such as an increase in heart-rate, could be achieved by stimulating certain nerves branching off from the first few thoracic spinal cord segments. Using this, and other similar studies, he then theorized that each system in the body was innervated with two sets of nerves that had opposing functions (Purves, Augustine et al., 2001). These two sets of nerves we now know to be the Sympathetic and Parasympathetic branches of the Autonomic Nervous System; though recently a third branch has been separated from the others and called the Enteric Nervous System, which is found solely in the gut and the supporting features such as the Pancreas (Barker and Barasi, 1999).

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Although the ANS plays an unconscious process, it can sometimes be over-ridden by conscious thought; one example of this is in breathing, although breathing happens automatically without any need for purposeful thought, it is possible to take control of your breathing, such as holding your breath, or exhaling forcefully (to inflate a balloon for example).

As the Autonomic Nervous System is such an important part of the nervous system, studying it and knowing how it works, and how it is controlled, is also very important; in this essay I am going to talk about the three different branches of the Autonomic Nervous System and their functions and differences, and then explain how they are regulated in the brain.

The Sympathetic Nervous System

When we are faced with what our bodies consider to be a threat we have to be prepared to, either stand and face the danger (fight), or get ourselves out the situation as quickly as possible (flight); this is the job of the Sympathetic Nervous System. These “Fight or Flight” responses are designed to give us as much of a physical advantage as possible, such as: making sure enough oxygen is reaching our muscles as possible by increasing heart-rate and dilating blood vessels, saving energy by slowing down lesser-needed processes such as digestion, dilating our pupils so we take in as much light as possible thus improving our vision, and makes our hairs stand on end so we look as big and ferocious as possible; all of these would have given us an edge over any predators we faced.

The main nerve-trunks of the Sympathetic Nervous System leave the Central Nervous System from the Thoracic and Lumbar segments of the spinal cord, this is also known as the thoraco-lumbar outflow (Bakewell,1995). The cell bodies of the Sympathetic pre-ganglionic neurones are found in the lateral horn of the spinal cord between segments T1 and L2. Some of these pre-ganglionic fibres of the Sympathetic Nervous System synapse with the post-ganglionic fibres in a long chain, known as the Sympathetic Chain, that runs the length of the spinal cord, although some have a separate ganglion outside this sympathetic chain; from the ganglion, the post-ganglionic neurones then synapse with their target-organ. Post-ganglionic Sympathetic neurones have a very extensive network of dendrites that are innervated by many pre-ganglionic neurones; but there are roughly 10 times as many post-ganglionic sympathetic neurones than pre-ganglionic neurones, this divergence is thought to aid with coordination of activity between neurones projecting from different segments of the spinal cord (Kandel, Schwartz, and Jessell, 2000).

The Parasympathetic Nervous System

The Parasympathetic Nervous System works in opposition to the Sympathetic Nervous System and therefore gives rise to “Rest and Digest” responses, which act to rebuild the energy stores that were depleted in the previous period of Sympathetic activity. These responses include: slowing down the heart-rate, stimulating digestion, and constricting the pupils.

The cell bodies of Parasympathetic Pre-ganglionic neurones are found mostly in the brainstem, with some in the sacral segments of the spinal cord. In the brainstem, these cell bodies are found in the EdingerWestphal nucleus in the midbrain, which is related to the Oculomotor nerve, the Superior and Inferior Salivary nuclei found in the Pons, and related to the Facial nerve and Glossopharyngeal nerve, and in the Dorsal motor Nucleus of the Vagus nerve, in the medulla and the Nucleus Ambiguus. Because of the location of the cell-bodies of the Parasympathetic Nervous System, the it can also be called the Cranio-Sacral outflow (Bakewell, 1995).

There are a few contrasts between the Sympathetic Nervous System and the Parasympathetic, apart from their opposing actions; the first and most obvious of these is that, in the sympathetic nervous system, all the preganglionic neurones are very short, most making synapses in a chain that runs very close to the spinal cord, meaning they have much longer post-synaptic neurones (as the pathway is disynaptic), whereas in the parasympathetic nervous system, the pre-ganglionic neurones are much longer and almost reach their target organ before making a synapse with a post-ganglionic neurone. This is an important difference as pre-ganglionic neurones are myelinated and therefore conduct action potentials a lot faster than the unmyelinated post-ganglionic neurones, suggesting the the parasympathetic nervous system works slightly quicker than the sympathetic (Kandel et al. 2000). Another difference is that there is a lot of divergence in the Sympathetic Nervous System, as previously discussed, whereas in the Parasympathetic Nervous System there is a lot less, with the ratio of Pre-Ganglionic Neurones to Postganglionic Neurones being 1:3, although this is highly dependant on the tissue as in some it is nearly a 1:1 ratio. Thirdly, both Pre-Gnaglionic and post-Ganglionic neurones in the Parasympathetic Nervous System use Acetyl Choline (ACh) as their Neurotransmitter, whereas Post-ganglionic neurones in the Sympathetic Nervous System use Noradrenaline; this is important as it has lots of clinical applications for drugs that target specifically the Sympathetic or Parasympathetic Nervous System.

The Enteric Nervous System

The Enteric Nervous System is a unique branch of the Autonomic Nervous System that is found lining the sides of the digestive system, specifically the oesophagus, stomach, intestines and secretory glands such as the pancreas (Bear, Connors, and Paradiso, 2007). The Enteric Nervous System is responsible for the control of the tension of the walls of the gut and monitoring the ever-chaning chemical balance within the digestive system, these are highly important functions in digestion. The special property of the Enteric Nervous System is that it can act reasonably independently from the rest of the Central Nervous System, leading to it sometimes being referred to as “The Little Brain”. The cell bodies of the Enteric Nervous System are connected together in two major structures known as plexuses; these are the myenteric plexus and the submucous plexus, these plexuses line the walls of the gut in two separate layers and control peristalsis, internal mucous levels and every other important aspect of the digestive system, including a role in secretion from the pancreas and gall bladder. Though the Enteric Nervous System is a separate function of the Autonomic Nervous System that deals almost solely with the digestive process, it contains as many neurones as the entire spinal cord does, meaning it has a very. Unlike the Sympathetic and Parasympathetic branches of the Autonomic Nervous System which are limited to either 1 or 2 Neurotransmitters, the Enteric Nervous System has been shown to have as many as 20 possible Neurotransmitters, though they have yet to all be proven to act as Neurotransmitters in this system (Gershon, Kirchgessner and Wade, 1994). The Enteric Nervous System has also been shown to play a role in Immune Responses (Hansen, 2003).

Higher Control of the Autonomic Nervous System

As the Sympathetic and Parasympathetic Branches of the Autonomic Nervous System work, for the most part, in opposition to each other, there must be some higher control coordinating them to work in harmony, and the first step of this is in the Nucleus of the Solitary tract (Kandel et al. 2000); here, the nucleus receives afferent input from the Facial, Glossopharyngeal and Vagal nerve, and firstly sends this information to both the brainstem and the spinal cord, where basic functions of the Autonomic Nervous System are carried out, but more cleverly, the Nucleus of the Solitary Tract also takes in other information and combines it, the Nuclei of the Solitary tract also project to the Periaqueductal grey, which also receives information from the hypothalamus, the periaqueductal grey takes all this information and then projects to the Reticular Formation of the Medulla, where it controls the co-ordination between behavioural activity and the autonomic nervous system, and example of this is that when doing heavy exercise (behavioural), it’s important your heart-rate increase so your muscles can get a greater oxygen supply from the blood (autonomic). Another important control of the Autonomic Nervous System is in homeostasis, there needs to be cooperation between the baroreceptors or chemoreceptors for example, and the most useful branch of the autonomic nervous system, so hair can stand on end on cold days to preserve heat, for example; this is the job of the hypothalamus (Kandel et al. 2000) as the hypothalamus receives input from pretty much every sensory pathway in the body; a dated study from Swanson and Sawchenko (1983) proved that the Paraventricular Nucleus of the Hypothalamus had descending pathways to the Autonomic Nervous System. As well as direct control over the Autonomic Nervous System, the Hypothalamus also has an indirect influence over it through the use of relays in other parts of the brain (Squire, Berg, Bloom, du Lac, Ghosh and Spitzer, 2008).

In conclusion, the Autonomic Nervous System is a diverse and widespread system that innervates nearly every aspect of the body, meaning that a threatening stimulus can elicit a whole range of responses throughout the body, from increasing heart rate to diverting blood from the gut and widening blood vessels, this means that the whole body is very quickly and efficiently prepared to face the danger, and then the entire body can quickly calm down again to replenish resources; and not only this, but because of its connections with the hypothalamus and other higher centres, the Autonomic Nervous System isn’t limited to reacting to physical changes, such as body temperature, but it can react to even mental stimuli such as emotions or the anticipation of something. This is important in evolutionary terms, there is little point in reacting to a danger once it has already causes damage so the heightened physical performance needs to come in anticipation of a danger, so that when the danger presents itself, the “fight or flight” response is at its strongest. As this is such an important process in the body, many studies have been carried out into it, and continue to be carried out, and much work is being done on diseases of the Autonomic Nervous System, as, obviously, they have such a dramatic effect on the sufferer.

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Gershon M. D., Kirchgessner A.L. and Wade P.R., (1994) Functional anatomy of the enteric nervous system, Springer Berlin Heidelberg
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