Neurological Snippets

The first in a series of news and reviews written by Dr Philip Parkin, Neurologist, Christchurch.

Autonomous Nervous System - the brain's unsung hero

On hearing the term ‘the nervous system’ we immediately think of its two principal components: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the motor and sensory nerves that transmit sensory information from the periphery to the brain and that also carry the motor signals from the spinal cord to the muscles). But there is a third component, largely unsung, without which most of our internal organs would fail – the autonomic nervous system.

As the name implies, this neurological system functions largely beyond our will and, therefore, with little awareness on our part of the crucial role it is playing behind the scenes. It is, quite simply, the control system that regulates such bodily functions as breathing, heart rate, digestion and the transportation of food through the gut, bladder and sexual function and the regulation of blood pressure – to name just some. It is also the generator of the familiar ‘fight-or-flight’ response.

It has two components – the sympathetic and the parasympathetic systems. The former arises from the lower part of the brain (brainstem) and the latter from the spinal cord, although both are ultimately controlled by the brain and particularly respond to our emotions and circumstances, While these two components of the autonomic nervous system used to be viewed as each having an opposite effect to the other on the organs they supply, this has proved to be an over-simplification. Instead, the sympathetic system tends to be responsible for making physiological changes quickly and the parasympathetic tends to be a more slowly-acting dampening system. The ‘fight or flight’ acute stress response to threat allows us the opportunity of experiencing what the sympathetic component of the autonomic nervous system does. It immediately diverts blood from the gastrointestinal tract and skin to the muscles and the lungs, where it is urgently needed. It activates the adrenal glands to release adrenaline and noradrenaline, increases the heart rate and the breathing rate, dilates the coronary arteries to increase blood flow to the heart, dilates the pupils and constricts the urinary sphincter. The sympathetic nervous system has prepared its owner to immediately face the threat.

By contrast, the parasympathetic nervous system has been described as promoting ‘rest and digest’. It increases blood flow to the digestive tract, increases salivary gland production, regulates contraction of the heart, constricts the pupils and increases the curvature of the lens to assist in close vision.

One of the most common symptoms reflecting an impairment in the working of the autonomic nervous system is when it fails to immediately constrict the blood vessels to maintain normal blood pressure when a person stands up from having been lying down. If this shortcoming is immediately corrected, a few seconds of light-headedness rapidly passes. If, however, it doesn’t, light-headedness rapidly progresses to collapse and brief loss of consciousness typical of syncope, or fainting.

Diseases that affect the autonomic nervous system on its own are rare but, when they do occur, the symptoms usually reflect impairment in blood pressure regulation, in temperature regulation through sweating, impairment in bowel function or alteration in bladder or sexual function. By contrast, impairment in autonomic nervous system function as part of another neurological condition is not uncommon. In many people with Parkinson’s disease, for example, constipation, dizziness on standing, urinary dysfunction, altered sexual function or impaired temperature regulation can arise through the additional involvement of the autonomic nervous system and they represent some of the ‘non-motor’ manifestations of the condition. Other neurodegenerative conditions may have autonomic symptoms associated with them. In the disorder multiple system atrophy (MSA), for example, symptoms resembling Parkinson’s or a cerebellar disorder are usually accompanied by similar symptoms reflecting the fact that one of those additional ‘multiple systems’ affected in the disease is the autonomic nervous system.

The health and wellbeing of our body beyond the central and peripheral nervous systems is therefore very reliant upon the smooth background functioning of our unsung nervous system hero, the autonomic nervous system.