The Enteric Nervous System

Comprises the plexuses in the wall of the GIT namely

  1. The myenteric plexus of Auerbach, and
  2. The submucosal plexus of Meissner and their efferent and afferent connections.

Well recognized subdivision of the autonomic nervous system because of the following reasons:

  1. It contains more than post-ganglionic and pre-ganglionic components of the sympathetic and parasympathetic divisions. It, in addition to these, also contains many interneurons and general visceral afferents.

  2. Extensive interconnections occur between the neuronal elements named above

  3. When isolated from the central nervous system, the enteric nervous system continues to function reflexly, controlling peristalsis, GIT gland secretion and blood flow modulation
    .
  4. This network of ganglionic plexuses is isolated from surrounding intercellular fluids in much the same manner, as the brain is isolated within the blood brain barrier.
Note: The enteric nervous system can operate in isolation. However, normally efferent signals from both the parasympathetic and sympathetic divisions drive it, even though these efferent signals are subject to more modulation within the enteric nervous system than in other peripheral ganglionic plexuses of the autonomic nervous system.
 

Comparison of sympathetic and parasympathetic systems
Sympathetic
Parasympathetic
Origin thoracolumbar Cranio-sacral
Ganglia Para-vertebral Peripheral (near target)
Neuro transmitters at target Adrenaline/ nor-adrenaline Acetyl-choline
Distribution Wide spread Localized
Predominant effect Fight and flight response calm body maintenance
     

 

Note:

  • Athough generally regarded as antagonistic, a delicate balance between the two systems maintains a more or less constant level of visceral activity (homeostatic) under conditions that usually prevail.

  • The functional effects of the parasympathetic division tend to be more localized while those of sympathetic system are more generalized, a difference which rests on two anatomical considerations:

    a) Pre-ganglionic parasympathetic axons usually terminate on very much smaller number of post-ganglionic neurons than do their sympathetic counterparts.

    b) The generalized character of sympathetic activity is reinforced by the release of adrenaline and noradrenaline into the circulation from the cells of the adrenal medulla, which are innervated directly by pre-ganglionic sympathetic neurons.
Regulation of autonomic function

 

NOTE:

Cerebellar influence on autonomic activity

The hypothalamocerebellar and cerebellohypothalamic pathways provide direct connections between the integrative centres for visceromotor and somatomotor activity

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Cortical Modulation of autonomic function

The human brain has the capacity to override many, if not all, visceromotor activities that normally are considered reflexive or involuntary. For example, many Hindu yogis are able to exert “voluntary” control over such functions as heart rate, blood pressure, respiration, gastrointestinal motility, and sphincter control through intense concentration and practice.