Autonomic Tracts

Autonomic pathways refer to the neuronal pathways used by the autonomic nervous system. The autonomic nervous system is a subdivision of the peripheral nervous system.

It performs functions that are not under the control of will power of the body. The autonomic nervous system comprises of sympathetic and parasympathetic nervous systems. Both these components of the autonomic nervous system use different pathways in order to perform their specified functions.

The various functions performed by the autonomic nervous system include control of breathing, control of blood pressure, control of heart rate, process of digestion and defecation, process of urination, etc. In this article, we will discuss the pathways used by the autonomic nervous system while performing these functions.

We will discuss the origin, location, course, and termination of these autonomic pathways. In the end, we will also discuss how the human body suffers when these autonomic tracts are injured. So, keep reading.

Divisions

As we have discussed earlier, the autonomic nervous system comprises of two divisions;

  • Sympathetic Nervous System
  • Parasympathetic Nervous System

We will discuss the autonomic tracts separately for each of these divisions of the autonomic nervous system.

Sympathetic Tracts

The term sympathetic tracts is used to represent the neuronal pathways used by the sympathetic nervous system to carry out its functions. 

The sympathetic tracts are discussed under three headings;

  • Inflow or Input Tracts
  • Sympathetic Ganglia
  • Outflow or Output Tracts

Inflow or Input Tracts

The inflow tracts of the sympathetic nervous system comprise of the nerve fibers that carry information from the central nervous system to the sympathetic ganglia. These are called the preganglionic fibers.

The preganglionic neurons of the sympathetic tracts from which the input or inflow tracts originate are located in the intermediolateral columns of the spinal cord.

Sympathetic Ganglia

The main center for the activity of the sympathetic nervous system is the sympathetic ganglia. The sympathetic ganglia are present in two groups; the paravertebral ganglia and the ganglia around the blood vessels.

The paravertebral ganglia are the collection of cell bodies present along the length of the spinal cord in the form of a chain. This chain of paravertebral ganglia is present in the thoracic and lumbar regions of the spinal cord.

In addition to the chain of paravertebral ganglia, some other sympathetic ganglia are also present associated with major vessels such as aorta, mesenteric arteries, etc.

Some sympathetic ganglia are also present in the nerve plexus such as pelvic plexus etc.

Outflow or Output Tracts

The outflow tracts comprise of the postganglionic fibers. These fibers originate from the sympathetic ganglia in the form of gray ramus. These postganglionic fibers merge with the spinal nerve. The spinal nerve, in turn, carries the sympathetic fibers to different organs of the body. 

Neurotransmitter

There are two neurotransmitters used in sympathetic tracts.

The neurotransmitter released by preganglionic fibers at the sympathetic ganglia is acetylcholine.

The main neurotransmitter released by the postganglionic fibers at the target organs is epinephrine. The only exception are sweat glands. In the case of sweat glands, the postganglionic sympathetic fibers release acetylcholine.

Example

Here we will discuss an example in order to fully understand the concept of the sympathetic tracts. Take an example of heart rate

The sympathetic pathways are responsible for increasing the normal heart rate. The preganglionic neurons in the intermediolateral nuclei of the spinal cord send signals to the sympathetic ganglia.

The output fibers from these sympathetic ganglia directly innervate the SA node, the musculature of heart, and vasculature of the heart. The release of epinephrine by these postganglionic fibers results in increased heart rate, increased contraction of the heart, and increased blood pressure. 

Parasympathetic Tracts

The parasympathetic tracts are the neuronal pathways used by the parasympathetic nervous system in order to perform its various functions. These tracts are different from the tracts used by the sympathetic system discussed earlier. 

The tracts parasympathetic system are also discussed under three headings;

  • Inflow or Input Tracts
  • Parasympathetic Ganglia
  • Outflow or Output Tracts

Inflow or Input Tracts

Just like the sympathetic nervous system, the inflow tracts of the parasympathetic nervous system carry information from the central nervous system to the parasympathetic ganglia. These tracts comprise of fibers called the preganglionic fibers.

The inflow tracts of the parasympathetic nervous system originate from the preganglionic neurons located at three different locations. These include;

  • Tectum of the Midbrain (in case of accommodation and pupil constriction)
  • Nuclei of Cranial Nerve
  • Preganglionic neurons in the Spinal cord

In case of the spinal cord, the preganglionic neurons of the parasympathetic system are located in the intermediolateral column of gray matter, in the sacral regions of spinal cord.

The preganglionic fibers arising from the neurons present at these three locations carry nerve impulses to the ganglia of the parasympathetic nervous system.

Parasympathetic Ganglia

The parasympathetic ganglia relay information received from the central nervous system to the target organs. They receive input in the form of the preganglionic fibers and send output via postganglionic sympathetic fibers.

The parasympathetic ganglia are located at different regions in the body, such as in the head and neck region, thorax, abdomen, and in the pelvic region.

The head and neck region have four pairs of parasympathetic ganglia. These are;

  • Otic ganglia
  • Pterygopalatine or Sphenopalatine ganglia
  • Submandibullary ganglia
  • Ciliary ganglia

The other parasympathetic ganglia are present in different plexus present in different parts of the body such as mesenteric plexus, celiac plexus, pelvic plexus, etc.

Outflow or Output Tracts

The outflow tracts of the parasympathetic system are also in the form of postganglionic fibers. These fibers join the cranial nerves in the case of head and neck region and supply the target organs.

In the case of other peripheral parasympathetic ganglia, the postganglionic fibers may join a spinal nerve or may directly innervate the target organ.

Neurotransmitter

The entire parasympathetic system uses only one neurotransmitter, and that is acetylcholine. This neuron is released by the preganglionic as well as the postganglionic fibers of the parasympathetic tracts. Thus, acetylcholine is used throughout the tracts of the parasympathetic nervous system.

Example

Let us discuss an example of the parasympathetic tracts so that we understand the concept completely. Take an example of the process of urination.

The parasympathetic system is responsible for the process of urination. The urge to urinate arises in the brain and the signals are carried by the descending autonomic tracts to the intermediolateral nuclei in the spinal cord. These nuclei send preganglionic fibers to the parasympathetic ganglia located in the pelvic plexus.

The postganglionic fibers from pelvic plexus enter the pelvic nerves. These nerves innervate the detrusor muscle and contract it. The unopposed contraction of detrusor muscle results in the relaxation of internal sphincter and the bladder is emptied.

Pathologies

Many vital functions of the body are controlled by the autonomic nervous system. Any injury or damage to the autonomic tracts can result in a number of pathological conditions. Severe damage to the autonomic pathways can result in fatal conditions. Here, we will mention some important pathological conditions arising from the injury to the autonomic tracts.

Ptosis and Anopthalmos

Ptosis means dropping of eyelid and Anopthalmos means dry eyes. It results from damage to the superior cervical ganglion that provides autonomic innervation to the eyelid.

Loss of Accommodation and Pupillary Constriction

This results from any damage to the autonomic fibers contained in the oculomotor nerve.

Cardiac Arrest or Cardiac Shock

The contraction of the heart is under the control of the autonomic nervous system. The sympathetic system speeds up the heart rate whereas the parasympathetic system decreases it. Any imbalance in the two innervations due to damage to the vagus nerve or sympathetic fibers can result in lethal cardiac shock leading to cardiac arrest.

Urge Incontinence

The process of urination is also under the control of the autonomic system. Any damage to the autonomic tracts that supply the bladder can result in urge incontinence or other abnormalities associated with the process of urination.

Summary

The autonomic tracts are the nerve fibers and pathways used by the autonomic nervous system in order to perform its functions.

The autonomic tracts consist of;

  • Sympathetic Tracts
  • Parasympathetic Tracts

The sympathetic tracts have preganglionic fibers in the thoracolumbar region of the spinal cord. They release acetylcholine.

The sympathetic ganglia are present in the form of a paravertebral chain of ganglia as well as individual ganglia around the major vessels and in different nerve plexes.

The postganglionic fibers join the spinal nerves or travel independently and supply the target organs. These fibers release epinephrine.

The parasympathetic tracts have preganglionic fibers arising from three different locations;

  • Tectum of Midbrain
  • Brain Stem
  • Sacral segment of Spinal Cord

The parasympathetic ganglia are present in different regions of the body such as pelvis, head and neck, mesenteric plexus, etc.

The postganglionic fibers arising from these ganglia join the cranial or spinal nerves, or travel independently and supply the target organs.

The only neurotransmitters used in the parasympathetic tracts is acetylcholine.

Any injury or damage to the autonomic tracts can result in a number of pathologies. The most common pathologies include;

  • Ptosis and Anopthalmos
  • Loss of accommodation
  • Loss of pupil reflex
  • Cardiac abnormalities
  • Urge incontinence

References

  1.  Schmidt, A; Thews, G (1989). “Autonomic Nervous System”. In Janig, W (ed.). Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370.
  2. Allostatic load notebook: Parasympathetic Function Archived 2012-08-19 at the Wayback Machine – 1999, MacArthur research network, UCSF
  3. Langley, J.N. (1921). The Autonomic Nervous System Part 1. Cambridge: W. Heffer.
  4. Jänig, Wilfrid (2008). Integrative action of the autonomic nervous system : neurobiology of homeostasis (Digitally printed version. ed.). Cambridge: Cambridge University Press. p. 13. ISBN 978052106754-6.
  5. Costanzo, Linda S. (2007). Physiology. Hagerstwon, MD: Lippincott Williams & Wilkins. p. 37ISBN 0-7817-7311-3.
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