Neurological & Special Senses › Autonomic Nervous System

Autonomics Overview

Notes

Autonomics Overview

Sections


Summary

Overview

  • Parasympathetic preganglionic neurons lie within cranial nerve nuclei 3, 7, 9, 10 and the sacral intermediolateral cell column of the spinal cord, from S2 to S4, collectively called "craniosacral."
  • Sympathetic preganglionic neurons lie within the thoracolumbar intermediolateral cell column, from T1 to L2: they lie in the between the cranial and sacral parasympathetic regions.

Pre- & Post-ganglionic fibers

Parasympathetic

  • Postganglionic neurons lie far from the preganglionic neuron: in the wall of the target organ (or they lie close to the target organ, instead).
  • Preganglionic parasympathetic axons travel a long distance to synapse on the postganglionic neuron.
  • Postganglionic parasympathetic fibers project from the postganglionic neuron to deep into the target organ: they travel a short distance.

Sympathetic

  • Postganglionic sympathetic neurons lie close to the preganglionic neurons (in the paravertebral chain or one of the prevertebral ganglia).
  • Preganglionic sympathetic axons travels a short distance to synapse within the postganglionic neuron.
  • Postganglionic sympathetic fibers travel a long distance to their target organ.

Parasympathetic vs Sympathetic

  • The parasympathetic nervous system brings the body into state of REST; whereas, the sympathetic nervous system brings it into a state of ACTION.

the neurobiology of the autonomic fiber pathways

The following neurons release acetylcholine:

  • Preganglionic parasympathetic.
  • Preganglionic sympathetic.
  • Postganglionic parasympathetic.
    • Thus, 3 of the 4 neuron classes release acetylcholine.
  • But show that most postganglionic sympathetic fibers release norepinephrine (noradrenaline) with the exception of:
    • The acetylcholine that sweat glands release.
    • The epinephrine (adrenaline) that adrenal medullary cells release.

neuropeptides

  • Neurotransmitters are small molecules with transient effects; whereas, neuropeptides are generally packaged into larger vesicles and have more wide-reaching and long-lasting effects.
  • Neuropeptides are organized into many different classes, which notably include pituitary and hypothalamic hormones, as well as many other categories.
  • Target organs of the autonomic nervous system include endocrine and exocrine glands, nonglandular body organs, blood vessels, and skin, thus the autonomic nervous system reaches targets of highly varying functions.
    Indicate that autonomic dysfunction can present with numerous clinical manifestations and pathologies such as:
    • Cardiac rhythm abnormalities (arrhythmias)
    • Respiratory failure
    • Gut dysmotility
    • Bladder dysmotility
    • Skin manifestations, such as hair fiber loss and sweating

the enteric nervous system

  • The digestive tract also contains its own, separate autonomic nervous system.
  • The enteric nervous system comprises numerous neurons distributed in myenteric and submucosal plexuses.

Physiology of digestion in brief:

  • Food bolus trigger peristalsis, which propels food through the digestive tract.
  • Enteric neural circuits increase intestinal blood volume: blood goes to our guts after we eat.
  • Enteric neural circuits adjust secretomotor activity within the GI tract to regulate absorption of nutrients into the blood stream.

Clinical correlate:

  • Psychopharmacologic drugs often affect enteric nervous system function.
    • Acetylcholine increases peristalsis.
    • Cholinesterase inhibitors, such as donepezil for dementia, increase circulating levels of acetylcholine, and can result in diarrhea, and anticholinergic drugs (such as tricyclic antidepressants) can cause constipation.

Full-Length Text

  • Here, we will draw a general layout for the autonomic nervous system.
  • Draw a representative parasympathetic preganglionic neuron.
  • Now, draw a representative sympathetic preganglionic neuron.
  • Next, draw target organs far from the preganglionic neurons.

Now, let's start a new table, so we can track – where the systems originate and the relative length of their pre- and post- ganglionic axons.

  • Indicate that parasympathetic preganglionic neurons lie within cranial nerve nuclei 3, 7, 9, 10 and the sacral intermediolateral cell column of the spinal cord, from S2 to S4, collectively called "craniosacral."
  • Then, indicate that sympathetic preganglionic neurons lie within the thoracolumbar intermediolateral cell column, from T1 to L2: they lie in the between the cranial and sacral parasympathetic regions.
  • Now, in our diagram, in the parasympathetic arrangement, draw a postganglionic neuron far from the preganglionic neuron: in the wall of the target organ (or they lie close to the target organ, instead).
  • Then, show a preganglionic parasympathetic axon synapse on the postganglionic neuron – it travels a long distance.
  • Next, show a postganglionic parasympathetic fiber project from it to deep into the target organ– it travels a short distance.
  • Now, in the sympathetic nervous system arrangement, draw a nearby postganglionic neuron; postganglionic sympathetic neurons lie close to the preganglionic neurons (in the paravertebral chain or one of the prevertebral ganglia).
  • Then, show a preganglionic sympathetic axon synapse within the postganglionic neuron– it travels a short distance.
  • Show a postganglionic sympathetic fiber travel to its target organ– it travels a long distance.
  • Finally, as review, write that the parasympathetic nervous system brings the body into state of REST; whereas, the sympathetic nervous system brings it into a state of ACTION.

With that anatomy established, let's turn our attention to the neurobiology of the autonomic fiber pathways.

  • Show that the following neurons release acetylcholine:
    • Preganglionic parasympathetic.
    • Preganglionic sympathetic.
    • Postganglionic parasympathetic.
    • Thus, 3 of the 4 neuron classes release acetylcholine.
  • But show that most postganglionic sympathetic fibers release norepinephrine (noradrenaline) with the exception of:
    • The acetylcholine that sweat glands release.
    • The epinephrine (adrenaline) that adrenal medullary cells release.

Now let's transition to neuropeptides.

  • Write that neurotransmitters are small molecules with transient effects; whereas, neuropeptides are generally packaged into larger vesicles and have more wide-reaching and long-lasting effects.
  • The neuropeptides are organized into many different classes, which notably include pituitary and hypothalamic hormones, as well as many other categories.

Lastly, let's address the target organs, themselves.

  • Indicate that target organs of the autonomic nervous system include endocrine and exocrine glands, nonglandular body organs, blood vessels, and skin, thus the autonomic nervous system reaches targets of highly varying functions.
  • Indicate that autonomic dysfunction can present with numerous clinical manifestations and pathologies such as:
    • Cardiac rhythm abnormalities (arrhythmias)
    • Respiratory failure
    • Gut dysmotility
    • Bladder dysmotility
    • Skin manifestations, such as hair fiber loss and sweating
  • Lastly, consider that the digestive tract also contains its own, separate autonomic nervous system: the enteric nervous system.
  • Indicate that the enteric nervous system comprises numerous neurons distributed in myenteric and submucosal plexuses.

Let's look at the physiology of digestion in brief.

  • Draw a schematic of the oral cavity, esophagus, stomach, and intestines.
  • Adjacent to it, draw representative vasculature.
  • Now, show a food bolus trigger peristalsis, which propels food through the digestive tract.
  • Then, show that enteric neural circuits increase intestinal blood volume: blood goes to our guts after we eat.
  • Next, indicate that the enteric neural circuits adjust secretomotor activity within the GI tract to regulate absorption of nutrients into the blood stream.
  • As a clinical correlate, psychopharmacologic drugs often affect enteric nervous system function.
    • For instance, write that acetylcholine increases peristalsis.
  • So write that cholinesterase inhibitors, such as donepezil for dementia, increase circulating levels of acetylcholine, and can result in diarrhea, and anticholinergic drugs (such as tricyclic antidepressants) can cause constipation.