Parasympathetic Nervous System

Aka: Parasympathetic Nervous System, Parasympathetic System, Parasympathetic Nerve, Parasympathomimetic, Acetylcholine, Cholinergic Activity, Muscarinic Receptor, Muscarinic Acetylcholine Receptor, Nicotinic Receptor, Nicotinic Acetylcholine Receptor, Choline

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II. Definitions

  1. Parasympathetic Nervous System
    1. Part of Autonomic Nervous System with activity that typically counters the Sympathetic Nervous System
    2. Energy conserving (anabolic) system activated in rest and relaxation situations
      1. Contrast with the Sympathetic Nervous System, an energy expending system activated in times of stress
  2. Parasympathomimetic
    1. Drugs that mimic parasympathetic activity
  3. Acetylcholine
    1. Neurotransmitter for autonomic preganglionic Neuron Synapses (CNS to autonomic Ganglion)
      1. Acetylcholine mediates both preganglionic sympathetic and Parasympathetic Systems
    2. Acetylcholine is Neurotransmitter for parasympathetic postganglionic Neurons (autonomic Ganglion to target organ)
      1. Acts at muscarinic and nictonic Acetylcholine receptors
    3. Acetylcholine is the Neurotransmitter at the Neuromuscular Junction
      1. Muscle Contraction is triggered by release of Acetylcholine into the synaptic cleft
  4. Choline
    1. Water soluble B Vitamin that is a precursor to Acetylcholine and phospholipids
  5. Choline Esters
    1. Acetyl derivatives bound to Choline by an ester bond
    2. Hydrolyzed by Cholinesterases, including Acetylcholinesterase
  6. Cholinomimetic Alkaloids
    1. Plant extracts with similar activity as Choline esters (e.g. Acetylcholine), but not susceptible to Cholinesterases
    2. Other than Nicotine (which acts at Nicotinic Receptors), most are Muscarinic Agonists

III. Physiology: Activity

  1. Muscarinic Acetylcholine Receptors (Parasympathetic System)
    1. M1: Increased Stomach acid pepsin
    2. M2: Decreases Heart Rate and Blood Pressure
    3. M2: Stimulates bronchoconstriction and Bronchial gland secretion
    4. M2: Increases gastrointestinal activity (peristalsis) and secretion
    5. M2: Relaxes gastrointestinal sphincters (e.g. anal sphincter in Defecation)
    6. M2: Stimulates Urethral relaxation and Bladder contraction
    7. M2: Mediates Erection (contrast with ejaculation mediated by the Sympathetic System)
    8. M3: Decreases pupil size (Miosis) and contracts ciliary Muscle (accommodation)
    9. M3: Stimulates Salivary WATERY secretion (contrast with viscous secretion by the Sympathetic System)
  2. Nicotinic Acetylcholine Receptors
    1. Skeletal Muscle Contraction
    2. CNS and Ganglion Neurotransmission

IV. Anatomy

  1. Craniosacral division of the Autonomic Nervous System
  2. Signals originate
    1. Sacral spinal cord (S2 to S4) supplies urinary and Gastrointestinal Systems
    2. Brain Stem nucleii (CN 3, CN 7, CN 9, CN 10)
      1. Vagus Nerve (CN 10) has broad, far ranging activity (lungs, heart, gastrointestinal)
      2. Limited roles by CN 3 (pupil/ciliary constriction), CN 7 (tears/Saliva) and CN 9 (Saliva)
  3. Preganglion Neurons Synapse at cranial autonomic ganglia or in terminal ganglia near target organs
    1. Transmitted to target organ
  4. Neurotransmitters
    1. Both preganglionic and postganglionic Neurons release Neurotransmitter Acetylcholine
    2. Acetylcholine acts at Muscarinic Receptors
      1. End organs (e.g. heart, lung) on release from postganglionic Neurons
      2. Central neurotransmission
    3. Acetylcholine acts at Nicotinic Receptors
      1. Neuromuscular Junctions (Neurons abutting Muscle fibers)
      2. Sympathetic and parasympathetic ganglia (between preganglionic and postganglionic Neurons)
  5. Images
    1. parasympatheticNervousSystem.jpg
    2. autonomicSynapse.png

V. Pathophysiology

  1. See Cholinergic Toxicity (e.g. Organophosphate Poisoning)
  2. See Anticholinergic Toxicity (e.g. Anticholinergic Medication)

VI. References

  1. Goldberg (2014) Clinical Neuroanatomy, Medmaster, p. 54-60
  2. Netter (1997) Atlas Human Anatomy, ICON Learning, p. 152-4

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