Parasympathetic Nervous System Notes

Parasympathetic Nervous System

  • Rest and Digest Division
  • Maintains functions like digestion and urine formation
  • Cranial Sacral Division: Uses cranial nerves and pelvic nerves from the sacral plexus

Cranial Nerves Involved

  • Oculomotor (III)

  • Facial (VII)

  • Glossopharyngeal (IX)

  • Vagus (X)

    • Main parasympathetic nerve
    • Innervates most thoracic and abdominal viscera
    • 90% of parasympathetic output

Parasympathetic Sacral Nerves

  • Innervate the last segment of the large intestine, urinary bladder, and reproductive organs
  • Sacral nerve branches form pelvic splanchnic nerves
    • Pass out of the spinal cord
    • Form plexuses in the pelvic floor at terminal ganglia or pass those plexuses and go into the wall of the organ where it will synapse

Neurotransmitters and Receptors

  • Neurotransmitter: Acetylcholine
    • Released by both pre- and postganglionic parasympathetic neurons
    • Usually excitatory
  • Receptors:
    • Nicotinic receptors:
      • Membranes of all postganglionic neurons
    • Muscarinic receptors:
      • Membranes of all parasympathetic neurons

Parasympathetic Effects (Rest and Digest)

  • Decreases heart rate and blood pressure via the vagus nerve (X)
    • Effect on cardiac muscle cells
  • Smooth muscle cells:
    • Constriction of pupil (III)
      • Sphincter pupillae muscle
      • Reduces amount of light
    • Accommodation of lens for near vision (III)
      • Contraction of the ciliary muscle
      • Changes the lens to a more rounded shape
    • Constriction of the bronchioles (X)
      • Reduces airflow
    • Smooth lining of the digestive tract (X)
      • Peristalsis to propel food
    • Digestive and urinary sphincters (X and sacral nerves)
      • Promotes urination and defecation
    • Engorgement of the penis or clitoris
      • Sacral nerves
      • Parasympathetic division innervates certain blood vessels for this
  • Glandular epithelium:
    • Increases secretion from other glands (not sweat glands)
    • VII: Tear production from lacrimal glands and mucus production from nasal mucosa
    • VII and IX: Increased production of saliva from the salivary glands
    • X: Secretion of enzymes and other products from the digestive tract cells

Dual Innervation

  • Sympathetic and parasympathetic divisions work together to maintain homeostasis
  • Both divisions innervate many of the same organs with antagonistic actions
  • Allows the sympathetic division to dominate during physically demanding periods
  • Parasympathetic division preserves homeostasis between periods of increased physical activity
  • Example: Salivary glands
    • Sympathetic: Mucus secretion
    • Parasympathetic: Watery secretion
    • Full robust saliva needs innervation from both

Single Innervation

  • Sympathetic division controls smooth muscle of most blood vessels (especially cutaneous and muscle)
  • Vasoconstriction: Increased frequency of action potentials causes contraction of smooth muscle which is going to cause vasoconstriction which is going to decrease the diameter of the blood vessel which is going to increase the blood pressure.
    (Vasoconstriction:Increasedfrequencyofactionpotentials→contractionofsmoothmuscle→decreasedvesseldiameter→increasedbloodpressure)(Vasoconstriction: Increased frequency of action potentials \rightarrow contraction of smooth muscle \rightarrow decreased vessel diameter \rightarrow increased blood pressure)
  • Vasodilation: Decreased frequency of action potentials relaxes smooth muscle
    (Vasodilation:Decreasedfrequencyofactionpotentials→relaxationofsmoothmuscle)(Vasodilation: Decreased frequency of action potentials \rightarrow relaxation of smooth muscle)
  • Sympathetic division prioritizes blood flow (e.g., to skeletal muscles and heart during fight or flight; away from skin to decrease bleeding)

Bee Swarm Example

  • Sympathetic response: Escape from bees
    • Muscles more active
    • Vasodilation to muscles, vasoconstriction to digestive/urinary organs and skin
    • Bronchioles dilate
    • Heart beats faster and harder (increased blood pressure)
    • Pupils dilate
  • Parasympathetic response: Restore resting conditions
    • Repair tissues (increased digestive activities)
    • Increased urinary processes
    • Blood flow to muscles decreases (vasoconstriction)
    • Blood flow to digestive/urinary organs increases (due to decreased sympathetic activity)
    • Bronchioles constrict
    • Heart rate slows (decreased blood pressure)
    • Pupils constrict, lens adjusts for near vision

Control Hierarchy

  • Cerebral cortex: Awareness of situation (bees sting)
  • Amygdala: Instills fear
  • Hypothalamus: Master regulator of the autonomic nervous system
  • Reticular formation: Basal functions
  • Autonomic centers controlled by sympathetic and parasympathetic neurons

Autonomic Reflexes

  • Glossopharyngeal nerve: Attracted by the carotid body baroreceptors
    *Rise in blood pressure
    *Synapse with the vagus nerve which will lower our heart rate
  • Sympathetic reflex (decrease in blood pressure)
    • Goes to the same place but our effect needs to be to decrease our heart rate, that has to go out through sympathetic stimulation out the cardio axillary nerve to go to the heart
    • Tell the heart to increase the heart rate and that's going to cause the blood pressure to increase
  • Cardioacceleratory is like blood pressure to help your body use the nutrients and its energy as effectively as possible in this fight or flight situation.

Enteric Nervous System

  • Autonomic; operates without conscious control
  • Local reflexes that don't involve the central nervous system
  • Regulates activity of the digestive tract (secretion of digestive juices, motility)
  • Involuntary responses to stimuli like stretching of the digestive tract wall (causes contraction and peristalsis)

Recap: Sympathetic vs. Parasympathetic

  • Sympathetic: Wide distribution (everywhere with skin)
  • Parasympathetic: Limited to head and thorax
  • Origin:
    • Sympathetic: Lateral gray horns of the spinal cord (thoracolumbar)
    • Parasympathetic: Cranial sacral nerves (cranial sacral division)
  • Ganglia:
    • Sympathetic: Sympathetic chain or prevertebral (collateral)
    • Parasympathetic: Terminal ganglia (abdominal plexuses or within the wall of visceral effectors)
  • Ganglia location:
    • Sympathetic: Distant from effectors
    • Parasympathetic: Close to or within effector
  • Axon links:
  • Neurotransmitters:
    • Both use acetylcholine
      • Sympathetic: Only for sweat glands
    • Sympathetic: Norepinephrine (generally)
      Actions of Sympathetic and Parasympathetic divisions are wide-ranging.