2.3 Autonomic Control of the Gastrointestinal Tract

• What type of stimulation increases the activity of the enteric nervous system?

  Parasympathetic stimulation
• How is the parasympathetic supply to the gut divided?

  Into cranial and sacral divisions
• Which cranial nerve fibers primarily innervate the gut?

  Vagus nerves
• What areas do the cranial parasympathetic fibers innervate?

  Esophagus, stomach, and pancreas
• Which part of the large intestine receives less innervation from cranial parasympathetic fibers?

  Intestines down through the first half
• Where do sacral parasympathetics originate?

  Second, third, and fourth sacral segments
• Through which nerves do sacral parasympathetics pass to reach the large intestine?

  Pelvic nerves
• Which regions of the intestine are better supplied with parasympathetic fibers?

  Sigmoidal, rectal, and anal regions
• What is the primary function of the parasympathetic fibers in the gastrointestinal tract?

  To execute defecation reflexes
• Where are the postganglionic neurons of the gastrointestinal parasympathetic system located?

  In the myenteric and submucosal plexuses
• What effect does stimulation of parasympathetic nerves have on the enteric nervous system?

  It causes a general increase in activity
• What does the enhanced activity of the enteric nervous system affect?

  Most gastrointestinal functions
• What is the effect of sympathetic stimulation on gastrointestinal tract activity?

  It usually inhibits gastrointestinal tract activity.
• Where do sympathetic fibers to the gastrointestinal tract originate?

  In the spinal cord between segments T5 and L2.
• What happens to most preganglionic fibers after leaving the spinal cord?

  They enter the sympathetic chains lateral to the spinal column.
• What are the outlying ganglia that sympathetic fibers pass through?

  Celiac ganglion and various mesenteric ganglia.
• Where are most postganglionic sympathetic neuron bodies located?

  In the outlying ganglia.
• How do postganglionic fibers reach the gastrointestinal tract?

  They spread through postganglionic sympathetic nerves.
• How does sympathetic innervation of the gastrointestinal tract compare to parasympathetic innervation?

  Sympathetic innervation is more extensive throughout the gut.
• What neurotransmitter do sympathetic nerve endings mainly secrete?

  Norepinephrine.
• What is the general effect of sympathetic nervous system stimulation on gastrointestinal activity?

  It inhibits gastrointestinal tract activity.
• How does norepinephrine affect intestinal tract smooth muscle?

  It inhibits smooth muscle except mucosal muscle.
• What is the major way norepinephrine exerts its effects on the enteric nervous system?

  By inhibiting neurons of the entire enteric nervous system.
• What can strong stimulation of the sympathetic system cause in the gastrointestinal tract?

  It can block movement of food through the tract.
• What type of nerve fibers innervate the gut?

  Afferent sensory nerve fibers
• Where do some afferent sensory nerve fibers have their cell bodies?

  In the enteric nervous system
• Where do other afferent sensory nerve fibers have their cell bodies?

  In the dorsal root ganglia of the spinal cord
• What can stimulate sensory nerves in the gut?

  Irritation of the gut mucosa
• What is another stimulus for sensory nerves in the gut?

  Excessive distention of the gut
• What is a third stimulus for sensory nerves in the gut?

  The presence of specific chemical substances
• What can signals transmitted through afferent fibers cause?

  Excitation or inhibition of intestinal movements
• Where do other sensory signals from the gut go?

  To multiple areas of the spinal cord and brain stem
• What percentage of nerve fibers in the vagus nerves are afferent?

  80 percent
• What do afferent fibers in the vagus nerves transmit signals from?

  From the gastrointestinal tract
• Where do the sensory signals from the gastrointestinal tract go?

  Into the brain medulla
• What do the signals in the brain medulla initiate?

  Vagal reflex signals
• What do vagal reflex signals control?

  Many functions of the gastrointestinal tract
• What supports the three types of gastrointestinal reflexes?

  The anatomical arrangement of the enteric nervous system
• What are the three types of gastrointestinal reflexes?

  1. Reflexes integrated within the gut wall enteric nervous system
  2. Reflexes from the gut to prevertebral sympathetic ganglia
  3. Reflexes from the gut to spinal cord or brain stem
• What do reflexes integrated within the gut wall enteric nervous system control?

  Gastrointestinal secretion, peristalsis, and mixing contractions