Neuro-Endocrine Regulation of GI Flashcards
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Submucosal Plexus
- layer of nerves found beneath the mucosa
- nerves within this plexus send out processes which synapse on other nerves within the plexus (inter-neurons), as well as, effecter cells (secretory, endocrine) located within the mucosa and submucosa, and also smooth muscle cells in the circular muscle layer.
Myenteric Plexus
- nerves reside between the circular and longitudinal layers of smooth muscle
- Output from these nerves primarily regulate and coordinate the contractility of the muscle layers. The innervation of muscle layers can either inhibit or stimulate muscle activity dependent on the neurotransmitter released. However, visceral smooth muscle contractility is generally under inhibitory control.
Inputs to the Enetric (Intrinsic) Neuronal Plexi
- CNS - sympathetic and parasympathetic autonomics
- walls of the alimentary canal itself - proprio- and chemo receptors located within the mucosa and submucosa
- neurons within wach plexus communicate via interneurons - reflex arcs
Outputs from the Enteric (Intrinsic) Neuronal Plexi
- Outputs from the neuronal plexi regulate all functions of the alimentary canal, and modulate function of the pancreas, gall bladder, as well as, provide information to the central nervous system.
- Interneurons within the plexi coordinate activities along the canal.
- Cholinergic neurons (acetylcholine) located in the submucosal plexus regulate secretory cell activity while cells in the circular muscle layer are regulated by neurons located within both plexi. This is particularly true for the sphincter regions, which are thickenings of the circular layer.
- On the other hand, the longitudinal muscle layers that drive movements along the canal are regulated primarily by neurons whose cell bodies are located in the myenteric plexus.
Extrinsic Regulation - Sympathetic Innervation
- The Salivary glands are innervated by fibers originating in the superior cervical ganglia.
- Thoracic and lumbar cord segments extend neurons to the celiac, superior and inferior mesenteric and hypogastric plexuses (the Sympathetic Chain).
- These ganglia send out axons that innervate the enteric plexi via adrenergic (norepinephrine) synapses, and directly innervate the pancreas, liver, gall bladder and vascular smooth muscle of feed arterioles.
- The primary consequence of sympathetic stimulation is a general inhibition of motor and secretory function that includes enhanced contraction of the alimentary sphincters via activation through the submucosal plexus.
***Possibly the most important response to sympathetic output is vasoconstriction through the direct innervation of blood vessels.
Extrinsic Regulation - Parasympathetic Innervation
- The 7th and 9th cranial nerves innervate the salivary glands.
- Vagal branches innervate from the esophagus through the ascending colon, and include the liver, pancreas and gall bladder.
- The descending colon to anus is innervated by pelvic nerves.
- All parasympathetic fibers synapse on neurons within the enteric plexi. The response to parasympathetic stimulation is a general increase in motor and secretory function within the alimentary canal, and ancillary organs.
Extrinsic Regulation - Central Afferents
•Afferents to the spinal cord travel primarily in vagus nerves, with fewer in sympathetic tracts.
-However, visceral pain signals, originating in nerve endings (receptors) located in the wall of the alimentary canal, are carried via sympathetic nerve fibers whose cell bodies reside in the “sympathetic chain” ganglia.
- Most afferent signals arise from proprio- and chemo-receptors located in the mucosa with their cell bodies located in the myenteric and submucosal plexi.
- Afferent signals are transmitted from the plexi to either the sympathetic chain or spinal cord where these signals are assimilated. Thus afferent sensory signals can elicit local responses within segments of the canal via direct enteric reflex, and can coordinate responses between different segments of the canal via long central reflex arcs.
- In addition, parasympathetic afferent fibers provide information to hypothalamus regarding moment-to-moment feeding behavior.
Reflex Arcs
- In general, reflex arcs are described by their point of origin and termination.
- For example, an enteric reflex that initiates in the intestine to regulate gastric acid secretion is referred to an entero-gastric (or -oxyntic) reflex.
- A classic reflex that originates from stretch of the stomach to enhance motility of the colon is called the gastro-colic reflex. This reflex travels from the stomach via the vagus nerve to the spinal cord, then back to the colon also via the vagus. Therefore, this is in the general class of a vago-vagal reflex.
Intergration of Signals
Signal integration occurs primarily within the enteric nervous system, but also through long reflexes coordinated in prevertebral ganglia and spinal cord.
Extrinsic Intergration
- Afferents signals originating from enteric receptors are relayed through the enteric neuronal network then feed into the parasympathetic and sympathetic arms of the ANS.
- These fibers synapse centrally where they integrate with incoming signals from other areas of the alimentary canal, as well as, with higher centers.
- The summation of signals at this level modifies subsequent feedback to the digestive tract. Since the pancreas and biliary tract receive no direct innervation from the alimentary canal, all sensory feedback is via these “spinal reflexes.”
Intrinsic Integration
- Integration within the canal is coordinated through enteric neurons.
- The strongest input comes from local alimentary receptors, but these intrinsic signals are influenced by long reflexes from central autonomics.
- When food resides in a specific region of the tract, local signals to the enteric plexi to drive local alimentary function.
- Long reflexes coordinate activity, but rarely are strong enough to override strong local signals such as distension or lumenal acidity.
Hormone
Chemical messenger produced and secreted by a specialized gland, released into the blood where it “circulates” to distant organs to elicit a physiological response. Effects are exerted relatively slowly, but for prolonged periods The GI tract is the largest endocrine organ in the body. It produces a wide variety of hormones and biologically active peptides
Gastrin Family
- Increases Intracellular Ca2+
- Gastrin
- CCK (neurotransmitter and hormone)