The Second Brain: A Groundbreaking New Understanding of Nervous Disorders of the Stomach and Intestine by Michael D. Gershon

The Author
The author is not a gastroenterologist, but a neurobiologist, whose interest in the serotonin neurotransmitter took him down into the bowels of medicine. This book is a history of the development of the understanding of the intestinal nervous system, a history in which the author played a major role.

How Science is Actually Done
The author describes numerous experiments he and other conducted to figure out the intestinal nervous system. There is a great deal of information and the writing gets rather technical at times. This would be a good book for a college undergraduate science major to read.

Autonomic Nervous System
The autonomic nervous system is motor not sensory and controls smooth muscles and glands, not skeletal muscles. It has three parts:
• Sympathetic Nervous System
• Parasympathetic Nervous System
• Enteric Nervous System

Sympathetic Nervous System
• The nerve cell bodies are located near the spine
• Its nerves exit the spinal chord near the thoracic and lumbar vertebra
• Controls eye pupil dilation
• Controls male ejaculation
• Its axons release norepinephrine at their synapses

Parasympathetic Nervous System
• Its nerve cell bodies are located near the innervated organ
• Nerves exit the spinal chord near the cranial and sacral vertebra
• The vagus nerve is part of it
• Its axons release acetylcholine at their synapses

Enteric Nervous System
The intestines contain large numbers of interneurons, that is, neurons that connect via synapses only to other neurons, and not to sensory, muscle or gland cells. One of the main neurotransmitters used by these enteric interneurons is serotonin. Serotonin stimulates the peristalsis of the smooth muscles of intestines.

The colon is also known as the large intestine. It probably evolved to help land animals conserve water. The body moves water from inside the lumen of the colon into the bloodstream by removing salt from the lumen of the colon, which causes the water to follow by osmosis. There are lots of benign, symbiotic bacteria in the colon, but there are few bacteria in the small intestines. The purpose of diarrhea is to cleanse colon of pathogenic bacterial.

Development of the Enteric Nervous System
During embryonic development, the neural crest is formed from cells that migrate from the nearby neural tube. These neural crest cells later migrate to the intestines, where they become the enteric nervous system.

Defects of Intestinal Innervation
• Chagas disease (American trypanosomiasis), a tropical parasitic infection common in Latin America, kills enteric ganglia of Auerbach’s myenteric plexus
• Hirschsprung’s Disease (congenital megacolon): babies lack enteric ganglia in the colon, due to a failure in the embryonic development of the enteric nervous system
• Neuronal Intestinal Dysplasia (NID): a congenital disease causing a lack of sympathetic or parasympathetic innervation of the intestines
• From Wikipedia I found that all newborns are kept in the hospital until they have their first bowel movement, in order to identify babies that have intestinal problems, and that in the United Kingdom there is a charity for NID called the Adele Chapman Foundation

Gastric Intrinsic Factor
Gastric glands in the stomach secrete a glycoprotein called gastric intrinsic factor that helps the small intestine absorb vitamin B12.

Liver and Gall Bladder
The liver secretes bile into the duodenum to emulsify fat, which is then digested by the lipase enzyme secreted into the duodenum by the pancreas. Excess bile is stored in the gall bladder. Gall bladder ganglia are part of the enteric nervous system.

Pancreas and the Vagus Nerve
The vagus nerve of the parasympathetic system stimulates the pancreas to increase secretions when eating a meal.

Pancreas and Bicarbonate
• Brunner’s glands of the pancreas produce a bicarbonate alkaline fluid to neutralize the stomach acid in the food that enters the duodenum from the stomach.
• Hormonal regulation: The secretin hormone produced by the duodenal endocrine cells tells the pancreas now much bicarbonate to produce.
• Nerve regulation: The pancreas receives serotonin-releasing neurons from the duodenum which inhibit the release of the bicarbonate.

Pancreas and Digestive Enzymes
• Cholecystokinin is released by endocrine cells in the stomach and travels through the bloodstream to the pancreas, where it stimulates the release of digestive enzymes.
• Gastrin is released by endocrine cells in the duodenum and travels through the bloodstream to the pancreas, where it also stimulates the release of digestive enzymes.

Enterochromaffin Cells
Enterochromaffin cells are found in intestinal lining and were named for the fact that they stain well with chromium dyes, because they have an affinity for chromium. Enterochromaffin cells contain 95% of the body’s serotonin. As was first proposed by the late Edith Bülbring of Oxford University, Fellow of the Royal Society, and pioneer in the study of the physiology of smooth muscle, enterochromaffin cells are sensory receptors that respond to pressure in the intestines by releasing serotonin into the connective tissue of the intestines. The serotonin in the connective tissue then stimulates:
• Secretomotor neurons that in turn stimulate the crypt cells to secrete salt into the lumen, and water then follows the salt into the lumen, due to osmotic pressure
• Enteric nervous system interneurons causing peristalsis
• Neurons that connect to the CNS and cause nausea and vomiting

Serotonin Transporter Protein
Serotonin transporter protein is in the outer membrane of neighbors of the enterochromaffin cells. It removes the serotonin after it has spent enough time stimulating the serotonin receptors.

Interstitial Cells of Cajal
• They are named after are named after the Spaniard Santiago Ramón y Cajal
• They are pacemakers in the enteric nervous system
• They amplify nerve signals
• They originate not in the neural crest, but from mesenchymal precursor cells, from which smooth muscle cells also develop
• Because they are not neurons, the body can produce more of them in adulthood, if some of them become damaged


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