Download Cell Bio 1- Functional Anatomy of GI Tract Introduction Terms

Cell Bio 1- Functional Anatomy of GI Tract
•
•
•
•
•
•
Introduction
Terms describing the gastrointestinal system are varied, and not always accurate
The term “gastrointestinal” refers, strictly speaking to the stomach and the intestines
– The term “digestive system” may be most accurate and encompassing regarding the contents of this
part of Block 5
• However, the “digestive system” does much more than “digest”
Anatomically, the digestive (gastrointestinal) system consists of two general components
– The digestive tract/gastrointestinal (GI) tract/alimentary canal
– Accessory organs
The digestive system is essentially a disassembly line
– Its primary purpose is to ingest nutrients and break (digest) them down into forms that can be used
(absorbed) by body cells
Absorbed nutrients have two basic functions
– Fuel
• Molecules provide the energy to drive cellular processes
– Repair
• Molecules represent the building blocks for cell growth and repair
The key processes of digestion and absorption are aided/made possible by secretion and motility
Digestion
• Digestion is the breakdown of food into components which can be absorbed by the body
• Digestion takes two forms
– Mechanical
– Chemical
• Mechanical digestion comprises the physical processes of crushing and shearing
• Mechanical digestion has two main purposes
– It eases the process of swallowing and subsequent movement
– It increases the surface area of ingested food thus facilitating subsequent chemical digestion
• The processes of crushing and shearing occur from the mouth to the small intestine
• Chemical digestion refers to a series of enzymatic reactions that break down macromolecules into their
monomers (which can then be absorbed)
– Some authors consider the term “digestion” to refer essentially to chemical digestion
• Chemical digestion occurs along the length of the GI tract but is predominant in the small intestine
Absorption
• Absorption is the uptake of molecules from the lumen across the digestive epithelium
– Absorption is truly the primary function of the GI tract itself
– Most absorption takes place in the intestines
Secretion
• Secretion is the addition of fluid to the GI tract by glands located in the tract and in associated organs
• There are two basic types of secretions
• Serous secretions occur from the mouth to the distal small intestine
• These watery secretions contain enzymes, and are also able to create a fluid environment for
• Mucous secretions occur from the mouth to the anus
• They provide lubrication and protection
Motility
• Motility is a general term that in GI physiology refers to rhythmic contraction and relaxation of smooth muscle
of the gut tube
• Motility serves three general purposes
• Mixing of food
• Propulsion of food
• Storage of food (using sphincters)
Cell Bio 1- Functional Anatomy of GI Tract
•
Functional Anatomy
The digestive tract (GI tract) is a hollow, muscular tube extending from mouth to anus
– The digestive tract is ~5m in life but may nearly double in length after death (loss of muscle tonus)
Oral Cavity
• The oral cavity is the site of food intake (ingestion)
– Analysis of ingested material is also performed in the oral cavity
• In the oral cavity, chewing initiates the breakdown of food
• The chemical digestion of both lipids and carbohydrates begins in the oral cavity
– Lipid digestion begins with lingual lipase, which is secreted by intrinsic salivary glands of the tongue
– Carbohydrate digestion is initiated by salivary amylase
• Chemical digestion is initiated by the sight, smell, and taste of food
Pharynx and Esophagus
• The pharynx and esophagus are muscular tubes that serve as conduits from the mouth to the stomach
– Muscular contraction permits swallowing
• No digestive enzymes are secreted by the pharynx or esophagus
– However, enzymes from the mouth do travel with the bolus of food
Stomach
• The stomach is a J-shaped muscular sac in the upper abdomen just inferior to the diaphgram
• It links the esophagus to the small intestine
• The stomach functions primarily as a temporary food storage organ, during which time it also continues the
processes of mechanical and chemical digestion
• The stomach initiates the chemical digestion of protein
• Stomach digestion results in chyme (kime), a semifluid mixture of partially digested food
• Very limited absorption takes place in the stomach
Small Intestine
• The small intestine is a coiled mass filling most of the abdominal cavity inferior to the stomach and liver
• The small intestine is the longest part of the digestive tract
• This length—and thus extensive surface area—is crucial for two reasons
• The small intestine is the principle, and final site of chemical digestion
• It is the principle site of nutrient, electrolyte, and water absorption
Large Intestine
• The large intestine begins at the terminal part of the ileum (small intestine) and ends at the anus
• It is ~5 feet long in the cadaver
• There are three main functions of the large intestine
• Reabsorption of water thus compacting the food residue into feces
• Absorption of key vitamins liberated by bacterial action
• Storage of fecal matter prior to defecation
•
•
Accessory Glands and Organs
The accessory glands and organs perform two general functions
– Add secretions to the hollow organs to aid in digestion and absorption
– Aid in mechanical digestion
• Teeth and tongue
Saliva has multiple functions
– Initiation of digestion
– Lubrication of food to facilitate swallowing
Cell Bio 1- Functional Anatomy of GI Tract
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
– Cleansing of mouth and inhibition of bacterial growth
The only digestive function of the liver is the production and secretion of bile
– Bile is a fat emulsifier
– Bile is stored and concentrated by the gallbladder
The digestive function of the pancreas is to secrete pancreatic juice
– ~1.2-1.5 L day
Pancreatic juice has two main functions
– It contains digestive enzymes for all the macromolecules
– It contains bicarbonate (HCO3-) which buffers stomach acids
Other Digestive System Functions
The primary role of the digestive system is the preparation of foodstuffs for use by body cells
In addition, the digestive system plays some other key roles
– Fluid and electrolyte balance
– Immune function
The GI tract represents the main source of fluid and electrolyte intake
As well, it represents a potentially significant route for loss of fluids/electrolytes
– Diarrhea
The GI tract represents a very large surface, open to the exterior, which must be closely guarded
Many lymphocytes and other immune cells are located along the GI tract in two main non-encapsulated forms
– Scattered diffusely
• MALT (Mucosa-associated lymphatic tissue)
– Organized into nodules/follicles
• Peyer patches
The lymphatic tissue has two main functions
– Defense against pathogens
– Immunologic tolerance to dietary substances and “friendly” bacteria
Intro to Regulation of GI Tract
Unlike the cardiovascular or respiratory systems, which are always “on”, the digestive system is relatively
quiescent between meals
– But when called to action, the digestive system must mount an appropriate response
First, food must be detected, and its composition determined, then it must be moved through the GI tract in a
coordinated manner
– Communication between parts of the tract, and communication between the tract and associated
organs is necessary to ensure appropriate secretory, and blood flow activity, for optimal digestion and
absorption
Digestive system activity is controlled in two general ways
– By the nervous system and associated neurotransmitters (neural regulation)
– By non-neural signaling molecules (endocrine and paracrine regulation)
Neural Regulation
The autonomic nervous system is responsible for neural regulation of the entire digestive system, including the
GI tract, and associated accessory organs
– However, the remainder of the present lecture deals with neural regulation of the GI tract specifically
The GI tract, like the rest of the digestive system, is innervated by the sympathetic and parasympathetic
divisions of the autonomic nervous system (ANS)
– In addition, the GI tract has its own nervous system, the enteric nervous system (ENS), which is
considered a component of the ANS
Cell Bio 1- Functional Anatomy of GI Tract
•
•
•
•
When talking about the GI tract, the three divisions of the ANS fall into two categories
– Intrinsic nervous system
• Enteric nervous system (ENS)
– Extrinsic nervous system
• Sympathetic and parasympathetic divisions of the ANS
The ENS lies entirely in the wall of the gut
– It has no cell bodies in the brain or spinal cord
The ENS begins in the esophagus and runs to the anus
The intrinsic, or enteric nervous system (ENS), can direct all functions of the GI tract even in the absence of
extrinsic innervation
– The ENS contains ~100 million neurons
• Equal to spinal cord or rest of ANS
– The location of the ENS in the gut creates advantages
• Gut location is a useful anatomical alternative to packing millions of neurons in the brain or
spinal cord
• Gut location may allow ENS to more quickly responsive to local needs
Intrinsic Nervous System
• Neurons of the ENS are organized into thousands of small ganglia
• The ganglia are primarily organized into two plexuses
• Submucosal plexus
• Myenteric plexus
• The submucosal (Meissner’s) plexus—located in the submucosa—is found only in the small and large intestine
• It is mainly concerned with controlling the function of the mucosa, tiny segment by tiny segment
• This may involve local secretion, local absorption, and local contraction of submucosal muscle
• The myenteric (Auerbach’s) plexus—located between the circular and longitudinal muscle layers—mainly
promotes muscle activity of the muscularis externa, from esophagus to rectum
• Increased rate and intensity of rhythmic contractions
• Some neurons of the myenteric plexus may synapse with neurons in the submucosal plexus, or directly
on secretory cells
• At least a dozen different neurotransmitters are known to be secreted by the enteric nervous system
• Acetylcholine, epinephrine, norepinephrine, ATP, serotonin, dopamine, substance P, and others
• Acetylcholine generally excites GI activity (rest and digest)
• Epinephrine and norepinephrine generally inhibit GI activity (fight or flight)
Extrinsic Nervous System
• As mentioned previously, the ENS can operate entirely independently of the brain or spinal cord, however,
certain GI functions are dependent on extrinsic innervation
– Both plexuses of the ENS are wired to the sympathetic and parasympathetic divisions of the ANS, which
can produce great enhancement or inhibition of GI function
Parasympathetic Nervous System
• All parasympathetic presynaptic nerve fibers arise from the brainstem or the sacral spinal cord
– Parasympathetic nerve fibers supplying the digestive system arise from both locations
• Cranial parasympathetic nerve fibers to the GI tract and associated organs are carried within three cranial
nerves
– CN VII- Supplies sublingual and submandibular salivary glands
– CN IX- Supplies the parotid gland
– CN X- Supplies GI organs from the esophagus to the first half of the large intestine
• 75% or more of all parasympathetic fibers are carried in the vagus nerve
• The sacral parasympathetics originate in the second, third, and fourth sacral segments of the spinal cord
Cell Bio 1- Functional Anatomy of GI Tract
•
•
•
•
These fibers are carried by the pelvic splanchnic nerves
– They supply the distal half of the large intestine all the way to the anus
Preganglionic parasympathetic fibers synapse with enteric (ENS) postganglionic fibers in the wall of the organ
There is no direct innervation of efferent parasympathetic fibers to effector cells
– Transmission pathway is always via one, or more neurons of the ENS
Stimulation of parasympathetic fibers—and the release of acetylcholine and/or other ENS neurotransmitters—
causes a general increase in the activity of the entire enteric nervous system
– In general this is reflected by increased glandular secretion and motility
Sympathetic Nervous System
• All sympathetic presynaptic nerve fibers arise from the intermediolateral cell columns (IMLs) of T1 to L2
• Sympathetic fibers to the GI tract originate in segments T5 to L2
• The sympathetic fibers innervate essentially all of the GI tract equally
• Parasympathetic innervation is much more concentrated nearest the oral cavity (swallowing) and anus
(defecation)
•
•
•
•
Preganglionic sympathetic fibers synapse with postganglionic fibers in the prevertebral ganglia
The postsynaptic fibers reach the end organ along the major blood vessels and their branches
Some fibers may innervate blood vessels and glandular structures directly, while others may synapse in the ENS
In the GI tract, as elsewhere, the sympathetic nervous system is an antagonist of the parasympathetic nervous
system
• As in other parts of the body it causes vasoconstriction
• In the GI tract it inhibits smooth muscle function, thus slowing down motility
• Sphincters are an exception (closed under sympathetic activation)
• Sympathetic activation inhibits glandular secretion
Cell Bio 1- Functional Anatomy of GI Tract
Sensory Nerves
• The GI tract is richly supplied with sensory (afferent) nerve fibers
• There are two general categories of sensory fibers
– Fibers conveying pain information
– Fibers conveying reflex information
Reflex Information
• The reflex information carried by visceral afferent fibers generally does not reach the level of consciousness
– Specific information regarding the gut results in specific motor commands
• Afferent fibers carrying reflex information accompany both sympathetic and parasympathetic visceral motor
fibers
• The fibers carrying reflex information have their cell bodies in the inferior vagal ganglion and the dorsal root
ganglia
– However, some of the reflex fibers have their cell bodies in the enteric nervous system
•
Afferent fibers carrying reflex information are stimulated in two general ways
– Mechanically
• Stretch of smooth muscle
– Chemically
• Changes in pH, osmolality, or concentration of specific nutrients
GI Reflexes
• The anatomical arrangement of the enteric, sympathetic, and parasympathetic nervous systems supports two
general types of gastrointestinal reflexes
– Short reflexes
– Long reflexes
Short Reflexes
• Short reflexes are integrated entirely within the ENS
– As elsewhere in the nervous system, the ENS circuitry includes afferent, efferent, and interneurons
• Short reflexes coordinate local responses to local stimuli
– For instance, the arrival of a food bolus may cause local peristalsis and glandular secretion
• Short reflexes provide most of the control required for normal functioning of the GI tract
Long Reflexes
• Long reflexes are those that utilize some aspect of extrinsic innervation
• Long reflexes take two general forms
– Reflexes involving other parts of the GI tract and Reflexes involving the CNS
Cell Bio 1- Functional Anatomy of GI Tract
•
These are reflexes from one area of the gut to the prevertebral ganglia (PVG), and back to another part of the
gut
– Intestinofugal neurons (afferents) synapse in the PVG and result in sympathetic motor response
• These reflexes are designed to protect against large increases in tone and intraluminal pressure
• Enterocolic reflex
– Signals from the colon to inhibit stomach motility and secretion
• Colonileal reflex
– Signals from the colon to inhibit emptying of ileal contents
CNS Reflexes
• These reflexes are from the gut to the spinal cord or brainstem, and then back to the GI tract
– Many such reflexes can be mediated entirely by the vagus nerve (vagovagal reflex)
• Examples
– Enterogastric reflex (vagovagal)
– The defecation reflex (spinal reflex)
Enterogastric Reflex
• Chyme from the stomach fills the duodenum (entero), stretching its walls
• Sensory signals are integrated in the brainstem resulting in diminishing parasympathetic impulses to the
stomach (gastro)
• Stomach peristalsis is inhibited and intestinal filling slows down
Higher CNS Centers
• The GI tract is also under control of higher CNS centers
– For instance, the flight-or-flight response reduces blood flow to the GI tract
– The sight and smell of food can both increase gastric acid secretion
•
•
•
Endocrine and Paracrine Regulation
Regardless of the controlling modality, regulation of GI function occurs via the release of substances, generally
peptides
– A true endocrine substance—a hormone—uses the bloodstream to travel to target cells
– A paracrine substance is not secreted into the blood, but instead is released into the interstitium, and
thus targets nearby cells
The digestive tract produces several hormones that affect almost every aspect of digestive function
These peptide hormones are produced and secreted by enteroendocrine cells of the epithelium in response to
various stimuli
– Gastrin, a stomach hormone, stimulates gastric HCl secretion in response to various stimuli
Cell Bio 1- Functional Anatomy of GI Tract
•
•
•
Like GI hormones, paracrines are synthesized in endocrine cells of the GI tract
– The paracrines act locally by diffusing over very short distances
Somatostatin is secreted by endocrine cells along the length of the GI tract and inhibits gastrin release and
various other secretions
Hormones/paracrines released from the GI tract have three general targets
– GI tract and accessory organ glands
• Gastrin and somatostatin
– GI tract and accessory organ smooth muscle
• Cholecystokinin-- is produced by endocrine cells of the duodenum and jejunum and among
other things, stimulates contraction of the gallbladder
– Organs not directly associated with digestion and absorption
• Ghrelin --Ghrelin is made by endocrine cells of the gastric mucosa and acts on the hypothalamus
to stimulate appetite