Advance Digestive Physiology
(Topics and program)
 The topics
 Classification of animals based on fermentation in digestive
tracts
 The properties of ruminant’s digestive tracts and functions
 Development of digestive system
 Salivation
 The receptors
 The salivary glands
 Control of salivation
 Mastication and swallowing
 Rumen and reticulum properties
 Characteristics of the preruminant stomach
 The wall structure
 Development and control of forestomach motility
 Blood circulation
 Receptors
 Rumination and its components
 Attempts to control ruminoreticulum fermentation
 Events associated with eructation
 Absorption
 Urea recycling
 The role of thermodynamics in controlling rumen
metabolism
 Omasum
 Omasal motility
 The properties of obomasum
 Glands and secretions
 Microscopic anatomy
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 Abomsal motility
 Migrating motor (myoelectric) complexes (MMCs)
 Small intestine
 Wall layers
 Neuronal network
 Blood circulation
 Movements control
 Transport systems in the epithelia
 Entrogastric inhibitory reflex
 Large intestine
 Wall properties
 Absorption
 Motor activity of cecum
 Evacuation contractions
 Defecation
1. Classification of animals based on fermentation in the gut
Based on fermentation, the animals are classified to two groups: Pregastric
fementers and large intestine fermenters. These differences have some
advantages and disadvantages for each group. The advantages of pregastric
fermentation are:
 Make better use of alternative nutrients (cellulose and non protein
nitrogen)
 Ability to detoxify some poisonous compounds (oxalates, cyanide,
alkaloids)
 More effective use of fermentation end-products including: volatile fatty
acids, microbial protein, and B vitamins
 Decrease in handling undigested residues
 In wild animals, it allows animals to eat and run
The disadvantages of pregastric fermentation are:
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 Fermentation is inefficient (energy loss 5-8 % of total caloric value as
methane and loss 5-6 % of total caloric value as heat of fermentation). In
the other hand some ammonia resulting from microbial degradation will
be absorbed and excreted and 20% of the nitrogen in microbes is in the
form of nucleic acids.
 Ruminants are susceptible to ketosis
 Ruminants are susceptible to toxins produced by rumen microbes
(nitrates, nitrites, urea, ammonia, lactic acid, methyl indole isoflavonoid
estrogens, and …)
2- The properties of ruminant's digestive tract and functions
Digestive system of animals has different functions:
 ingestion (eating)
 chewing (mastication)
 swallowing (deglutition)
 absorption of nutrients
 elimination of solid wastes (defecation)
Different species of animals have digestive systems adapted to the most
efficient use of the food they consume. The anatomy and physiology of the
digestive systems of herbivores, carnivores, and omnivores all differ.
The digestive tract extends from the lips to the anus. It includes the mouth,
pharynx, esophagus, stomach, and the small and large intestines. Accessory
glands include the salivary glands, the liver, and the pancreas. The length and
complexity of the digestive system depends on the species. In herbivores, it is
very long and complex.
The digestive tract has the architecture of a typical hollow organ. It has a
lumen and a wall consisting of several layers: mucosa, submucosa, muscularis
externa and serosa/adventitia. The mucosa is made up of an epithelial lining, a
lamina propria of loose connective tissue and blood vessels and muscularis
mucosa containing one or two thin layers of smooth muscles. In an organ that
has not muscularis mucosae, the lamina propria and the submucosa blend
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together to form the propria-submucosa. The mucosa of the digestive tract is the
surface across which most substances enter the body. It has many functions
including:
 Secretion of enzymes, acid, mucin, hormones and antibodies
 Absorption of the break down products of digestion, water, vitamins and
etc
 Barrier to prevent the entry of antigens, pathogenic organisms
 Immunologic protection via the lymphoid tissue in the submucosa.
The submucosa contains loose or dense connective tissue, blood and
lymphatic vessels and the autonomic parasympathetic submucosal plexuses
(Meissner). Glands may be present in the submucosa of some parts of GI: oral
cavity, esophagus, stomach and the intestines. Variable amounts of lymphoid
tissue are seen here and are known a gut associated lymphoid tissue (GALT).
The muscularis externa contains two layers. The internal layer is generally
circular and the external layer mostly longitudinal. Between these two layers are
the autonomic parasympathetic myenteric plexuses (Auerbach). In certain
areas, skeletal muscles may be present. The serosa is made up of loose
connective tissue, blood and lymphatic vessels, some adipose tissue and an
external covering of simple squamous epithelium (mesothelium). The adventitia
is similar to the serosa but does not have a mesothelial covering.
The epithelium of digestive tract serves as a selective barrier between the
contents of the lumen in the digestive tract and the tissues of the body, an area
for the digestion and absorption of food as well as the production of hormonal
factors. The abundant lymphoid tissue in the lamina propria and the submucosa
serve a protective function against bacteria and viruses in the lumen. The IgA
produced in the GI tract is resistant to proteolytic enzymes and is functional in
the lumen.
The muscle fibers propel and mix the food in the digestive tract.
Parasympathetic and sympathetic fibers coordinate the contraction of the layers
(peristalsis).
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2-1. Different parts of ruminant's digestive system
Digestive system of ruminant animals includes the different parts:
 Mouth - grasps the food
 Teeth - grind the food
o Ruminants have only one set of teeth in the front of the mouth
(incisors), and two sets in the back (molars).
 Tongue - covered with finger-like projections (papillae) that contain taste
buds.
 Salivary glands - secrete saliva that moistens food and is mixed with the
food material to aid in swallowing.
 Pharynx - funnels food into the esophagus, preventing food material from
entering the lungs.
 Esophagus - food tube that leads from the mouth to the stomach.
 Multi-chambered stomach
o Reticulum - honeycomb-like interior surface, this part helps to
remove foreign matter from the food material.
o Rumen - the organ that allows for bacterial and chemical
breakdown of fiber.
o Omasum - section that is round and muscular.
o Abomasum - very similar to the stomach of non-ruminants.
 Small intestine - where most of the food material is absorbed into the
bloodstream.
 Large Intestine - begins to prepare unused food material for removal from
the body.
o Colon - collects the unused food material that is to be removed
from the body
o Rectum - “poop chute”
 Anus - opening through which the waste is removed.
2-1-1. Oral cavity
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Oral cavity or mouth which initially receives the food entering the gut may be
specialized in different animals. The mouth is made of lips, tongue, palate,
pharynx, and teeth. The salivary glands and lymphoid tissues are also located
in the oral cavity.
Salivary glands: Secretions from the salivary glands contain enzymes, water
and glycoproteins. These working together and help swallowing. Secretory IgA,
lactoferrin and lysozyme in the saliva perform its protective functions. Amylase
is found in the saliva of omnivores such as rats and pigs, but is absent in
carnivores like dogs and cats. Lipases may be found in some young animals that
are nursing or on a high-milk diet (e.g. calves). Saliva can serve a neutralizing
function if it contains a high concentration of sodium bicarbonate and phosphate
(e.g. cattle). The salivary glands have different function:
 Preparing enzymes
 Moistens and lubricates feed
 Water balance
 Bloat prevention
 Recycling of N and minerals including Na, P, and S (A 700 kg dairy cow
fed a hay-grain diet will secrete about 190 liter saliva/day containing, 3080 g total N, 1100 g NaHCO3, 350 g Na2HPO4, and 100 g NaCl)
 Buffer secretion (normal rumen, pH 5.5 – 7.0, without salivary buffers,
pH 2.8 – 3.0)
Surrounding each major salivary gland is a connective tissue capsule. Septa
of connective tissue from the capsule extend into the gland and divide the organ
into lobes and then lobules. A rich vascular and nerves plexus surrounds the
secretory units and the ducts. The small tubes go into ducts. Those ducts go into
larger ducts that have little stripes on them, called striations. Those go into ducts
between the lobes of the gland called interlobar or excretory ducts. The main
duct of the salivary glands then goes into the mouth. Actually each salivary
gland contains the different components:
 Secretory units made up of serous, mucous or a combination of these cells
types. The cells are arranged as acini or tubulo-acini. Serous cells are
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shaped like a pyramid. They are joined together in a group that is shaped
like a ball. The ball is called acimus, with a small lumen in the centre.
These cells are eosinophilic with centrally located nuclei. Mucous cells
are usually shaped like a cube and have flat or oval nucleus. They are
joined together to make a tubules, which are very small tubes. These cells
make glycoproteins that make saliva wet and slippery. The mucous
material (mucinogen granules) in the cytoplasm stains palely in the H&E
preparation.
 Ducts can be classified (from small to large) into intercalated (small ducts
leading away from the secretory units), striated intralobular, interlobular
and finally the main excretory duct. Goblet cells may be present in the
epithelium.
 Myoepithelial cells wrap around the secretory units and the ducts.
Contraction of these cells, which have numerous microfilaments in their
cytoplasm. They can squeeze the saliva gland so the saliva comes out
faster.
 Lymphocytes and plasma cells are found in the connective tissue
surrounding the acini. Immunoglobulin A (IgA) is synthesized by the
plasma cells. The secretory IgA complex, resistant to proteolysis, is then
released into the saliva.
There is different kind of salivary glands. Parotid gland, mandibular
gland, sublingual gland and some minor salivary glands. Purely serous acini
are found in the parotid glands of most domestic animals.
The parotid glands which are a serous gland are specific in structure as the
end piece cells are arranged in spherical form (Edgar & Dawes, 2004). In
mucous glands however, the arrangement is of a tubular nature resulting in large
central lumen. The peripheral branches of the facial nerve (VII) are related
closely with the parotid gland. The walls of the parotid duct are thick due to the
unification of the ductules which is also responsible for the drainage of lubules
of the gland (Edgar & Dawes, 2004). The duct is situated anterior to the border
of the gland and on the surface of the masseter muscle. The duct then curves
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over the anterior border of the masseter muscle and opens within the oral cavity
in papilla adjacent to second upper molars.
Mandibular gland are mixed seromucous gland contains a combination of
tubules and terminal acini with mucous cells. In cows, sheep and pigs,
sublingual gland contains mainly mucous cells. In dogs and cat, this is a mixed
seromucous gland. Innervation to the sublingual gland derives from two
important sources: 1) Sympathetic innervation from the cervical chain ganglia
and 2) Parasympathetic innervation, like the submandibular gland, is derived
from the submandibular ganglion.
A number of minor salivary glands are also present. These seromucous
glands include labial, buccal, molar, palatine and (only in carnivores)
zygomatic glands. Minor salivary glands do not have connective tissue capsules.
Some salivary glands are embedded in the tongue. They are called lingual
salivary glands. Don't confuse them with the "sublingual salivary gland," a
discrete organ in and of itself. Lingual glands may be of the serous or mucous or
mixed type. Some of them open out via ducts onto the surface, some of them
have ductwork leading to the "moat" that surrounds the large tongue papillae.
Unlike the major salivary glands, the minor salivary glands lack a branching
network of draining ducts. Instead, each salivary unit has its own simple duct.
Most of the minor glands receive parasympathetic innervation from the lingual
nerve, except for the minor glands of the palate, which receive their
parasympathetic fibers from the palatine nerves.
The production of saliva is an active process that occurring in two phases: 1Primary secretion that occurs in the acinar cells. This results in a product similar
in composition and osmolality to plasma and 2- ductal secretion that results in a
hypotonic salivary fluid. It also results in decreased sodium and increased
potassium in the end product.
The salivary ducts rely heavily on the Na/K/2Cl cotransporter. The duct cells
maintain a negative resting membrane potential, and these cells hyperpolarize
secondary to the efflux of potassium and influx of chloride with autonomic
nervous stimulation. This is unusual, and is referred to as the “secretory
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potential”, because most excitable cells depolarize (rather than hyperpolarize)
with stimulation. The degree of modification of saliva in the ducts turns heavily
on salivary flow rate. Fast rates result in a salivary product more like the
primary secretion. Slow rates result in an increasingly hypotonic and potassium
rich saliva.
The parasympathetic nervous system is the primary instigator of salivary
secretion. Interruption of parasympathetic innervation to the salivary glands
results in atrophy, while interruption of sympathetic innervation results in no
significant change in the glands. It was once thought that the sympathetic
nervous system antagonizes the parasympathetic nervous system with respect to
salivary output, but this is now known not to be true. Stimulation by the
parasympathetic nervous system results in an abundant, watery saliva.
Acetylcholine is the active neurotransmitter, binding at muscarinic receptors in
the salivary glands. Stimulation by the sympathetic nervous system results in a
scant, viscous saliva rich in organic and inorganic solutes. For all of the salivary
glands, these fibers originate in the superior cervical ganglion then travel with
arteries to reach the glands:
 External carotid artery in the case of the parotid
 Lingual artery in the case of the submandibular
 Facial artery in the case of the sublingual
In about 50% of total saliva is secreted by the paired parotid glands. Parotid
weight of CS, again irrespective of body size, is more than three times that of
GR. This means, salivary glands have regressed as ruminants increased fiber
digestion. The question arises; do CS and IM then need so much more saliva for
buffering purposes? Because as will be seen, all these selective species also have
a much denser, evenly distributed rumen papillation than GR. This results in a
greater internal surface enlargement facilitating faster absorption of SCFA;
hence: little danger of pH depression. First of all, these bigger glands supply
more diluting liquid, which reduces retention time. Secondly, CS produce a
much higher proportion of thin, proteinaceous serous saliva to carry away much
of the soluble plant cell contents set free by puncture crushing of dicots (GR
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grind fibrous food sideways). There is reason to believe that some of these
nutrients (e.g. sugars) are absorbed already in loco, while more solutes are
washed, together with excessive serous saliva, down the ventricular groove into
the abomasum. This would lead to a certain loss of salivary bicarbonate and to
CO2 formation in reaction to the acidic gastric juice. It would, however, initially
explain the considerable surplus of HCl-producing parietal cells. There is
another reason for much more (and more serous) saliva production in CS and
IM: it is a counter-adaptation to overcome the plants' chemical defenses. The
phenolic compounds produced by plants form insoluble complexes with protein
(tanning effect). Moreover, as protein feed protection experiments have shown,
the undigestible tannin-protein complex will be dissolved in the acidic abomasal
environment this would be a vital second reason for so much more HClproduction in that thicker abomasal mucosa of selective ruminants.
Ruminants produce a high daily output of saliva (6 to 16 L/d in sheep; 60 to
160 L/d in cattle). The secretions from parotid glands are isotonic with blood
plasma, have no significant amylase content, change their composition in
response to salt depletion, contain urea and alkali. The characteristics of the
various salivary glands of sheep are summarized in Table 1. Their secretion
responds strongly to mechanical stimulation of the mouth, esophagus, and
ruminoreticulum. In contrast, the submaxillary, sublingual, and labial glands
produce small quantities of hypotinic, mucous, weakly buffered saliva. The
submaxillary and labial glands are strongly stimulated only by feeding and give
no response to esophageal or ruminoreticular stimulation.
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Table 1. Salivary glands and their properties (sheep)
Salivary glands
Total salivary
Characteristics
Site of reflexogenic
volumes (L d)
Parotids
3-8
stimuli
Serous, isotonic, strongly Mouth,
buffered
Inferior molars
Palatine, buccal,
0.7-2
2-6
esophagus,
buffered
ruminoreticulum
Isotonic, strongly buffered
Mouth,
esophagus,
ruminoreticulum
0.4-0.8
Mucous, hypotonic, weakly Mouth
buffered
Sublingual, labial
ruminoreticulum
Serous, isotonic, strongly Mouth,
pharyngeal
Submaxillary
esophagus,
0.1
during
feeding, not cudding
Very mucous, hypotonic, Mouth
weakly buffered
Total volume
6-16
Although parotid saliva is isotonic with blood plasma, there are much
higher concentrations of K+ (13mM), HCO3- (112 mM), and HPO4-- (48 mM)
and correspondingly lower concentrations of Na+ (170 mM) and Cl- (11 mM).
In salt-depleted animals there is a replacement of Na+ by K+ because of the
action of aldosterone.
The high salivary content of HCO3- and HPO4-- account for its high
alkalinity (pH 8.1) and is an important mechanism for neutralization of about
one-half of VFAs in the forestomach. The pK value for HCO3- and HPO4-systems, being 6.1 and 6.8 respectively, help to buffer the ruminal contents in
the normal pH range of 5.5 to 7.0. The high content of phosphate also represents
a form of recycling, the microbes having a high demand for phosphate to
synthesize nucleoproteins, phospholipids, nucleotide coenzymes, etc. Salivary
nitrogen, 77% of which comes from urea, provides a useful additional source of
NPN for microbial protein synthesis. The high urea content of ruminant saliva
may be a critical factor for survival in situations of severe protein deficiency,
when even the kidneys can increase renal tubular reabsorption of urea to
facilitate its recycling.
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A basal level of parotid secretion occurs even in the totally denervated or
atropinized gland. Reflex-evoked increases in salivation are due to the excitation
of the secretory (acinar) cells by acetylcholine liberated by the parasympathetic
nerve endings, and this may be blocked by atropine. Electrical stimulation of the
sympathetic nerve supply after atropinization produces a transient increase in
salivary output followed by a compensatory reduction. This effect is due to the
norepinephrine induced contraction of the contractile myoepithelial (basket)
cells that surround the acini and small ducts. It leads to an expulsion of stored
saliva rather than to an increase in secretion. The increase in parotid blood flow
does not exactly parallel the increase in parotid secretion and depends on a
noncholinergic parasympathetic mechanism, which is not affected by atropine.
Salivary reflexs are integrated in salivary centers located in the hindbrain.
The
major
reflex
excitatory
input
arises
from
postulated
buccal
mechanoreceptors located in or near the tooth sockets, and the sensory pathways
project mainly to the salivary center on the same side as the receptor stimulation.
Thus chewing of ingesta or cud causes a large increase in salivary secretion, one
of which in cattle may increase its rate from 2 ml/min to 30 to 50 ml/min.
Experimentally, the parotid and other major glands also increase their secretory
rates in response to distension of the esophagus, reticulum, reticuloomasal
orifice, and ruminoreticular fold as a result of excitation of tension receptors
located in these sites. In contrast, little increase is evoked by lightly stroking the
ruminoreticular epithelium. Such stimulation primarily excites epithelial
receptors and has a lesser effect on tension receptors. Tension receptor induced
reflex effects account for the small increases in salivation that occur at the time
of each reticular contraction and for the transient large increase in salivation that
occurs when the cardia and esophagus are distended. Reflex increases in
salivation may be inhibited by concurrent stresses and excitement.
The lymphoid tissues of the oral cavity include the ring of tonsils and the
diffuse lymphoid tissues in the connective tissues. These are part of the body’s
first line of immune response (the lumen of the GI tract, starting from the oral
cavity, is physically continuous with the outside world).
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