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Digestion
and Absorption in Ruminants
(and
particularities in Goats)
Author:
C.G. Ciappesoni Ph.D
student
The oral cavity (mouth) is protected by a pair of lips. The lips are
highly vascularized, skeletal muscle tissue with an abundance of sensory nerve
endings. Lips help retain food as it being chewed and play a role in phonation (the
modification of sound).7
The initial step in digestion is prehension,
the conveyance of food into the
mouth. In the goat, the lips, teeth
and tongue are the primary prehensile
organs. The lips of goats and sheep are most important in
seizing feed, while the cow makes more use of the tongue for grasping and tearing of grasses
and forbs. Thus, the goat is capable of greater selectivity in its
grazing.5
The
teeth function in providing the first process of mechanical digestion, the
grinding of large food particles into smaller particles (vanLoon, 1976 in 2).
Herbivores have compound teeth called Hypsodont
teeth and other animals have simple or Brachydont teeth. The dentition of
herbivores must allow for continuous and considerable wear at the masticatory
surfaces.3
Goats, like other
ruminants, lack an upper set of incisor teeth. Instead they possess a
hardened dental pad, against which they bite.5
Mastication,
or chewing, is necessary for the reduction
of feed to smaller particle sizes. Goats, exhibit an accentuated lateral
movement of the jaws while chewing, which greatly increases the
grinding action of the teeth. These lateral
movements result in the molars developing a
sharp, pointed surface on the inner edge of the
lower teeth and on the outer edge of the upper teeth. Due to the fact that the upper jaw is wider
than the lower jaw, only one side of the mouth can
be used at a time to grind the feed.5
Food
is not supported in a tetrapod's oral cavity by a current of water, so a
muscular tongue has an important role in digesting, transporting and swallowing
the food. The tongue is also used in many other ways. Aside from variations in
tongue shape, various regions of the tongue have elevations of the mucosa called
papillae which serve both mechanical
and gustatory functions.3
Taste Bud of the Mammal:
Although taste and smell have much in common, we think
of smell as chemical
information carried in the air and taste as chemical information from material
in contact with parts of the mouth. Taste is detected by barrel-shaped clusters
of cells called taste buds. Although taste buds are used primarily to find and
recognize food, they are also important in sexual and other behavioral
interaction. Taste buds are abundant in mammals. Most are associated with
papillae on the tongue, but some are found on the palate, pharynx and epiglottis.3
Epithelial
cell layer: this layer, in
contact with and lining the lumen of the digestive tract,
is composed of
epithelial cells whose morphology and function vary along the course of the
tract. The epithelium may be composed of stratified
squamous epithelial cells in regions where the tract functions mainly in
transport of ingesta (e.g., esophagus); or it may be composed of secretory
cells (e.g., in portions of the stomach); or absorptive cells (e.g., duodenum), or both.3
While grazing or browsing, the feed is only chewed
enough to be mixed with saliva to
form a bolus of suitable size to
swallow. Saliva,
which is a mixture of serous and mucous fluids, is released by the
stimulation of chewing from five pairs of glands.
The parotids, which are located behind the angle of
the jaw; the submaxillaries, on the inner sides of the parotids; the sublinguals,
on each side of the tongue base; and the inferior molar and buccal
glands in the cheeks.1 The basic secretory units of salivary
glands are clusters of cells called an acini. These cells secrete a fluid that
contains water, electrolytes, mucus and enzymes, all of which flow out of the
acinus into collecting ducts.1 (see
figure)
The salivary glands differ in the type of secretion they produce:
· parotid glands produce a serous, watery secretion
· submaxillary (mandibular) glands produce a mixed serous and mucous secretion
· sublingual glands secrete a saliva that is predominantly mucous in character 1
Within
the ducts, the composition of the secretion is altered. Much of the sodium is
actively reabsorbed, potassium is secreted, and large quantities of bicarbonate
ion are secreted. Bicarbonate secretion is of tremendous importance to ruminants
because it, along with phosphate, provides a critical buffer that neutralizes
the massive quantities of acid produced in the forestomachs. Small collecting
ducts within salivary glands lead into larger ducts, eventually forming a single
large duct that empties into the oral cavity.1
- serous
cells, which secrete a watery fluid, essentially devoid of mucus
- mucous
cells, which produce a very mucus-rich secretion
Acini
in the parotid glands are almost exclusively of the serous type, while those in
the sublingual glands are predominantly mucus cells. In the submaxillary glands,
it is common to observe acini composed of both serous and mucus epithelial cells.1
In
the histological sections of canine salivary gland shown above, the cells stained
pink are serous cells, while the white, foamy cells are mucus-secreting cells.1
Secretion
of saliva is under control of the autonomic nervous system, which controls both
the volume and type of saliva secreted.1
Saliva functions primarily as an
aid in mastication and swallowing. 1 Some
animals secrete salivary amylase,
which converts starch into maltose. Amylase is most abundant in the saliva of
the domesticated pig.2 Saliva contributes
very importantly to the buffering of rumen fermentation, provides
nutrients (nitrogen, phosphorous, and sodium) to rumen microorganisms and
certain anti-frothing properties.5
Ruminants produce abundant amounts of saliva with mature cows producing up to 150 liters of saliva per day. Sheep can produce up to 10 liters a day (Pond et al, 1995 cit. in 2 ).
From the mouth, the food enters the esophagus, a
muscular tube composed of striated skeletal muscle fibers. The particles
travel down the esophagus by peristalsis or propulsive movements (Cunningham,
1997 cit. in 2).
The pharynx
connects the oral cavity and the esophagus. It is the common passageway for food
and air. The following structures open into the pharynx: the caudal nares, the
mouth, two auditory tubes, the larynx, and the esophagus.9
The final step in pregastric
digestion is swallowing, also known as deglutition.
This is really a very complex process that can be thought of as occurring in
three steps:
1.
First, a bolus of food is pressed backward into the pharynx by the tongue. This
is the only step that is voluntary - the remaining steps occur by reflex.
2.
Once the bolus reaches the pharynx several actions are initiated, which
basically involve shunting the bolus into the esophagus while at the same time
closing alternative routes of escape. The lumen of the larynx is squeezed shut
and the epiglottis swings backward to cover the larynx. The larynx is also
pulled forward and down making the opening to the esophagus larger.
3. Finally, the tongue presses
backward and a peristaltic contraction in the pharynx propels the bolus into the
esophagus, where the actual act of swallowing takes place.1
The muscles of the pharynx are voluntary striated (skeletal) muscles and are controlled by nerves from the brain. The swallowing center in the medulla of the brain coordinates swallowing in between respiratory movements.9
The esophagus is
a musculo-membranous passageway connecting the pharynx to the rumen.1 There
are two muscle layers in the esophagus: circular and longitudinal. When these
muscle layers relax and contract, they aid in the movement of food boluses. Peristalsis
consists of relaxation followed by contraction. Peristalsis results in forward
movement of food from the contracted segment into the relaxed segment. Stretch
of the esophagus wall by the bolus of food causes peristalsis to occur.9
(see Gastrointestinal
Motility)
The esophagus is usually closed at the pharyngeal end (upper esophageal sphincter) and the stomach end (cardiac sphincter or lower esophageal sphincter).9
References