Page 412 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of Digestion / 397

not large enough to impair movement of almost entirely regulated by the autonomic
nervous system, whereas gastrointestinal
the other contents.
VetBooks.ir gastrointestinal tract provides the force to hormones are primary in the initiation of
The smooth muscle in the wall of the
bile secretion.
move digesta through the tract; gastroin-
testinal motility is the general term used
to describe the activity of this smooth Pregastric Physiology
muscle. Gastrointestinal motility is primar-
ily regulated by three mechanisms: (1) Prehension and Chewing
autonomic nervous system; (2) gastroin-
testinal hormones; and (3) enteric nerv- The act of bringing food into the mouth
ous system. is prehension. The teeth, lips, and tongue
Gastrointestinal hormones are released are used as prehensile organs by domes-
from endocrine cells in the epithelial lining tic animals. The lips of the horse, the
of the gastrointestinal tract (enteroendo- tongue of the cow and sheep, and the
crine cells) and may stimulate or inhibit snout of the pig are used extensively in
gastrointestinal smooth muscle. The obtaining food.
release of these hormones is usually in The type of teeth, arrangement of jaws,
response to digesta in the lumen of the and chewing (mastication) habits vary
tract. Thus, these hormones are a means of with the species and the food. Carnivorous
local regulation that is coordinated with the animals have simple teeth and tear their
ingestion and digestion of food. These hor- food but do little grinding. Herbivorous
mones are also capable of signaling to the animals have at least some hypsodont
hypothalamus and pituitary for appetite teeth; the upper jaw is wider than the lower
regulation and other endocrine feedback jaw; and chewing of the food is thorough.
mechanisms. Mastication can be controlled voluntarily,
The enteric nervous system consists of but the presence of food in the mouth will
neural plexuses between layers of smooth stimulate reflex chewing.
muscle in the wall of the tract (Fig. 21‐1).
These plexuses contain complete neurons
(dendrites, cell bodies, and axons) that can Saliva and Salivary Glands
form complete neural and reflex circuits in
the wall of the tract so that neural regula- Saliva consists of water, electrolytes, mucus,
tion can be independent of external inner- and enzymes. The water and mucus soften
vation. The presence of food and distension and lubricate the ingesta to facilitate masti-
of gastrointestinal tract segments act as cation and swallowing. Lysozyme is a sali-
stimuli to initiate activity of the enteric vary enzyme with antibacterial actions.
nervous system. The three regulatory The starch‐digesting enzyme amylase is
mechanisms (autonomic nervous system, present in the saliva of omnivores (pig) and
gastrointestinal hormones, and enteric to a limited degree in horses but absent in
nervous system) also regulate secretions ruminants and carnivores (dog).
from glands in the wall of the gastrointesti- Adult cattle may secrete up to 200 L of
nal tract (Fig. 21‐1) and the intestinal saliva per day as compared to 1 to 2 L per
accessory organs (salivary glands, liver, and day for humans. This large volume main-
pancreas). tains the fluid consistency of the rumen
All three of these mechanisms may contents, and components of the saliva may
regulate a given intestinal segment or also prevent frothing of the rumen fluid.
accessory organ, but the relative impor- Ruminant saliva has a relatively high pH
tance of each varies among the segments of and contains high concentrations of bases
the gastrointestinal tract and the accessory (bicarbonate and phosphate). These bases
organs. For example, salivary secretion is neutralize acids produced by fermentation