Lecture 21: The Digestive System

Question 1

What are the major methods of obtaining food

Answer 1

Filter feeding:

• Amphioxus and tunicates
• Some fishes
• Baleen whales
• Bite-tear-swallow:
• Create suction-swallow
• Rasping
• Impaling with beaks or teeth
• Picking up with beaks
• Sucking (blood or milk)
• Cropping
• Snap-bite-tear

Question 2

Describe the four digestive system functions

Answer 2

• Transportation:
• Peristalsis (myenteric reflex)
• Reverse peristalsis
• Physical treatment:
• Chewing
• Propulsive peristaltic contractions
• Local intermittent constrictive contractions
• Chemical treatment:
• Digestive enzymes
• Digestive acids
• Bile
• Absorption:
• Movement from lumen into blood or lymph

Question 3

Describe the overall anatomy of the digestive system

Answer 3

• Location:
• Pleuroperitoneal cavity: Fishes, Amphibians, Many tetrapods
• Peritoneal cavity (beyond esophagus): Some reptiles, Birds, Mammals
• Coelom:
• Cavity derived by splitting of early lateral plate mesoderm
• Somatic mesoderm
• Splanchnic mesoderm
• Visceral peritoneum:
• Covers most of digestive tract
• Derived from splanchnic mesoderm
• Parietal peritoneum:
• Lines peritoneal cavity
• Derived from somatic mesoderm

Question 4

Describe the overall anatomy of the digestive tract

Answer 4

• Early embryonic gut tract:
• Midgut: Part containing yolk or to which yolk sac is attached
• Foregut: Part anterior to midgut. Forms pharynx, esophagus, stomach, and much of small intestine
• Hindgut: Part posterior to midgut. Forms remainder of intestine and cloaca
• Length of digestive tract: Correlates mostly with which ingested material is readily absorbed.
• Shape of digestive tract: Correlates to some extend with shape of animal’s body
• Size of stomach: Correlates with whether or not food supply is constant

Question 5

Describe the primary tongue

Answer 5

• Primary tongue:
• Elevation of floor in oropharyngeal cavity:
• • Has skeleton of basihyal and ceratohyal cartilages
• • Has no musculature
• • Found in elasmobranchs, bony fishes, and amphibians with persistent gills
• Primary tongue with extension:
• Tongue extension can be flipped out of the mouth:
• • Terrestrial urodeles and anurans
• • Glandular field

Question 6

Describe the tongue of reptiles, mammals, and birds

Answer 6

• Tongue of reptiles and mammals:
• Derived from:
• • Paired lateral lingual swellings from mandibular arch
• • Hyoid arch derivative that develops a glandular field
• • Mesenchyme from third pharyngeal arch.
• Tongue of birds:
• In most birds tongue is almost entirely devoid of intrinsic musculature.
• • Movement of tongue related to movement of hyoid skeleton.

Question 7

List the layers of the gut tract wall

Answer 7

• Outer to inner:
  1. Serosa:
  2. Longitudinal smooth muscle layer
  3. Myentericplexus (of Auerbach)
  4. Circular smooth muscle layer
  5. Meissner’s plexus (submucosal plexus)
  6. Submucosa
  7. Mucosa
• See slide 10-11

Question 8

Describe the serosa

Answer 8

• Outer layer of connective tissue and simple squamous epithelium
• Continuous with mesentery:
• Pathway for blood vessels to gut tube
• Missing in some parts (i.e., esophagus):
• Replaced with adventitia connecting to abdominal wall
• Produces thin layer of lubricating fluid

Question 9

Describe Myenteric and Meissner’s Plexuses

Answer 9

• Collectively form the enteric nervous system of the gut tract.
• Can operate autonomously via intrinsic regulation and sensory reflexes.
• Work with sympathetic and parasympathetic (mostly vagus nerve) systems.
• Parasympathetic system generally increases GI tract activity.
• Sympathetic system generally decreases GI tract activity.
• Myenteric (Auerbach’s) plexus:
• Located between longitudinal and circular smooth muscle layers.
• Primarily controls intestinal smooth muscle and participates in tonic and rhythmic contractions.
• Extends the entire length of the G.I. tract.
• Effects:
• • Increased tonic contraction of gut wall
• • Increased intensity of rhythmical contractions
• • Slightly increased rate of rhythmical contraction
• • Increased velocity of excitatory waves
• Meissner’s (submucosal) plexus:
• Effects:
• • Controls mainly GI secretion, absorption and local blood flow
• • Mainly concerned with controlling function within inner wall of each minute segment of the intestine
• • Helps in control of local secretion, local absorption, and local contraction of submucosal muscle

Question 10

Describe the submucosa

Answer 10

• Thick connective tissue layer that supports bases of compound alveolar glands
• Incorporates rich bed of arterioles and venules that supply mucosal capillary beds.

Question 11

Describe the mucosa

Answer 11

• Derived from endoderm
• Single cell layer forming a continuous sheet lining entire G.I. tract.
• Includes lamina propria:
• Loose CT with sensory nerves, blood vessels and some glands
• Includes muscularis mucosa:
• Thin layer of smooth muscle
• Creates mucosal ridges and folds

Question 12

Describe the esophagus

Answer 12

• Distensible muscular tube
• Variable in length which may be correlated with length of neck
• Esophageal musculature:
• Early part is mostly striated muscle
• Latter part is smooth muscle
• Early part is lined with stratified squamous epithelium which transitions to simply columnar epithelium.
• In terrestrial turtles, birds and some mammals it may be keratinized.
• Crop:
• Paired diverticula, mostly developed in grain-eating birds
• Used to hoard seeds and grain
• May produce “pigeon’s” milk for nestlings

Question 13

Describe the stomach shape and function

Answer 13

• Muscular chamber
• Functions:
• Receiving chamber from esophagus
• Secretes digestive enzymes and mucus
• Different lining of stomachs:
• Esophageal-like epithelia
• Glandular epithelia
• Shape:
• May remain straight throughout life
• May be bent to assume a “J” shape:
• • Lesser curvature = concave border
• • Greater curvature = convex border:
• – May be attached to a double fold of mesentery referred to as the greater omentum

Question 14

Compare the ruminant stomach with the human stomach

Answer 14

• Ruminant stomach
• 4 chambers: rumen, reticulum, omasum, abomasum
• Human stomach
• Cardiac sphincter-esophagus
• meets stomach
• Mostly lined with gastric epithelium

Question 15

Describe the stomach comparatively across vertebrates.

Answer 15

• No definite stomach in:
• Agnathans:
• • Entire gut tract lined with simple epithelium consisting of goblet cells and cells secreting proteolytic enzymes.
• Fishes:
• Varies from no definable stomach (gars) to a straight stomach to a “J”-shaped stomach (sharks)
• Frogs:
• Similar to esophagus and highly distensible
• Crocodilians and birds:
• Proventriculus secretes digestive enzymes
• Gizzard serves as a grinding mill
• See Slide 25-32

Question 16

Describe Absorptive Surfaces

Answer 16

• Absorption is a major function of the gut track.
• Most absorption occurs in the small intestine.
• Anatomical modifications for absorption include:
• Increasing length of intestine
• Plicae:
• • These are folds of the mucosa, each with a submucosal core.
• Crypt-villus system:
• • Each villus contains a core of lamina propria.
• • The crypts extend down between the villi below the bases of the villi.
• • Crypts and villi constitute glands of the small intestine.
• Microvilli
• See Slide 34

Question 17

Describe the types of teeth found in the various vertebrate groups and be familiar with the nomenclature describing types of teeth.

Answer 17

• Types of teeth may be correlated with vertebrate evolution and with types of food eaten.
• Categories based on types of food eaten:
• • Plankton (filter feeders)
• • Insects (insectivorous)
• • Shellfish
• • Flesh (carnivorous)
• • Piscivorous
• • Vegetation (herbivorous)
• • Omnivorous
• Teeth are normally attached to the bones of the upper and lower jaws.
• Teeth may also be found on the palate or other pharyngeal areas.
• Teeth are usually constructed of dentine.
• Mesodermal derivative formed by odontoblasts.
• Dentine may be capped by enamel or vitrodentine.
• Enamel is an ectodermal derivative formed by ameloblasts.
• Teeth may be attached to surface of jaw or may be set down into sockets.
• Teeth may all be the same in a given individual (homodont).
• Teeth may come in several different shapes in a given individual (heterodont).
• Polyhyodont-permanent replacement of teeth
• Diphyodont-two sets of teeth
• Monophyodont-one set of teeth
• Bunodont-all teeth on single plain
• See Slide 39-40

Question 18

List some of the types of teeth in heterodont mammals

Answer 18

• Heterodonty in Mammals
• Incisors
• • For cutting, Ex: elephant tusks
• Canines:
• • For piercing, Ex: walrus tusks
• Premolars: To masticate food
• Molars: To masticate food
• Diastema: Space without teeth; e.g., no canines

Bunodont: All teeth on single plain
Sectorial teeth: Carnassials; e.g., upper premolar and lower molar in carnivores

Question 19

What is the Dental Formulae

Answer 19

• Catarrhines and humans have 2-1-2-3=16 x 2 = 32 total teeth.
• Canines: 3-1-4-2 and 3-1-4-3 43
• If 0 is present, diastema is present
• Diastema present in rabbits, cows, and hamsters

Question 20

Describe the mammalian set of teeth

Answer 20

• The incisors are used for biting and nipping.
• The canines are used for for stabbing.
• The premolars and molars are used for grinding and chewing.
• The molars are characterized by cusps.
• • Each cusp in the upper molar = a cone.
• • Each cusp in the lower molars = aconid.
• • A minor cusp is a conule.
• • The prefixes: proto-, para-, meta-, hypo-, and ento-are used to designate cones and conids within each molar.
• • The cingulumis an accessory ridge of enamel around the margins of the cheek teeth. The style is a vertical ridge around the margin of a cheek tooth.
• The primitive lower molar:
• Well-developed triangular area =the trigonid.
• Posterior to the trigonid is a region called the talonid.
• Proper occlusion occurs when the protocone of the corresponding upper molar bites into the talonid of the lower molar.
• The primitive generalized cusp pattern:
• Consisted of three main upper cusps and five main lower cusps.
• The cheek teeth become squared up for a more effective grinding surface.
• • Adding an additional cusp (the hypocone) to the upper molar
• • Eliminating a conid (the paraconid) from a lower molar
• This alteration is common in herbivorous, grazing mammals.
• See Slide 46