14. Forestomach Motility

Hindgut vs Foregut Fermentors

• Hindgut fermentors are less efficient than foregut fermentors.
• Examples of hindgut fermentors: horse, elephant, rhinoceros.
• Examples of foregut fermentors: ruminants, camelids, kangaroo.
• Advantage of foregut fermentation: bacterial products are formed before the monogastric tract.
• Bacterial protein, B vitamins, and Vitamin K are produced.

Anatomy of the Forestomach

• The forestomach includes the reticulo-rumen, omasum, and abomasum.

Reticulo-rumen

• The Reticulum is structure 3 in the diagram
• The Ruminoreticular fold is structure 9 in the diagram
• The Cranial sac of the rumen is structure 4
• The Dorsal sac of the rumen, which includes blind sacs, is structure 5
• The Ventral sac is structure 7

Sacs

• Sacs are separated by muscular pillars.
• The Cranial pillar (10) forms a muscular ring.
• The Longitudinal pillar (11) is between the dorsal and ventral sacs.
• The Caudal pillar is structure 12.

Omasum

• The Omasum is structure 15.
• The Omasum is connected to the reticulo-rumen by the reticulo-omasal orifice (ROO) (2) and the omasal canal (16).
• The Omasum is connected to the abomasum by the omaso-abomasal orifice (OAO) (18).
• Leaves inside the omasum, are like book pages, gains surface area for VFA

Abomasum

• The Abomasum (20) is the true stomach.
• The Abomasum produces acid chyme.

Epithelium

• The forestomach epithelium is stratified squamous epithelium.
• There are no gastric glands in the forestomach.
• The epithelium absorbs mainly VFA, electrolytes, and water.
• Surface area is increased by honeycomb upfoldings of the reticulum and ruminal papillae.
• The forestomach can be viewed as a specialized esophageal structure.
• It is located before the true stomach, like the crop in birds.
• It has the same epithelium as the esophagus.
• Motility is dominated by the extrinsic autonomic nerve system.

Forestomach Motility

• The functions of the forestomach include providing continuous mixing of bacteria and feed constituents, retaining ingesta for microbial digestion and to downsize particles (kneading and rumination), provide an environment for microbial digestion, and removing fermentation gases (1-2 L/min).

Fate of Feedstuffs

• Dense feedstuffs (grain, cud) and reswallowed boluses (cud) entering from the esophagus drop directly to the cranial sac of the rumen from the cardia.
• Light feedstuffs (hay and grass) float in the dorsal sac of the rumen.
• Several days of fermentation, soaking, and rumination are required to make feedstuffs denser.
• Dense material moves to the cranial sac, then to the reticulum, and finally to the reticulo-omasal orifice.

Reticulo-rumen Cycles

• Reticulo-rumen cycles are controlled by gastric centers (CNS).
• Primary Contraction Cycle: occurs at a frequency of 1-3 per minute.

Primary Contraction Cycle

• Cycle initiated by double reticular contraction
• 1st Reticular contraction = Mixing
• 2nd Reticular contraction = Evacuation: Dense material drops down.
• Dense particles move through the ROO into the omasal canal.
• Contraction of the cranial sac and upward movement of the cranial pillar occur.
• This separates Dense particles, which go to → Reticulum, and Light particles, go to → Dorsal sac, via strong muscle contraction
• Dorsal sac contracts (cranial to caudal), followed by ventral sac contraction (cranial to caudal).
• Ventral sac contracts (caudal to cranial) and cranial pillar relaxes.
• Denser material moves forward into the cranial sac.
• A single reticulo-rumen contraction cycle concludes the "primary contraction cycle."

Eructation Reflex

• Eructation reflex (also known as the Secondary Contraction Cycle) is related to the primary cycles depending on diet
• Purpose: Expelling of fermentation gases.
• Gas produced at 1-2 L/min includes CH4, H2, and CO2.
• Stimulus: Accumulation of ruminal gas in the dorsal sac (ruminal wall tension).
• The reflex event is integrated by the CNS; the event is rumen tension with gas
• As a primary cycle is concluding, the cranial pillar does not relax (remains elevated) and retains solid digesta in the rumen.
• The dorsal sac contracts (caudal to cranial) and Moves gas cap forward.
• The reticulum relaxes, and displaced gas cap forces fluid and digesta in the reticulum ventrally.
• The exposed cardia opens, and gas enters the esophagus, causing a "burp."
• Rumen gas is mostly aspirated into the trachea and then exhaled.
• Ventral sac contracts (caudal to cranial), and the cranial pillar relaxes.
• Dense material moves forward into the cranial sac
• The secondary contraction cycle concludes in the same way as the primary contraction cycle.

Rumination Reflex

• The Rumination Reflex is integrated by the CNS. The cycle happens ~8 hours/day.
• The Function is to downsize dry matter when not grazing.
• The Reflex is typically initiated just before a primary cycle.
• The rumination reflex consists of four events (4 R's).

Regurgitation

• Extra reticular contraction precedes normal double contraction: Rumen content moves to cover the cardia
• Animal inspires against closed glottis to create a retching maneuver.
• Decreases intrathoracic pressure (more negative pressure).
• Cardia opens, content enters the esophagus, and a reverse peristaltic wave of the esophagus moves content to mouth.
• Primary cycle goes on while chewing the cud.

Remastication

• Fluid is squeezed out of the bolus and swallowed.

Reinsalivation

• Reinsalivation of cud

Redeglutition

• Cud is swallowed about 1 sec before the next 1º cycle.

Omasal Canal

• The omasal canal is where content passes through the ROO and enters the canal.

Aspiration of reticular contents into the omasal canal

• During the 2nd reticular contraction of the 1* cycle (evacuation), the omasal canal relaxes.
• Decreased pressure in the omasal canal occurs, and reticular content is sucked into the omasal canal.

Omasal pump

• After the ROO closes, the omasal canal contracts and the content is forced into omasal leaves.

Omasum

• Absorbs electrolytes, volatile fatty acids (VFA), and water.

Omasal Motility

• Reduce the size of particulate material
• The omasum periodically contracts when distended with ingesta.
• Omasal contents pass to the abomasum via the OAO.

Control of Forestomach Motility

• Forestomach motility is controlled by gastric centers in the CNS via the vagus
• It is centrally controlled like esophageal motility
• The content of the forestomach and abomasum affects the frequency of reticulo-ruminal contractions to provide signals.

Afferent Receptors

• Sensation is sent to the gastric centers (CNS).
• Tension Receptors (mechanoreceptors):
• Reticular and ruminal tension receptors are excitatory (feed forward) and increase cycling.
• Abomasal tension receptors are inhibitory (feedback), creating a long inhibitory absomasoruminal reflex
• Results in decreased cycling Chemoreceptors
• Monitor pH and osmolarity:
• Forestomach receptors slow motility if pH and osmolarity decrease cycling
• Allows more time for alkaline salivation
• Abomasal receptors increase rumen motility if pH = empty abomasum, increasing cycling
• Afferent signals are integrated in the medullary gastric centers.

Efferent Pathways

• Vagal parasympathetic outflow to effector neurons:
• Necessary for effective rumen motility.
• The enteric neural system is ineffective.

Sympathetic Nervous System

• Outflow inhibits forestomach motility.

Abomasum

• Peristaltic patterns are controlled similarly to the monogastric stomach.

Esophageal Groove

• Also called the reticular groove
• Anatomical groove at the reticulum-rumen interface:
• begins at the cardia and passes to the reticulo-omasal orifice.
• Lips of groove contract and form a closed tube when a lamb or calf suckles.
• Allows passage of milk directly to the abomasum
• Bypasses the forestomach avoids milk putrefying in rumen

Suckling Reflex

• Closes lips of esophageal groove

Afferent Path

• Receptors in the mouth and pharynx
• Mechanoreceptors - Suckling
• Chemoreceptors - Salt content (NaCl, NaHCO3); milk has salt

Efferent Pathway

• Vagal parasympathetic pathway to effector neurons
• Causes a contraction of the esophageal groove, forming a tube

Development of Reticulo-rumen

• The reticulo-rumen as a percentage of the weight of the entire "stomach" increases with age (40 to 70%), while the abomasum decreases (50 to 15%).
• Early creep feeding will hasten solid feed development
• Creep feeding at 1 week will result in a functional rumen by 6 weeks Development increases faster with grain because VFA stimulates the growth of rumen papillae.

Microflora Changes

• Development of rumen microbial population is also dependent upon solid feed.
• Types of microbes which digest fiber are different from those which digest soluble carbohydrate.
• Normally, a two-week period is necessary to change from one diet to another

Explore the distinctions between foregut and hindgut fermentation in animals, highlighting examples such as ruminants and horses. Learn about the anatomy of the forestomach, including the reticulum, rumen, omasum, and abomasum. Discover the roles of sacs and muscular pillars in ruminant digestion.