Sunday 27 May 2012

The Rumen Stomach and Vomiting in Monogastrics

Hi :) In this post we'll be discussing the structure and development of the ruminant stomach, the structure and function of the oesophageal groove as well as the forestomach motility patterns in a ruminant.We'll also go over the sequence of events and neural pathways involved with vomiting in monogastric animals.

Structure and Development of the Ruminant Stomach

The forestomach of the rumen consists of the rumen, reticulum and omasum. The forestomach is lined with non-glandular mucosa. The fourth chamber is the abomasum and this acts like a monogastric stomach as it is lined with glandular mucosa and secretes acid. The structure is represented in the diagram below:
The Ruminant Stomach
Source. Please see this website if you'd like to use the diagram :)

The walls of all the chambers of the stomach contain smooth muscle.

At birth, the young ruminant derives all its nutrition from milk. Thus milk bypasses the immature forestomach and goes straight to the abomasum where digestion is initiated. As the animal gets older, the forestomach develops and becomes populated with microorganisms and it begins to feed off plant based foods instead of milk. The forestomach becomes fully functional at 2-3 months of age.

The Reticular Groove

In order to bypass the forestomach, the reticular groove acts as a tube to guide milk to the abomasum. This reflex is initiated by the act of suckling or drinking as well as by the presence of milk in the pharynx or even through conditioning. The reflex is lost when the ruminant stops suckling. The reflex occurs through several steps. First afferent impulses travel to the coordinating centre in the brain stem via the trigeminal nerve. Next, efferent impulses travel via the vagus nerve to the reticular groove and smooth muscle contraction turn the groove into a tube. This allows milk to pass from the oesophagus to the abomasum.

Forestomach Motility Patterns

The forestomach plays an important role in storage, maintaining a suitable environment for microbes, mixing eructation, further breakdown of plant material through rumination as well as the absorption of VFAs water and electrolytes.

There are several types of contractions which occur and these are quite important to remember.

A Word of Warning: The following lists of steps involved in these contractions may seem scary and intimidating! But don't worry :) I have only included the steps in this way because it breaks down the process into smaller steps and helps you to understand the processes a bit better. It's really not so bad :)

This website shows you the anatomy of the rumen. Its helpful to go through the following processes with this in mind.

Primary Contractions: Emptying:

  1. These start in the reticulum at a rate of 1-3/min. 
  2. The reticular wall contracts twice within 5-10 seconds. 
  3. The first reticular contraction reduces the reticular volume by 50% and this forces coarse material from the top of the reticulum to the central and dorsal parts of the rumen. 
  4. The reticulum then relaxes for one second.
  5. A second reticular contraction occurs and this almost completely empties the reticulum of the remaining fine material.
  6. Most of this fine material is transferred back into the atrium
  7. During the second contraction, the reticulo-omasal orifice opens for a few seconds and well digested fluid flows into the omasum. 
Primary Contractions: Mixing:

  1. These begin in the rumen after the biphasic contractions of the reticulum. 
  2. These contractions begin in the atrium and atrial pillar and spread caudodorsally to the dorsal sac. 
  3. The cranial pillar then contracts, placing it in a vertical position. This moves digesta into the relaxed reticulum.
  4.  The dorsal sac contracts and this moves digesta to the caudodorsal blind sac. 
  5. The contraction of the caudodorsal blind sac and caudal pillar pushes the digesta upwards and forwards and this completes the circular movement of the digesta. 
  6. Contractions in the ventral sac now begin. 
  7. These start cranially and move caudally to the caudoventral blind sac.
  8. This caudoventral blind sac contracts and completes the circular movement of the digesta. 
  9. Mixing now occurs between the dorsal and ventral sacs with the well digested material remaining in the ventral part of the rumen. 
  10. However, when the ventral sac contracts some of the digesta moves over the cranial pillar into the atrium. 
  11. This digesta is shuttled back and forth between the atrium and reticulum and eventually empties through the reticular-omasal orifice.  
Secondary Contractions: Eructation

  1. Secondary contractions usually follow a variable number of primary contractions.
  2. Usually, 2 or 3 primary contractions occur for every secondary contraction.
  3. The secondary contraction starts in the caudodorsal blind sac and moves cranially forcing the dorsal ruminal gas cap in the same direction. 
  4. At the same time, cranially-moving contractions occur in the ventral part of the rumen. This starts in the caudoventral blind sac and displaces gas dorsally.
  5. As the dorsal rumen contraction reaches the atrium, the cranial pillar elevates and this moves fluid away from the oesophageal opening, allowing gas to enter the oesophagus. 
  6. After entering the oesophagus, gas moves towards the oral cavity through retroperistalsis. This is followed by a peristaltic wave (towards the stomach) to clear any residual fluid from the oesophagus. 
 Rumination:

  1. The first act of rumination is regurgitation.
  2. It starts with a reticular contraction (which lasts 2-4 seconds) that occurs just before the primary contractions begin.
  3. The contraction removes recently-ingested material from the oesophageal opening and replaces it with semi-digested material that has undergone some fermentation. 
  4. Simultaneously with this contraction, the lower oesophageal sphincter opens and there is an inspiratory excursion against a closed glottis. In other words, the animal tries to breathe in but it can't because the opening of the trachea is closed.
  5. This creates a negative pressure in the thoracic cavity which 'sucks' the digesta into the oesophagus. An antiperistaltic wave then propels the cud into the oral cavity through the upper oesophageal sphincter. 
  6. Once in the mouth, excess fluid is squeezed out of the cud by the tongue. This fluid is then swallowed. 
  7. The cud is masticated, saliva is added and is then swallowed. 
   
Regulation of Forestomach Motility

The forestomach contractions which I mentioned above are mainly regulated through long vagovagal reflexes. Sensory information from receptors in the forestomach are transmitted to the integrating centres in the medulla of the brain through the vagus nerve. The vagus nerve also conveys efferent signals from the dorsal vagal nucleus in the brain back to the smooth muscle of the forestomach.The stretch receptors in the forestomach are arranged in series with smooth muscle cells. Moderate distention of these receptors stimulates contractions while over-distension inhibits contractions.

There are also chemo receptors located in the epithelium of the forestomach. These respond to changes in pH, osmolarity, and the concentration of volatile fatty acids. In general, the activation of these receptors leads to the inhibition of contractions.

Vomiting in Monogastic Animals

Several events are associated with vomiting. The early signs include hypersalivation, cardiac rythm changes, and possibly defecation. Salivation stimulates swallowing and this is associated with the relaxation of the lower oesophageal sphincter. Antiperistalsis then begins in the small intestine. At first, the duodenum in relaxed, it then undergoes 'retrogade giant contractions' (RGCs) which move the intestinal contents into the stomach. Immediately after this RGC, duodenal motor activity is inhibited. Retching (which is forceful contractions of the abdominal muscles and diaphragm against a closed glottis, this produces an increased pressure in the GIT) begins with the onset of RGC.

These events are  followed by retrograde contractions of the antrum and relaxation of the corpus, oesophagus and the upper and lower oesophageal sphincters. The gastric contents are forcefully expelled by the contractions of the abdominal muscles and diaphragm. As the vomit passes through the pharynx, the glottis and nasopharyngeal openings close and this prevents aspiration and nasal regurgitation. 

Neural Pathways: 

The vomiting reflex involves neural pathways which synapse in the 'emetic'centre of the medulla in the brain. This 'emetic centre' is not a distinct object or area of the brain, instead it is pharmacologic entity that receives a range of information. It receives info from the peripheral visceral receptors, the chemoreceptor trigger zone in the brain, the vestibular apparatus and from other cortical centres in the brain. 

The emetic centre can be stimulated by several different things and once stimulated it triggers a series of reactions which lead to vomiting (as mentioned above). The emetic centre can be stimulated directly, by afferent nerves from the GIT acting on this centre. Examples of this type of stimulation include pharyngeal stimulation, bacterial toxins, upper GI irritation or distension. The emetic centre can also be stimulated indirectly via stimulation of the chemoreceptor trigger zone, located in the fourth ventricle of the brain. Various drugs stimulate the centre in this way, for example those which are used in the treatment of cancer. In addition, the centre can be stimulated indirectly via other pathways, such as the cerebral cortex, brain stem and vestibular apparatus.


That's it for this post, please feel free to ask questions in the comments section below :)



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