Reticulorumen Motility Is Controlled by the Central Nervous System and Affected by Intraluminal Conditions
In the dorsal vagal nucleus of the brainstem, there is a motility control center for the regulation of Feticuloruminal motility. This center sends action potentials along afferent fibers to the forestomach by way of the vagus nerve.
There is an extensive enteric nervous system within the reticulorumen, but vagal innervation is necessary for coordination of normal motility patterns. When the vagal nerves are destroyed, motility of the rumen musculature ceases initially but returns within several days; however, the motility that develops after vagotomy is erratic, uncoordinated, and incapable of supporting the normal flow of ingesta through the reticulorumen. Vagotomized ruminants do not survive.'Γhe dorsal vagal nucleus receives afferent stimuli that affect the control of forestomach motility. Important afferent signals come from the lumen of the reticulorumen and monitor distention, ingesta consistency, pH, VTA concentration, and ionic strength. Rumen volume, or distention, appears to be monitored by stretch receptors in the walls and especially in the pillars. Moderate distention increases rumen motility and rumination. Increased motility and rumination have the effect of raising the rate at which particles are broken down, leading to a higher passage rate. Thus, rumen throughput is enhanced when increased intake expands rumen volume. Severe distention, as occurs pathologically in bloat, causes cessation of rumen motility.
The consistency of ingesta also has an important influence on rumen motility. Consistency is determined largely by diet type. When the diet consists of succulent plants, grain, or finely chopped forage, there is little material in the solid zone, or rumen mat, and the slurry zone is fluid. This type of fluid ingesta offers little resistance to the movement of the rumen pillars; thus the rumen musculature has to apply relatively little force to mix and circulate the rumen contents.
Tension receptors in the reticuloruminal muscle appear to monitor the force necessary to move the pillars through the ingesta. Highly fluid rumen ingesta are associated with low muscle tension and have a negative influence on reticulorumen motility. At the other dietary extreme, when animals are eating dry, long-stem hay, the rumen contents are solid and create a large and highly interwoven rumen mat. Resistance to movement of the pillars through the solid mass of ingesta is high and leads to stimulation of tension receptors, resulting in a positive feedback on motility. The motility rate is directly related to the rate of particle breakdown; this arrangement appears to be a self-regulatory mechanism that increases the rate of particle comminution when animals consume diets with large particle size.Chemoreceptors in the walls of the rumen and reticulum monitor pH, VFA concentration, and ionic strength (or osmolality). The pH of the reticulorumen is normally slightly acid, reflecting the acidity of the VFAs, but extreme acid conditions are undesirable. Increasing VFA concentrations or decreasing pH results in a suppression of rumen motility. The normal rumen pH is in the range of 5.5 to 6.8, depending on the type of diet. When the rumen pH falls much below 5.0, motility is severely depressed. This response appears to be protective because fermentation tends to be enhanced by motility-induced mixing; thus suppression of motility slows down fermentation, allowing VFA absorption to catch up with VFA production.
Osmolality may influence rumen motility as well, although motility appears less sensitive to osmotic changes than it does to pH changes. Normal osmolality in the rumen is about 280 mθsm, but the osmolality increases during active fermentation. Osmotically active solutes in the rumen include organic acids as well as salivary and dietary electrolytes. As organic acid formation increases during fermentation, osmolality also increases, tending to reduce motility. The rumen epithelium creates a relatively impermeable barrier to water, so wide swings in rumen osmolality can occur without large shifts in water between the rumen and the vascular compartment. At abnormally high osmolalities, however, water can be drawn into the rumen.