VPB 121: Veterinary Physiology - II (2+1)

REGULATION OF SALIVARY SECRETION

• When food enters the mouth, a copious secretion of saliva takes place by reflex stimulation of the salivary glands through the buccal receptors and secretory centers.
• Normal foodstuffs causes salivary secretion rich in mucin and enzymes to facilitate easy swallowing, while dry or noxious materials causes watery secretion with little mucin.
• The types and volumes of secretion varies with diverse range of stimuli. The chewing of meat cause a flow of saliva from its  sub maxillary and sublingual glands but not from parotid gland. While dry bread promoted abundant parotid watery saliva which contains little mucin.

• The saliva produced from one gland differs from that of the other.  These may be mixed together in differing proportions. A single gland may be composed of different type of cells  and their relative contributions to the saliva produced by that gland may vary. The secretion from a gland composed of only one type of secretory cell also varies in composition at differing rates of secretion .There is a general mechanism by which change in the composition of saliva can occur.
• The secretion of saliva in response to food in the mouth is an unconditioned reflex,and the stimulus producing it is termed as unconditioned stimulus .It is not necessary for food to enter the mouth to provoke a flow of saliva because the sight,smell or even the thought of food  may stimulate salivation .The extent to which this psychic secretion occurs varies in different species and it is particularly marked in dog. The nature of the secretion produced varies,the sight of meat evokes a thick,stringy, mucus  secretion from the sub maxillary and sublingual glands, while the sight of dry bread results in an abundant secretion of watery parotid saliva.
• Psychic reflexes were termed as conditioned reflexes by Pavlov [1910] because they were dependent on certain  conditions for their development  known as  acquired reflexes.The conditioned reflex is that when a stimulus which in itself does not provoke secretion. 
•  The optic and olfactory nerves function as afferent/sensory pathways for this reflex activity. Psychic salivary secretion is exhibited by goats and pigs, but it is feeble or absent in horse and sheep. Food is a natural stimulus for salivary secretion and salivation during feeding is an inherited reflex. When a dog is conditioned by a neutral stimuli (bell sound at the time of feeding) for certain period of time, only the bell sound without food is sufficient for increased salivary secretion by way of acquired reflex.