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Regulation of Salivary Secretion

Salivary secretion is exclusively under neural control; both sympathetic and parasympathetic nerves supply the salivary glands.

A. Sympathetic nerves

These fibres arise from T1 to T4 segments of the spinal cord and reach the superior salivary ganglion. Post- ganglionic fibres start here and run along the blood vessels and supply all the salivary glands. The sympathetic fibres supply the blood vessels, the myoepithelial (or basket) cells and acinar cells.


B. Actions

1. Sympathetic stimulation causes secretion of small amounts of thick, viscous saliva rich in organic constituents from the submandibular glands. Increased salivary secretion

a. Mechanism of action

Sympathetic stimulation /circulating catecholamines stimulate salivary secretion rich in enzymes through alpha and beta receptors.  Alpha receptor action is mediated through calcium ions and beta receptor action ismediated through cyclic AMP.


The norepinephrine released brings about vasoconstriction and decreased blood flow; thus, the water content of the secretion is less.

2. The myoepithelial (or basket) cells contract under the influence of norepinephrine and cause expulsion of the already secreted saliva from the acinus.

C. Parasympathetic nerve fibers

These come from a nucleus present at the junction of the medulla and pons near the tractus solitarius. The nucleus has got two parts :

  1. The caudal part (called inferior salivary nucleus) :
This provides parasympathetic nerve fibres to acinar cells and blood vessels of parotid gland. These fibres pass via the IX (i.e. glossopharyngeal ) nerve
  1. The superior part (called superior salivary nucleus) :
This provides parasympathetic fibres to the acinar cells and blood vessels of the submaxillary and sublingual glands. These fibres pass through the VII (i.e. facial ) nerve and the chorda tympani nerve.

Pain afferents from these glands run in the parasympathetic nerves.


D. Actions

Stimulation of the parasympathetic nerve fibres results in profuse secretion of watery saliva with a relatively low content of organic material.  Along with this secretion, there is a significant vasodilation in the salivary glands; the vasodilation is likely to be due to VIP. (VIP is a co-transmitter with acetylcholine in some of the post-ganglionic parasympathetic neurons).  Atropine and other cholinergic blocking agents reduce salivary secretion.


E. Mechanism of action of parasympathetic nerves

  1. Acetylcholine (Ach) released from the nerve endings directly act on the acinar cells and stimulate enzyme secretion
  2. Ach activates kallikrein; kallikrein in turn activates kininogen and converts it into bradykinin. Bradykinin produces vasodilatation and increases the blood flow to the gland
  3. VIP is a co-transmitter released along with Ach and causes vasodilatation.

Types of salivary secretion

1. Spontaneous or resting salivary secretion

Saliva is secreted continuously even in the absence of any known stimulus; this is called spontaneous or resting salivary secretion. This is possibly due to release of minute amounts of acetylcholine into the gland. However, spontaneous secretion cannot be blocked by atropine. It can, however, be blocked by metabolic poisons like cyanide. This indicates that the spontaneous salivary secretion depends upon the metabolic activity of the salivary glands. This type of secretion is responsible for keeping the oral cavity moist all the time.


2. Reflex secretion

 This occurs in response to a stimulus. The reflex secretion can be further subdivided into

a. unconditioned (or inherent)reflex secretion

Food (and other substances) in the mouth causes reflex secretion of saliva (also, stimulation of the vagal afferent fibres at the gastric end of the oesophagus results in reflex secretion of saliva)


G. Pathway

Food in the mouth à stimulates the trigeminal, glossopharyngeal and vagal nerves à impulses in them are carried to the superior and inferior salivary nucleus à reflex salivation

1. conditioned (or acquired) reflex secretion

2. Salivary secretion can be easily conditioned (as shown by Pavlov); the sight, smell or even thought of food causes salivary secretion.

3. Deglutition or Swallowing

4. This is the process by which the chewed food is emptied form the mouth into the stomach. It is initiated voluntarily but is completed reflexly.

H. Pathway

Deglutition is initiated by afferent impulses in V, IX and X cranial nerves à impulses from there are carried and integrated in the nucleus of the tractus solitarius and the nucleus ambiguus à from here, efferent fibres pass to the muscles of the pharynx and tongue via the V, VII and XII cranial nerves.


I. Mechanism

Swallowing is initiated by the voluntary action of passing the bolus of food with the help of the tongue to the pharynx; thereafter, the process occurs reflexly. When food reaches the pharynx, it starts a wave of involuntary contraction in the pharyngeal muscles; this pushes the food into the oesophagus.  Peristalsis in the oesophagus pushes the food down the oesophagus at a rate of about 4 cm/second. However, in the upright position, gravity pulls the liquid/semisolid food to the lower end of the oesophagus faster than the wave of peristalsis.


J. Other components of the swallowing reflex

1. inhibition of respiration

2. closure of glottis

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