L2-Physio Salivation & Swallowing (Deglutition)

Introduction

The digestive system consists of:

A-The alimentary canal or the gastrointestinal tract (GIT).

B-Various digestive glands associated with it e.g. the salivary glands, liver, and pancreas

The GIT

-It is a muscular tube about 4.5 meters long extending from the mouth to the anus.

-It includes the buccal cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

General functions of the digestive system

1-Motility

2-Secretion of digestive juices

3-Digestion of food

4-Absorption

Mouth and Esophagus

-In the oral cavity, the process of food mastication occurs which cuts down the large food particles into small pieces and mix them with saliva forming what is called a bolus.

-The bolus is propelled backwards into the pharynx and downwards through the esophagus to the stomach by the process of swallowing.

Salivation

Salivary secretion

-Saliva is secreted by 3 pairs of salivary glands: the parotid, submandibular and sublingual glands as well as by several small glands scattered in the mucous membrane of the buccal cavity.

-Each gland is formed of a group of secretory  acini, the secretion of which is carried by a system of ducts, that unite to form one large duct which opens into the buccal cavity.

The Salivary Glands

The salivary acini (salivons)

There are two types of acini:

1-Serous cells: These secrete thin saliva rich in the enzyme ptyalin(amylase)

2-Mucous cells: These secrete viscid saliva rich in mucin

The parotid glands contain only serous cells (25%), while the sublingual glands have only mucous cells (5%). The submandibular glands contain both secretory cells and secret 70% of the total secretion

Secretion of Saliva

-The secretion of saliva is an active secretory process.

-The initial salivary secretion from the acini is nearly isotonic

-In the duct : Some Na+ and small amount of Cl- and HCO3- are actively reabsorbed, while K+ ions are secreted. This process is potentiated by the aldosterone hormone.

Mechanism of salivary secretion

Composition of Saliva

The amount secreted daily is about 1.5 liters

The pH of saliva is about 7. Why?

It is hypotonic relative to saliva

Its composition is:

1-Water 99.5%

2-Solids 0.5%

a-Inorganic ions

b-organic ions

Innervations of salivary glands

The salivary glands are supplied by both sympathetic and parasympathetic nerves

Sympathetic stimulation produces a little secretion of viscous saliva rich in organic constituents e.g. mucin and ptyalin.

Parasympathetic stimulation Produces

A-Secretion of large amount of watery saliva

B-Marked vasodilatation in the salivary glands

Mechanism of salivary secretion

-Salivary secretion is a rapid process which is under nervous control only.

-It passes in 3 phases:

1-Cephalic phase (Conditioned reflexes)

2-Buccal phase (Unconditioned reflexes)

3-Gastrointestinal phase

Swallowing (Deglutition)

It is the act of transferring the food from the buccal cavity to the stomach.

It can be studied from successive X-ray films taken while swallowing a barium meal which is opaque to the X-ray

Phases of swallowing

The process of swallowing is divided into:

1-Buccal stage

2-Pharyngeal phase

3-Esophageal phase

Buccal Stage

-This stage is voluntary

-After mastication, and the formation of the bolus of food, the tongue is voluntarily elevated against the hard palate mainly by contraction of the mylohyoid muscles.

-The bolus is rolled backwards then is forced into the pharynx.

-The mouth must be closed.

The Pharyngeal Stage

-This stage is involuntary

-It occurs as a result of a reflex called the swallowing reflex

-As the bolus is pushed backwards it stimulates certain receptors around the opening of the pharynx

-Impulses which arise are transmitted via 5, 9, 10th cranial to the deglutition center in the medulla

From the deglutition center, impulses are discharged via efferent fibers in the 5, 7, 9, 10, 12th cranial nerves leading to:

I-protective  reactions: This prevents food entrance into the nose , trachea or back into the buccal cavity.

II-Propulsive mechanisms: This pushes the bolus into the esophagus

The protective mechanisms

A-Elevation of the soft palate

B-Elevation of the larynx against the epiglottis

C-Approximation of the vocal cords

D-Approximation of palato-pharyngeal folds at the fauses

E-Continued contraction of the mylohyoid muscles

The propulsive mechanisms

-The superior constrictor muscle of the pharynx contracts initiating a rapid peristaltic movement that passes downwards through the pharyngeal muscles.

-This raises the intra-pharyngeal pressure from atmospheric to about 100 cm water which pushes the bolus into the esophagus

-The entire process occurs only in 1-2 seconds.

Esophageal stage

-The esophagus is a muscular tube about 20cm long which is lined by a mucous membrane containing many simple mucus glands.

-The esophagus can be divided into 3 parts according to the nature of muscle in its wall.

-Its upper esophageal end is guarded by the upper esophageal sphincter while lower end is  guarded by lower esophageal sphincter

-During rest, these 2 sphincters are normally closed while the pressure inside the esophageal lumen is negative like the intra-thoracic pressure.

-The esophageal stage of deglutition is also an involuntary stage in which peristaltic movement occurs in the esophageal wall propelling the bolus to the stomach.

Two types of peristaltic movements  normally occur in the esophagus:

1-Primary peristalsis

2-Secondary peristalsis

Neural control of swallowing

Stages of Swallowing

Swallowing Disorders

Dysphagia

Achalasia

Aerophagia

Clinical significance

Swallowing becomes a great concern for the elderly since strokes and Alzheimer’s disease can interfere with the ANS. Speech therapy is commonly used to correct this condition since the speech process uses the same neuromuscular structures as swallowing.

Esophageal sphincter due to cancer

A list of things that can be done to make swallowing easier

1. Chew all food well

2. Slow the rate of eating

3. Alternate food with liquids to “wash” food down

4. Take smaller bites

5. Swallow everything before the next bite

6. Swallow several times per bite

7. Eat when less fatigued

8. Eat in a relaxed atmosphere

9. Eat the most difficult food first (when swallowing muscles are less fatigued)

10. Avoid problematic food or prepare it in a way that is easier to swallow (e.g., peel apple, blend carrots, etc.)

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