Digestion First Stage

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Questions and Answers

What is the term for the contractions that start when the bolus reaches the upper part of the esophagus?

Lower esophageal sphincter contractions

Primary peristaltic contractions (correct)

Secondary peristaltic contractions

Deglutition reflex contractions

What is the primary function of the first stage of deglutition?

Passage of food material from the pharynx into the oesophagus

Passage of material through the oral cavity into the pharynx (correct)

Contraction of the superior pharyngeal constrictor

Mastication and respiration

What happens to the pressure in the upper part of the esophagus initially when a bolus enters?

The pressure remains constant

The pressure becomes positive

The pressure becomes negative (correct)

The pressure increases suddenly

Which muscles are involved in the upward and backward movements of the tongue during the first stage of deglutition?

Mylohyoid, styloglossus, and hypoglossus Signup and view all the answers

What is the role of the lower esophageal sphincter?

To relax and allow the bolus to enter the stomach Signup and view all the answers

What occurs during the second stage of deglutition?

The passage of the bolus from the pharynx into the oesophagus Signup and view all the answers

What induces secondary peristaltic contractions?

The distention of the upper esophagus by the bolus Signup and view all the answers

What is the maximum pressure developed during primary peristaltic contractions?

10 to 15 cm of H2O Signup and view all the answers

Which nerves carry parasympathetic action potentials to the stomach?

Vagus nerves Signup and view all the answers

What is the effect of cocainisation on the swallowing reflex?

It inhibits and abolishes the swallowing reflex Signup and view all the answers

What is the primary function of postganglionic neurons in the stomach?

To stimulate secretion by parietal and chief cells Signup and view all the answers

What is the stimulus for the deglutition reflex?

The receptors present in the oropharyngeal region Signup and view all the answers

What occurs during the elevation of the soft palate during the second stage of deglutition?

The nasopharynx is closed off from the rest of the pharynx Signup and view all the answers

What is the primary function of the primary peristaltic wave during the second stage of deglutition?

To propel the bolus down the pharynx and into the oesophagus Signup and view all the answers

What is the role of gastrin in the stomach?

To stimulate the secretion of HCl and pepsin Signup and view all the answers

What is the sequence of events in the lower esophageal sphincter during swallowing?

Constriction, relaxation, and then constriction again Signup and view all the answers

What type of act is the beginning of swallowing?

Voluntary act Signup and view all the answers

What is the role of the cricopharyngeus muscle during the second stage of deglutition?

It contracts to keep the oesophagus closed until the bolus approaches Signup and view all the answers

What is the primary mechanism by which chyme in the duodenum with a pH less than 2 inhibits gastric secretions?

Sensory input to the medulla from the duodenum inhibits the motor input from the medulla to the stomach Signup and view all the answers

What is the primary function of the peristaltic wave in the stomach?

To mix and grind food Signup and view all the answers

What happens to the pharynx after the bolus enters the oesophagus?

It reopens Signup and view all the answers

What is the role of pacemaker cells in the stomach?

To induce slow waves on which the spike potentials that initiate the peristalsis contraction are superimposed Signup and view all the answers

What is the final phase of the peristaltic contraction of the antrum?

Phase of retropulsion and grinding Signup and view all the answers

What happens when the peristaltic wave moves over the proximal antrum?

The previously contracting terminal antrum relaxes Signup and view all the answers

What happens to the pylorus when the peristaltic wave moves over the middle of the antrum?

It opens Signup and view all the answers

How does the gastric chyme get swept into the duodenum during the emptying phase?

By the sieving effect of the antral pump Signup and view all the answers

What happens to the viscous and solid mass of the chyme during the emptying phase?

It is retained in the stomach Signup and view all the answers

What happens to the parts of the chyme that flow across the central opening of the peristaltic wave?

They are swept into the proximal antrum Signup and view all the answers

What happens during the contraction of the terminal antrum?

The pylorus closes and the transpyloric flow is stopped Signup and view all the answers

What is the result of the jet-like retropulsion during the contraction of the terminal antrum?

A forceful mixing of the chyme with grinding of particles Signup and view all the answers

What is the primary function of the peristaltic wave in the antrum?

To propel the gastric chyme through the antrum Signup and view all the answers

What happens to the chyme in the proximal antrum during the phase of emptying and mixing?

It accumulates in the relaxed proximal antrum Signup and view all the answers

Which type of muscle forms the External Sphincter around the anal canal?

Voluntary striated muscle Signup and view all the answers

What is the primary function of the rectum during defecation?

To expel feces Signup and view all the answers

What is the typical intrarectal pressure that triggers the desire for defecation?

20 to 25 cm H2O Signup and view all the answers

What is the role of the stretch receptors in the rectal wall during defecation?

To make the individual aware of the urgency of defecation Signup and view all the answers

Which nerve branch innervates the External Sphincter?

Pudendal nerve Signup and view all the answers

What is the result of increasing intra-abdominal pressure during defecation?

Relaxation of the external anal sphincter Signup and view all the answers

What is the role of the myenteric plexus in defecation?

Mediation of intrinsic defecation reflexes Signup and view all the answers

What is the effect of prolonged distension of the rectum on the internal anal sphincter?

It is transiently relaxed Signup and view all the answers

Study Notes

First Stage or Oral Stage

The first stage of deglutition involves the passage of material through the oral cavity into the pharynx, which is under voluntary control.
The contraction of the mylohyoid, styloglossus, and hypoglossus muscles causes upward and backward movements of the tongue, throwing the bolus of food back through the pillars of the fauces into the pharynx.
During this phase, mastication ceases, and respiration is inhibited reflexly.

Second Stage or Pharyngeal Stage

The second stage involves the passage of the bolus from the pharynx into the esophagus, which is a reflex process known as the swallowing reflex.
Key events in this stage include:
- The contact of food material with pharyngeal and peripharyngeal structures initiates and completes the second and third stages of deglutition.
- The soft palate is elevated, and the nasopharynx is closed off from the rest of the pharynx.
- The larynx and hyoid bone are elevated, and the vocal cords are adducted, causing a momentary stoppage of respiration and speech.
- The contraction of the superior pharyngeal constrictor induces a rapid pharyngeal peristaltic wave, which propels the bolus into the esophagus.

Peristalsis and Swallowing

Primary peristaltic contractions occur when the bolus reaches the upper part of the esophagus, propelling the bolus towards the stomach.
Pressure developed during primary peristaltic contractions is important for propelling the bolus.
Initially, the pressure becomes negative in the upper part of the esophagus due to the stretching of the closed esophagus by the elevation of the larynx.
Secondary peristaltic contractions occur if the primary peristaltic contractions are unable to propel the bolus into the stomach, pushing the bolus into the stomach.

Role of Lower Esophageal Sphincter

The distal 2-5 cm of the esophagus acts as a sphincter, known as the lower esophageal sphincter, which is constricted always.
When the bolus enters this part of the esophagus, the sphincter relaxes, allowing the contents to enter the stomach.
After the entry of the bolus into the stomach, the sphincter constricts, closing the lower end of the esophagus.

Deglutition Reflex

Though the beginning of swallowing is a voluntary act, it becomes involuntary and is carried out by a reflex action called the deglutition reflex.
The deglutition reflex occurs during the pharyngeal and esophageal stages.
Stimulus: When the bolus enters the oropharyngeal region, the receptors present in this region are stimulated, carrying parasympathetic action potentials to the stomach.

Gastric Phase

The gastric pump is characterized by peristaltic waves, which are induced by pacemaker cells in the stomach body.
The peristaltic waves are propagated from the body to the antrum, eventually closing the pyloric sphincter.
The peristaltic contraction of the antrum can be divided into three phases: propulsion, emptying and mixing, and retropulsion and grinding.
The antral pump is associated with a sieving effect, where liquids with small suspended particles are swept across the pylorus into the duodenum, while viscous and solid materials are retained in the stomach.

Intestinal Phase

Chyme in the duodenum with a pH less than 2 or containing lipids inhibits gastric secretions by three mechanisms:
- Sensory input to the medulla from the duodenum inhibits the motor input from the medulla to the stomach.
- Local reflexes inhibit gastric secretion - enterogastric reflex, secretin, and cholecystokinin produced by the duodenum decrease gastric secretions in the stomach.

Defecation

Voiding of feces is known as defecation, which involves a series of coordinated motor activities.
The rectum has two primary functions: (1) to serve as a storage site for feces and (2) to expel feces during defecation.
When the rectal capacity is exceeded, intramural stretch receptors are activated, making the individual aware of the urgency of the forthcoming event and relaxing the internal anal sphincter.
The individual can inhibit defecation by voluntarily increasing the tone of the external anal sphincter, which transiently increases internal sphincter tone.

Defecation Reflexes

Defecation reflexes are classified into two types: intrinsic and extrinsic.
Intrinsic defecation reflex is mediated through the myenteric plexus, where the distention of the rectal wall by the arriving feces initiates peristaltic waves that spread through a local circuit in the descending and sigmoid colon and the rectum.

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Description

The first stage of digestion, also known as the oral stage, involves the passage of food through the oral cavity into the pharynx, controlled by voluntary muscles. This stage includes tongue movements, mastication, and respiration.