Principles of Viscera

Question 1

What are viscera?

Answer 1

• Organs that regulate the internal environment
• Occupy cavities in the body: thorax. Abdomen, pelvis – also in the head & neck
• Involved in secretion, excretion, digestion, absorption

Question 2

Mention the two types of viscera.

Answer 2

Hollow (Tubular) and Solid Viscera

Question 3

Mention the characteristics of hollow viscera

Answer 3

Hollow viscera are usually tubular, with a cylindrical wall and a lumen. It is enclosed in three layers:

• Serosa (external layer)
• Muscularis (middle layer)
• Mucosa (internal layer)

Question 4

Describe serosa layer (serous membrane)

Answer 4

It’s subdivided into parietal and visceral layer of the serosa. THe parietal layer lines the body wall and receives nerve and blood supply from body wall.

The visceral layer covers the external layer of hollow viscera an receives the same nerve and blood supply as the viscera. Its main function is to minimize friction through the production of fluid that lubricates the cavity between the layer.

Question 5

Describe the muscularis layer.

Answer 5

The middle layer of hollow viscera. It contains circular and longitudinal smooth muscles, providing motility (e.g. peristalsis) and expansion (e.g. stomach food storage) respectively.

Question 6

Describe the muscosa layer.

Answer 6

The innermost layer of a hollow viscera. May have folds that increase the surface area for absorption (such as villi of the small intestine)

Question 7

Describe the features of the lumen of hollow viscera.

Answer 7

Hollow viscera may have a sphincter - which may either be anatomical or functional sphincter.

• Anatomical sphincter controls the passage and prevents the reflux of contents. This is achieved through the localised muscle thickening around the wall of tubular viscus usually at distal ends of ducts, near an external orifice
• Functional Sphincter differs as it has no localised muscular thickening. Its function is dependent on another structure’s function and movement

Question 8

Anatomical sphincter can be either _______ or ________. Explain the difference.

Answer 8

• Involuntary Sphincter:
  
  • Made of smooth muscle, innervated by autonomic NS
  • 1^st line of defence
• Voluntary Sphincter
  
  • Made of skeletal muscle, under voluntary control
  • Supplied by somatic NS
  • 2^nd line of defence

Question 9

Describe the features of a solid viscus.

Answer 9

Usually a gland, a cluster of secretory (or excretory) cells. I thas an external capsule (for protection) and serosa (reducing friction). It may have subdivision:

• outer cortex
• inner medulla
  *

Question 10

Mention the types of solid viscus.

Answer 10

• Exocrine gland
  
  • Glands that secrete into a duct or systems of ducts
  • Ducts emerge from hilum of viscera
  • Open into hollow viscera
  • E.g. liver and bile duct

• Endocrine gland
  
  • Glands that secrete directly into the bloodstream
  • Usually, secrete hormones that act in other parts of the body
  • Rich blood supply
  • e.g. adrenal gland

Question 11

Explain how the two layers of serosa arise.

Answer 11

Early in development, viscera develop in body wall behind the cavity. As the cavity enlarges, the serous membrane surrounding the space starts to engulf the viscera. This leads to formation of a double-layered membrane: where one covers the body wall and the other the viscera.

Question 12

Describe the peritoneal cavity.

Answer 12

Peritoneal Cavity refers to a closed cavity in the abdomen lined by a serous membrane called peritoneum. Abdominal viscera are suspended in the body cavity by a mesentery. During development, alternating parts of the gut retain or lose a mesentery along their course.

Question 13

Viscera suspended on the end of a mesentery are in danger of _______.

Answer 13

Twisting (torsion)

Question 14

What is paired viscera?

Answer 14

• Paired Viscera
  
  • Each develops and are located on one side of the body
  • Examples:
    
    • Ejaculatory ducts
    • Ureter
    • Kidneys
  • Receive nerve and blood supply for each site

Question 15

What is referred pain?

Answer 15

• Referred Pain
  
  • perception of pain in areas other than the site of stimulation, usually due to a common spinal segmental origin of the nerves to the site of stimulation and the site to which the pain is referred
  • probably due to convergence of the two inputs to the same population of neurons at a given spinal segmental level
  • Visceral pain is usually referred to somatic regions.

Question 16

What is unpaired viscera?

Answer 16

• Unpaired Viscera
  
  • Develop in the midline but migrate away from it
  • e.g. GI tract + associated glands and ducts
  • Nerve supply from both sides of spinal cord

Question 17

How does referred pain differ between paired and unpaired viscera?

Answer 17

For paired viscera, referred pain is isolated to one side of the body as each of the viscera has its own independent nerve supply. For unpaired viscera, since it has nerve supply from both sides of the spinal cord, the visceral pain will be referred to the midline (origin of viscus). The pain stimuli reach both sides of the spinal cord simultaneously.