Digestive system overview

Question 1

1. …Discuss the role of the intrinsic (enteric) nervous system in gut motility and the advantage of having extrinsic nervous system input.

Answer 1

The enteric nervous system is subdivision of the ANS located in the walls of the GI tract: made of two interconnected plexuses, the Meissners Plexuses of the
submucosa and the Auerbach Plexuses between the 2 layers of the muscularis externa. These plexuses
are made of sensory neurons, motor neurons and interneurons plus pre-ganglionic parasympathetic and
post-ganglionic sympathetic fibers. These neurons of the enteric nervous system are able to cause contraction
and give rise to local peristaltic waves without CNS input. Peristalsis can happen based only on local
changes of Meissner’s Plexuses because these plexuses have reflex action: sensory neuron senses
food and reacts with contraction (or secretion) locally. However, CNS input is required for smooth and
coordinated peristalsis throughout the entire GI tract. Additionally, modulation of motility requires CNS action.

Question 2

2. …Describe the structural properties of smooth muscle in the gut that contribute to its performance as a single functional unit and what factors influence the behavior of smooth muscle in the gut.

Answer 2

Smooth muscle cells are connected mechanically and also via gap junctions. This allows cell-to-cell communication of electrical, ion and small molecule signals. Additionally, smooth muscle of the gut is innervated by autonomic nerves which are capable of innervating many muscle cells from one axon. This allows gut smooth muscle to act as a single unit. Pacemaker cells of the gut depolarize
spontaneously and are capable of firing action potentials without nervous influence, and since these cells
are all connected, the AP can result in contraction of many muscles at once. This gives rise to the “tone” of the smooth muscle of the gut.
Smooth muscle tone can be modulated by the autonomic nervous system and neurotransmitters to either
increase contraction and generate larger tension forces or relax to allow food to move through.

Question 3

3. …Identify the four layers of the GI tract wall and the basic structural components of each.

Answer 3

1

Question 4

4. Discuss the role of sphincters along with their location in the GI tract.

Answer 4

1

Question 5

5. …How does the digestive system protect itself from auto-digestion and pathogenic bacterial invasion?

Answer 5

bacterial peptides and mucous layer

Question 6

6. Understand the 4 modes of communication in the GI tract: endocrine, paracrine, neurocrine and immune (juxtacrine).

Answer 6

1

Question 7

smooth muscle contraction review

Answer 7

Smooth muscle cells of the gut use actin and myosin to contract just like skeletal muscle. ca2+ enters the cell due either to influx via VSCC or the sarcoplasmic reticulum and binds to calmodulin. Binding of Ca to calmodulin allows for the activation of myosin light chain kinase (MLCK)
which phosphorylates myosin allowing crossbridge cycling. Smooth muscle cells, unlike skeletal muscle
cells, can maintain contraction for long periods without use of ATP. This is called the “latch state” and is
achieved by maintaining high intracellular calcium concentrations.