Overview of GI Function

Question 1

Explain Splanchnic Circulation

Answer 1

Blood from the gut, spleen and pancreas flow to the liver via the hepatic portal vein. The blood is filtered by the liver sinuses and leaves via the hepatic vein into the vena cava.

Question 2

What do reticuloendothelial cells do?

Answer 2

Remove bacteria and particulate matter from the blood that may have entered in the GI tract. Prevents direct transport of potentially harmful pathogens into the body.

Question 3

What can increase blood flow in the GIT?

Answer 3

• vasodilators released from GI mucosa (cholecystokinin, vasoactive intestinal peptide, gastrin, secretin)
• vasodilators released from GI glands (bradykinin)
• decreased oxygen conc in gut wall due to increased metabolism.
• subsequent adenosine release

Question 4

Parasympathetic Innervation of GIT?

Answer 4

Vagus, Pelvic and Splanchnic Nerves - increases blood flow and glandular secretion - affects stomach and lower colon.

Question 5

Sympathetic innervation of the GIT?

Answer 5

Celiac, Superior Mesenteric, Inferior Mesenteric and Hypogastric Plexuses - vasoconstriction of arterioles, happens during exercise or circulatory shock.

Question 6

What is the Enteric Nervous System?

Answer 6

Part of the nervous system which can regulate and control movement and secretion of the gut on its own without autonomic input. Consists of sensory, inter, and motor neurons but is mainly concerned with motor control. Myenteric Plexus lies between circular and longitudinal smooth muscle layers. Submucosal plexus lies between the smooth muscle layer and mucosa, innervates glandular epithelium, endocrine glands, and blood vessels and controls GIT secretion and local blood flow.

Question 7

GIT Smooth Muscle Synctium

Answer 7

Smooth muscle fibres are connected by gap junctions and adherens junctions. The actin and myosin filaments are diagonally aligned along a long axis. The filaments are anchored at dense bodies (alpha-actinin-rich) throughout the sarcoplasm. Contractile arrays anchored to sarcolemma by dense plaques.

Question 8

Where is the continual slow intrinsic electrical activity of the GIT initiated?

Answer 8

Interstitial Cells of Cajal

Question 9

How does slow wave potential affect spike potentials?

Answer 9

Higher the slow wave potential, then greater frequency of spike potentials.

Question 10

Why does GIT’s electrical potential fluctuate?

Answer 10

Mainly due to opening of calcium ion channels leading to influx of Ca2+ into the cell and then terminated by K+ efflux.

Question 11

Fed State GIT Motility?

Answer 11

During/after meal. Peristalsis and Mixing Movements.

Question 12

Interdigestive State GIT Motility?

Answer 12

Between meals. Clears out gut to make room for next meal. Migrating Motor Complexes.

Question 13

Peristalsis

Answer 13

automatic reflex response to stretch of GIT wall. Occurs during fed state in all parts of the GIT. Moves oral-rectal direction.Needs an active myenteric plexus. Can be reversed e.g. vomiting.

stretch initiates circular muscle behind bolus to contract it (ascending excitatory reflex) and circular muscle in front of bolus to relax (descending inhibitory reflex).

Question 14

Segmentation

Answer 14

Mixes digested food. Contraction of circular muscle layer at short intervals along GIT, pushes food back and forth to mix.

Question 15

Migrating Motor Complexes (MMC)

Answer 15

Occurs during periods of fasting or between meals. Clears the stomach and small intestine of debris and bacteria. Consists of 4 phases:

• 1- 45-60mins - period of slow waves and inactivity
• 2- 30-35mins - action potential activity on slow waves but sporadic contractions of circular muscle.
• 3- 2-12mins - action potential activity on slow waves but regular contractions of circular muscle.
• 4- period in which action potential activity and contraction reduce and merge into phase 1.