Lecture 24 - Introduction to physiology and motility

Question 1

Functions of the GI tract

Answer 1

Obtain nutrients required for growth and energy needs

Replace foods and salts lost in the urine and faeces, from sweating and form breathing

Question 2

Flow chart

Answer 2

Food ingested - mechanical digestion which is essential for chemical digestion which is essential for absorption and then give off the material we do not need as waste that is excreted

CNS/ENS and hormones regulates motility and regulates secretions. Motility carries out mechanical digestion and aids in chemical digestion and absorption. Secretion is essential for chemical digestion but also aids in mechanical digestion and aids in absorption

Question 3

Basic principles of the regulation of GI function

Answer 3

Regulate conditions in intestinal lumen
Designed to control the composition of the luminal contents and this regulation of conditions creates optimal conditions for digestion and absorption in the lumen which is carried out by receptors in the GI tract that are stimulated to tell it there is food present.

Receptors in the wall of the GI tract:
Respond to stretch (food in the lumen) and change in composition (such as pH, osmolarity, amino acids, sugars and fats)

Effectors are smooth muscle and glands:
Reflexes stimulated by receptors stimulate - smooth muscle contraction, gland secretion

Question 4

Nervous regulation of GI function - CNS

Answer 4

Coordinates activity over long distances
Thinking about food, making food or actually eating it - the CNS is stimulating the stomach so it can prepare for the arrival of food

Parasympathetic nervous system - stimulates motility and secretion

Sympathetic nervous system - inhibits motility and secretion

Modulates activity of enteric nervous system
CNS does not directly innervate the gastrointestinal system and what it actually does is modulate the activity of the enteric nervous system which does directly innervate the smooth muscle cells of the GI system etc.

Question 5

Nervous regulation of GI function - ENS

Answer 5

Enteric nervous system
Submucosal plexus - regulation of secretion - just below epithelial layer
Myenteric plexus - regulation of motility - amongst smooth muscle cells and regulates the contraction of the smooth muscle cells

Involved in local reflexes - peristalsis and segmentation i.e. the ENS controls things over small distances

Totally self contained - can control the activities locally without any hormonal or CNS input

Question 6

Hormonal regulation of GI function

Answer 6

GI tract is the largest endocrine organ in the body

Endocrine and paracrine function - some of them are true endocrine secreting cells which secrete there hormones into the blood and arrives at its targets and initiates it’s response whereas some of the cells are paracrine in terms of their function and secrete hormones into the interstitial fluid and it acts on the local environment just on the surrounding cells

Critical hormones 
Gastrin 
Gastric inhibitory hormone (GIP) 
Secretin 
Cholecystokinin

Question 7

Functions of motility in the GI tract

Answer 7

Moving it through and breaking it up

Movement at a controlled rate - a rate that allows digestion and absorption to occur
Propulsion
Storage
Aids in chemical digestion and absorption

Mechanical digestion - break food down into particles
Increase in surface area
Essential for chemical digestion - provides more surface area for the enzymes to act on

Mixing - mix it with digestive enzymes and other secretions
Aids in chemical digestion

Exposure to absorptive surfaces
Aids absorption
Once chemical digestion has occurred, the products are rapidly transported to the absorptive surfaces so that they can be absorbed

Question 8

Basis of GI motility

Answer 8

There is some skeletal muscle in the mouth for example but majority of the GI tract is smooth muscle and this muscle drives motility

Properties
Spontaneously active - contracts without external input
Frequency of contraction property of region - stomach 3 per min, duodenum 12 per min, illeum 9 per min
Strength of contraction regulated by nervous and hormonal output

Question 9

Generalised motility patterns - fasting

Answer 9

Migrating motor complex
4 hours after ever meal
1 hour of doing virtually nothing, 50 mins of uncoordinated motility and 10 mins of quite coordinated motility
Repeats every 2 hours until you eat again - sweeping out undigested food
Housekeeping

Question 10

Generalised motility patterns - feeding

Answer 10

Storage
Stomach and colon
Relaxation of smooth muscles allows volume to increase without change in pressure

Propulsion
Esophagus, stomach, small and large intestine
Perstalisis

Mixing
Stomach - reteropulsion
Small and large intestine - segmentation

Propulsion/peristalsis = series of wave like muscle contractions behind the bolus of food, forcing it through the GI tract (esophagus, stomach, small intestine and large intestine). Proximal contraction with distal relaxation 
Segmentation/mixing = occurs in only in the small intestine and the large intestine. Contraction and relaxation of circular muscle in order to mix the food and allow for contact with the surface of the intestine 
Storage = mainly occurs in the stomach where there is relaxation of the smooth muscle = distension

Question 11

Peristalsis

Answer 11

series of wave like muscle contractions behind the bolus of food, forcing it through the GI tract (esophagus, stomach, small intestine and large intestine). Proximal contraction with distal relaxation

It is like squeezing out toothpaste

Question 12

Segmentation

Answer 12

This is how mixing occurs in the small intestine, sloshes food backwards and forwards within the lump and mixing it with the secretions of the associated glands, alternate contraction and relaxation connected to the GI tract

Contraction and relaxation of circular muscle in order to mix the food and allow for contact with the surface of the intestine

Question 13

Chewing

Answer 13

Reducing size of food - important for swallowing and starts off mechanical digestion

Allows ingestion and reduces size
Mixes with saliva - taste which is important to determine if we should be eating something or not
Voluntary - skeletal muscle
Reflex control of strength, frequency, side
Create a bolus that is then swallowed

Question 14

Swallowing

Answer 14

Rapid transfer of material from mouth to stomach

Initated at will, proceeds reflexly - you can’t stop it once it has started

Question 15

Gastric motility - functions

Answer 15

Storage
Fundus and body of stomach

Mechanical digestion
Antrum - creates surface area for chemical digestion to act on
The stomach is the most important site for mechanical digestion and it involves muscular contractions in the antrum regions of the stomach which also carry out mixing

Mixing
Antrum - once the food is mixed with the secretions it is known as chyme

Controlled delivery to duodenum
Pyloric sphincter

Deliver food material to the stomach very rapidly, it is stored within the funds and body and then it is processed by the stomach in the antrum and then delivered to the small intestine at a rate at which the small intestine can handle it

Question 16

Fasting gastric motility

Answer 16

The stomach shrinks to a small volume and the migratory motor complex occurs

Shrinks to approximately 50mL in volume

Migrating motor complex 
Occurs 4 hours after a meal 
Continues until you eat 
One hour of inactivity 
50 minutes of uncoordinated activity 
10 minutes of coordinated activity 

Functions
Housekeeping - residual secretions and undigested materials

Question 17

Feeding gastric motility

Answer 17

Storage
Receptive relaxation and gastric accommodation - nervous regulation via vagus nerve
Increase in volume with minimal change in pressure - so it can accommodate the arrival of food through relaxation in smooth muscle without the increase in pressure
Fundus and body - increase in volume primarily occurs here because they are the sit of storage of the food material

Perstalsis 
Initiated on greater curvature and spreads to antrum (and then spreads to the pyloric sphincters) 
3 contractions per min 
First 60 min following meal is gentle 
60-300 min more intense activity 

Receptive relaxation - involves receptors that respond to stretch in the lower esophagus and the passage of food down the esophagus as you swallow and stimulates these receptors and by the vagus nerve going up to the CNS and back to the stomach relaxes the smooth muscle upon the arrival of food

Gastric accommodation - it responds to stretch receptors in the body of the stomach and these signals go up the vagus nerve and back down again and relax the smooth muscle and does this through inhibiting the activity of the ENS not inhibiting the activity of the smooth muscle cells themselves

Perstalsis in stomach
Propulsion
Retropulsion
Combined with pyloric sphincter - mechanical breakdown and mixing

Question 18

Gastric emptying

Answer 18

Rate matches digestive capacity of intestine

Regulated by feedback from the duodenum

Factors affecting gastric emptying…
Size of meal - larger the meal faster the emptying
Composition of meal - fluids are faster than solids, related to role of stomach in digestion which is to cause a reduction in size of food particles and increase the surface area of food, fats slow gastric emptying and fats are difficult to digest

Question 19

Small intestine motility

Answer 19

Function
Mixing with secretions from pancreas, biliary system (liver and gallbladder) and intestine
Controlled movement
Exposure of products of digestion to absorptive surfaces

Motility patterns
Between meals - migrating motor complex
After meal - segmentation for mixing and exposure to absorptive surfaces (contributes to movement also), limited peristalsis (in humans) for movement

Question 20

Colonic or large intestinal motility

Answer 20

Storage of faeces

Large periods of inactivity

Segmentation
Exposure to absorptive surfaces
Absorption of fluid and salt from faeces

Mass movement
1-2 times a day following meals
Perstaltic wave - drives faeces into rectum, intiates defecation