Module 6 Section 1

Question 1

anatomy of the gastrointestinal system

Answer 1

• the GI system is a continuous series of hollow, muscular tubes that extends from the mouth to the anus, where the movement of muscles, along with the release of various secretions, allows for the digestion and absorption of food
• many organs and structures form the GI system

Question 2

anatomical structures and organs of GI system (12)

Answer 2

• nasal passages
• mouth
• salivary glands
• esophagus
• liver
• stomach
• gallbladder
• pancreas
• small intestine
• colon
• rectum
• anus

Question 3

the role of the digestive system

Answer 3

• the primary role of the digestive system is to take the food and drinks that we consume and transfer the nutrients, water and electrolytes into the body
• this is essential for body function, as the foods we eat are necessary for the continual production of ATP, as well as renewal and growth of the body’s tissues

Question 4

5 stages of nutrient breakdown

Answer 4

1. ingestion: this is the physical act of eating
2. digestion: the breakdown of food into small molecules that can be absorbed
3. absorption: the movement of the nutrients from the digestive tract into the circulatory system
4. distribution: the movement through the circulatory system for delivery to the tissues and cells
5. usage: the movement of the nutrients into cells for metabolism or anabolic processes

Question 5

4 basic digestive processes

Answer 5

• motility
• secretion
• digestion
• absorption

Question 6

motility

Answer 6

• refers to the muscular contractions that both mix and propel the contents of the digstive system
• similar to the smooth muscle cells of the walls of the circulatory system, the smooth muscle cells in the walls of the digestive system have a constant level of contraction, or tone, to prevent too much stretching of the walls and to keep pressure on the contents
• propulsive movements push the contents forward
• the rate of movement depends on the segment of the digestive system as contents are moved at an appropriate speed to allow each segment to do its job
• mixing movements help to mix the food eith digestive enzymes, and to promote absorption by making sure all of the contents eventually make contact with the walls of the digestive system

Question 7

digesitve organs or structures that use skeletal muccle rather than smooth muscle

Answer 7

• mouth
• early part of esophagus
• external anal sphincter

Question 8

secretion - exocrine

Answer 8

• along the entire digestive system are exocrine galnds that secrete digestive juices into the digestive tract lumen
• the contents of the digestive juices vary throughout the digestive system depending on regional functionm but they all contain water and electrolytes
• each section of the digestive system also secretes a unique mixture of enzymes, bile salts, and mucus
• the secretory cells of the glands get their water and raw materials from the plasma to produce their particular secretion
• on the appropriate neural or hormonal signal, the secretions re released into the lumen of the digestive tract
• normally, all of these secretions are absorbed back into the body after they have compelted thier role in digestion

Question 9

secretions - endocrine

Answer 9

in addition to exocrine secretions, there are endocrine cells in the digestive tract that secrete gastrointestinal hormones into the blood to help control motility and exocrine gland secretion

Question 10

digestion

Answer 10

• digestion is the biochemical process of the enzymatic breakdown of foods that are structurally complex into the small, absorbable units
• each of the 3 biochemical families of food (carb, proteins, fats) undergoes different processes

Question 11

carbohydrates

Answer 11

• the majority of carbs ar eingested as polysaccharides
• plant based polysaccharides are mainly starches, while meats contain the polysaccharide glycogen
• some carbs are ingested as disaccharides (two sugars) such as sucrose and lactose
• during digestion, all starch, glycogen, and disaccharides are broken down into monosacharides, mainly glucose with some fructose and galactose
• only monosaccharides can be absorbed

Question 12

fats

Answer 12

• dietary fat are usually consumed as triglycerdies
• during digestion, the triglycerides are broken down by releasing two of the fatty acids, leaving a monoglyceride, a glycerol molecule with one fatty acid molecule attached
• both the free fatty acids and the monoglyceride can be absorbed

Question 13

proteins

Answer 13

• dietary proteins come from meats, legumes, eggs, grains, and dairy products
• during digestion, proteins are broken down into single aa, as well as some small polypeptides for absorption

Question 14

absorption

Answer 14

• most absorption occurs in the small intestine, which is also where digestion is completed
• through the process of absorption, the small absorbable units produced as a result of digestion, along with water, vitamins, and electrolytes, move from the digestive tract lumen into the blood and lymph

Question 15

4 major layers of digestive tract wall

Answer 15

• mucosa
• submucosa
• muscularis externa
• serosa

Question 16

mucosa

Answer 16

• this is the innermost layer and lines the luminal surface
• its surface is highly folded to increase its surface area but this is variable
• in the esophagus, there is little folding whereas in the small intestine, where absorption occurs, there is the highest amount of folding

Question 17

submucosa

Answer 17

• this layer is thick and made of connective tissues that support the mucosa, and provides distensibility and elsaticity to the digestive tract
• it contains larger blood and lymph vessels that send smaller branches to the mucosa and to the outer layers
• it also contains the submucosal plexus, which is a network of nerves

Question 18

muscularis externa

Answer 18

• this is the smooth muscle layer that surrounds the submucosa
• it generally has two muscle layers, an inner circular layer and an outer longitudinal layer
• contraction of the inner layer decreases the lumen diameter while contraction of the longitudinal layer shortens the tube
• together, both layers provide the propulsive and mixing contractions
• between the two muscle layers is another nerve network, the myentric plexus, along with hormones and local chemical mediators, help to regulate gut activity

Question 19

serosa

Answer 19

• this is the outer connective tissue layer that covers the digestive tract
• throughout most of the digestive tract, the serosa is continuous with the mesentery
• this helps to suspend the digestive organs from the inner wall of the abdominal cavity, yet allows them to move for mixing and propulsion

Question 20

defining characteristics of layers of digestive tract wall

Answer 20

Question 21

regulation of digestive function

Answer 21

• digestive motility and secretion are regulated in order to maximize digestion and absorption of ingested food
• there are 4 factors that regulate digestive system function:
  1. autonomous smooth muscle function
  2. intrinsic nervous plexuses
  3. extrinsic autonomic nerves
  4. GI hormones

Question 22

intrinsic nerve plexuses

Answer 22

• also called the enteric nervous system(ENS), the intrisic nerve plexuses are the submucosal plexus and the myentric plexus
• although the primary purpose of the ENS is self-regulation, it does receieve significant input from the autonomic nervous system
• the ENS is involved in all aspects of the digestive tract function
• there are sensory neurons that recieve information on local stimuli within the digestive tract
• other nerves will innnervate smooth muscle cells, exocrine glands, and endocrine glands to affect motility and secretion of digestive juices
• like the central nervous system, neurons of the ENS are connected by interneurons

Question 23

what would occur if a large piece of food was stuck in the esophagus

Answer 23

• sensory receptors within the enteric nervous system would detect this blockage
• these sensory neurons would relay their signal to interneurons, which would coordinate a response in local motor neurons to induce contraction of smooth muscle cells, thus increasing motility to push food forward

Question 24

extrinsic autonomic nerves

Answer 24

• these are the nerve fibres from both the sympathetic and parasympathetic nervous systems that innervate various digestive organs
• the ANS influences motility and secretion by modifying the activity of the enteric nervous system, regulating GI hormone secretion, and some direct innervation on smooth muscle and glands
• the sympathetic system tends to slow down the digestive tract conditions and secretions whereas the parasympathetic system increases smooth muscle cell contractility and promotes secretion of digestive juices and hormones
• unique to the parasympathetic nerve supply of the digestive system, the postganglionic parasympathetic nerve fibres are actually part of the intrinsic nervous system

Question 25

gastrointestinal hormones

Answer 25

- in the mucosa of some parts of the digestive tract are endocrine glands which release their hormones into the blood where they are carried to other parts of the digestive system - they can have either excitatory or inhibitory actions on motility and exocrine gland secretions - many of the gastrointestinal hormones are found in other locations throughout the body such as the brain - in the brain they function as NT and neuromodulators

Question 26

autonomous smooth muscle function

Answer 26

- some smooth muscle cells have pacemaker-like rhythmic, spontaneous changes in membrane potnetial - this type of electrical acitvity is called slow-wave potentials and originates from the non-contractile interstitial cells of Cajal - the interstitial cells of Cajal (ICC) are located at the boundary of the circular longitudinal muscle layers - the slow-waves themseleves are not action potentials in that they cannot induce contractions

Question 27

control of smooth muscle contraction

Answer 27

- the membrane potential of the smooth muscle cells oscillates moving closer to threshold then further from threshold - if these waves reach threshold at the peak of deplarization then they trigger a series of action potentials that cause contraction - because smooth muscle cells are connection by gap junctions, slow-wave potnetials can move between the cells and allow a sheet of smooth muscle cells to act as a functionla syncytium and contract as a unit

Question 28

what determines whether an action potential is initiatied

Answer 28

- mechanical, neural, and hormonal factors modulate the membrane potential around which the slow-waves oscillate - if the starting membrane potential is closer to threshold, then the peak of the oscillations can trigger an action potential - this is what happens when food is in the digestive tract, so there is increased muscle acitvity for mixing and propulsion