5: Digestive System Flashcards
4 basic processes of DIGESTION
- motility
- secretion
- digestion
- absorption
motility
muscular contraction that MIX and MOVE FORWARD the contents of the tract
secretion
secretion of digestive juices by DIGESTIVE EXOCRINE GLAND CELLS
location of digestive endocrine gland cells
lining the digestive tract AND in acessory digestive organs
type of cell
specialised epithelial cells
mechanism of action
secrete also a range of signal proteins classified either as GI HORMONE or GI PEPTIDES –> enter blood –> carried to target organs, regulating digestive function
digestion
the breakdown of starch, glycogen and disaccharides into their constituent monosaccharides
absorption
transfer of vitamin, electrolytes, nutrients and some water from digestive tract lumen into the blood OR lymph
what makes up the digestive system
- digestive tract
2. accessory digestive organs
accessory digestive organs
1.the biliary system [liver and gallbladder]
2.exocrine pancreas
3.salivary glands
they lie outside the digestive tract but empty their contents into the digestive tract
length of digestive tract
4.5 m in its NORMAL CONTRACTILE STATE
organs comprising the digestive TRACT
- mouth
- pharynx (throat)
- oesophagus
- stomach
- small intestine (DJI - duodenum, jejunum, ileum)
- large intestine [CACR - cecum, appendix, colon, rectum]
- anus
why these considered separate entities despite being continous w one another
because of their regional modifications allowing them to carry out their special function
when is a substance considered INSIDE the body
only after it moves from the lumen of digestive tract into absorptive epithelial cells (everything IN the DT is considered outside the body as the DT is continous w the external environment)
4 layers of digestive tract from innermost layer outward
- mucosa
- submucosa
- muscularis externa
- SEROSA
1.mucosa’s innermost layer (1st)
mucous membrane
- serves as protective surface
- epithelial layer
- modified in some areas for absorption/secretion
- contains exocrine gland cells for secretion of DIGESTIVE JUICES
- contains endocrine gland cells for secretion of blood-borne GI hormones
- contains epithelial cells specialised in ABSORBING nutrients
1.mucosa’s middle layer
lamina propria
- thin middle layer of connective tissue
- contains GALT which is important in defence against disease.
- GALT = gut-associated lymphoid tissue
1.mucosa’s 3rd layer
muscularis mucosa
- outermost layer of smooth muscle
- highly folded in some parts of the GIT (eg. small intestine) for greater surface area for absorption
- pattern of surface folding can be modified by contraction
2.Submucosa
- thick layer
- connective tissue
- contains submucosal nerve plexus
- contains large blood and lymph vessels
- provides distensibility and elasticity
- Muscularis Externa
MAJOR smooth muscle coat of the GIT
two layers
1.Inner Circular Layer
2.Outer Longitudinal Layer
Inner circular layer
muscle fibres running circularly around the tube
contraction = decrease in diameter of tube at poin of constriction
Outer longitudinal layer
muscle fibres running longitudinally along the length of the tube
contraction = shortening of GIT
combined effects of contraction of these two
creates propulsive and mixing movements of GIT
Myenteric Nerve Plexus
Between the inner circular & outer longitudinal layer.
-help regulate local gut activity
- Serosa
- outer connective tissue layer (covering)
- secretes SEROUS fluid -lubricates &prevents friction of GIT w neighbouring organs
- throughout much of the tract, SEROSA is continuous w Mesentry (suspends digestive organs from inner wall of abdominal cavity like a sling)
4 factors involved in REGULATION of Digestive System Function
- Autonomous SMOOTH MUSCLE function
- intrinsic nerve plexuses
- extrinsic nerves
- GI hormones
1.Autonomous Smooth Muscle Function
- SLOW-WAVE POTENTIALS: self-induced electrical activity in digestive smooth muscle
- by pacemaker cells “Interstitial Cells of Cajal”
- located in the layers of MUSCULARIS EXTERNA
- slow waves are NOT action potentials, only induce rhythmic cycles of depolarisation and repolarisation close to the threshold potential.
- if waves REACH threshold potential at peak of depolarization–> rhythmic cycles of MUSCLE CONTRACTION
another name for Slow-wave Potentials
BER (Basic Electrical Rhythm) of the digestive tract
2.Intrinsic Nerve Plexuses
consists of:
1. The Submucosal Plexus
2. The Myenteric Plexus
collectively, referred to as ENTERIC NERVOUS SYSTEM. Has a considerable degree of self-regulation.
-the only body system to have its own INTRAMURAL NERVOUS SYSTEM
Intrinsic Nerve Plexus subdivision
Intrinsic Afferent Neurons (respond to local stimuli in digestive tract)
Intrinsic Efferent Neurons: control smooth muscle.
-endocrine and exocrine cells of digestive tract
Intrinsic EFFERENT Neurons affect…
- digestive tract motility
- secretion digestive juices
- secretion GI hormones
- LOCALLY
eg. if large piece food stuck in oesophagus, local response to push the food forward
- Extrinsic Nerves
- formed from both branches of the autonomic NS (Sympathetic Division and Parasympathetic Division)
- originate outside the digestive tract
Extrinsic Nerves affect…
- activity of intrinsic nerve plexuses
- level of GI hormone secretion
- act directly on Smooth Muscle AND glands
Sympathetic division effect…
- dominates in “fight or flight” situations-inhibit /slow down digestive tract CONTRACTION and SECRETION
- digestive processes not of highest priority when faced w emergency situation
Parasympathetic division effect…
- dominates in quiet “rest and digest” situations
- promotes smooth muscle motility
- promotes secretion of digestive enzymes & hormones
role of vagus nerve reflexes before food gets in the stomach (extrinsic nerve)
increases salivary secretion, stomach, pancreatic and liver secretion in anticipation of arrival of food
4.GI Hormones
inhibitory or excitatory influences on digestive smooth muscle AND exocrine glands
-GI hormones produced by SPECIALISED ENDOCRINE cells in certain regions of the mucosa
3 types of SENSORY RECEPTORS to LOCAL changes in GIT
- Chemoreceptors
- sensitive to chemical components within the lumen - Mechanoreceptors
- stretch or tension within the wall - Osmoreceptors
- sensitive to osmolarity of luminal contents
what does stimulation of either of these elicit
a. neural reflexes
b. secretion of hormones
