colonic phase Flashcards
Function of colon
Absorption of water. Absorption of ions. Storage.
Colon is separated from the small intestine by the………….. sphincter. Once beyond this sphincter chyme is called fecal material!
ileocecal
The colon is subdivided into multiple anatomical zones. Along the length of the colon are 3 bands of longitudinal muscle called……………
taeniae coli
Rectum function is storage of feces until it is appropriate to defecate. The rectum lacks………….. muscle. When………………. muscle contracts, rectal valves contract to aid in keeping feces in the rectum.
circular longitudinal
The anal canal is surrounded by smooth muscle forming the internal sphincter. This is continuous with musculature of the rectum. Around the internal sphincter is the external sphincter which is made of…………. muscle.
skeletal
Colon exhibits what two forms of mixing motility?
haustral contraction and short-duration contraction
Mass movement propels feces caudally. The………………….. reflex enables packing of feces until appropriate time to defecate.
rectoanal
Describe haustral contraction:
When the taeniae coli contract, the colon is squished into a series of bulging pouches called haustra. This is Long-duration (~60 sec) contraction of circular muscle, which kneads the feces to extract salt and water. Contraction moves the fecal material both retrograde and anterograde!
Describe short-duration contraction:
Short-duration contraction (~8 sec) are contractions of circular muscle that also serve to mix feces.
What system regulates haustral and short-duration contractions?
Both are locally regulated by the enteric system.
Mass movement is a propulsive movement of fecal material toward the anus occurs 1-3 times daily. The…….. frequency, ………..amplitude contraction keeps fecal material in the colon for an extended period of about 48 hrs.
low high
Muscle contraction in Mass movement is very large with wide bands of muscle strips involved in…………………. Each band is contract for an extended period of time.
cleaning sections of the colon fully
Are slow waves involved in Mass movement?
No
What induces mass movement?
Locally coordinated by the enteric nervous system for the most part. It can also be induced by long reflexes!
Motility is modulated by what?
The ANS.
………………………. stimulation stops movement. Stimulation of the vagal nerve induces greater motility in the……….. colon. Stimulation of the pelvic nerves does the same to the………….. colon.
Sympathetic proximal distal
How do long reflexes influence mass movement?
- Orthocolic reflex is activated upon waking, leading to an urge to defecate in the morning. - Gastrocolic reflex, a long reflex, increases mass movement frequency in the colon, particularly in the sigmoid colon. Distention of the stomach as well as digestive products elicit gastrin release that induce movement. Additionally, CCK induces this movement. This give rise to the urge to defecate following a meal. - Colonocolonic reflex: contraction of one part of the colon induces relaxation of another.
Describe colonocolonic reflex
When contraction of one part of the colon induces relaxation of another.
Which reflex is activated upon waking, leading to an urge to defecate in the morning?
Orthocolic reflex
The rectoanal reflex is responsible for what?
It prevents leakage and optimizes defecation to a point where the rectum has collected stool, and the time is appropriate.
Describe the rectoanal reflex in detail:
- Normally the rectum contracts more frequently than the sigmoid colon and fecal matter is pushed retrogradely.
- Filling of rectum via mass movement transport relaxes the smooth muscle of the internal anal sphincter via VIP & NO from enteric inhibitory motor neurons (this is a local reflex).
- The external sphincter responds oppositely with increased rectal pressure increasing its tone.
- This rectoanal reflex prevents leakage and optimizes defecation to a point where the rectum has collected stool and to an appropriate time.
Very important! What kind of control is the gastrocolic reflex?
Also, what is it due to?
Hormonal control AND a long reflex loop
Gastrin & CCK
The rectum is not heavily innervated by sensory neurons, but the anus is! What does this mean?
The anus can distinguish between gaseous, solid, and liquids, and expulsion is regulated appropriately.
Voluntary opening of the external sphincter is accompanied by……………. which changes/straightens the intestine. Furthermore, relaxation of the……………. muscle reduces the rectoanal angle.
squatting
puborectalis
in pic: 1 = puborectalis muscle
Describe voluntary expulsion of flatus (gas):
the puborectalis muscle does not relax and there is no change in the rectoanalangle, and thus gas can pass without loss of feces.
Additionally, defecation may be accompanied by a deep breath and exhalation against the………. glottis. This increases abdominal pressure.
What else can further increase pressure?
closed
Abdominal muscles can be contracted to further increase pressure.
What release peptide YY?
Enterendocrine cells of the colon
Enterendocrine cells of the colon release peptide YY in response to what?
Detection of unabsorbed nutrients particularly fat in the fecal material.
What does peptide YY do?
- This hormone slows small intestine motility.
- This reflex is called the ileal break due to its function to give more time for nutrients to be absorbed by the intestine.
Goblet cells in the colonic crypts secrete…………
mucus
Enterocytes within the crypt secret……………via………….., and water follows.
What is the function of this?
Cl- via CFTR
Function is to induce movement of fluids in crypt and avoid stagnation.
When it comes to NaCl absorption in the proximal colon, Na+ absorption is mediated by……………….. while Cl- absorption is mediated by…………………
Operation of the two results in maintenance of pH and electroneutrality. Export of both on the…………………. side draws water.
Na/H exchange
Cl-/bicarbonate exchange
basolateral
Colonic bacteria convert residual dietary carbohydrates not absorbed in the small intestine into short chain fatty acids (SCFA). Then what happens?
- SCFA is pumped into enterocytes via SMCT1.
- SCFA feed enterocytes.
The ENaC Na+ transporter is what brings Na+ into the cell (absorption). Describe this process.
- ENaC is regulated by aldosterone with acute high plasma K+ inducing release of aldosterone and insertion of ENaC. Consequent increase Na transport leaves the lumen negative driving higher secretion of K through apical K channels.
- Na/K pump transports Na+ out of cell on basolateral side.
- EMF moves Cl- paracellularly.
- Water follows.
- Mechanism active in distal portion of colon.
