physiology of the large intestine Flashcards
what does the large intestine comprise of
>caecum >appendix >colon - ascending, hepatic flexure, transverse, splenic flexure descending and sigmoid >rectum >anal canal and anus
large intestine is lined with longitudinal smooth muscle layer in the caecum and colon - true or false
true
what is the taeniae coli
the three strands of smooth muscle layer in the caecum and the colon
muscle layers at the:
internal anal sphincter
> smooth muscle is thickened
>surrounded by skeletal muscle of the external anal sphincter
what is the function of the taeniae coli and circular muscle layers in the colon
> they cause ‘sac-like’ bulges - the haustra
how does material travel from small intestine to large intestine
from the terminal ileum to the caecum via the gastroileal reflex
gastroileal reflex
> responds to gastrin and CCK, the presence of foodstuff in the stomach and gastric peristalsis
initiation of the reflex causes peristalsis in the ileum and opens the ileocecal valve
does the ileocaecal valve do
> maintains a positive resting pressure
>relaxing in response to distension of the duodenum
>contracting in response to distension of the ascending colon
being under the control of the vagus nerve, sympathetic nerves, enteric neurones and hormonal signals
what is the appendix
a blind-ended tube with extensive lymphoid tissue connected to the distal caecum via the appendices orifice that may be obstructed by faecalith, potentially causing appendicitis
primary functions of the colon
> absorption of sodium chloride and water to condense ileocaecal material to solid, or semi-solid stool
absorption of short chain fatty acids
Secretion of potassium, bicarbonate and mucus
reservoir -storage of colonic contents
periodic elimination of faeces
fluid and electrolyte movement in the colon
> possesses colonic folds, crypts and microvilli that increases surface area
-surface epithelial cells (colonocytes) mediate electrolyte absorption which drives absorption of water
crypt cells mediate ion secretion
goblet cells secrete -copious mucus containing glucosamineglycans - hydrated to form a slippery surface gel
trefoil proteins involved in host defence
what are the 3 main patterns of motility in the large intestine
- haustration
- peristaltic propulsive movements
- defaecation
what is haustration
-non-propulsive segmentation
>haustra are saccule caused by alternate contraction of the circular muscle
-disappear before and reappear after a mass movement
>generated by slow wave activity
>mixes content -allows time for fluid and electrolyte reabsorption
what is the benefit of mass movement in digestion
> mass movement is simultaneous contraction of large sections of the circulate muscle of the ascent and transfer colon and it powerfully drives faeces into distal regions
triggered by the gastrocolic response involving gastrin and extrinsic nerve plexuses
> > mass movement in the distal colon propels faeces in to the rectum triggering the defection reflex in response to rectal stretch
defaecation
> mass movement - the rectum fills with faecal matter > this activates the rectal stretch receptors > this leads to the activation of afferents to the brain and also leads to the activation of afferents to the spinal cord»_space; this respectively leads to the altered firing in efferent to the spinal cord and also the activation of parasympathetic efferents
> > > leads to the contraction of smooth muscle of the sigmoid colon and rectum - the internal anal sphincter relaxes this leads to either
1) relaxation of skeletal muscle of external anal sphincter - defection assisted straightening of the anorectal angle, abdominal skeletal muscle contraction and expiration against closed glottis
2) defection delayed - rectal wall gradually relaxes
