tonic

segmental

propagated

inhibited

• slow wave frequency rhythm that is responsible for timing, speed, direction of GI contractions
• dependent on gap junctions
• excitation controlled by interstitial cells of Cajal

• NOT caused by BER
  • they govern the rate at which they occur
• on an upstroke of BER, AP's can occur that cause contraction
  • associated with increase in intracellular calcium
  • amplitude of contraction depe3nds on number of spike potentials
    • number of spike potentials determined by muscle stretch, release of NT's, certain hormones

• longitudinal
  • thinner with few gap junctions
  • primary stimulus excitatory via ACh
• circular
  • thicker, with many gap junctions
    • function as electrical syncytium
  • controlled by many neurotransmitters, but main neural input is inhibitory
    • ENS controls it

• location
  • fundus of stomach
  • gallbladder
  • sphincters
• characteristics
  • BER
  • always slightly contracted
  • can relax or conract based on excit./ inh. NT release

• location
  • circular smooth muscle
• characteristics
  • determined by BER frequency
  • facilitate mixing of chyme, net movement away from the mouth
    • due to gradient of BER along stomach and small intestines

• storage
• mixing
• emptying

NO ABSORPTION (unless aspirin, alcohol)

• this is during fasting
• begins in lower 2/3 of stomach and ends at terminal ileum
• phases
  • I- motor inactivity (physiologic ileus)
  • II- intermittent motor activity
  • III- housekeeper (5-15 minute duration)

Phase II and III represent the activity front

• period of regular, intense, repetitive conractions
• associated with increases in motilin that happen right before phase III (M cells of duodenum)
  • stimulated to be released by ACh acting on M3 receptor
  • inhibited by duodenal acidification and serotonin antagonists
  • stronger vagal activity induces irregular MMC cycle
• function
  • clears the small intestines of all undigested residue, debris (ex: mucus, sloughed cells)
  • prevent bacterial overgrowth (we dont want bacteria in small intestines)

1. upon degluttination, receptive relaxation signal transmitted by vagal efferents via VIP/NO (inh. signal)
   1. mainly at the fundus (storage part of the stomach)
2. when food enters stomach, upper 1/3 shows slow sustained contractions due to vagovagal reflexes that cause ACh release in response to the distension
   1. phase I- propolsion via proximal antral contraction (terminal antral filling)
   2. phase II- emptying via midantral contraction (transpyloric flow or retropulsion)
   3. phase III- retropulsion via terminal antral contraction (retropulsion/trituation)
      1. narrowing of pyloris only allows liquid like particles through (prevents reflex of duodenal contents in stomach)
      2. force of retropulsion increases in terminal antrum as antral contraction approaches closed pylorus
3. propogated peristaltic contractions

• vagovagal reflexes
  • inhibitory input causing the receptive relaxation
  • stimulatory input causes slow sustained contractions along with contraction of pylorus

• controls hormones, refelxes that will control slow delivery of nutrients

If it were up to the stomach, every time it gets distended, it would stimulate contraction of all its contents into the intestines.

• volume (increase stomach volume means fater rate of emptying)
• particle size (liquids empty faster than solids)
• osmolarity (isotonic empty faster than hypotonic which empties faster than hypertonic)
• caloric content (fat slowest because calorically dense)
• pH- increases in pH (decrease H) lead to slower gastric emptyin
• hormones
• neural mechanisms (enterogastric reflexes)

CCK

GIP

secretin

gastrin

ALL SLOW GASTRIC EMPTYING WHILE INCREASING STRENGTH OF CONTRACTION

• increase in gastric emptying of liquids
  • impair relaxation of upper 1/3 of stomach
• decrease gastric emptying of solids
  • decrease force of antral contractions, so grinding of particles is prolonged

• SI has
  • faster BER (12/min in duodenum and 8/min in ileum)
  • peristaltic waves do not travel entire intestines
  • no storage of food

• allows for movement of content away from mouth

• segmenting contraction (allows for most exposure to absorptive suface)
• peristalsis
  • propulsive segment- contracted circular muscle and relaxed longitudinal muscle
  • receiving segment- relaxed circular muscle and contracted longitudinal muscle (contracting longitudinal shortens the distance)

• spike activity
• generation of MMC
• slow wave frequency/velocity
  • increased in hyperthyroidism, leads to diarrhea

• Large intestines has:
  • no MMC
  • no prox./distal BER gradient
  • much slower transit (days)
  • storage function
  • mass peristalsis
  • longitudinal muscle is 3 discrete bands
  • mass movement peristalsis
  • frequent retrograde movement

• mass movements- powerful movements of contractions of long segments of colon
• mixing movements (mainly in right colon)- contents shuttled back and forth between haustra (associated with short bursts in spike activity)
• haustral migration- mov't of haustra in either direction (peristalsis)(long bursts of spike activity)

• decrease mixing movements
• increases haustral number and migration
• increases/initiates mass movements

• tonically contracted smooth muscle
• parasymp. stim. release VIP/NO, leading to relaxation of IAS

• excitatory innevation via pudendal N.

1. rectospincteric reflex- contents in rectum cause reflexive distension of material
2. voluntary activity- contract EAS, and urge goes away, but you can voluntarily relax, and defecation occurs
3. but at some level of increased distension, you must deficate

In aging, EAS get stiffer and hypertrophied, leading to incontenance.