Gastrointestinal Motility Flashcards
(3 cards)
Movements of GI tract - slow waves, spike potentials
Smooth muscles of GI tract function as a syncytium.
- individual smooth muscles are 200-500 um in length and 2-10 um in diameter and arranged in bundles
- within each bundle the mucles fibers are electrically connected with one another through large numbers of gap junctions that allow low resistance movement of ions from one muscle cell to the next
- each muscle layer represents a branching latticework of smooth muscle bundles and functions as a syncytium; that is when an action potential is elicited anywhere within the muscle mass, it generally travels in all directions in the muscle.
- the distance it travels depends on the Excitability of the muscle
- transmission of these electrical signals is faster length wise than sideways
Two types of electrical activities are seen in these muscles - slow waves and spikes
in addition the voltage of rmp of the gi smooth muscle can change to diff levels which can also have imp effects in controlling motor activity of the gi tract
slow waves caused by undulating changes in rmp
- intensity varies b/w 5-15 mv
- frequency ranges from 3-12 / min. 3 in body of stomach, as much as 12 in the duodenum and about 8 or 9 in the terminal ileum
- precise cause is not completely understood although they appear to be caused by complex interactions among the smooth muscle cells and specialized cells called the interstitial cells of cajal which are believed to act as electrical pacemakers for smooth muscle cells
- do not cause muscle contraction by themselves but mainly excit the appearance of intermittent spike potentials, which in turn actually excite the muscle contraction
Spike Potentials
- true action potential
- occurs when rmp becomes more positive than about -40 mv ( normal is between -50 and -60 mv avg -56 mv)
- the higher the slow wave potential rises the greater the frequency of the spike potentials becomes usually ranging between 1-10/ second.
- larger number of ca2+ ions and some na+ ions influx (calcium - sodium channels)
factors that depolarize the membrane -
• stretching of the muscle
• stimulation by ach released from parasympathetic stimulation
• specific gi hormones
factors that hyperpolarize
• effect of norepinephrine and epinephrine
• stimulation of sympathetic nerves (norepinephrine)
tonic contractions
- continuous; it is not associated with the basic electrical rhythm of the slow waves but often lasts several minutes or even hours
- caused by
1) continuous repetitive spike potentials
2) by continuous depolarization (partial depolarization by some hormones or other factors)
3) by continuous entry of ca2+ ions
functional types of gi movements
1) peristalsis / propulsive movements
- forward propulsion - sm contraction proximal to the food bolus and simultaneous relaxation distal to the food bolus
- distension of rhe gut wall by a food bolus triggers reflexive contractions of smooth muscles (mainly inner circular and outer longitudinal)
- a contractile ring appears aroun the gut and then moves forward.
- myenteric plexus is almost entirely responsible for coordination of peristalsis
- in its absence, peristaltic movements either are severely impaired or entirely absent
- law of the gut
peristaltic reflex plus the anal direction of movement of the peristalsis
segmentation contractions or mixing movements
- these are local intermittent segmentation contractions which occur every few centimetres in the gut wall. these constrictions promote the chopping and shearing of the contents and facilitates proper mixing of the of the chyme with pancreatic and intestinal secretions.
chewing (mastication)
swallowing (Deglutition)
