Outline of Lecture 28 (05-13 B; Ravich)
- The GI tract can be seen as a series of pumps, conduits, reservoirs, and gates. This allows the
tract to perform several functions such as propulsion, grinding, mixing, and compartmentalization.
- There are 2 types of enteric smooth muscle contractions. Phasic contractions result from
momentary stimulation and are used for pumping. Tonic contractions result in continuous pressure such as
in sphincters, and must be relaxed to allow gating.
- Neural input for most of the GI tract is mostly autonomic. The parasympathetic side stimulates
the GI tract in general, and its nerves enter through the vagus and sacral plexus. The sympathethic side is
inhibitory in general, and nerves enter through the sympathetic ganglion.
- The myenteric plexus is the intrinsic nervous system of the gut and while it is modulated by the
CNS it can operate independently.
- Some GI muscle is directly innervated and others are indirectly innervated.
- The pattern of contraction determines the result. A smooth contraction wave gives propulsion; a
smooth but partial contraction wave gives mixing and propulsion; uncoordinated contractions give mixing
- Slow waves are rhythmic undulations in the membrane potential of myocytes. Spike potentials,
and hence contractions, only occur at the peaks of the slow waves, and this helps control the timing of
- Sphincters act as one-way valves. Hence a proximal distention causes relaxation to allow
forward movement of material, but a distal distention causes increased contraction to prevent retrograde
- The oropharynx is common to both the GI and respiratory system. In swal