39. General principles of gastrointestinal function. Motility. Enteric nervous
system. Autonomic and hormonal control of the gastrointestinal tract.
General principles of gastrointestinal function.
The intestinal wall is composed of the following layers (from the outer surface inward):
serosa, longitudinal smooth muscle layer, circular smooth muscle layer, submucosa,
and mucosa.
The smooth muscle fibers in the longitudinal and circular muscle layers are electrically
connected through gap junctions that allow ions to move from one cell to the next. Each
muscle layer functions as a syncytium; when an action potential is elicited in the
muscle mass, it generally travels in all directions in the muscle. The distance it travels
depends on the excitability of the muscle.
Electrical activity of gastrointestinal smooth muscle
● Rhythm of most gastrointestinal contractions is determined by the frequency of
slow waves in the smooth muscle membrane potential.
○ These waves are not action potentials; instead, they are slow, undulating
changes in the resting membrane potential.
● Spike potentials are true action potentials that cause muscle contraction.
○ They occur when the resting membrane potential becomes more positive.
The channels responsible for the action potentials allow particularly large
numbers of calcium ions to enter along with smaller numbers of sodium
ions; they therefore are called calcium-sodium channels.
● The basic level of resting membrane potential of gastrointestinal smooth muscle
can be increased or decreased.
○ Factors that depolarize the membrane include
■ stretching the muscle
■ stimulation by acetylcholine
■ stimulation by parasympathetic nerves that secrete acetylcholine at
their endings
■ stimulation by gastrointestinal hormones.
○ Factors that hyperpolarize the membrane include
■ the effect of norepinephrine or epinephrine on the muscle
membrane
■ stimulation of the sympathetic nerves that secrete norepinephrine
at their endings.
Enteric nervous system
system. Autonomic and hormonal control of the gastrointestinal tract.
General principles of gastrointestinal function.
The intestinal wall is composed of the following layers (from the outer surface inward):
serosa, longitudinal smooth muscle layer, circular smooth muscle layer, submucosa,
and mucosa.
The smooth muscle fibers in the longitudinal and circular muscle layers are electrically
connected through gap junctions that allow ions to move from one cell to the next. Each
muscle layer functions as a syncytium; when an action potential is elicited in the
muscle mass, it generally travels in all directions in the muscle. The distance it travels
depends on the excitability of the muscle.
Electrical activity of gastrointestinal smooth muscle
● Rhythm of most gastrointestinal contractions is determined by the frequency of
slow waves in the smooth muscle membrane potential.
○ These waves are not action potentials; instead, they are slow, undulating
changes in the resting membrane potential.
● Spike potentials are true action potentials that cause muscle contraction.
○ They occur when the resting membrane potential becomes more positive.
The channels responsible for the action potentials allow particularly large
numbers of calcium ions to enter along with smaller numbers of sodium
ions; they therefore are called calcium-sodium channels.
● The basic level of resting membrane potential of gastrointestinal smooth muscle
can be increased or decreased.
○ Factors that depolarize the membrane include
■ stretching the muscle
■ stimulation by acetylcholine
■ stimulation by parasympathetic nerves that secrete acetylcholine at
their endings
■ stimulation by gastrointestinal hormones.
○ Factors that hyperpolarize the membrane include
■ the effect of norepinephrine or epinephrine on the muscle
membrane
■ stimulation of the sympathetic nerves that secrete norepinephrine
at their endings.
Enteric nervous system
