However, where in the thoracic cavity an increase in sympathetic activity as a consequence of inhalation yields increased activity (i.e., an increase in heart rate), below the diaphragm in the abdominal cavity, the same increase in sympathetic activity yields an inhibitory effect on the enteric nervous system, which slows digestive action (i.e., the secretion of conducting fluids), locomotion, and a myriad of other complex digestive activities. The opposite is also true – whereas in the thoracic cavity an increase in parasympathetic emphasis as a consequence of exhalation results in slowing of the heart rate, in the abdominal cavity, increased parasympathetic emphasis (vagal nerve) stimulates the enteric nervous system which responds by increasing digestive activity.
This opposite action above and below the diaphragm has to do with how sympathetic and parasympathetic nervous signals are interpreted. In the thoracic cavity (above the diaphragm), upon inhalation pressure decreases (becomes more negative) and heart rate accelerates to the net influence of increasing sympathetic action involving increased secretion of nor-epinephrine acting on adrenergic receptors of the heart, and decreasing parasympathetic action involving decreased secretion of acetylcholine serving to deactivate cholinergic receptors of the heart, thereby diminishing vagal inhibitory affect – the net effect is increased heart rate. Below the diaphragm, upon inhalation pressure increases and all of the organs of the abdomen are compressed. The increased secretion of nor-epinephrine that results in activation of adrenergic receptors in the heart, has the opposite effect on the enteric nervous system, inhibiting its action and “slowing” digestion.