Physiology
The oesophagus has striated or skeletal muscle at the upper end, smooth muscle at the lower end nearing the stomach, and a mixture of both types of muscle in the middle.13
The myenteric plexus is one of the systems of nerves stimulating smooth muscle along the gastrointestinal tract and is responsible for peristalsis, the wave-like contractions that propels GI contents along. It lies between the longitudinal and circular muscle layers.14
The LOS is controlled by parasympathetic (vagus) nerves and sympathomimetic (mainly splanchnic) nerves. It has two core muscle fibre types, one of which contracts in response to cholinergic stimulation to open the sphincter. While nicotinic transmission predominates at nerve ganglia, muscarinic and serotonergic receptors are also involved.12,13
The main neurotransmitter in the smooth and striated muscle contraction is acetylcholine, with muscarinic-type receptors, along with tachykinins. Nitric oxide is the main relaxant or inhibitor, along with substance P. Vasoactive intestinal peptide (VIP) causes contractions in the oesophagus and is the main compound opening the LOS. Other neurotransmitter GI relaxants include purine.12,14,15
The central nervous system is also involved in regulating smooth muscle oesophageal peristalsis, initiating the swallowing-induced ‘primary’ peristalsis and the oesophageal distension-induced ‘secondary’ peristalsis.12
In achalasia, damage to the oesophageal nerves (possibly due to viral infection or an autoimmune reaction) can eventually lead to oesophageal paralysis. Food can accumulate and ferment in the oesophagus before being washed back up into the mouth.16