Parasympathetic autonomic nervous system concepts

The concept of chemical neurotransmission originated in the 1920s with the classic experiments of Otto Loewi (which were themselves inspired by a dream), who demonstrated that by transferring the ventricular fluid of a stimulated frog heart onto an unstimulated frog heart he could reproduce the effects of a (parasympathetic) nerve stimulus on the unstimulated heart (Loewi Navratil, 1926). Subsequently, it was found that acetylcholine was the neurotransmitter released from these parasympathetic nerve fibers. As well as playing a critical role in synaptic transmission in the autonomic nervous system and at vertebrate neuromuscular junctions (Dale, 1935), acetylcholine plays a central role in the control of wakefulness and REM sleep. Some have even gone as far as to call acetylcholine a neurotransmitter correlate of consciousness (Perry et al., 1999). 

The concept of chemical transmission in the nervous system arose in the early years of the century when it was discovered that the functioning of the autonomic nervous system was largely dependent on the secretion of acetylcholine and noradrenaline from the parasympathetic and sympathetic nerves respectively. The physiologist Sherrington proposed that nerve cells communicated with one another, and with any other type of adjacent cell, by liberating the neurotransmitter into the space, or synapse, in the immediate vicinity of the nerve ending. He believed that transmission across the synaptic cleft was unidirectional and, unlike conduction down the nerve fibre, was delayed by some milliseconds because of the time it took the transmitter to diffuse across the synapse and activate a specific neurotransmitter receptor on the cell membrane. 

Studies of neuromuscular junctions of the autonomic nervous system as early as 1904 led to the suggestion that adrenaline might be released at the nerve endings. Later it was shown that, while adrenaline does serve as a transmitter at neuromuscular junctions in amphibians, it is primarily a hormone in mammals. Nevertheless, it was through this proposal that the concept of chemical communication in synapses was formulated. By 1921, it was shown that acetylcholine is released at nerve endings of the parasympathetic system, and it later became clear the motor nerve endings of the somatic system also release acetylcholine. 

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