Consciousness acetylcholine

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). 

Perry, E., Walker, M., Grace, J. 8r Perry, R. (1999). Acetylcholine in mind a neurotransmitter correlate of consciousness Trends Neurosci. 22, 273-80. 

The available data are consistent with the present thesis that cholinergic inputs to cerebral cortex mediate intradendritic events fundamental to conscious activity as a primary role, and that cholinergic modulation of electrophysiological activity may be secondary, even epiphenomenal. Transduction pathways exist whereby muscarinic receptors (and possibly nicotinic receptors acting presynaptically to inhibit acetylcholine release) may lead to actions on the cytoskeleton directly relevant to consciousness. The thesis presented here describes these pathways and also suggests a possible explanation for the diversity of neuromodulators and metabotropic receptors. Accordingly, qualitative aspects of our consciousness would be finely tuned by a number of neurochemicals, prominent among which is acetylcholine. 

This book adds to numerous preceding texts on consciousness the relatively new concept that particular neurotransmitters may be central to the process. As outlined in the Preface, communication between neurons is essential for consciousness and such communication, on the timescale applicable to conscious perception, is principally mediated by chemical neurotransmission. As Susan Greenfield (2000) points out in The Private Life of the Brain , acetylcholine may enable a whole population of cells to become more important than individual units, a kind of neuroscientific Marxism If the concept of transmitter NCC is incorporated into future discussions of the neurobiology of consciousness, or adds a further dimension to the neuropharmacology of disorders of the brain which affect conscious awareness, this book will have more than served its purpose. 

How can enhancement and blockade of the same system result in such dissimilar alterations in consciousness In the case of acetylcholine, we find the answer not only in spatial differentiation, but also in the fact that acetylcholine activates the cortex in both waking and REM sleep but has quite different effects on consciousness because of the other neuromodulators serotonin and norepinephrine that are (in waking) or are not (in REM sleep) co-released. 

Collier, B. and Mitchell, J. F. (1967) The central release of acetylcholine during consciousness and after brain lesions. J. Physiol. (Load.), 188,83-98. 

In humans, the symptoms of acetylcholine poisoning resemble on the one hand the syndrome of nicotine poisoning (muscle twitch, excitability, followed by muscular paralysis) and, on the other hand, the syndrome of muscarine poisoning (nausea, pain, exudation, increased diuresis, dyspnoea, pulmonary oedema). Owing to the effect on the central nervous system, the loss of sense of orientation, ataxy, tremor, derangement of consciousness and fainting occur simultaneously. 

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