A neuroscientific approach to consciousness.

For a neuroscientist, consciousness currently defies any formal operational definition. However, the phenomenon is distinct from self-consciousness: after all, one can "let oneself go," when experiencing extreme emotion, but still be accessing a sentiment, subjective, conscious state. This raw, basic subjective state does not appear to be an exclusive property of the human brain. There is no obvious qualitative transformation in either the anatomy or the physiology of the central nervous system of human or non-human animals, no phylogenetic Rubicon in the animal kingdom. Similarly, there is no clear ontogenetic line that is crossed as the brain grows in the womb, no single event or change in brain physiology, and certainly not at birth, when consciousness might be generated in an all-or-none fashion. A more plausible, and scientific, view of consciousness might be therefore that it is not a different property of the brain, some magic bullet, but that it is a consequence of a quantitative increase in the complexity of the human brain: consciousness will grow as brains grow. Hence, consciousness is most likely to be a continuously variable property of the brain, in both phylogenetic and ontogenetic terms. Here, we describe how modern techniques may be utilized to determine the physiological basis of consciousness.

The role of muscarinic receptors and intracellular Ca2+ in the spectral reflectivity changes of squid iridophores.

In this paper we describe changes in spectral reflectivity of the light reflectors (iridophores) of the squid Alloteuthis subulata. The spectral changes that can be seen in living squid, can also be brought about by superfusing whole skin preparations with acetylcholine (ACh) (20 micro mol l(-1)) and muscarine (30 micro mol l(-1)) but not nicotine (up to 50 mmol l(-1)), suggesting that cholinergic muscarinic receptors are involved. Changing the osmolarity of the external solution had no effect on spectral reflectivity. To study the iridophores at the cellular level, iridophores were isolated enzymatically. Lucifer Yellow filled the iridophores uniformly, showing cellular individuality. Isolated iridophore cells were loaded with Fura-2 AM and cytoplasmic Ca(2+) was recorded ratiometrically. Intracellular Ca(2+) (resting concentration at 66.16 nmol l(-1)) increased transiently after addition of ACh (50 micro mol l(-1)), muscarine (25 micro mol l(-1)), but not nicotine (up to 5 mmol l(-1)). Ca(2+) also increased when superfused with potassium chloride (10 mmol l(-1)) and caffeine (2.5 mmol l(-1)). Hypo- and hyperosmotic solutions had no effects on the cytoplasmic Ca(2+). By presenting direct evidence that iridophores are polarised cellular structures containing Ca(2+) stores and that they are activated via cholinergic muscarinic receptors, we demonstrate that Ca(2+) is involved in the reflectivity changes of the iridophores of A. subulata. Specimens were prepared for transmission electron microscopy. It was found that the orientations of the plates with respect to the skin surface are in good agreement with the expected orientations based on the prediction that the iridophores act as multilayer reflectors.