In this paper the pattern of innervation of one of the simplest forms of skin, the integument of amphioxus (covering the dorsal fin in the middle third of the animal) is compared and contrasted with that of the cornea and of the skin in man.
An account is given of the results of stimulation experiments in which the integument in this region was irradiated with infra-red rays.
The observations which have been made can be summarized briefly as follows:
1. No neuro-epithelial cells were encountered in the epidermis in the region chosen for study.
2. Evidence is given which suggests that the nuclei along the course of the nerve-trunks which combine to form the ‘dorsal roots’ of the nerve-cord are not those of the first sensory neurone. They appear to be analogous to the Schwann-cell nuclei of vertebrates.
3. The ratio of epidermal cells to parent sensory axons in randomly selected metameres of integument in the selected region is as low as 7:1. From this it has been calculated that the density of innervation is comparable with that in the cornea and in the skin of vertebrates.
4. Every epidermal cell has two or more terminal filaments ending in relationship to it.
5. Pre-terminal axons from parent axons of different diameters are widely and apparently randomly scattered beneath the epidermis.
6. Each and every epidermal cell is related to terminals which approach from different directions and serve more than one parent axon.
7. The patterned arrangement of the nerves serving the integument of amphioxus is comparable with that observed in the skin of teleost fish, in the cornea of a number of vertebrate species, and in the skin of man.
8. The nerves in the integument in the selected region can transduce non-injurious infra-red stimuli, although the stimuli were of a kind not normally encountered by the animal in its natural habitat.
9. In the light of these observations and of experimental observations in man (Weddell, 1955; Lele and Weddell, 1956; Weddell, 1957), it is difficult to subscribe to the notion that information concerning the environment is transmitted to the central nervous system from the skin by a restricted series of nerves having terminals which only transduce stimuli having quite specific physical attributes. Rather, it seems likely that information reaches the central nervous system in the form of a space-time pattern of action potentials from endings which are either more or less available to a range of stimuli having different physical characteristics. In other words, different stimuli must evoke different patterns of activity, which are analysed by the central nervous system acting in a role of an analogue as opposed to a digital computer.