An Analysis of Fibre-size in the Human Optic Nerve.

DATA on the fibre-size " spectrum " in the optic pathways must form an essential anatomical preliminary to electro-physiological studies of the visual neural system. Extensive studies on peripheral nerves have brought to light some of the factors on which the differences of calibre of nerve fibres appear to be based. Studies of fibre-size in the central nervous system have been carried out by Haggquist (1936) on the spinal cord, and by Szentagothai-Schimert (1941) on several of the important pathways of the brain. It has been recognised from the time of Gudden (1886) that the optic nerve contains myelinated fibres of graded size. Arey and his co-workers in their extensive investigation on the quantitative (and to a certain extent qualitative) composition of the optic nerve in a series of vertebrates, observed that in man all the fibres are myelinated and are of widely varying size. A considerable body of electro-physiological work on the optic nerve has been completed on frogs, rabbits and cats, and attempts have been made to group the fibres in this nerve on a physiological basis using conduction rate, threshold to electric stimuli, refractory period, time to maximum, etc., as criteria for grouping. In the histological study which this work entailed the authors arbitrarily distinguished certain fibre-sizes and attributed to them certain physiological properties. As yet, however, no systematic analysis of the range and frequency of the fibre-sizes in the optic nerve has been attempted. The importance of this investigation is emphasised by the findings of Granit that the electrical activity in the optic nerve (as recorded from its constituent fibres by his micro-electrode technique) indicates a specific sensitivity of the related receptors and their associated neurons in the retina to monochromatic lights. Such elements in the retina, possessing specific sensitivity to different wave-lengths of light, will give rise to impulses which are probably conducted along nerve fibres of one particular type in preference to others. The qualitative variations of nerve fibres so far recognised are the varying thickness of the axis cylinder and of the myelin sheath, and the presence or absence of the sheath of Schwann and the nodes of Ranvier. The present investigation deals with fibre-sizes, which are analysed in detail. Material and methods Human optic nerve sections of 5A thickness were prepared and several standard silver and myelin stain techniques were employed.