No abstract

Most of the C fibre thermoreceptors hitherto found responded only to rather extreme heating or cooling (Iggo, 1959 a, b) and therefore were possibly concerned with pain. They were relatively insensitive to mechanical stimulation. There are also numerous C fibres which respond both to mild mechanical stimulation and to sudden cooling of the skin Iggo, 1959a). They have been classified as mechanoreceptors by Iggo (1960).In the present investigation sensitive specific cold and warm receptors with afferent C fibres have been found and their behaviour has been studied quantitatively. The opportunity has also been taken to examine quantitatively the responses of C heat receptors (Iggo, 1959b) and of a few C mechanoreceptors to thermal stimulation. A preliminary account of these results has been published (Hensel, Iggo & Witt, 1959). METHODSThe cats used were anaesthetized with chloralose (60 mg/kg intramuscular) and urethane (250 mg/kg intramuscular). After depilation the leg was fixed by drill pins inserted through each end of the tibia. The skin over the saphenous nerve was incised, the nerve was ligated proximally and dissected free from connective tissue. The cut edges of the skin were tied to a metal ring and the trough formed was filled with liquid paraffin B.P. Fine strands of the nerve were dissected on a black Perspex plate and laid on silver electrodes. A pair of stimulating electrodes was placed under the nerve about 3 cm distal to the recording electrodes and an earthed plate was placed between the two pairs of electrodes. The amplified impulses in the fine nerve strand were recorded by means of two separate systems, each consisting of a cathode-ray oscilloscope and a recording camera. The first system recorded the impulses in the usual way with a stationary cathode-ray beam and moving bromide paper (Fig. 1 A). The expanded sweep of the second oscilloscope was triggered by the square-wave pulse

A photosystem I (PS I) reaction center complex was isolated and purified from the cyanobacterium Synechococcus sp. The complex has a molecular mass of about 600 kDa and contains 120 Chl a molecules per photoactive Chl a, (P-700). Electron micrographs show that the PS I complex has the shape of a disk with a diameter of about 19 nm and a thickness of 6 nm. Computer analysis reveals that the complex is composed of three similar units.

The integral membrane protein complex of the photosystem I reaction center from the cyanobacterium Synechococcus sp. has been isolated and highly purified. At extremely low protein complex concentrations (50 pg/ml), crystallization occurred. The crystals are dichroic, photoactive and show an X-ray powder diffraction pattern.