SUMMARY1. Isosmetric contractions of cat flexor digitorum longus whole muscles and of functionally isolated motor units have been measured under conditions similar to those used by Buller & Lewis (1965a).2. Motor unit twitch time to peak was inversely related to axonal conduction velocity. The logarithm of tetanic tension was directly related to conduction velocity. These relationships suggest that each motoneurone has an influence on the muscle fibres which it innervates.3. The ratio of twitch to tetanic tension was directly related to the time to peak of the motor unit. This fact might be explained by variation between motor units of the duration of 'active state '.4. The muscle length at which tension was maximal varied between motor units and the optima were found over the range of muscle lengths which could occur in the body. Slow motor units had longer optimal lengths.5. The sample of motor units was considered to be unbiased because the distribution of axon conduction velocities was compatible with reported motor fibre diameter spectra of the muscle nerve. The mean motor unit tetanic tension gave a reasonable estimate of the number of ac-motor axons in the muscle nerve. Twitch tensions gave a value that was 40 % higher.6. Motor unit and whole muscle data were in good agreement for lengthtetanus tension curves, for times to peak and for twitch-tetanus ratios at long muscle lengths.

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Afferent impulses in 61 fine myelinated or unmyelinated vagal fibers arising from endings in large blood vessels ("vascular endings") were recorded in anesthetized dogs and cats. The endings had a sparse, irregular spontaneous discharge, or they were quiescent. They were stimulated by capsaicin, phenyl diguanide, or veratridine injected into the bloodstream but not by sodium cyanide or hypoxia. Unlike arterial baroreceptors and atrial receptors, which are concentrated in localized areas, vascular endings were widely distributed in the thoracic aorta, the pulmonary artery, the brachiocephalic artery, the splenic artery, the atriovenous junctions, the atrial appendages, the inferior vena cava, and the hepatic vein. Conduction velocity in 28 fibers ranged from 0.8 to 11.0 m/sec (mean 3.1 m/sec), and in 19 of these fibers the velocity was less than 2.5 m/sec (i.e., the fibers were C-fibers). Vascular endings were stimulated by distending the vessels with balloons. Endings in the pulmonary artery and the aorta did not respond to pressures within the physiological range, but they were stimulated by abnormally high pressures (pulmonary artery 60-110 mm Hg, aorta 200-215 mm Hg). The physiological role of the vascular endings is unknown. Downloaded from 35. FIDONE, S.J., AND SATO, A.: Study of chemoreceptor and baroreceptor A-and C-fibers in the cat carotid nerve.

This paper describes experiments in which impulses have been recorded from afferent vagal fibres whose endings were located in the lung. These fibres differed from the pulmonary stretch fibres described by Adrian (1933) and subsequent workers in that their spontaneous activity was sparse and irregular and showed no respiratory modulation at normal ventilation volumes. The fibres were stimulated, however, by inflating the lungs with larger volumes. The fibres appeared to be of small diameter, having a mean conduction velocity of 16 m/sec and a blocking temperature of 1-6°C.These slowly conducting fibres were identified in the following way. Capsaicin, a decylenic acid amide of vanillylamine, injected intravenously produces bradyeardia, systemic hypotension and apnoea (Toh, Lee & Kiang, 1955; Porsz6asz, Such & Porszasz-Gibiszer, 1957). Porsztasz et al. (1957) suggested that this triad of effects was evoked reflexly by stimulation of baroreceptors in the extrapulmonary parts of the pulmonary artery.Subsequently, Coleridge, Coleridge & Kidd (1964) found that although pulmonary arterial baroreceptors undoubtedly played a part in mediating the reflex effects, some other afferent endings within the vascular bed of the lung itself were probably involved. A search was therefore made for intrapulmonary receptors which were stimulated by capsaicin. The afferent fibres described in the present paper were encountered in the course of this work. They were the only pulmonary fibres consistently stimulated by capsaicin, the drug having little or no effect on the activity of pulmonary stretch fibres of the type described by Adrian (1933). METHODSTwenty-seven dogs (9-32 kg) were anaesthetized with morphine sulphate (15 mg subcutaneously) followed either by chloralose (0-08 g/kg intravenously) or by a 1:1 mixture (0-25 ml./kg intravenously) of Dial Compound (allobarbitone 100 mg/ml., urethane 400 mg/ ml., Ciba) and Nembutal (sodium pentobarbitone 60 mg/ml., Abbott Laboratories) solutions.