Differences in Variability of Discharge Frequency between Primary and Secondary Muscle Spindle Afferent Endings of the Cat

Abstract

THE frequency of action potentials in a nerve fibre has long been known to be a function of the input intensity. Fluctuations in frequency when the stimulus is held constant have often been considered merely as unwanted ‘noise’ limiting the accuracy of the experimental results. However, these fluctuations may also limit the amount of information which can be transmitted down a nerve fibre and so are of interest per se. In a receptor or an isolated axon, these fluctuations may result from the basic processes of action potential initiation and are of interest in this context. If the variability had the same basis in different cells (for example, resistive noise), one would expect that it would be smaller, the larger the cell or axon, and this is often the case¹. However, one of us (P. B. C. M.) noticed that the discharge initiated from single secondary endings of muscle spindles of the cat seemed to be considerably more regular than that from the primary endings, though these latter have the larger afferent nerve fibres²; later this was also found to have been noticed by others^3,4.