# The Sizes of the Nerve Fibres of Muscle Nerves

V. S. V. Fernand, J. Z. Young

## Abstract

The diameters of the fibres in the nerves innervating twenty-five muscles in the rabbit have been measured and two types of muscle nerves are distinguished. (1) The 'bimodal' nerves of nearly all limb muscles and the extrinsic eye muscles show two peaks in the size-frequency distribution and contain many large fibres ($\text{up to}$ 20 $\mu$) as well as small ones. (2) The 'unimodal' nerves to muscles of the face, larynx, infrahyoid region and diaphragm have no large fibres and show a single sharp peak at about 10 $\mu$, with few fibres either smaller or larger. It is suggested that these unimodal nerves contain only a single class of fibre. The muscles they innervate have relatively simple actions, not involving weight-bearing, and they contain few or no spindles. The size of nerve cells and fibres is determined by (1) general morphogenetic factors, (2) the extent of the peripheral field innervated, (3) the amount of influence received by the neuron from afferent and central sources. The nerve fibres to the more proximal limb segments are larger than those reaching more distally. This is held to be a result of the fact that the first neurons to differentiate under general morphogenetic influences come to innervate the more proximal muscles and thus receive peripheral stimulation for a longer time than the neurons that develop somewhat later and must therefore innervate more distal muscles. The small size of the nerve fibres of the unimodal muscle nerves is due to the relatively small amount of afferent and central stimulation that their cells receive, because their muscles have simple actions and few or no proprioceptors. There is no connexion between the colour of muscle fibres or speed of their contraction and the size of the nerve fibres innervating them. The small size of the nerve fibres to the soleus is due to the fact that the muscle lies far distally and is innervated late. There is no close connexion in the adult between nerve-fibre size and the amount of muscle tissue or number of muscle fibres innervated.