A Histological Problem Concerning the Conditions at the Nerve Endings in Skeletal Muscle

Abstract

Until the recent appearance of the chemical transmission hypothesis 1 it was generally assumed that the excitation of skeletal muscle was effected electrically by the action current of the nerve ending. At present neither the chemical nor the electrical hypothesis can be considered to be well established. It seems likely from the nature of the problem that much of the decisive evidence must come from a demonstration that the structures and arrangements at the neuro-muscular junction are more suited to the one type of transmission than to the other. In any case, the point of view must be consistent with respect to the structure of the junction and its function. It is our purpose to discuss a serious inconsistency which exists at present in regard to the electrical polarization of the membrane of nerve ending.

The polarization of the membranes∗ of both nerve and muscle is believed to be a consequence of semi-permeabilities. The membranes are impermeable to anions while permeable to certain cations, notably to K which is the principal cation within the cell but which exists in relatively low concentration in the extracellular fluid. The K ions are retained within the cell for the most part in association with the impermeable negative ions thus providing a concentration difference from inside to outside which accounts satisfactorily for the observed membrane potential. 2 , 3 It is believed that in the case of muscle the polarized membrane is associated with the sarcolemma or a plasma membrane immediately underlying it. The nerve according to the most generally accepted histological description penetrates the muscle membrane after losing its sheaths and lies embedded in the specialized sarcoplasm of the end plate.