IT was discovered more than a century ago by Matteucci and by du Bois-Reymond ( 1843) that electric currents can be obtained from isolated, living (striated) muscle and also that contraction of a muscle is associated with a brief electric potential change. The electric currents were eventually shown to be not merely incidental phenomena in muscle activity, but the means by which activation of the contractile substance is normally initiated in the animal body. The arrival of a nerve impulse at a muscle leads to the production of electric changes there, thus providing a means by which the nerves can initiate muscular contraction. The basis of this phenomenon will now be considered in some detail.
Historical. If a long strip is cut out of a muscle which has a regular structure and parallel fibres (e.g. frog's sartorius or semimembranosus), a cylinder of muscle is obtained with an undamaged central portion (if the dissection has been done carefully) and neat cuts at both ends. The undamaged portion may be regarded as normal living muscle at the time of excision, dying only slowly under good conditions. The terminal portions are already dead, and death will gradually extend inwards from the dead zones. There is a rough line of demarcation between living and dead parts of the muscle, so when a potential difference was observed between a cut end and the centre it was at first referred to as the demarcation potential. A similar potential can be recorded from an excised muscle or one in situ between an uninjured region and a region damaged chemically or by crushing. It was not unreasonable to suppose that the damage was itself responsible for the development of the potential difference and so it was also referred to as an injury potential.