Muscle: Electrical Activity Of Contraction (1969)

e [Music] all body movements simple and complex are the result of the contractions of specialized structures [Music] muscles all muscular contractions are initiated by nerve pulses in the laboratory this can be easily demonstrated applying an electrical stimulus directly to the muscle has the same result contraction of the whole muscle is due to the contraction of its parts the muscle itself is made up of many individual strands or fibers each fiber when stimulated electrically contracts so the basic unit of muscular contraction is the single fiber the contraction of the single fiber must be regulated if the work of the whole muscle is to be effective obviously a relationship exists between nerve impulses and the subsequent contraction of individual muscle fibers the exact nature of this relationship can be determined through the use of highly specialized equipment each muscle fiber is a single cell surrounded by a very thin membrane what we want to know is what happens to this single muscle fiber when we stimulated it electrically in this apparatus I have a muscle bundle made out of about 25ers sitting in a tissue bath in the B also I have two very fine micro electrodes sitting very close to the surface of the muscle fiber these micro electrodes are attached to the input of an oscilloscope the tip of these micro electrodes is so fine that I will not injure the cell when I penetrate the micro electrode inside so I can record actually the electrical event within one single f fiber in spite of the fact that there are other cells around this muscle fiber I am now going to penetrate the micro electrode into the fiber now you see the upper line indicates a zero potential or no potential at all between the two micro electrod as I am penetrating the microelectrode I watch the oscilloscope beam aha there is a sudden deflection in the beam of the oscilloscope this indicates that I have penetrated into the muscle fiber and further indicates that the inside of the membrane is negatively charged with respect to the outside this difference is approximately 90 M and it is called the resting membrane potential now let us see what happens as I start electrically stimulating this fiber okay now I will start increasing the stimulus strength slowly and watching the scull nothing is happening I increase the stimulus strength further still nothing happening and as I increase it a bit more aha there it is a rapid event on the face of the oscilloscope the muscle fiber has fired and this rapid event seems to have occurred when the difference between the outside and the inside of the fiber has decreased 260 Mt let's try this again I will now again start increasing the strength of the stimulus nothing happen I stimulate it further still nothing okay I increase a bit more ah there it is a spike again and it seemed to have fired at around - 60 Mt again this firing of the muscle is called the action potential this action potential is the signal for muscle to contract it travels rapidly all over the muscle fiber and activate the muscle fiber instantaneously this is an all or nonone response in other words a muscle fiber either contracts fully or not at all now let us see how this action potential relates to contraction now I'm going to put a second Beam on the oscilloscope the upper beam as previous viously is going to indicate the electrical activity the lower beam the contraction so let us start by again stimulating the fiber and recording the electrical event and the mechanical contraction simultaneously the muscle fiber fires and the contraction follows let me do this again notice that the electrical event precedes the contraction by a very brief interval and that the electrical event is over before the contraction begins what this means is that the stimulus only triggers the contraction it's little bit like turning on a light something happens between throwing of the switch and the coming on of the light it's in fact during this short time interval that the mechanism of the actual contraction itself is activated keep in mind that in the living body muscles are stimulated by nerves one nerve cell May stimulate one single muscle fiber or it may stimulate many cells if only a few fibers are stimulated the muscle contracts weakly if a lot of fibers are stimulated the muscle contract strongly and it's really the contraction of the single fiber multiplied many times that gives us the wide variety of movement [Music]