On electromyography, all of the following features suggest deneravation, except:
Ans. is 'b' i.e. Small short-duration polyphasic action potential Small short-duration polyphasic action potentials are seen in myopathy & myaesthemia gravis.In denervation: early in the course, the number of MUPs, (motor unit potential) appearing during contraction is reduced and the configuration of the remaining ones are quite normal.in time, the remaining MUPs often increase in size and in amplitude and become longer in duration and sometimes polyphasic.after about 2 weeks of denervation, fibrillation potentials can be recorded from the resting muscle. (Fibrillation is the spontaneous contraction of a single muscle fibre. It is ordinarily not visible through the skin.) another type of spontaneous activity sometimes recorded from resting denervated muscle is that of positive sharp waves also called as 'saw tooth' potentials. in chronic denervation, spontaneous and repetitive fasciculation potentials may be seen (Fasciculation represents the spontaneous firing of an entire motor unit, causing contraction of a group of muscles and in contrast to fibrillation is visible through the skin. Fasciculation potentials are evidence of motor nerve fibre irritability, most often the result of reinnervations following nerve or motor neuron damage)in chronic denervation sometimes, 'giant potential' can be seen. This is because surviving axons often produce collateral sprouts which re-innervate denervated muscles, thus some motor units become much larger than normal in both amplitude and duration.Just to recapitulate basic physiology of a Motor unit:A motor unit consists of a motor neuron together with all the muscle fibres which it innervates.The muscle fibres belonging to a motor unit are scattered throughout a portion of the muscle.In any given portion of a muscle, muscle fibres belonging to 20-50 different motor units intermingle with each other.The unit of activation of a muscle is a motor unit, and the unit of activation within a motor unit is a muscle fibre.
Unrgulated firing of individual muscle fibers
Small short-duration polyphasic action potentials
Presence of positive sharp waves
Spontaaneous firing of motor units
Nerve Physiology