Changes in motor units properties in a rat model of amyothropic lateral sclerosis (ALS)
ALS is a fatal neurodegenerative disorder characterized by developing muscle weakness and paralysis which is an effect of selective and progressive death of motoneurons. The experiments were carried out on motor units (MUs) of the medial gastrocnemius muscle in transgenic rats with hSOD1(G93A) mutation. The animals were divided into three groups: 3 months old, 4 months old and approximately 200 days old in nearly terminal state, when nerve stimulation revealed on average 6 MUs in the studied muscle. Because of considerable disturbances in contraction time (CT) and disappearance of sag effect, usually applied for MUs classifi cation as fast and slow, the division of MUs was based on 20 Hz tetanus index. Meanwhile, to distinguish fast fatigable (FF) from the fast resistant (FR) MUs the fatigue index (FatI) was applied. Results showed that MUs proportion changed radically in the course of the disease and in terminal state an increase of S and a decrease of fast MUs was observed. Moreover, CT increased gradually in all MU types. FatI of FF MUs increased but, at the same time, for FR and S gradually decreased. Twitch force as well as tetanus force in terminal group increased especially for FR and S MUs what indicates plastic changes in examined muscle (sprouting of surviving axons and collateral reinnervation), leading also to increased amplitude of MUs action potentials. EMG analysis additionally showed slowing of axonal conduction velocity or/and motor plate transduction.