Figure 5. Loss of repetitive firing in a subpopulation of cells of FUS mice.

(A) Example of an ankle flexor FUS^P525L motoneuron (recorded at P180) that was unable to fire repetitively in response to a slow ramp of current (A₁), despite being able to generate a single spike in response to a short pulse of current (A₂), and despite being still connected to its muscle fibers, as shown by the presence of a motor unit action potential and a motor unit twitch following the spike. Top red trace: force developed by the motor unit. Grey trace, second from the top: EMG recording showing the motor unit action potentials. Green trace, second from bottom: membrane potential. Bottom blue trace: injected current. A₂ is an average of 15 sweeps. (B) Example of an ankle flexor FUS^P525L motoneuron (recorded at P180) that was able to fire repetitively in response to a slow ramp of current. Same organization as in A. (C) Contractile properties of motor units innervating ankle extensor muscles at P30. Filled symbols represent motor units recorded in FUS^WT mice, colored according to their physiological type (FF: red squares; FR: yellow diamonds, S: green circles). WT motor units that were unable to fire repetitively are represented with a black four-pointed star. Empty symbols represent motor units recorded in FUS^P525L, colored according to their physiological type (FF: red squares; FR: yellow diamonds, S: green circles). Mutant motor units that were unable to fire repetitively are represented with a black empty five-pointed star. The dashed lines at 8 mN and 20 ms represent the limits used to classify the motor units, and the dotted line and 1.3 mN represent the separation line between Large and Small motor units. (D) Contractile properties of motor units innervating ankle extensor muscles at P180. Same legend as in C. (E) Contractile properties of motor units innervating ankle flexor muscles at P30. Same legend as in C. (F) Contractile properties of motor units innervating ankle flexor muscles at P180. Same legend as in C. Motoneurons indicated by an arrow correspond to the two examples in panels A and B. (G) Comparison between FUS^WT (blue) and FUS^P525L (red) mice of the proportion of non-firing cells innervating ankle extensor muscles at P30 and P180. (H) Comparison between FUS^WT (blue) and FUS^P525L (red) mice of the proportion of non-firing cells innervating ankle flexor muscles at P30 and P180.