Fig. 5. Discrimination of the M-spike with extracellular recordings in caudal hindbrain. (A) Schematic representation of the experimental arrangement, superimposed drawings of the M-cells onto the medulla oblongata with simultaneous paired intra- and extracellular recording were used for B-E. CB, cerebellum; VII, facial lobe; X, vagal lobes (B,Ci,Cii) Extracellular responses recorded close to the left M-axon. Stimulus intensity was gradually increased from subthreshold for both M-axons (black), to above threshold for one (blue) or both (red). Asterisk indicates M-spike(s). (Ci,Cii) Amplitudes of the surface M-spike fields in B as functions or recording distance from the midline of the caudal hindbrain (Ci) and the amplitude of the single spike at the midline as a function of depth from the surface (Cii). Negative depths are in the saline superfusate. (D) Hindbrain field potentials evoked by orthodromic M-cell activation. Suprathreshold current injection in the left M-cell soma (top), and the corresponding M-axon spike and compound action potentials in the hindbrain (middle). After moving the intracellular electrode to the left M-soma (not shown), the directly activated left M-axon spike and compound field potentials was recorded instead (asterisk) (bottom). Asterisk and horizontal broken line indicate M-axon spike and M-triggered activity, respectively. (E) Compound fields correlated with sound evoked M-cell spike activity. The sound stimulus (top) evoked an EPSP recorded from the M-soma before (red) and after (blue) Cl^- injection; (bottom) corresponding fields recorded in the hindbrain. Note the two M-spikes (asterisks) are separated temporally by M-triggered activity (broken line).