^""^^"i^^^ r\ r^ 



— . - — ■'^ 



B 



FIGURE 3. — Diagram summarizing the organization of white muscle fibres in the myotome of Pacific herring ( A), compared with those 

 of rainbow trout and carp (B). It is not known whether a single axon may supply both large and small white fibres in herring (though 

 both are focally innervated), nor is it known which of the two alternative innervation patterns for rainbow trout and carp are actually 

 present. An overlap of innervation between small and large fibres seems most likely. Note the presence ofintermediate fibres between 

 red and white zones of the myotome in carp and rainbow trout; they are absent m herrmg. (B partly after Patterson et al. 1975 and 

 Johnston et al. 1977.) 



myotome (though on the same side of the fish and 

 fairly close to each other), the appearance of occa- 

 sional spikelike potentials in the white zone was 

 not always reflected directly in the record from the 

 red. At lower swimming speeds, when the elec- 

 trodes in the white zone did not pick up spikelike 

 potentials at every tail beat, higher recording 

 speed (Figure 4D) showed the variety of response 

 from the white zone of the same myotome at suc- 

 cessive tail beats. 



Spikelike potentials were present (although 

 usually <0.5 mV in amplitude) and were often 

 reflected at lesser amplitude by the electrodes in 

 the red portion of the myotome, but there were also 

 much smaller irregular potentials from the white 

 region of the myotome, resembling the smaller 

 irregular potential bursts from the multiply in- 

 nervated red fibres. In carp, both red and white 

 muscle fibres are multiply innervated and there 



694 



are intermediate fibres lying between red and 

 white fibre zones ( Figure 3B ). The electrodes in the 

 white portion of the myotome were placed close to 

 the spinal column so that they did not lie near the 

 intermediate zone recently described by Johnston 

 et al. (1977). 



Our results clearly indicated that the white 

 fibres were active even at low swimming speeds, 

 and that the activity at these speeds did not re- 

 semble the spikelike muscle potentials observed 

 when the fish are swimming faster. 



Rainbow Trout 



Rainbow trout were examined last of the three 

 fish studied and, to our surprise, gave results com- 

 parable with those from the herring, although in 

 salmonids the white fibres are multiply inner- 

 vated, as they are in carp. At speeds below 2 BL/s, 

 no activity was detectable from the white (mosaic) 



