Fig. 8. Alternative neural circuit models for the S-start startle response. The left-hand circuit depicts a possible S-start mechanisms involving multiple reticulospinal pathways. Commissural and ipsilateral reticulospinal cells cause motoneuron and muscle activity regionally in the spinal cord. The right-hand circuit depicts a possible C-start model involving a combination of reticulospinal and local circuits. Anterior activity is determined through descending reticulospinal commands, while posterior activity is generated through local spinal cord circuits. The spinal cord interneurons are presumed to be the same as those used in the C-start and are depicted as such. The numbers of cells indicated in the diagrams do not reflect the numbers or distributions in the fish, nor do they reflect the relative numbers of different cell types. Instead, they simply illustrate regional differences along the body