The effects of head and tail stimulation on the withdrawal startle response of the rope fish (Erpetoichthys calabaricus).

While most actinopterygian fishes perform C-start or S-start behaviors as their primary startle responses, many elongate species instead use a withdrawal movement. Studies of withdrawal have focused on the response to head-directed or nonspecific stimuli. During withdrawal, the animal moves its head back from the stimulus, often resulting in several tight bends in the body. In contrast to C-start or S-start behaviors, withdrawal to a head stimulus generally does not involve a subsequent propulsive stage of movement. We examined intraspecific diversity in withdrawal behavior and muscle activity patterns of the rope fish, Erpetoichthys calabaricus, in response to stimulation of the head and the tail. In addition, we describe the anatomy of the Mauthner cells and their axon caps, structures that are generally absent in species with a withdrawal startle. We recorded high-speed video (250 Hz) and electromyograms (EMGs) from 12 electrodes in the axial muscle during the behavioral response. We used Bodian silver staining techniques to visualize Mauthner cell and axon cap morphology. We found that E. calabaricus responds with a withdrawal to both head and tail stimulation. Tail stimulation elicits a stronger kinematic and muscle activity response than head stimulation. While withdrawal movement generally constitutes the entire response to head stimuli, withdrawal was followed by propulsive movements when the tail was stimulated, suggesting that withdrawal can both act alone and serve as the first stage of a propulsive startle. Unexpectedly, bilaterality of muscle activity was variable for responses to both head and tail stimuli. In addition, we were surprised to find that E. calabaricus has a distinct axon cap associated with its Mauthner cell. These data suggest that the withdrawal response is a more diverse functional system than has previously been believed.

Hox gene misexpression and cell-specific lesions reveal functionality of homeotically transformed neurons.

Hox genes are critical for establishing the segmental pattern of the vertebrate hindbrain. Changes in their expression can alter neural organization of hindbrain segments and may be a mechanism for brain evolution. To test the hypothesis that neurons induced through changes in Hox gene expression can integrate into functional neural circuits, we examined the roles of ectopic Mauthner cells (M-cells) in the escape response of larval zebrafish. The activity of the paired Mauthner cells in rhombomere 4 (r4) has been shown to be critical for generating a high-performance startle behavior in response to stimulation of the tail (Liu and Fetcho, 1999). Previous studies have found that misexpression of particular Hox genes causes ectopic M-cells to be generated in r2 in addition to the r4 cells (Alexandre et al., 1996; McClintock et al., 2001). With calcium imaging, we found that the homeotically transformed neurons respond to startle stimuli. To determine the roles of ectopic and endogenous M-cells in the behavior, we lesioned the r2, r4, or both M-cells with cell-specific laser lesion and examined the effect on startle performance. Lesion of the normal M-cells did not decrease escape performance when the ectopic cells were present. These results indicate that the homeotically transformed Mauthner cells are fully functional in the escape circuit and are functionally redundant with normal M-cells. We suggest that such functional redundancy between neurons may provide a substrate for evolution of neural circuits.

Strikes and startles of northern pike (Esox lucius): a comparison of muscle activity and kinematics between S-start behaviors.

S-starts are a major class of fast-start behaviors that serve diverse locomotor functions in fishes, playing roles in both feeding strike and escape startle events. While movement patterns are similar during strike and startle, their motor control mechanisms have not been compared. To investigate heterogeneity in S-start responses and to test the hypothesis that S-starts are generated by the same patterns of muscle activity regardless of the behavioral context in which they function, we examined kinematic and muscle activity patterns of northern pike (Esox lucius) performing feeding and escape S-starts. Movements were recorded with high-speed video (250 Hz). Muscle activity was recorded from seven electrodes, one in the left adductor mandibulae and bilaterally in the anterior, midbody and posterior epaxial white muscle. Although S-shaped movements are produced in both feeding and escape, kinematics and electromyogram (EMG) patterns differ. Stage 1 (pre-propulsive movement) is significantly slower and more variable during feeding strikes and involves caudal bending with less rostral movement than recorded for startle behaviors. Correspondingly, there is strong caudal muscle activity prior to rostral activity during strikes, whereas in startles caudal muscle activity had near simultaneous onset with contralateral rostral activity. Onset of jaw muscle activity occurred significantly after the onset of axial muscle activity during feeding strikes. By contrast, during startles, jaw activity onset was nearly simultaneous with the onset of axial muscle activity. Stage 2 kinematics generally did not differ between the strike and startle; however, EMGs indicate that stage 2 movements are generated by different patterns of muscle activity for the two behaviors. Although strikes and startles are similar in their propulsive performance, they appear to be initiated and driven by fundamentally different motor control mechanisms. We suggest that S-start startle behavior is mediated by a simple system of descending reticulospinal input to spinal neurons while the S-start strike involves a more complex neural circuit, allowing greater modulation of stage 1 movements while maintaining high stage 2 performance.