209
octaval nuclei, including dDON. Therefore, convergence of otolithic inputs in the
dDON may be common among fishes, including those with different peripheral
anatomy.
3.2.3 Medulla to Midbrain
Injections of neurobiotin into physiologically characterized auditory sites in the
midbrain nucleus centralis (NC in Figs. 2b, c and 3b) revealed the sites in the
medulla that are components of the ascending auditory pathway. Inputs to the auditory midbrain originate primarily in the contralateral DON, with smaller contributions from the ipsilateral DON and secondary octaval (SO) nuclei in the medulla,
which may receive input from both the ipsilateral and contralateral DONs (EddsWalton and Fay 2005a) (Figs. 2c and 3b).
The function of AON in the auditory circuit of toadfish remains to be revealed.
Cells in AON receive substantial saccular input (Edds-Walton et al. 1999) and
respond to auditory frequencies (Edds-Walton, unpublished data), but the connectivity of AON is not known. A small number (<5) of retrogradely filled cells were
present in AON in some, but not all, investigations of inputs to nucleus centralis
(Edds-Walton 1998a; Edds-Walton and Fay 2005a). The scarcity of these fills suggests either that AON contributes little to the ascending auditory pathway or that
AON contributes indirectly, via intermediate nuclei, such as the secondary octaval
nuclei. For the second scenario, the retrograde fills in AON would have been due to
trans-synaptic retrograde spread of the low molecular weight label (3000 mw dextran amine) from the dorsal SO nucleus (SOdor) to AON during incubation. Thus
far, those two possibilities have not been investigated. Injections of label into AON
(without involving other medullary nuclei or tracts) for anterograde transport that
would reveal the target(s) of AON projection cells have not been successful. In addition, evaluating the origins of inputs to the SO nuclei is challenging due to the location and small size of the nuclei, which make discrete labeling of only those nuclei
extremely difficult. However, it is critical to characterize the role that each may have
in binaural processing of sound.
Although the MON and TON do not send projections to the auditory processing
regions of the midbrain, the possibility remains that saccular projections to MON/
TON reflect inputs for orientation and reflex responses to sound (see Physiology
below). The utricle also provides input to these two nuclei, along with substantial
inputs from the semi-circular canal cristae (Highstein et al. 1992). Extracellular
recording followed by injection of neurobiotin at the recording site revealed that a
subset of MON cells (in M2, M3 subnuclei designated by Highstein et al. 1992)
respond to particle motion stimuli (mostly ≤100 Hz) and others (in M3) respond to
both particle motion and lateral line stimuli (water motion), consistent with lateral
line input to M3 (Highstein et al. 1992). The auditory and bimodal cells responded
best to sounds in the horizontal plane, potentially reflecting inputs from the rostral
or caudal saccule (Fig. 1b) or from the horizontally oriented utricle (Edds-Walton
et al. 2013). In addition, ipsilateral label injections into auditory sites in MON also
What the Toadfish Ear Tells the Toadfish Brain About Sound
octaval nuclei, including dDON. Therefore, convergence of otolithic inputs in the
dDON may be common among fishes, including those with different peripheral
anatomy.
3.2.3 Medulla to Midbrain
Injections of neurobiotin into physiologically characterized auditory sites in the
midbrain nucleus centralis (NC in Figs. 2b, c and 3b) revealed the sites in the
medulla that are components of the ascending auditory pathway. Inputs to the auditory midbrain originate primarily in the contralateral DON, with smaller contributions from the ipsilateral DON and secondary octaval (SO) nuclei in the medulla,
which may receive input from both the ipsilateral and contralateral DONs (EddsWalton and Fay 2005a) (Figs. 2c and 3b).
The function of AON in the auditory circuit of toadfish remains to be revealed.
Cells in AON receive substantial saccular input (Edds-Walton et al. 1999) and
respond to auditory frequencies (Edds-Walton, unpublished data), but the connectivity of AON is not known. A small number (<5) of retrogradely filled cells were
present in AON in some, but not all, investigations of inputs to nucleus centralis
(Edds-Walton 1998a; Edds-Walton and Fay 2005a). The scarcity of these fills suggests either that AON contributes little to the ascending auditory pathway or that
AON contributes indirectly, via intermediate nuclei, such as the secondary octaval
nuclei. For the second scenario, the retrograde fills in AON would have been due to
trans-synaptic retrograde spread of the low molecular weight label (3000 mw dextran amine) from the dorsal SO nucleus (SOdor) to AON during incubation. Thus
far, those two possibilities have not been investigated. Injections of label into AON
(without involving other medullary nuclei or tracts) for anterograde transport that
would reveal the target(s) of AON projection cells have not been successful. In addition, evaluating the origins of inputs to the SO nuclei is challenging due to the location and small size of the nuclei, which make discrete labeling of only those nuclei
extremely difficult. However, it is critical to characterize the role that each may have
in binaural processing of sound.
Although the MON and TON do not send projections to the auditory processing
regions of the midbrain, the possibility remains that saccular projections to MON/
TON reflect inputs for orientation and reflex responses to sound (see Physiology
below). The utricle also provides input to these two nuclei, along with substantial
inputs from the semi-circular canal cristae (Highstein et al. 1992). Extracellular
recording followed by injection of neurobiotin at the recording site revealed that a
subset of MON cells (in M2, M3 subnuclei designated by Highstein et al. 1992)
respond to particle motion stimuli (mostly ≤100 Hz) and others (in M3) respond to
both particle motion and lateral line stimuli (water motion), consistent with lateral
line input to M3 (Highstein et al. 1992). The auditory and bimodal cells responded
best to sounds in the horizontal plane, potentially reflecting inputs from the rostral
or caudal saccule (Fig. 1b) or from the horizontally oriented utricle (Edds-Walton
et al. 2013). In addition, ipsilateral label injections into auditory sites in MON also
What the Toadfish Ear Tells the Toadfish Brain About Sound