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3.2 Circuits: What Goes Where
The “acoustic nerve,” cranial VIII, includes afferents from the sensory epithelia in
the endorgans of the ear: the three canal cristae and the otolithic endorgans (the
lagena, saccule, and utricle). Efferent axons (from the paired efferent nuclei in the
midline of the medulla) are also found in VIII as they travel to their peripheral projection sites. Afferent axons from the peripheral endorgans send their input to the
octaval column in the medulla (Fig. 2b, c). The octaval column has five nuclei in
toadfish (and all teleosts): the anterior octaval nucleus (AON), the MON, the DON,
the tangential octaval nucleus (TON), and the posterior octaval nucleus (PON), as
reported in detail by Highstein et al. (1992).
3.2.1 Saccular Inputs
Labeling of the entire saccular nerve bundle revealed projections to four of the five
octaval nuclei in the medulla, in order of greatest to fewest projections: DON, AON,
MON, and minimal input to TON. Distinct neuron bundles from the rostral, middle,
and caudal saccule were labeled individually and in pairs to evaluate the organization of input from hair cells with different orientations (compare regions in Fig. 1b)
in the two nuclei believed to be involved in auditory processing, the DON (dorsal
division, which includes both medial and lateral subdivisions) and AON (EddsWalton 1998a, b). The hypotheses tested included (1) a topographic organization
with afferent input in DON and/or AON reflecting the rostral-caudal organization of
the saccule; (2) vector organization, representing hair cell orientations, and (3)
extensive overlap of inputs consistent with analyses of the pattern of activity across
the entire saccule.
The regional labeling of saccular afferents as they exited the sensory epithelium
was designed to provide comparable projection data from (1) the afferents of the
vertically oriented hair cells (90°, dorsal and ventral with regard to the fish) in the
middle of the saccule, (2) from the rostral sweeping hair cell orientations (0–90° in
the vertical plane), and (3) from the caudal sweeping orientations (also 0–90° in the
vertical plane; Fig. 1b). The regional analyses revealed that there is great overlap in
the primary afferent projections that represent different hair cell orientations (but
see Fig. 6 in Edds-Walton 1998a for parallel input from the middle saccule). In other
words, there was no evidence for a simple topographic or vector map in either the
elongate DON or the much smaller AON. The data indicate that primary afferents
from different hair cell orientations on the saccule converge in the lateral to medial
axis and in the rostral to caudal axis of the dorsal region of the DON (= dDON,
above the descending tract of cranial V), as well as throughout the medial AON. From
the viewpoint of determining the direction of a sound source, the computations in
the DON are likely to consist of “weighting” of multiple inputs that converge onto
the dendrites of the principal cells (although some axosomatic endings were seen in
dDON; Edds-Walton 1998a).
P.L. Edds-Walton
3.2 Circuits: What Goes Where
The “acoustic nerve,” cranial VIII, includes afferents from the sensory epithelia in
the endorgans of the ear: the three canal cristae and the otolithic endorgans (the
lagena, saccule, and utricle). Efferent axons (from the paired efferent nuclei in the
midline of the medulla) are also found in VIII as they travel to their peripheral projection sites. Afferent axons from the peripheral endorgans send their input to the
octaval column in the medulla (Fig. 2b, c). The octaval column has five nuclei in
toadfish (and all teleosts): the anterior octaval nucleus (AON), the MON, the DON,
the tangential octaval nucleus (TON), and the posterior octaval nucleus (PON), as
reported in detail by Highstein et al. (1992).
3.2.1 Saccular Inputs
Labeling of the entire saccular nerve bundle revealed projections to four of the five
octaval nuclei in the medulla, in order of greatest to fewest projections: DON, AON,
MON, and minimal input to TON. Distinct neuron bundles from the rostral, middle,
and caudal saccule were labeled individually and in pairs to evaluate the organization of input from hair cells with different orientations (compare regions in Fig. 1b)
in the two nuclei believed to be involved in auditory processing, the DON (dorsal
division, which includes both medial and lateral subdivisions) and AON (EddsWalton 1998a, b). The hypotheses tested included (1) a topographic organization
with afferent input in DON and/or AON reflecting the rostral-caudal organization of
the saccule; (2) vector organization, representing hair cell orientations, and (3)
extensive overlap of inputs consistent with analyses of the pattern of activity across
the entire saccule.
The regional labeling of saccular afferents as they exited the sensory epithelium
was designed to provide comparable projection data from (1) the afferents of the
vertically oriented hair cells (90°, dorsal and ventral with regard to the fish) in the
middle of the saccule, (2) from the rostral sweeping hair cell orientations (0–90° in
the vertical plane), and (3) from the caudal sweeping orientations (also 0–90° in the
vertical plane; Fig. 1b). The regional analyses revealed that there is great overlap in
the primary afferent projections that represent different hair cell orientations (but
see Fig. 6 in Edds-Walton 1998a for parallel input from the middle saccule). In other
words, there was no evidence for a simple topographic or vector map in either the
elongate DON or the much smaller AON. The data indicate that primary afferents
from different hair cell orientations on the saccule converge in the lateral to medial
axis and in the rostral to caudal axis of the dorsal region of the DON (= dDON,
above the descending tract of cranial V), as well as throughout the medial AON. From
the viewpoint of determining the direction of a sound source, the computations in
the DON are likely to consist of “weighting” of multiple inputs that converge onto
the dendrites of the principal cells (although some axosomatic endings were seen in
dDON; Edds-Walton 1998a).
P.L. Edds-Walton