Single-unit labeling of medial olivocochlear neurons: the cochlear frequency map for efferent axons

Abstract

Medial olivocochlear (MOC) neurons are efferent neurons that project axons from the brain to the cochlea. Their action on outer hair cells reduces the gain of the "cochlear amplifier," which shifts the dynamic range of hearing and reduces the effects of noise masking. The MOC effects in one ear can be elicited by sound in that ipsilateral ear or by sound in the contralateral ear. To study how MOC neurons project onto the cochlea to mediate these effects, single-unit labeling in guinea pigs was used to study the mapping of MOC neurons for neurons responsive to ipsilateral sound vs. those responsive to contralateral sound. MOC neurons were sharply tuned to sound frequency with a well-defined characteristic frequency (CF). However, their labeled termination spans in the organ of Corti ranged from narrow to broad, innervating between 14 and 69 outer hair cells per axon in a "patchy" pattern. For units responsive to ipsilateral sound, the midpoint of innervation was mapped according to CF in a relationship generally similar to, but with more variability than, that of auditory-nerve fibers. Thus, based on CF mappings, most of the MOC terminations miss outer hair cells involved in the cochlear amplifier for their CF, which are located more basally. Compared with ipsilaterally responsive neurons, contralaterally responsive neurons had an apical offset in termination and a larger span of innervation (an average of 10.41% cochlear distance), suggesting that when contralateral sound activates the MOC reflex, the actions are different than those for ipsilateral sound.

Keywords: acoustic protection; cochlear amplifier; descending system; masking; outer hair cell.

Fig. 1.

Tuning curves from labeled medial olivocochlear (MOC) neurons. Response type and characteristic frequency (CF) are indicated next to each tuning curve. Each neuron was from a different preparation. SPL, sound pressure level; Ipsi, ipsilateral.

Fig. 2.

Photomicrograph of the termination of a labeled MOC branch in the organ of Corti, from the second turn of the cochlea. A total of 44 outer hair cells (OHCs) were innervated by this branch (15 in row 1, 23 in row 2, and 6 in row 3), and it formed a total of 64 endings (26 in row 1, 30 in row 2, and 8 in row 3). Two other branches of this axon terminated out of the field of view in smaller patches apically. This labeled axon was from an MOC Ipsi unit with a CF of 4.21 kHz.

Fig. 3.

Camera lucida drawing of the labeled axon of a single MOC neuron (Ipsi unit, CF 14.0 kHz) that terminates in the lower first (basal) turn of the cochlea. The drawing is a surface view of the organ of Corti, osseous spiral lamina, and intraganglionic spiral bundle. From the bottom of the drawing, the axon emerged from the modiolus and crossed the spiral ganglion. This crossing position for all axons of the database (avg. 15.13% distance from basal end) was independent of CF and did not differ significantly between Ipsi and contralateral (Contr)a axons (avg. 13.95 vs. 17.34%, t-test, P = 0.21). For the illustrated axon, the injection site (indicated by the axon swelling, nearby red blood cells, and graphite from the pin used to access the ganglion) was found in the spiral ganglion. At the peripheral edge of the ganglion, the axon ran briefly in the intraganglionic spiral bundle, where it gave off 2 branches that crossed the osseous spiral lamina (in our database the average number was 2.0 and the range was 1–4). The thicker branch had periodic constrictions (indicated by tick marks). These constrictions were interpreted as nodes of Ranvier (Liberman and Oliver 1984). In contrast, the thinner branch (in this case and in others) lacked nodes and was apparently unmyelinated or lightly myelinated. This thin branch formed one tunnel-crossing branch to innervate a small patch of OHCs (shading) basally, whereas the thicker branch formed 6 tunnel-crossing branches to innervate a larger patch of OHCs apically (in the database, the average number of tunnel-crossing branches per axon was 6.5 and the range was 2–14). The fine terminal branches are not indicated on the drawing, but they formed endings on OHCs. Each innervated OHC is indicated by a dot, and the hair cell rows are indicated by the numerals. A total of 30 OHCs were innervated (8 in row 1, 11 in row 2, and 11 in row 3) by a total of 35 endings (9 in row 1, 12 in row 2, and 14 in row 3). The arc indicates the span of innervation (497 μm, which is 2.6% of the total cochlear distance), and arrows indicate the innervation midpoint (located at a position that was 22.40% of total cochlear distance from the base to the apex) and center of gravity (a fraction 0.729 of the distance from the basal-most to apical-most innervated OHC, which is at a position 22.99% of the total cochlear distance, see materials and methods). This axon was labeled with horseradish peroxidase but the appearance of MOC axons labeled with biocytin was qualitatively similar. This was the only MOC neuron injected in this cochlea and this was the only axon that was recovered. An auditory-nerve fiber was recorded at the same site and its CF was 17.6 kHz, but it was not injected.

Fig. 4.

Drawings of labeled axons of 4 other MOC neurons, positioned onto templates of a portion of the cochlear spiral (thin gray shading indicates position of OHCs). The templates are aligned such that the 13.18-kHz point is uppermost on the drawing (an orientation similar to Fig. 3). The MOC unit CFs are given next to the drawings and the midpoints of innervation are indicated by the arrows. The unit ID numbers, response types, numbers of innervated hair cells, and spans are as follows: A: MCB 460.6, Ipsi unit, 24 OHCs, 230 μm (1.28% cochlear distance); B: MCB 432.5b, Contra unit, 14 OHCs, 3,334 μm (21.24%); C: MCB 454.2, Ipsi unit, 69 OHCs, 1,567 μm (9.47%); D: MCB 521.7, Contra unit, 56 OHCs, 816 μm (4.25%). Labeled auditory-nerve fibers and ganglion cells (arrowheads, with their CFs given in italicized numbers) in the same cochleas are shown. The positions of these nerve fibers, along with the formula for nerve-fiber mappings (see legend of Fig. 7B), were used to calculate the location of the cochlear amplifier for the CF of the MOC axons (gray shading, also see Fig. 9). The cochlear amplifier location (for a nerve fiber) begins at the CF position and extends basally one-fourth octave in the cochlear base (A and B). The extent of the amplifier may be larger apically (C and D), since there is an increase in 2-tone suppression (Prijs 1989) and calculated gain functions (Shera 2007). A doubling in extent of the amplifier is assumed but the “?” symbols on the figure indicate the amount of increase is unknown.

Fig. 5.

Number of outer hair cells innervated (A) and endings formed (B) for MOC axons, plotted as a function of CF. Symbols code for response type (see key).

Fig. 6.

Number of OHCs innervated by MOC axons in row 1 (A), row 2 (B), and row 3 (C), plotted as a function of CF. Symbols code for response type (see key).

Fig. 7.

Mappings for labeled MOC axons and auditory-nerve fibers onto the organ of Corti as a function of CF. Innervation midpoint or center of gravity (e.g., Fig. 3) are organ of Corti distance from basal end as a percent of total distance from base to apex. A: innervation midpoint for axons of MOC neurons (see key for response type), with best-fit lines separately for Ipsi units [dashed line, %Dis = 75.53 − 43.85 × log(CF), R = 0.93] and Contra units [dotted line, %Dis =80.50 − 44.92 × log(CF), R = 0.95]. B: innervation midpoint for MOC axons compared with innervation point for auditory-nerve fibers, with best-fit regression for all MOC axons [solid line, %Dis = 78.2 − 46.1 × log(CF), R = 0.95] and for nerve fibers [dashed line, %Dis = 70.2 − 40.0 × log(CF), R = 0.99]. Dotted line shows the mapping for auditory-nerve fibers from Tsuji and Liberman (1997). An adjusted version of the MOC mapping (see results), using the positions of nerve fibers of similar CFs in individual cochleas (Fig. 4), was %Dis = 78.73 − 45.96 × log(CF), R = 0.93 (not plotted), which differed little from the unadjusted best-fit line above. C: innervation center of gravity position mapping for MOC units [points, dashed line: %Dis =77.5 − 46.3 × log(CF), R = 0.95], which is similar to the innervation midpoint line (solid line from B).

Fig. 8.

A: fractional center of gravity of MOC terminal arbors (see Fig. 3 and materials and methods) as a function of CF. B: innervation span for MOC axons as a function of CF. Span (in %cochlear distance) is measured from the most basal to the most apically innervated OHC (see Fig. 3).

Fig. 9.

Comparison of MOC axon midpoints (symbols) and spans (bars) with the extent of the cochlear amplifier. Innervation midpoints are plotted as a function of CF, with bars showing the innervation span for each axon. Gray shading shows the approximate extent of the OHCs involved in the cochlear amplifier, which occupies a one-quarter octave extent basal to the auditory-nerve fiber CF position in the cochlear base. The graphed extent increases apically, since there is an increase in two-tone suppression (Prijs 1989) and calculated gain functions (Shera, 2007). On the graph, the amount of increase is a doubling in the apex, but the “?” signifies that the extent of this increase is unknown.