1Department of Biomedical Engineering and Hearing Research Center and 2Department of Otolaryngology, Boston University, Boston, Massachusetts 02215-2407
The Journal of Neurophysiology Vol. 87 No. 5 May 2002, pp. 2520-2530.
Hancock, Kenneth E. and
Herbert F. Voigt.
Intracellularly Labeled Fusiform Cells in Dorsal Cochlear Nucleus
of the Gerbil. II. Comparison of Physiology and Anatomy.
J. Neurophysiol. 87: 2520-2530, 2002.
cells represent the major class of dorsal cochlear nucleus (DCN)
projection neuron. Although much is understood abouttheir physiology
and anatomy, there remain unexplored issues withimportant functional
implications. These include interspeciesdifferences in DCN physiology
and the nature of the cell-to-cellvariations in fusiform cell
physiology. To address these issues,a quantitative examination was
made of the physiology and anatomyof 17 fusiform cells from a
companion study. The results suggestthat the basal dendrites
of gerbil fusiform cells may be electrotonicallymore compact than
those of the cat. This relative decrease inthe filtering of excitatory
inputs might account for the lowerincidence of type IV units in that
species. These data also suggestthat the gerbil DCN lacks the
high-frequency specialization describedin the cat, because the
tonotopic arrangement of the gerbil fusiformcells quantitatively
matches the place-frequency map for the gerbilcochlea. Certain
physiological properties have anatomical correlates.First, the basal
dendrites of low spontaneous rate cells are directedaway from the soma
only in the caudal direction, while the highspontaneous rate cells
have basal dendrites extending rostrallyand caudally. Second, input
resistance was dominated by the surfacearea of the apical dendrite.
Third, the discharge pattern wascorrelated with apical dendrite
orientation. Finally, there wasa spatial gradient of sensitivity to
broadband noise organizedat least partially within an isofrequency
axis. Such trends indicatethat neighboring fusiform cells are endowed
with different signalprocessingcapabilities.
The cochlear nuclei are the sole target of the auditory nerve (AN) and as such represents an obligatory processing stage inthe ascending auditory pathway. The laminated dorsal cochlearnucleus (DCN) contains a variety of morphological cell types exhibitingdiverse physiological responses. The outputs of the DCN arisefrom fusiform cells and giant cells, which project via the dorsalacoustic stria to the contralateral inferior colliculus (Adamsand Warr 1976). Fusiform cells are readily identified by largecell bodies and bipolar dendritic fields (Brawer et al. 1974;Lorente de Nó 1981). In the superficial layer, spinous apicaldendrites interact through a network of granule cells and cartwheelcells (Berrebi and Mugnaini 1991; Golding and Oertel 1997; Mugnainiet al. 1980) with somatosensory (Itoh et al. 1987; Weinberg andRustioni 1987; Wright and Ryugo 1996), vestibular (Burian andGstoettner 1988; Kevetter and Perachio 1989), and descending auditoryinputs (Benson and Brown 1990; Weedman and Ryugo 1996). The distalportion of the basal dendrite is excited by the descending branchof the auditory nerve (Smith and Rhode 1985), while the soma andproximal dendrites are likely inhibited by vertical cells (Saint-Marieet al. 1991; Voigt and Young 1980, 1990) and possibly by stellatecells of the posteroventral cochlear nucleus (PVCN) (Oertel etal. 1990; Zhang and Oertel 1994).
There are several theories regarding fusiform cell function. For example, strong sideband inhibition may serve to enhancethe representation of spectral peaks (Rhode and Greenberg 1994)or to extend dynamic range in the presence of noise (Palmer andEvans 1982). DCN neurons better code the envelopes of amplitude-modulatedstimuli than do auditory nerve fibers (Backoff et al. 1999; Kimet al. 1990), leading to the postulation of a "second axis" thatcodes for envelope frequency (Kim et al. 1990) or periodicitypitch (Langner and Schreiner 1996). Finally, recent evidence hasled to the theory that the DCN extracts spectral cues relevantfor sound localization. The head-related transfer function (HRTF)of the cat contains a prominent notch whose center frequency variesbetween 8 and 30 kHz according to the elevation of the sound source(Musicant et al. 1990; Rice et al. 1992). This frequency rangehas an enlarged representation in the cat DCN as compared withthe cochlea (Spirou et al. 1993). Type IV units, an importantsubset of DCN projection neurons, show sensitivity to both thewidth and center frequency of notches in broadband stimuli (Nelkenand Young 1994; Spirou and Young 1991), as do type III units ingerbils (Parsons et al. 2001).
The present report describes the physiology and anatomy of 17 intracellularly recorded and labeled fusiform cells from theDCN of anesthetized gerbils. The fusiform cells in gerbils andcats differ in their physiological response properties in thedecerebrate preparation. In particular, the incidence of typeIV units in the gerbil is less than one-third that reported inthe cat (Davis et al. 1996; Shofner and Young 1985). Antidromicstimulation studies in the cat indicate that at least a portionof the type IV unit population corresponds to fusiform cells (Young1980). Direct intracellular recording and labeling studies, however,suggest that gerbil fusiform cells are not type IV units (Dinget al. 1999). This difference across species in the response propertiesof an important projection neuron motivates one aim of this study:to make a detailed quantitative comparison of gerbil fusiformcell anatomy to that of the cat. Golgi studies in the cat providea suitable database of anatomical measurements for comparison(Blackstad et al. 1984), including the dimensions of each dendriticarbor. The anatomical analysis of this study suggests, in part,that gerbil fusiform cells may be electrotonically more compactthan those of the cat, and that this difference may account forsome of the observed differences in acoustic responseproperties.
Another issue is that fusiform cells exhibit a variety of response properties. In the decerebrate preparation, they have typeIII unit and type IV unit response maps (Ding et al. 1999; Young1980). In anesthetized preparations, fusiform cells exhibit pauser/buildup,chopper, or onset discharge patterns, depending on stimulus conditions(Hancock and Voigt 2002; Rhode et al. 1983; Rhode and Smith 1986;Smith and Rhode 1985). The existence of such variations must berelated to the complexity of the neural circuits with which fusiformcells interact. But are the cell-to-cell differences merely theresult of random "wiring" differences, or do they reflect underlyingprinciples of organization? This question motivates a second aim:to make a quantitative comparison of fusiform cell physiologyand morphology in cats and gerbils. Certain physiological characteristicswere indeed found to have specific anatomical correlates. Spontaneousrate (SR) was related to the disposition of the basal dendrites,input resistance was correlated with apical dendrite total length,and the discharge pattern at best frequency (BF) was correlatedwith fusiform cell orientation. It appears that neighboring fusiformcells may have different physiological properties and hence differentsignal processing capabilities by virtue of cell-to-cell variationsinmorphology.
This work represents part of the doctoral dissertation of K. E. Hancock.
Cell location was quantified as suggested in Fig. 1. The bottom of the figure shows a series of coronal sections, one of whichcontains a hypothetical neuron, indicated by the dark circle.The position z corresponds to the distance between the cell bodyand the rostral pole of the nucleus, while L indicates the totallength of the nucleus in the rostral-caudal direction. Near therostral end of the nucleus, the number of layers typically decreasedfrom three to two; the section where layering disappeared altogetherwas selected as the rostral pole.
The section containing the soma is shown rotated at the top of Fig. 1 and illustrates measurements made within the coronalplane. The position y is the depth along a line perpendicularto the ependymal surface. The value H is the length of this lineextended to the bottom edge of the nucleus. The position x isthe length of the arc along the bottom edge measured from theventrolateral side to the intersection with the line used to measuredepth. The width W is the total arc length of the bottom edgemeasured from ventrolateral to dorsomedial. The positions x,y, and z were normalized by W, H, and L, respectively, to givethe relative positions Px, Py, and Pz.
The fusiform cells were reconstructed in three dimensions working from cameral lucida drawings using custom-designed software.The dendritic structure was analyzed quantitatively from the reconstructiondata using a set of MATLAB (Mathworks) scripts. Total dendriticlength was computed by approximating each dendrite as a sequenceof small cylinders and summing the cylinderlengths.
Measurements were made individually on each of the apical and basal arbors. The methods follow those detailed by Blackstadet al. (1984) and will be described briefly here. The first stepwas to determine the long axis of the arbor. Blackstad et al.performed this task manually, whereas in this study an automaticmethod was adopted that consisted of computing the line betweenthe cell body and the center of mass of the dendritic terminals.The arbor was then rotated about its long axis in 1° steps. Ateach step the span of the arbor perpendicular to the long axiswas computed. The arbor thickness was defined as the narrowestspan, while the arbor width was defined as the widest span. Thedegree of planarity was quantified by computing the width to thicknessratio. The arbor height was measured as the extent of the arboralong its longaxis.
Geometry of the dendritic arbors
The fusiform cell dendritic arbors were quantified by a series of measurements inspired by the work of Blackstad et al. (1984)in the cat and described fully in METHODS. The results serve asa basis for comparison of the gerbil to the cat and as a basisfor a subsequent quantitative examination of the relationshipbetween physiology and anatomy. It is important to note that Blackstadet al. limited their consideration to the central third of theDCN and thus avoided effects due to the significant curvatureof the nucleus at its edges. It was not possible to do likewisein our case due to the sample size limitations inherent to anin vivo intracellular survey of thiskind.
The anatomical properties of the apical arbors are summarized in Table 1. Estimates were made of the total dendritic lengthsby summing the lengths of the individual segments comprising eachthree-dimensional (3-D) reconstruction. The mean apical lengthwas 3,359 µm, which is very close to the mean value of 3,212 µmreported by Blackstad et al. (1984) in the cat. The width andthickness measurements were obtained by rotating the arbor aboutits long axis to find the widest and narrowest projections, respectively.Both measures are smaller in the gerbil than in the cat, althoughthe ratio, which reflects the degree of arbor planarity, is approximatelythe same in both cases (2.56 in the gerbil, roughly 3 in the cat).The mean height of the apical arbor, measured parallel to thelong axis, was 157 µm in the gerbil as compared with 267 µm inthe cat.
Table 2 lists the anatomical properties of the basal dendrites in this study. The mean total length was 1,925 µm, as comparedwith 2,728 µm in the cat. The mean arbor width is smaller in thegerbil (314 µm) than in the cat (392 µm), but the thickness islarger (112 vs. 72 µm). The result is that the width to thicknessratio in the gerbil (3.07) is about half that in the cat (5.70).The mean basal arbor is 250 µm in height as compared with 389 µm in the cat.
A qualitative illustration of the tonotopic organization is presented in Fig. 2A, in which the 17 fusiform cells of this studyhave been mapped onto a single DCN. Only the basal dendrites havebeen drawn because that is presumably where fusiform cells receiveacoustic input (Smith and Rhode 1985). The dendrites are colorcoded according to BF, as shown. Figure 2A qualitatively suggeststhat the gerbil DCN has a substantial volume devoted to relativelow BFs. The orderly arrangement of cells by BF is also apparent,with the lowest BFs in the rostral-most and ventrolateral-mostaspect of the nucleus.
Figure 2A shows that BF increases in both the dorsomedial and caudal directions. This observation is quantified in Fig. 2Bwhere the BF is plotted as a function of the relative X-positions(ventrolateral-to-dorsomedial location) and the relative Z-positions(rostral-to-caudal location) of the fusiform cell somata. Thestraight line fits to these data indicate that BF is indeed highlycorrelated with these twodimensions.
The tonotopic axis is described by the equation log(BF) = 1.45Px + 1.01Pz (r = 0.81, P < 0.001), obtained by performing multiplelinear regression on log BF using both the relative X- and Z-positions.A tonotopic position was determined for each neuron by projectingits location onto this axis and computing the distance from theorigin (Px = Pz = 0). The resulting positions are plotted in Fig.3 as a function of BF. For comparison, the place-frequency mapfor the gerbil cochlea is also plotted, where position indicatesthe relative distance from the base of the cochlea. In general,Fig. 3 shows that the spatial distribution of BFs in the DCN closelyfollows the distribution of characteristic frequencies (CFs) alongthe cochlea. For comparison, the place-frequency map computedfor the cat DCN by Spirou et al. (1993) is also plotted.
Fusiform cell orientation varies with location
Figure 4 summarizes a systematic shift in the orientation of the long axis as a function of rostral-caudal position. In therostral end of the nucleus, the apical dendrites tend to be positionedon the rostral side of the soma while the basal dendrites extendin the caudal direction. Near the center of the DCN, the orientationis roughly vertical (dorsal to ventral). Toward the caudal pole,fusiform cells have caudally directed apical dendrites and rostrallydirected basal dendrites.
It would appear that this trend is a consequence of the shape of the DCN itself. The DCN can be visualized as a series ofthin shells, with the molecular layer wrapped around the fusiformcell layer, which in turn is wrapped around the deep layer. Thusfrom any position within the fusiform cell layer, the apical dendritesradiate outward to fill the molecular layer while the basal dendritesconverge inward to occupy the deep layer. A similar finding wasreported by Rhode et al. (1983), who further describe the cellsin the rostral end having more numerous apical dendrites thatmore frequently emerge directly from the soma. Neither this studynor the previous one finds any obvious physiological sequellaeto this trend, which possibly is simply a consequence of the overallcurvature of thenucleus.
Spontaneous rate depends on basal dendrite orientation
The spontaneous discharge rates of the fusiform cells in this sample range from 0 to 54 spikes/s. These can be separated intoa low SR group (rate <2.5 spikes/s, 8/17 cells) and a high SRgroup (rate >7.5 spikes/s, 9/17 cells). The creation of two SRcategories may, in fact, represent an artificial division of acontinuously distributed property, but is useful here as a convenientmeans of visualizing a related anatomical trend. Specifically,there is a correlation between spontaneous rate group and thedisposition of the basal dendrites, as depicted in Fig. 5. Thebasal dendrites of the low SR cells (top row) are primarily directedcaudally away from the soma. The high SR cells (bottom row) tendalso to have branches oriented in the rostral direction, givingthe distribution of the basal dendrites a more symmetrical appearance.
These observations were put on a more quantitative basis by computing the centroid of each basal arbor with respect to thesoma. The trend is specifically captured by the rostral-caudalcomponent, ZC, as shown in Fig. 5. Note that the value of ZC ispositive for positions caudal to the soma. For every low SR cell(ZC 63.2 µm), the basal arbor centroid is caudal to that ofevery high SR cell (ZC 48.8 µm). No correlation was found betweenthe absolute position of the basal arbor centroid and spontaneousactivity.
The nine cells shown in Fig. 5 come from the same general region of the DCN, reflected by the similarity of their best frequencies.This is an important consideration when examining the arrangementof the basal dendrites, because as described above, the orientationof the fusiform cell long axis changes as a function of position(Fig. 4).
Input resistance is a function of total apical dendritic length
Figure 6 shows that fusiform cell input resistance is correlated with apical dendritic length, but not with basal dendriticlength. The best line fit to the apical length was computed afterremoving the two points indicated by triangles in Fig. 6A. Thecorrelation with apical length is negative, so that larger lengthscorrespond to smaller resistances. This is consistent with a passivemodel in which membrane conductance is proportional to surfacearea, insofar as dendritic surface area is proportional to totallength. Input resistance was not correlated with the sum of theapical and basal lengths (r = 0.06). The presence of spines,however, greatly increases the surface area per unit length ofthe apical dendrite and so a simple sum of total lengths probablyunderstates the contribution of the apical dendrite to the overallpassive membrane conductance. We did not consider the effectsof surface area more directly because of difficulties estimatingit that arose from inconsistencies in the quality of spine labeling.
Regularity histogram shape depends on orientation of apical arbor
Regularity histogram shapes were quantified using three slope measurements as described in the companion paper. Briefly, thetransient slope, mT, most closely follows the classification schemeof Gdowski (1995). It is particularly sensitive to the changein interspike interval over the initial 5-10 ms of response andsince nearly all of the cells in this study had decreasing intervalsover this range, regardless of their subsequent behavior, thetransient slope was not necessarily an effective means of characterizingrate trends over the entire stimulus duration. To better capturethese rate trends, the slope measurements m1 and m2 were computedby performing a two-line fit to the interspike interval data afteromitting the firstbin.
Figure 7 shows that for the 10 fusiform cells with BFs less than 2 kHz, the slope m1 is correlated with the orientation ofthe apical dendrites. The angle apical corresponds to the rotationrequired to find the narrowest dendritic profile after makingthe long axis of the arbor vertical. A value of zero correspondsto an arbor oriented perpendicular to the coronal plane. The apicaldendrites in Fig. 7 are drawn looking down on their tops, suchthat their long axes project out of the page. The values of m1have been divided into three groups: high, medium, and low, indicatedin Fig. 7 by circles, triangles, and squares, respectively. Thedata indicate that the fusiform cells with apical arbors orientedmost nearly perpendicular to the coronal plane have the most positivem1 values (circles), while those oriented closest to parallelto the coronal plane have the most negative m1 values (squares).
Correlation of cell properties with location
An effort was made to identify fusiform cell characteristics that vary systematically with location. To do this, a wide varietyof physiological response metrics (see companion paper) were systematicallycorrelated with the normalized X-, Y-, and Z-position measurements(see METHODS).
Two statistically significant trends were identified, roughly orthogonal to the tonotopic axis. The relative noise index decreasedin the X-direction (ventrolateral to dorsomedial), meaning thatthe responses to noise became progressively weaker in this direction(Fig. 8A). A gradient in wideband inhibitory strength is sufficientto account for this observation, as detailed in the DISCUSSION.The second trend was that input resistance also tended to decreasein the X-direction (Fig. 8B). This is consistent with the factthat total apical dendritic length increases in the same direction(Fig. 8C), since the inverse relationship between these two propertieshas already been described.
Aside from tonotopy, the only statistically significant spatial trends were found in the X-direction. Since neither trendwas accompanied by a significant dependence on Z-position, itwas not possible to compute an axis trajectory for quantitativecomparison with the tonotopic axis computed above. It can, however,be said that while best frequency is strongly correlated withboth X- and Z-position, input resistance and relative noise responseare strongly correlated only with X-position. So, although theseresults cannot resolve the issue of an orthogonal axis per se,there is at least a qualitative suggestion that the spatial gradientsof these two properties are not parallel with the frequencyaxis.
A detailed quantitative examination of fusiform cell physiology and anatomy was made in this study. The following findingswill be discussed. 1) The basal dendrite in the gerbil is shorterthan in the cat. 2) The gerbil DCN appears to lack the high-frequencyspecialization of the cat DCN. 3) High spontaneous firing ratesare correlated with basal dendrites having both rostrally andcaudally directed branches, while low spontaneous rates are correlatedwith basal dendrites having only caudally directed branches. 4)The inhibition reflected in the slope of the regularity histogramis related to the orientation of the apical arbor. 5) Noise responsestrength appears to be systemically organized within an isofrequencysheet.
Consideration of the basal arbor total length
The basal dendrites in the gerbil have about 70% the total length that they do in the cat and thus may be electrotonicallymore compact. The equivalent cylinder model for a dendritic arborhas a characteristic electrotonic length, which is proportionalto the ratio of the physical length of the cylinder to the squareroot of its diameter (Rall 1977). Consider a hypothetical catfusiform cell that has three primary basal dendrites giving riseto identical branching structures. One possibility is that the"gerbil" fusiform cell retains all three branches, but each branchis shortened by a third. The equivalent cylinder for such a cellwould be physically shorter than its counterpart in the cat, buthave the same diameter and hence a shorter electrotonic length.A gerbil fusiform cell could also be produced by removing oneof the three hypothetical branches, giving the observed one-thirdreduction in total length. The equivalent cylinder would havethe same physical length as the cat fusiform cell, but its diameterwould be smaller, resulting in a longer electrotoniclength.
Which case best represents the actual situation? Blackstad et al. did not report branch order statistics for the cat, so itis not possible to make a comparison in this regard. But, as shownin Table 2, the height of the basal arbor is about one-thirdsmaller in the gerbil than in the cat. This would seem to rejectthe second scenario above, since the pruning of one branch wouldnot affect the size of the remaining branches and hence the arborheight should remain about the same. This leaves the first scenarioas the most viable and suggests that the effect of dendritic shorteningin the gerbil is to make the basal arbor electrotonically morecompact than in thecat.
Assuming that inputs from the auditory nerve are distributed toward the distal ends of the basal dendrites (Smith and Rhode1985), a possible effect of this electrotonic shortening wouldbe to decrease the attenuation of excitatory synaptic potentials.To the extent that inhibitory inputs are received at more proximallocations (Berrebi and Mugnaini 1991; Saint-Marie et al. 1991),they are relatively unaffected by the overall length of the dendrite.Hence, an electrotonically more compact basal dendrite might preferentiallyenhance excitatory drive relative to inhibition. Indeed, previousresults suggest that the proportion of DCN units having type IVunit properties is smaller in the gerbil than in the cat (11%vs. 32-45%) and the proportion of type III units is larger (62%vs. 23%) (Davis et al. 1996). Ding et al. (1999) reported on 13 labeled fusiform cells from the decerebrate gerbil, none of whichhad classic type IV unit response properties. Interestingly, thetype IV unit incidence in rabbit (23%) and chinchilla (25%) appearsto follow the same size-related trend (Davis et al. 1996; Huiand Disterhoft 1980; Kaltenbach and Saunders 1987).
This species-related difference in unit incidence was considered by Davis and Voigt (1996) using a point neuron model. Theirhypothesis was that a weakened contingent of type II unit inhibitiononto DCN projection neurons was responsible for the lack of typeIV units found in the gerbil relative to the cat. They showedthat a 40-50% reduction in the number of type II unit inputs wassufficient to turn a model type IV unit into a type IV-T or typeIII unit. A hypothesis based on the present finding in regardto basal dendrite length, in contrast, suggests that the balanceis shifted toward excitation by an enhancement of the excitatoryinputs, rather than by a reduction of the inhibitory inputs. Theseissues might effectively be explored using a compartmental modelof a fusiformcell.
Tonotopic organization of the gerbil DCN
The qualitative picture of tonotopy presented in Fig. 2 agrees well with the description of frequency organization in gerbilDCN obtained using measurements of 2-deoxyglucose uptake (Ryanet al. 1982). On a more quantitative basis, the fusiform cellBFs were plotted in Fig. 3 as a function of the cell positionalong the tonotopic axis. The data plotted in this manner correspondwell with the cochlea place-frequency map determined by Müller(1996) in the Mongolian gerbil (Fig. 3). Such quantitative correspondencebetween frequency representation in the cochlea and tonotopicorganization in central auditory structures is frequently observed(Müller 1990).
A notable exception is the cat DCN, whose representation of the 8- to 30-kHz range is disproportionately larger than the representationof the same frequency range in the cochlea (Spirou et al. 1993).The head-related transfer function (HRTF) in the cat containsspectral notches that may serve as cues for determining soundsource elevation (Rice et al. 1992). These notches fall in the8- to 30-kHz range, suggesting that the enhanced representationof these frequencies in the cat DCN is a functional specializationrelated to coding those particular spectral features (Spirou etal. 1993).
The place-frequency analysis shown in Fig. 3 is limited to the extent that it is a one-dimensional description of the tonotopy;it does not consider variations in cell density or the possibilityof a two-dimensional frequency gradient. The comparison with thecat data of Spirou et al. (1993) is appropriate insofar as thecat data are also based on a one-dimensional frequency axis. Thatstudy, however, was based on BF estimates made at finely spacedlocations from a large number of electrode tracks. Furthermore,Spirou et al. (1993) accounted for variations in fusiform celldensity when drawing conclusions about BF representation in theDCN.
The present data therefore should be regarded as preliminary but appear to underscore the fact that the tonotopy of the gerbilDCN (and cochlea) is largely devoted to a lower frequency range.Frequencies below 10 kHz occupy about 60% of the length of bothstructures, and no specialized frequency representation in thegerbil DCN is apparent. It is currently unknown whether or notthe gerbil HRTF contains spectral elevation cues similar to thoseidentified in the cat HRTF. Measurements of the HRTF in gerbiland a more detailed examination of frequency representation inthe DCN are essential to a comparative evaluation of DCN functionin the twospecies.
The results indicate that the disposition of the basal dendrites is correlated with spontaneous rate for the fusiform cellswith best frequencies less than 2 kHz. The basal dendrites oflow SR cells are directed away from the soma only in the caudaldirection, while the high SR cells have basal dendrite branchesextending both rostrally and caudally. The basal arbors of thelow SR units, relative to those of the high SR units, have morecaudally located centroids. It is important to note that thisobservation is based on relative comparisons of basal arbor structureamong cells having similar orientations. No absolute metric wasfound to allow for a global quantification of the relationshipbetween SR and basal dendrite position. It was thus necessaryto limit consideration to this BF range because of the generalshift in long axis orientation with rostral-caudal position (andhence BF) and because there were not sufficient numbers in anyother frequency band for cell-to-cell comparisons to bemade.
It is not immediately clear how the orientation of the basal dendrites may influence spontaneous activity. Liberman (1993)suggested that auditory nerve inputs to the cat DCN may be segregatedbased on spontaneous rate, with the high SR fibers terminatingdeeper in the nucleus than the low SR fibers. This appears tobe inconsistent with our results, since the rostrally directedbasal dendrites of the high SR fusiform cells tend to be moreshallow in depth than the caudally directed branches common toboth SRgroups.
Another possibility is that the rostrally directed branches access some other set of inputs that modulate spontaneous activity.It also may be that the orientation characteristic of the highSR cells has electrotonic consequences that emphasize excitatoryinputs over inhibitory inputs. Regardless of the mechanism, theresults suggest that the distribution of fusiform cell spontaneousactivities is not entirely due tohappenstance.
It is difficult to say what functional consequences may arise from having a subset of the fusiform cell population preferentiallyreceive low SR input. It has been suggested that low SR auditorynerve fibers are recruited to improve intensity discriminationat high sound levels or are useful for signal in noise problems(Viemeister 1983). Extending such interpretations to the DCN isproblematic, because they are based on the fact that low SR ANfibers typically have high acoustic thresholds (Liberman 1978),a trend not generally characteristic of DCN units. It has beenshown that low SR AN fibers better synchronize to AM tones (Jorisand Yin 1992), but there remains disagreement over the generalsuitability of DCN projection neurons for encoding such stimuli(Joris and Smith 1998).
Shape of regularity histogram
For the fusiform cells in the 0- to 2-kHz band, a correlation was observed between the orientation of the apical dendriticarbor, quantified by the rotation, apical, about the long axisyielding the narrowest projection of the arbor, and the shapeof the regularity histogram, as quantified by the slope, m1 (Fig.7). The slope values m1 and m2 are obtained by simultaneouslyfitting two lines to the interspike interval plot after omittingthe first bin. For the data of this study, the value of m1 appearsto be a more useful measure of inhibition than the monotonicityof the BF rate-level curve, because the m1 values are more evenlydistributed over a wider interval. A positive value of m1 reflectsa rate trend similar to the underlying excitatory drive from auditorynerve fibers and hence suggests relatively weak inhibitory input.Negative values are taken as an indication of a relatively strongerinhibitorycontribution.
One interpretation of this result is that the apical dendrite orientation determines the cross-sectional area of the arborperpendicular to the trajectory of the parallel fibers and henceaffects the pattern of activity in the apical arbor. For example,those presenting a narrow face to the parallel fiber network maybe more strongly influenced by inhibitory inputs from the cartwheelcell population. Although cartwheel cells are known to be acousticallyresponsive, their activity is relatively weak and of high threshold(Ding et al. 1999; Parham and Kim 1995) and would not be expectedto account for the relatively strong inhibition represented bynegative values of m1.
A second possibility is that since the neurons represented in Fig. 7 come from the ventrolateral third of the nucleus, thechange in apical arbor orientation may result from curvature ofthe strial axis in this region (Blackstad et al. 1984). In thiscase, the variations in orientation and in slope m1 may, in fact,be functions of position. That no such dependence was apparentin our position data may indicate limitations in the accuracyof mapping position across different tissuesamples.
Possible significance of physiology-morphology correlations
Figures 5-7 demonstrate that certain morphological features are correlated with specific physiological properties. The geometriesof the apical and basal arbors influence input resistance andspontaneous activity, respectively, while some aspect of cellorientation apparently contributes to the inhibition measuredin the regularity histogram. It is thus possible that fusiformcells along the isofrequency axis have graded physiological properties,and hence different signal processing characteristics. In thisway, the fusiform cell population might be capable of performingdifferent operations on the same set ofinputs.
A chance example from the present study illustrates that significant morphological differences may indeed exist between nearbycells. Figure 9 is a sagittal view of two fusiform cells, labeledin a single electrode track, their cell bodies separated by lessthan 10 µm. They were located in the caudal DCN, and hence, asdescribed earlier, their long axes have a rostral-to-caudal orientation.The apical arbor of the black-colored neuron is sparser than thatof the gray-colored neuron. Furthermore, the two basal arborsdo not overlap completely, but follow slightly different trajectories.The results of Fig. 9 support the notion that neighboring fusiformcells might have markedly different morphologies.
Organization of sensitivity to broadband noise
The notion that the DCN contains a functional axis orthogonal to the frequency axis is suggested by anatomical features. First,there is the network of parallel fibers oriented perpendicularto the underlying auditory nerve fibers. If systematic variationsin activity within the parallel fiber network contribute to thevariations in fusiform cell physiology, then fusiform cell propertiesmight vary in a spatially dependent manner. Another anatomicalsubstrate for a second axis in the DCN is a progressive decreasein the number of vertical cells in the dorsomedial direction withinthe coronal plane (Lorente de Nó 1981). Since vertical cellspresumably inhibit fusiform cells, the magnitude of inhibitoryresponse features (tone slope, interspike interval slope, etc.)should also decrease in the dorsomedialdirection.
The results show that the relative noise index is negatively correlated with position in the ventrolateral to dorsomedialdirection (Fig. 8A). In other words, fusiform cell responses tonoise tend to become weaker in the same general direction in whichLorente de Nó (1981) reported the vertical cell layer becomingthinner. A decrease in the number of vertical cells, however,represents a loss of inhibition and so cannot account for thediminishing noise responses. The noise sensitivity of DCN principalcells is thought to be shaped by a source of wideband inhibition,possibly originating in the PVCN (Nelken and Young 1994). Theobserved noise responses in the current study might be accountedfor by an increasing contribution of this inhibitory source inthe dorsomedialdirection.
Earlier studies have identified two inhibitory components to the response maps of type IV units, as schematized in Fig. 10.The first arises from a band of type II units, centered belowthe BF of the type IV unit (Voigt and Young 1990), while the secondcomponent, wideband inhibition, possibly arises from the PVCNand is centered above the type IV unit BF (Nelken and Young 1994;Spirou and Young 1991). This organization might be consistentwith wideband inhibitory input strengthening in the dorsomedialdirection, as suggested by the present results, and the verticalcell layer thinning in the same direction, as described by Lorentede Nó. If, for example, each type IV unit receives input froma band of type II units (presumably vertical cells) centered spatiallyon BF, then the resulting inhibitory band would have a lower BFbecause the vertical cells are more dense in that direction. Similarly,if the wideband inhibition is stronger in the dorsomedial direction,inhibition from a band spatially centered on BF will itself havea higher BF. The idea that these two inhibitory sources mighttrade for one another across the width of the DCN is consistentwith data presented by Nelken and Young (1994). Their Fig. 8Bsuggests that the influence of inhibition by type II units, asmeasured by the maximum driven BF rate, is negatively correlatedwith the influence of wideband inhibition, as measured by theminimum inhibitory notch width.
This report has described differences between cats and gerbils in the anatomical properties of DCN fusiform cells and in theirtonotopic arrangement. The report also described systematic distributionsof functional properties within the fusiform cell population.How such variations contribute to DCN function is yet to beunderstood.
We thank committee members H. S. Colburn, L. H. Carney, M. C. Liberman, and A. M. Berglund for helpful comments and suggestions. We also thank D. Oertel for comments and P. Patterson for processing some of the tissue.
K. E. Hancock was supported by a fellowship from The Whitaker Foundation. This work was funded by Grant DC-01099 from the National Institute on Deafness and Other Communication Disorders.
Present address of K. E. Hancock: Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, 243 Charles St., Boston, MA 02114.
Address for reprint requests: H. F. Voigt, Dept. of Biomedical Engineering, Boston University, 44 Cummington St., Boston, MA 02215-2407 (E-mail: [email protected]).
Received 27 April 2001; accepted in final form 20 December 2001.
|Table 1. Properties of apical dendritic arbors|
|Cell||TL, µm||Width, µm||Thickness, µm||Ratio||Height, µm|
|TL, total length.|
|Table 2. Properties of basal dendritic arbors|
|Cell||TL, µm||Width, µm||Thickness, µm||Ratio||Height, µm|
|TL, total length.|
Figure 1 Summary of position measurements.
Figure 2 Tonotopic organization of the gerbil dorsal cochlear nucleus (DCN). A: the basal dendrites of the 17 fusiform cells from this study mapped onto a single DCN. A patch has been removed from the surface of the DCN to show the interior. Dendrites are color-coded according to best frequency (BF) as shown. D, dorsal; R, rostral; L, lateral. B: scatterplots showing BF vs. relative X-position (top) and relative Z-position (bottom). Regression line for X-position given by log(BF) = 2.38x 0.72, r = 0.62, P < 0.01. Regression line for Z-position given by log(BF) = 1.30x 0.26, r = 0.74, P < 0.001.
Figure 3 Comparison of frequency-place maps for the gerbil cochlea and DCN. Place-frequency data for fusiform cells obtained by projecting soma locations onto frequency axis computed in the text. Place-frequency map for the gerbil cochlea from Müller (1996). Place-frequency map for the cat DCN from Spirou et al. (1993).
Figure 4 Fusiform cell orientation changes with rostral-caudal position. DCN is represented by a series of coronal slices. Arrows point from soma locations to renderings of fusiform cells in a sagittal view. Thick gray lines indicate the long axis of each cell. D, dorsal; C, caudal.
Figure 5 Spontaneous activity and basal dendrite arrangement. Neurons are from the 0- to 2-kHz region and are drawn in a parasagittal view. Apical dendrites are shown in gray; basal dendrites in black. Basal dendrites of low spontaneous rate (SR) cells (top row) are oriented primarily in the caudal direction. Those of the high SR cells (bottom row) have both rostrally and caudally directed branches. This difference is quantified using the parameter ZC, which is the rostral-caudal component of the basal arbor's centroid, measured with respect to the soma. The low SR units have larger ZC values (centroids located more caudally) than the high SR units.
Figure 6 Relationship between input resistance and arbor length. A: input resistance is highly correlated with apical dendrite length. Regression line: y = 10.5x + 48.2, r = 0.79, P < 0.001. Triangles indicate data points omitted as outliers. B: input resistance is uncorrelated with basal dendrite length. Regression line: y = 0.6x + 16.8, r = 0.08, ns.
Figure 7 Relationship between regularity histogram shape and orientation of apical arbor. Top: slope m1 from the regularity histogram plotted vs. the orientation of the apical arbor. The orientation apical is the rotation about the long axis giving the narrowest arbor profile. See text for a detailed description of how these values are measured. Regression line: y = 1.1x + 21.8, r = 0.82, P < 0.01. Data points indicated by circles, triangles, and squares represent high, middle, and low m1 values, respectively. Bottom: each group corresponds to the data points above and shows the fusiform cell apical arbors after superimposing the somata. The view is looking down on top of apical arbors, so that the long axes project out of the page. R, rostral; M, medial.
Figure 8 Physiological properties correlated with position. Relative noise index (A) and input resistance (B) are negatively correlated with position in the X-direction. Regression line for noise index: y = 1.16 × 103x +1.30, r = 0.53, P < 0.05. Regression line for input resistance: y = 50.0 × 103x + 50.7, r = 0.67, P < 0.01. C: total apical dendritic length increases with X-position. Regression line: y = 3.01 × 103x + 1.17, r = 0.56, P < 0.025. Triangle indicates an outlier removed from the regression analysis.
Figure 9 Two neighboring fusiform cells from the caudal end of the DCN. Long axes are nearly horizontal in accordance with the general rostral-caudal shift in orientation. At the top, the cells are drawn in their correct relative positions. At the bottom, they are drawn separately to show individual details clearly.
Figure 10 Inhibitory components of a type IV unit response map. Gray, inhibition from type II units that probably correspond to vertical cells. The peak of this inhibitory area is typically below the BF of the type IV unit. Lorente de Nó (1981) observed a decrease in the height of the vertical cell layer from ventrolateral (VL) to dorsomedial (DM). If a type IV unit draws input from a spatially symmetrical band of vertical cells, the center frequency of that band will be lower than the type IV unit BF because the verticals cells are more dense in the low-frequency direction. White, inhibition from a band of wideband inhibitors, possibly stellate cells in the posteroventral cochlear nucleus (PVCN). The peak of this inhibitory band is above the type IV unit BF. The results of this study suggest that the strength of wideband inhibition increases in the dorsomedial direction. A band of wideband inhibitors distributed symetrically in space with respect to a type IV unit will be centered higher in frequency because the wideband inhibitory strength increases in that direction.