Morphology and unbiased stereology of the lateral superior olive in the short‐beaked common dolphin, Delphinus delphis (Cetacea, Delphinidae)

Abstract

In all mammals, the superior olivary complex (SOC) comprises a group of auditory brainstem nuclei that are important for sound localization. Its principal nuclei, the lateral superior olive (LSO) and the medial superior olive (MSO) process interaural time and intensity differences, which are the main cues for sound localization in the horizontal plane. Toothed whales (odontocetes) rely heavily on hearing and echolocation for foraging, orientation, and communication and localize sound with great acuity. The investigation of the SOC in odontocetes provides insight into adaptations to underwater hearing and echolocation. However, quantitative anatomical data for odontocetes are currently lacking. We quantified the volume, total neuron number, and neuron density of the LSO of six common dolphins (Delphinus delphis) using the Cavalieri principle and the unbiased stereology optical fractionator. Our results show that the LSO in D. delphis has a volume of 150 + (SD = 27) mm3, which is on average 69 (SEM = 19) times larger than the LSO in human, or 37 (SEM = 11) times larger than the human LSO and MSO combined. The LSO of D. delphis contains 20,876 ± (SD = 3300) neurons. In comparison, data reported for the human brainstem indicate the LSO has only about ¼ that number but about the same number for the LSO and MSO combined (21,100). LSO neurons range from 21 to 25 μm (minor axis) and from 44 to 61 μm (major axis) in transverse sections. The LSO neuron packing density is 1080 ± (SD = 204) neurons/mm3, roughly half of the LSO neuron density in human. SMI‐32‐immunohistochemistry was used to visualize projection neurons in the LSO and revealed the presence of principal, marginal, and multipolar neurons in transverse sections. The distinct morphology of the LSO likely reflects the common dolphin's superb sensitivity to ultra‐high frequencies and ability to detect and analyze sounds and their location as part of its underwater spatial localization and echolocation tasks.

Bidirectional remodeling of the central auditory system caused by unilateral auditory deprivation

“…The superior olivary complex (SOC) is a major computational center in the auditory brainstem that mainly processes inter-aural-difference cues for sound localization. In all mammals, it comprises a group of auditory brainstem nuclei: the lateral superior olive (LSO), the medial superior olive (MSO), the trapezoid body, and peri-olivary nuclei ( 58–60 ).…”

Neuroanatomy of the Cetacean Sensory Systems

“…For instance, Breathnach (1960) observed that Monakow's striae acousticae, which connect the posterior cochlear nucleus neurons with the lateral lemniscus and the central nucleus of the inferior colliculus, are not apparent, while Held's striae (connecting the VCN and bilateral ventral nuclei of the trapezoid body, superior olivary and perio-livary nuclei) [114] are clearly visible. More recent studies specify morphological cell types (e.g., spherical neurons in the VCN or principal neurons in the lateral superior olive) comparative to terrestrial mammals [115] and provide estimates for the number and density of neurons, and the volume of the VCN and the lateral superior olive [116,117]. See Figure 1 for an overview of the auditory pathways.…”