Nathaniel k at calibre woods11/20/2023 ![]() ![]() The GFP expression associated with GCaMP6f expression is shown in green. E, Maximum intensity projection image of a whole mount preparation of the basal turn of a Drd2 Cre GCaMP6f mouse. ![]() D, Image taken through a dissecting microscope of an excised, otic capsule preparation with access to the basal turn “b” indicated the end of the basal turn of the cochlea. The overlay with Tuj1 ( c3, green) shows Th 2A-CreER GCaMP6f expression in neurons in the OHC region of the organ of Corti as indicated by the white circles. GCaMP6F expressing type II afferents are labeled in cyan ( c1), the pan-neuronal marker Tuj1 is labeled in yellow ( c2). C, Maximum intensity projection image of Th 2A-CreER GCaMP6f expression in the peripheral dendrites of apical type II afferent neurons. The GFP expression associated with GCaMP6f expression has been pseudo-colored cyan. B, Maximum intensity projection image of a whole mount preparation of the apical turn of a Th 2A-CreER GCaMP6f mouse. A, Image taken through a dissecting microscope of an excised, otic capsule preparation with access to the apical turn “a” indicates the location of the tip of the apical turn of the cochlear epithelium. Type II afferent-associated genes drive GCaMP6f expression. The small, unmyelinated type II afferents make a 90° turn at the level of the outer hair cells (OHCs) and then extend a long dendritic arbor toward the cochlear base, contacting an average of 24 OHCs (range 9–100 Smith, 1975 Kiang et al., 1982 Berglund and Ryugo, 1987 Weisz et al., 2012 Martinez-Monedero et al., 2016 Ghimire and Deans, 2019). Each myelinated type I afferent synapses with one inner hair cell (IHC) and each IHC is contacted by 10–30 unbranched type I afferent dendrites ( Liberman, 1982). The two types of SGNs differ not only in proportion but also in innervation pattern as illustrated in Figure 1 H, I. Type I afferents constitute 90–95% of all SGNs, with type II afferents making up the remaining 5% ( Perkins and Morest, 1975 Berglund and Ryugo, 1987). The spiral ganglion neurons (SGNs) of the cochlea are divided into type I afferents that sensitively encode sound intensity, timing, and frequency, and type II afferents that respond only to loud, broadband sound ( Robertson, 1984 Brown, 1994 Robertson et al., 1999 Flores et al., 2015 Weisz et al., 2021). Overall, calcium imaging can report type II afferent responses to damage even in mature and noise-exposed animals and reveals previously unknown tissue hyperactivity subsequent to acoustic trauma. In addition, days after acoustic trauma, acute photoablation triggered a novel response pattern in type II afferents and surrounding epithelial cells, delayed bursts of activity occurring minutes after the initial response subsided. Mature type II afferents responded to acute photoablation damage less often but at greater length compared with prehearing neurons. Two known marker genes ( Th, Drd2) and one new marker gene ( Tac1), expressed in type II afferents and some other cochlear cell types, drove GCaMP6f expression to reveal calcium transients in response to focal damage in the organ of Corti in all turns of the cochlea. To explore this hypothesis further, calcium imaging was used to determine the impact of acoustic trauma on the activity of type II cochlear afferents of young adult mice of both sexes. Altogether, these lines of evidence suggest that type II afferents sense loud, potentially damaging levels of sound. Limited recordings of type II afferents from cochlear apex of prehearing rats reveal they are activated by widespread outer hair cell stimulation, ATP, and by the rupture of nearby outer hair cells. While type I afferents convey information about the frequency, intensity, and timing of sounds, the role of type II afferents remains unresolved. Auditory stimuli travel from the cochlea to the brainstem through type I and type II cochlear afferents. ![]()
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