Innervation of the Cochlea. Reading: Yost Ch. 8

Transcription

1 Innervation of the Cochlea Reading: Yost Ch. 8

2 Fine Structure of the Organ of Corti

3 Auditory Nerve Auditory nerve (AN) is a branch of the VIII th cranial nerve (other branch is vestibular). AN is composed of cochlear afferent and efferent fibers. AN is cochleotopic: apical fibers form the core, more basal fibers added to outer layers of the nerve. AN leaves cochlea via the internal auditory meatus.

4 Afferent Innervation of the Cochlea Spiral ganglion cells are the neuronal afferents of the cochlea. Afferents innervate both inner and outer hair cells, project to the CNS. 30,000 in humans (~10x IHCs). 50,000 in cats (~20x IHCs). >250,000 in some cetaceans.

5 Spiral Ganglion Cells Two populations of neurons have been described in the spiral ganglion. The classification is based on somatic size, abundance, cytology, and myelination. Type I ganglion cells are large, bipolar, and represent 90-95% of the population. The nucleus is pale, and the cytoplasm is prominent characterized by plentiful ribosomes, endoplasmic reticulum, and Golgi bodies. Processes are heavily wrapped in myelin (faster conduction). 10 mm Ryugo Type II ganglion cells are small, bipolar or pseudomonopolar, and represent 5-10% of the population. The nucleus is pale and often eccentric, and the cytoplasm lacks the usual organelles. Processes are thinly myelinated if at all (slower conduction).

6 Type I Spiral Ganglion Cells Each type I spiral ganglion cell (SGC) innervates only one inner hair cell (IHC). Each IHC connects with type I SGCs.

7 Type I SGC Innervation of IHC Each IHC connects with type I SGCs. When examined under an electron microscope, synapses are asymmetric; that is, they are characterized by rounded vesicles in the presynaptic cell, and a prominent postsynaptic density. Asymmetric synapses are typically excitatory (glutamate). Pillar IHC Modiolar Horizontal section through base of IHC (Spoendlin 1985). a: afferent terminals; Syn: asymmetric afferent synapse; N: axons of other afferents Anatomical/physiological studies suggest that ganglion cells that synapse on the modiolar side of the IHC have thinner axons, fewer mitochondria, and have low rates of spontaneous activity. Conversely, SGCs that synapse on the pillar side have thicker axons, more mitochondria, and higher spontaneous discharge rates (Liberman 1982).

8 Type II Spiral Ganglion Cells Each type II spiral ganglion cell (SGC) sends a distal process (called an outer spiral fiber: OSF) into the tunnel of Corti to innervate only OHCs. Each OSF turns and runs longitudinally toward the base before crossing the tunnel and innervating OHCs. Each OHC connects to multiple type II SGCs.

9 Type II SGC Innervation of OHC Each OHC connects with multiple type II SGCs. When examined under an electron microscope, synapses are asymmetric; that is, they are characterized by rounded vesicles in the presynaptic cell, and a prominent postsynaptic density. Asymmetric synapses are typically excitatory (glutamate). Note the presence of large, efferent syaptic endings. Efferent synaptic ending (presynaptic) OHC Afferent synaptic ending (postsynaptic)

10 Tonotopic Organization Organ of Corti: sharply tuned, overlapping bandpass filter bank organized by CF. CF of hair cell depends on its location along the basilar membrane Tuning depends on mechanical properties of the basilar membrane (envelope of the traveling wave) and activity of OHCs Spectrum of a sound is topographically decomposed into separate frequencyselective channels (like Fourier analysis). Usually for mammals, there is log-linear relationship of frequency vs. distance along BM (Greenwood 1961). For humans, the function is: CF = 165.4*[ (35-x) -1] Hz, where x is in mm from the base.

11 Efferent Innervation Both inner and outer hair cells receive efferent innervation from the central nervous system. Lateral olivocochlear (LOC) fibers project to IHCs, where they usually synapse on the type I spiral ganglion cells innervating the hair cells. LOC fibers originate predominantly (but not exclusively) from ipsilateral structures (uncrossed olivocochlear bundle: OCB). Medial olivocochlear (MOC) fibers project to OHCs, where they synapse directly on the hair cells. MOC fibers originate predominantly (but not exclusively) from contralateral structures (the crossed OCB).

12 Sources of Cochlear Efferent Innervation The neurons that give rise to the efferent innervation of the cochlea are located in the superior olivary complex (SOC) in the brainstem. The LOC fibers originate from neurons in/around the S-shaped lateral superior olive (LSO). These neurons project to IHCs primarily on the ipsilateral (same) side; are part of the uncrossed OCB. The MOC fibers originate from neurons medial to the medial superior olive (MSO). These neurons project to OHCs primarily on the contralateral (opposite) side of the head and form part of the crossed OCB.

13 MOC Efferent Innervation MOC neurons receive inputs from the cochlear nucleus neurons, which in turn, receive inputs from type I spiral ganglion cells. The hypothesized functions of MOC efferents fall into three main categories: cochlear protection against loud sounds development of cochlea function, and detection and discrimination of sounds in noise.

14 Summary Cochlea receives afferent and efferent innervation. Afferents originate from spiral ganglion cells in the cochlea. Efferents are from the superior olivary complex in the brainstem. IHCs innervated by type I SGCs (10-30 per IHC). OHCs innervated by type II SGCs (many). Efferents innervate both IHCs (LOC efferents) and OHCs (MOC efferents). Type I SGC afferents carry nearly all information about sound to the brain. Type II SGC function not certain. Tonotopic organization of cochlea is preserved in projections to the brainstem.