The Morphology And Functional Characteristic Of Bushy Cell And Stellate Cell In Anteroventral Cochlear Nucleus In Rat

Our world is abundant of sounds. It’s necessary for livings to contact the outside world by receiving the acoustic information. The sound travel in the air as the form of mechanical wave, and spread to the cochlear which is regarded as peripheral auditory system. Hair cells work out the transmission into electrochemical signals in the spiral ganglion. Auditory nerve fibers carry electrical signals to the cochlear nuclear complex, which is the first synaptic station along the central auditory pathway.The anteroventral cochlear nucleus (AVCN)is one of the subdivisions of the cochlear nucleus complex. Two type of main projection neurons, bushy cell and stellate cell, are intermingled in the anteroventral cochlear nucleus. They receive direct input from the auditory nerve fibers, and their axons form distinct ascending pathways and project to nuclei in the auditory brainstem. Two type of bushy cell in the anteroventral cochlear nucleus, spherical bushy cells and globular bushy cells respectively, axons of spherical bushy cells terminate in the ipsilateral lateral superior olive (LSO),the lateral part of the ipsilateral medial superior olive (MSO), and the medial part of the contralateral medial superior olive. Most axons of the globular bushy cells terminate in the primary cell in the contralateral medial nucleus of the trapezoid body(MNTB) forming the largest synaptic endings-the calyces of Held,in the mammalian brain. Two groups of stellate cells can be distinguished in the ventral cochlear nucleus based on their projection patterns. Studies in slices revealed that the axons of some stellate cells project through the Trapezoid body named T stellate cell whereas others project Dorsal ward named D stellate cell。T stellate cells are a major source of input from the cochlear nucleus to the contralateral inferior colliculus(IC).The same neurons that project to the inferior colliculus also send collateral branches to the dorsal cochlear nucleus (DCN). Targets of D stellate cell are the contralateral dorsal and ventral cochlear nuclei. Bushy cells have distinct biophysical membrane properties that result in little temporal summation of synaptic inputs. They are well suited for conveying and processing the information about the timing and fine structure of sounds. In contrast, stellate cells are insensitive to the precise temporal information, thus they are more likely to integrating auditory signals coming at different times.As what have been stated above, bushy cells and stallate cells play different roles in conveying auditory information in the anteroventral cochlear nucleus. In our study, we explored the characteristic and the functions of these two type of neurons in terms of morphology and biophysiology. In these experiments, to examine the morphology of the the bushy cells and stallate cells in the anteroventral cochlear nucleus, biocytin was included in the pipette solution and slices were fixed with paraformaldehyde immediately after the recording. Current injections were carrred out from hyperpolarization to depolarization into the target cells in whole-cell patch-clamp recording model, we then recorded their action potentials and analyzed their membrane properties. In these ways, we were able to reveal the different functions of two type of neurons in processing the auditory information.A Total of 42 neurons were recorded,including 25 of bushy cells、11 of T-stallete cells and 6 of D-stallate cells. We find that bushy cells usually have a relatively large, round cell body characteristically with one or two primary dendrites which are derived from The two opposite poles of the cell body and proceed away from each other. The dendrites of bushy cells end in a spray of thinner and shorter braches. Stellate cells, or known as multipolar cells in some other studies, have a relatively small cell body. They general have more than three primary dendrites which are slender and sparsely branched, and more likely to cross the midline. The two classes could be separated purely on morphological grounds. electrophysiology can also be applied to assist in classifying AVCN neurons. After given current injections from hyperpolarization to depolarization, Bushy cells fired a single or a few action potentials at the onset of a suprathreshold depolarizing current step, whereas stellate cells fired multiple action potentials sustained throughout the period of current injection. To further illustrate bushy cells were sensitive to the beginning of the stimulus, trains of the current injection were carried out. We find that bushy cell consistently fired one action potential at the onset of stimulus, which reflected that bushy cell mainly encoded the information about the timing.