HST.723 - Neural Coding and Perception of Sound

Spring 2004

Atlas of the Subcortical Central Auditory System of the Cat

 This atlas created by Joe Adams includes three sets of images:

The Nissl stain shows nucleic acids (DNA and RNA), which means that cell nuclei and ribosomes (both free and those attached to rough endoplasmic reticulum) are visualized.  Staining of RNA is useful for identification of gross structures such as whole nuclei, as shown in the low magnification images, and of distinctive features of individual cells, as shown in the high magnification images.  While the Nissl technique shows all cells, immunostaining only reveals those cells containing a specific protein and is therefore useful for identifying cytochemically distinct types of cells.

Low-magnification Nissl-stained sections

There are low magnification views of 24 Nissl-stained sections spanning the entire subcortical auditory pathway from the dorsal cochlear nucleus (DCN) to the medial geniculate body (MGB) of the thalamus.  The successive images are spaced about 1-mm apart and are numbered beginning caudally with the DCN and progressing rostrally down the columns of images.  Each figure contains 6 images.

The numbers of individual figures in these low magnification views indicate the sections from which the high-magnification micrographs were taken.  For example, all the high-magnification images shown in the anterior periolivary region (APO) figure were taken from the tissue shown in Section Number 9 of the low- magnification series (the area labeled APO in the NLL series), as indicated by all images in this figure being numbered 9A, 9B, etc...

High-magnification Nissl-stained sections

High magnification images (500X) were obtained with a 40X objective in combination with a 12.5X photo ocular.  For cells in auditory nuclei caudal to the inferior colliculus, this magnification is quite informative for showing distinctive features of particular cell classes, e.g., their size, shape, and pattern of Nissl substance.

Immunostained sections

Other distinctive features of particular cell classes are revealed by immunostaining.  Three figures show different examples.

1. Comparison of Nissl stain vs. immunostaining for calbindin in the DCN.  For example, in Plate 1D, cartwheel cells stand out when stained for calbindin.  Calbindin (CaBP) is a calcium-binding protein whose precise functional role in cartwheel cells is unknown.  It is shown here as an illustration of the fact that cytochemical traits can be used to identify particular cell classes.  Note how conspicuous the calbindin immunostained cells are when compared to the adjacent Nissl stained section of the same region (Plate 1A).  The other 4 Nissl-stained plates (1B-C, 1-EF) show high-magnification views of cells in Plate 1A.

2. Cochlear-nucleus cells immunostained for GAD, the enzyme directly responsible for production of the inhibitory neurotransmitter GABA.  Each of the 6 plates is described below.

1. The DCN is a layered structure.  The outermost, ependymal layer (which contains no neurons) is unstained, while the second, molecular layer layer stains darkly due to the presence of GABAergic processes that terminate upon the dendrites of cartwheel cells and the apical dendrites of pyramidal cells.  The somata of pyramidal cells form the third, pyramidal layer, which is largely unstained.  The less darkly stained deep DCN contains large cells and vertical cells, whose processes receive many GABAergic terminals, but fewer than those in the molecular layer.

2. In the nerve root region (where the AN axons enter the CN), there are considerable differences in the density of GABAergic processes, but there are no distinctive layers of cells or stained processes, as in the DCN. 

3. The AVCN is more uniformly stained than the nerve root region.  The numerous small clear dots surrounded by dense staining are spherical bushy cells, which have a dense GABAergic innervation on their somata (which appears as a dense, dark band around the unstained, clear cells).  This innervation appears to be almost as dense as the innervation that these cells receive from auditory-nerve fibers.  The origins and functions of this GABAergic innervation are unknown.

4. High-magnification view of several cells in the nerve root region demonstrates the differences in GABAergic innervation in this small field of view.  At the top of the micrograph are two neurons that are covered with coarse, dense GABAergic terminals.  In contrast, in the lower half of the field, several cells receive fine, sparse innervation.  The latter are probably Type 1 stellate cells.  The former are either Type 2 stellate cells or globular bushy cells.

5. Similar comparison in the octopus cell region of the PVCN.  In the left half of the field are octopus cells, which have barely any GABAergic input.  In the right half are a few Type 2 stellate cells, which are densely innervated.

6. PVCN cells receive a relatively dense covering of fine GABAergic terminals. The size, density and placement of GABAergic inputs contribute to the physiological properties of the target neurons.  It is therefore expected that physiologically-distinct classes of cells will  have characteristic staining patterns in their inputs, as shown in these images.

3. Cytochemical traits of SOC neurons distinguished by immunostaining for AchE, GAD or calbindin.  Each of the 6 plates is described below.

1. Staining for acetylcholinesterase (AChE, the enzyme that degrades acetylcholine and is a useful marker for cholinergic neurons) in the hilus (dorsal indentation) of the LSO.  The dark staining shows the locations of the lateral olivocochlear (LOC) neurons which project to the inner hair cell region in the cochlea.

2. AChE staining in the anterior periolivary region (APO).  The dark staining shows the medial olivocochlear (MOC) neurons which project to outer hair cells in the cochlea.

3. The hilus of the LSO has a relatively dense GABAergic innervation as shown by GAD staining.

4. Sparse, large multipolar cells located along the margins of the MSO also have a dense GABAergic innervation.

5. Dense GABAergic innervation surrounding MNTB principal neurons (much like that seen on spherical bushy cells in AVCN) and the VNTB neuropil.

6. Calbindin immunostaining of MNTB cells (lower left of the image) and the dense terminals of the projections of these cells to both MSO (center of the field) and LSO (upper right) cells.  The MNTB cells are glycinergic and provide contralateral inhibitory inputs to MSO and LSO.  Humans lack a pronounced MNTB and LSO, but have a large MSO.  Consequently, the human MSO probably has little inhibitory input from the contralateral ear, as is present in cat.