It now looks like they have their lateral
geniculate nucleus (LGN)-- part of the thalamus in the middle of the brain — to thank for this «blindsight».
Searching for answers, Shatz decided to focus on the lateral
geniculate nucleus (LGN), a clump of tissue shaped like a piece of elbow macaroni, set behind each eyeball, that serves as a relay station shunting visual signals to the ocular dominance columns.
When they tracked the cells» axons from the eye into the brain, they saw that many of them terminated in the lateral
geniculate nucleus (LGN), the first relay station in the brain for visual information.
At lower levels, in the retina and in
the geniculate, cells in these two subdivisions differ in their color selectivity, contrast sensitivity, temporal properties, and spatial resolution.
Specifically, they have found evidence of processing in the human lateral
geniculate nucleus (LGN), a small node in the thalamus in the middle of the brain that relays nerve impulses from the retina to the primary visual cortex.
This image shows the localization of the human lateral
geniculate nucleus, using high - resolution functional MRI (orange) and anatomical MRI (green).
They hooked a potential receptor, serotonin 3, to green fluorescent protein and showed that the nerve terminals in the mouse tongue have serotonin 3 receptors, and these receptors are made by the nerve cells located in
the geniculate ganglion.
In both primates and rodents, messages from the two eyes enters the brain through a small structure called the lateral
geniculate nucleus or LGN, which is made of slivers of nerve cells, arranged like sponge in a layer cake.
C, D, Ca2 + signals from GFP +
geniculate ganglion cells in response to 10 μm ATP, 10 μm ATP + 1 μm ODS, 10 μm 5 - HT, 10 μm 5 - HT + 1 μm ODS, and 55 mm KCl (C) or 10 μm ATP, 10 μm CPG, 10 μm CPG + 1 μm ODS, and 55 mm KCl (D).
A, Confocal Z - projection of 10 0.7 μm optical slices of
geniculate ganglion from a 5 - HT3AGFP mouse showing expression of GFP (green) and P2X3 (magenta).
Approximately 25 % of all WT
geniculate neurons (5/19; Fig. 5) responded to exogenous 5 - HT with an increase in intracellular calcium, whereas no neurons from 5 - HT3AKO mice responded (0/25, p = 0.011, Fisher's exact test; Fig. 5).
A, RT - PCR showing the presence of 5 - HT3A transcripts in brain and
geniculate ganglion of WT mice, but the lack of 5 - HT3A mRNA in
geniculate ganglion of 5 - HT3AKO mice.
Z - stack images of taste buds and
geniculate ganglia were collected on an Olympus Fluoview FV300 laser scanning confocal microscope with a 60 × oil - immersion objective [numerical aperture (NA) 1.3] and 20 × oil - immersion objective (NA 0.7) or on a Leica TCS SP5 laser scanning confocal microscope with a 63 × oil - immersion objective (NA 1.4).
Consistent with GFP expression in
the geniculate ganglion, most taste buds innervated by
the geniculate ganglion contained some GFP - labeled intragemmal nerve fibers.
In mice expressing GFP under the control of the 5 - HT3A promoter, a subset of cells in
the geniculate ganglion and nerve fibers in taste buds are GFP - positive.
Geniculate ganglia were rapidly excised from CO2 - euthanized mice and placed in enzyme solution [minimum essential medium with Earle's balanced salts (MEM / EBSS; Hyclone) containing 1.25 mg / ml trypsin (Sigma - Aldrich) and 2.5 mg / ml collagenase A (Roche Diagnostics)-RSB- for 30 min.
We next tested the responsiveness of GFP - expressing and GFP - nonexpressing
geniculate ganglion neurons to exogenously applied 5 - HT.
Because all taste fibers express the P2X3 purinergic receptor (Ishida et al., 2009; Vandenbeuch et al., 2015), we immunoreacted
geniculate ganglion sections from 5 - HT3AGFP mice with antibodies against GFP (to amplify 5 - HT3AGFP signal) and against P2X3.
Our data demonstrate that a subset of
geniculate ganglion neurons expresses 5 - HT3 receptors and innervates taste buds, including serotonergic type III cells.
In situ hybridization on sections of gustatory ganglia (
geniculate, petrosal / nodose) and a somatosensory ganglion (trigeminal) were performed to reveal serotonin receptor gene expression at the cellular level.
F, G,
Geniculate ganglion.
Somatosensory innervation of these taste fields arises from the trigeminal nerve; therefore, cells of
the geniculate ganglion that innervate taste fields are gustatory rather than somatosensory in function.
RT - PCR shows 5 - HT3A transcripts in
geniculate ganglia as well as in brain (Fig. 1A).
In our study, pentobarbital inhibited 5 - HT evoked calcium signals in
geniculate ganglion neurons and, at anesthetic levels in mice, significantly inhibits 5 - HT3 function in vivo.
Interestingly, when nonbarbiturate anesthetics are used, a concentration - dependent increase in sideband excitation is observed in a subset of
geniculate ganglion neurons (Wu et al., 2015).
E, ATP (circles) and 5 - HT (triangles) dose — response curves for 5 - HT3AGFP - expressing
geniculate ganglion cells.
RT - PCR and in situ hybridization confirmed the presence of 5 - HT3A mRNA in
the geniculate ganglion.
GFP immunoreactivity was detected in a subset of
geniculate ganglion neurons (169/682, 24.8 %; Fig. 2B, C), whereas the P2X3 signal was present in nearly all ganglion cells (664/682, 97.4 %; Fig. 2B, C).
RNA was extracted from
geniculate ganglia of 5 - HT3AKO and WT mice (3 mice each) according to manufacturer's instructions using the RNeasy Micro kit (Qiagen), including a 30 min DNase I treatment at room temperature for removal of genomic DNA.
Normalized responses of Fura -2-AM loaded
geniculate ganglion neurons revealed that a subset (solid circles; 5/19) of WT neurons were activated by 10 μm 5 - HT; no neurons from 5 - HT3AKO animals (open circles; 0/25) responded to 5 - HT, yet all responded to 10 μm ATP and 55 mm KCl.
To first determine whether gustatory afferents express GFP in 5 - HT3AGFP mice, we examined
the geniculate ganglion and taste buds.
Biophysical properties and responses to glutamate receptor agonists of identified subpopulations of rat
geniculate ganglion neurons
The majority of cells of
the geniculate ganglion are gustatory, contributing to the chorda tympani and greater superficial petrosal nerves (Foley and DuBois, 1943; van Buskirk, 1945) innervating, respectively, fungiform papillae and palatal taste fields.
In addition, we tested whether other 5 - HT receptors were upregulated after genetic deletion of 5 - HT3 by measuring 5 - HT responses in isolated
geniculate ganglion neurons of WT and 5 - HT3AKO mice.
D, Example Fura -2-loaded
geniculate ganglion neuron response to 10 μm ATP, 10 μm 5 - HT, 10 μm 5 - HT + 800 μm pentobarbital, and 55 mm KCl.
D, Confocal Z - projections of
geniculate ganglion from 5 - HT3AGFP mice 5 d after FluoroGold injection showing expression of GFP (green) and FluoroGold label (FG; magenta).
Here the axons going down the optic nerve, across the optic chiasm, and into the expected targets in the brain: the lateral
geniculate and the superior colliculus.
Abbreviations: DLL, Nucleus dorsolateralis anterior thalami, pars lateralis; Gld, dorsolateral
geniculate complex; LdOPT, Nucleus lateralis dorsalis nuclei optici principalis thalami; Rt, Nucleus rotundus; SPC, Nervus superficialis parvocellularis; SpRt, Nucleus suprarotundus; TSM, Tractus septomesencephalicus.
The anterograde (forward) tracing results of retinal projections in the garden warbler demonstrated virtually identical connections between the eye and the brain as known from other lateral - eyed bird species [e.g. 15]--[18]: fibers either projected onto the contralateral optic tectum (part of the tectofugal system), to the nucleus of the basal optic root (part of the accessory pathway)(data not shown) or curved into the thalamus innervating the dorsal
geniculate complex (Gld, for details, see Fig. 1C, 2A, D).
The stem is erect or
geniculate at the base, the sheathes being smooth and cylindrical.