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The structural and functional characteristics of tectospinal neurons in the golden hamster
Insights
This study characterizes tectospinal cells in hamsters, revealing their structure, function, and connectivity. Tectospinal neurons primarily process somatosensory information, with some integrating visual input, and project to the spinal cord.
Area of Science:
- Neuroscience
- Cell Biology
- Spinal Cord Research
Background:
- The superior colliculus (SC) plays a crucial role in integrating sensory information and motor control.
- Tectospinal (TS) cells are a key output pathway from the SC to the spinal cord, but their detailed characteristics are not fully understood.
Purpose of the Study:
- To delineate the structural and functional properties of hamster superior collicular cells projecting to the first cervical spinal cord segment.
- To correlate anatomical features with physiological responses in these identified tectospinal neurons.
Main Methods:
- Intracellular recordings combined with horseradish peroxidase (HRP) injections in hamsters.
- Antidromic activation from the first cervical spinal cord segment to identify TS cells.
- Morphological analysis of HRP-filled neurons and physiological characterization of their responses.
Main Results:
- Fifty-two tectospinal cells were analyzed, with somata located across deep SC layers (SAI, SGP, SAP) and superficial layers (SGI).
- TS cells exhibited uniform morphology with large cell bodies and extensive dendritic trees, projecting axons with collateral branches in the SC and brainstem.
- Physiologically, most TS cells were somatosensory, with a subset showing bimodal (visual-somatosensory) or complex receptive fields; conduction velocity correlated negatively with axon diameter.
Conclusions:
- Tectospinal cells in hamsters are morphologically and physiologically diverse, primarily involved in somatosensory processing, with some integrating visual information.
- Dendritic extent into superficial layers correlates with bimodal sensory input, and complex receptive fields are associated with inhibitory inputs from the contralateral SC.
Abstract:
Intracellular recording and horseradish peroxidase (HRP) injection techniques were used to delineate the structural and functional characteristics of the superior collicular cells in the hamster, which could be antidromically activated from the first cervical segment of the spinal cord. Thirty-one such neurons were characterized, filled with HRP, and recovered. Complete physiological data were obtained from another 21 tectospinal cells for which anatomical data were sufficient only to define the laminar location of the cell body from which recordings were made. Of the total sample of 52 cells, 7.7% had their somata in the stratum griseum intermediale (SGI), 50% were in the stratum album intermedium (SAI), 36.5% were in the stratum griseum profundum (SGP), and 5.8% were in the stratum album profundum (SAP). The tectospinal cells were fairly uniform morphologically. They had large (27.7 +/- 5.5 microns diameter) cell bodies, which gave rise to an average of 6.7 +/- 1.2 primary dendrites. These were generally smooth and extended up to 500 microns away from the cell body. In many cases, they ascended out of the deep laminae into the stratum opticum (SO) and/or stratum griseum superficiale (SGS). The axons of TS cells averaged 3.4 +/- 0.8 microns in diameter, and they generally coursed radially to the SAP where they curved around the periaqueductal gray and entered the predorsal bundle. These axons often gave rise to collaterals that arborized in the deep laminae of the ipsilateral superior colliculus and subjacent reticular formation. The tectospinal cells were also fairly uniform physiologically. Their average conduction latency was 2.0 +/- 2.3 ms, and this variable had a strong negative correlation (-.81) with axon diameter for the recovered cells. Most (63.5%) of the TS cells were exclusively somatosensory and gave rapidly adapting responses to deflection of vibrissae and/or guard hairs; 7.7% were bimodal (visual-somatosensory); 11.5% had complex (Rhoades et al., '83) somatosensory receptive fields; 1.9% were discharged only by a noxious pinch, and 15.4% were unresponsive. A common feature of all bimodal tectospinal neurons was dendrites that extended at least as far dorsally as the SO. Whereas there were no other clear-cut correlations between the structural and functional characteristics of these tectal neurons, we did note that all of the cells with complex somatosensory receptive fields received inhibitory input from axons that either originated from, or passed through, the contralateral superior colliculus.