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J Neurophysiol 64: 1791-1800, 1990;
0022-3077/90 $5.00
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Journal of Neurophysiology, Vol 64, Issue 6 1791-1800, Copyright © 1990 by APS

ARTICLES

Intracellular records of the effects of primary afferent input in lumbar spinoreticular tract neurons in the cat

Y. Sahara, Y. K. Xie and G. J. Bennett
Neurobiology and Anesthesiology Branch, National Institute of Dental Research, National Institutes of Health, Bethesda, Maryland 20892.

1. The afferent-evoked synaptic input to lumbar spinal cord (L5-S1) neurons that were activated antidromically from the medial pontomedullary reticular formation (nucleus reticularis gigantocelluaris and vicinity) was investigated with the use of intracellular recordings in pentobarbital sodium-anesthetized cats. 2. Spinoreticular tract (SRT) neurons (n = 33) were categorized into three types ("deep-inhibited," "deep-complex," and "intermediate") on the basis of their locations and of their responses to natural and electrical stimulation. 3. The deep-inhibited-type neurons, located in the medial part of the deeper laminae (approximately VI-VIII), comprised a large component of the sample (20/33). They had no demonstrable excitatory receptive field (RF). However, electrical stimulation of low-threshold cutaneous afferents of hindlimb nerves evoked inhibitory postsynaptic potentials (IPSPs) via an oligosynaptic linkage. High-threshold cutaneous and muscle afferents also evoked IPSPs. 4. In the deep-complex-type neurons (8/33), electrical stimulation of low-threshold cutaneous afferents evoked complex IPSP-excitatory postsynaptic potential (EPSP) sequences. With intense stimuli, long-latency C-fiber-like EPSPs were evoked. Two of these eight neurons were characterized as wide-dynamic-range (WDR) neurons with large, excitatory and inhibitory cutaneous RFs. 5. Intermediate-type neurons (5/33) were concentrated in the lateral spinal gray and relatively superficially (approximately lamina V). These neurons had convergent low- and high-threshold cutaneous inputs (WDR neurons). Electrical stimulation of low-threshold cutaneous afferent fibers from within the excitatory RF evoked mono- or disynaptic EPSPs followed by IPSPs. High-threshold muscle and cutaneous afferents also evoked EPSPs. 6. These results show that SRT neurons have a variety of response characteristics resulting from various degrees of spatial and temporal summation of primary afferent input. Neurons with widespread inhibitory responses but no excitatory drive from the periphery comprise a surprisingly large component of the SRT: the function of these cells is unknown. It is apparent that the spinoreticular projection has considerable functional heterogeneity.




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