PubMed · 1944403
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M R Swenson, L Cornacchia. 1991. Dispersion or block?. https://doi.org/10.1002/mus.880141016
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Turtle visual cortex has three layers and receives direct input from the dorsolateral geniculate complex of the thalamus. The outer layer 1 contains several populations of interneurons, but their physiological properties have not been characterized. This study used intracellular recording methods followed by filling with Neurobiotin to characterize the morphology and physiology of two populations of layer 1 interneurons. Subpial cells have somata positioned in the outer third of layer 1 and dendrites confined within the band of geniculate afferents that runs from lateral to medial across visual cortex. Their dendrites are composed of a sequence of many beads or varicosities separated by intervaricose segments. They have membrane time constants of tau(o) = 45.5 +/- 5.2 ms and electrotonic lengths of 1.1 +/- 0.2. Subpial cells show spike rate adaptation in response to intracellular current pulses. Stellate cells have somata located in the inner two-thirds of layer 1 and, less frequently, in layers 2 and 3. Their dendrites extend in a stellate configuration across the cortex. They are smooth or sparsely spiny, but never bear distinct varicosities. They have membrane time constants of tau(o) = 155.1 +/- 12 ms and electrotonic lengths of 3.8 +/- 0.5. They show little spike rate adaptation in response to intracellular current pulses. The positions of the two populations of cells in visual cortex and their physiological properties suggest that subpial cells may participate in a feedforward inhibitory pathway to pyramidal cells, whereas stellate cells are involved in feedback inhibition to pyramidal cells.
A previous study reported that a peptide, sensorin-A, is expressed exclusively in mechanosensory neurons having somata in central ganglia of Aplysia. The present study utilized in situ hybridization, staining by nerve back-fill and soma injection, and electrophysiological methods to characterize the locations, numbers, and functions of sensorin-A-expressing neurons and to define the relationships between soma locations and the locations of peripheral axons and receptive fields. Approximately 1,000 cells express sensorin-A mRNA in young adult animals (10-30 g) and 1,200 cells in larger adults (100-300 g). All of the labeled somata are in the CNS, primarily in the abdominal LE, rLE, RE and RF, pleural VC, cerebral J and K, and buccal S clusters. Expression also occurs in a few sparsely distributed cells in most ganglia. Together, receptive fields of all these mechanosensory clusters cover the entire body surface. Each VC cluster forms a somatotopic map of the ipsilateral body, a "sensory aplunculus." Cells in the pleural and cerebral clusters have partially overlapping sensory fields and synaptic targets. Buccal S cells have receptive fields on the buccal mass and lips and display notable differences in electrophysiological properties from other sensorin-A-expressing neurons. Neurons in all of the clusters have relatively high mechanosensory thresholds, responding preferentially to threatening or noxious stimuli. Synaptic outputs to target cells having defensive functions support a nociceptive role, as does peripheral sensitization following noxious stimulation, although additional functions are likely in some clusters. Interesting questions arise from observations that mRNA for sensorin-A is present not only in the somata but also in synaptic regions, connectives, and peripheral fibers.
Persistent discharge of wide dynamic range (WDR) neurons was recorded from lumbar dorsal horn of anesthetized rats following subcutaneous bee venom injection into the receptive field. To quantitatively describe the complexity of this nociceptive activity, we computed the approximate entropy (ApEn) for each sampled interspike interval (ISI) series. A larger value of ApEn indicates higher complexity or less regularity and vice versa. The ApEn value varied across different WDR neurons tested, and for each neuron the ApEn remained constant through the 1-h discharge though the average ISI of the sampled data increased progressively with time (16 neurons). A low dose of intravenous morphine (0.3 mg/kg) depressed the activity of WDR neurons differentially, and the degree of this inhibition showed a significant correlation with the value of ApEn (P<0.001, 27 neurons, Spearman's correlation test). The present results suggest that the complexity feature of WDR neurons is various under tissue injury state, and for each single WDR neuron the complexity feature is relatively independent of the strength of peripheral noxious input and cannot be fully described in terms of average firing rate. Moreover, the response of the nociceptive discharge to analgesics may be related to the nonlinear dynamics feature of nociceptive neurons, which can be quantitatively characterized by the degree of complexity.