SUMMARY1. Combining intracellular recording and dye-injection techniques permitted direct correlation of neuronal soma size with peripheral nerve conduction velocity in individual neurones of the L4 dorsal root ganglion (d.r.g.) of the anaesthetized 5-8-week-old rat.2. The conduction velocities fell into two main groups; those > 14 m/s (Ax and ft fibres) and those < 8 m/s (Ad and C fibres).3. Fibres with conduction velocities in the Ad range (2-2-8 m/s) in the sciatic nerve between the sciatic notch and the neuronal soma in the d.r.g. often conducted more slowly, that is in the C-fibre range (< 1-4 m/s), in the periphery from the tibial nerve to the sciatic notch.4. For the fast-conducting myelinated afferents, there was a loose positive correlation between cell size and the conduction velocity of the peripheral axon, whereas a clearer positive correlation existed between neuronal cell size and axonal conduction velocity both for Ad-and for C-fibre afferents.5. The relationship of the cell cross-sectional area (measured at the nucleolar level), to the cell volume for each neuronal soma was similar for the different conduction velocity groups.6. The somata of the fast-conducting myelinated Ax and A,8 fibres had a similar mean and range of cross-sectional areas to those of the large light cell population.7. The somata with Ad and C fibres were of a more uniform size and were restricted to the smaller cells within the ganglia. The mean and range of cross-sectional areas of the C cells was similar to those of the small dark cell population. Ad somata had a larger mean and range of cell sizes than those of the small dark cell population.8. The relationships of peripheral axon type to the morphological cell types are discussed.

Spontaneous pain, a poorly understood aspect of human neuropathic pain, is indicated in animals by spontaneous foot lifting (SFL).

SUMMARY1. Intracellular recordings were made in dorsal root ganglia in vitro at 37 'C. The L4, L5 and L6 ganglia from 46-to 51-day-old female Wistar rats were used. In each neuron conduction velocity (CV) was measured and fluorescent dye was injected. Later the intensity of the immunoreactivity to RT97 (a monoclonal antibody to the phosphorylated 200 kDa neurofilament subunit) as well as the cell size (crosssectional area at the nuclear level) were measured in the dye-injected neurons. RT97 was used to distinguish between the L (light, neurofilament-rich) and the SD (small dark, neurofilament-poor) neuronal somata.2. Neurons were classified as C neurons (CV < 1-3 m/s), C/Ad neurons (1-3-2 m/s),A neurons (2-12 m/s) or Aa//? neurons (> 12 m/s).3. All A-fibre somata were RT97 positive (L) and all C-fibre somata were RT97 negative (SD), although in the C/M group both positive and negative neurons were seen. Thus, RT97-negative somata had C (unmyelinated) or C/Ad fibres, while RT97-positive somata had A (myelinated) or C/Ad fibres. 4. The size distributions of A neurons and C neurons were consistent with their classification as L-and SD-cell neurons respectively. The size distribution of Ad cells was skewed with a peak of small cells and a tail of medium-sized cells.5. There was a loose positive correlation between cell size and fibre CV. 6. RT97 intensity was positively correlated with CV if all neurons were considered together, but no correlation was seen within the C, Ad or Aoc/, CV groups.7. RT97 intensity was positively correlated with cell size when all neurons were considered together. Although no correlation was seen within the C or the Ad CV groups, a clear positive correlation was seen for Atz// neurons. 8. The relationship of RT97 intensity to cell size was not demonstrably altered by axotomy, time in vitro or the presence of intracellular dye in control experiments.9. RT97-negative and -positive neurons could be seen in neonatal rat ganglia. Their size distributions resembled, respectively, the SD-and L-neuron populations at this age. RT97 immunoreactivity may therefore be a useful predictor of the cell type and myelinated state which a sensory cell is destined to reach in the adult rat.

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We have examined the distribution of the sensory neuron-specific Na + channel Na v 1.8 (SNS/PN3) in nociceptive and non-nociceptive dorsal root ganglion (DRG) neurons and whether its distribution is related to neuronal membrane properties. Na v 1.8-like immunoreactivity (Na v 1.8-LI) was examined with an affinity purified polyclonal antiserum (SNS11) in rat DRG neurons that were classified according to sensory receptive properties and by conduction velocity (

No abstract

Binding to isolectin-B4 (IB4) and expression of tyrosine kinase A (trkA) (the high-affinity NGF receptor) have been used to define two different subgroups of nociceptive small dorsal root ganglion (DRG) neurons. We previously showed that only nociceptors have high trkA levels. However, information about sensory and electrophysiological properties in vivo of single identified IB4-binding neurons, and about their trkA expression levels, is lacking. IB4-positive (IB4ϩ) and small dark neurons had similar size distributions. We examined IB4-binding levels in Ͼ120 dye-injected DRG neurons with sensory and electrophysiological properties recorded in vivo. Relative immunointensities for trkA and two TTX-resistant sodium channels (Nav1.8 and Nav1.9) were also measured in these neurons. IB4ϩ neurons were classified as strongly or weakly IB4ϩ.All strongly IB4ϩ neurons were C-nociceptor type (C-fiber nociceptive or unresponsive). Of 32 C-nociceptor-type neurons examined, ϳ50% were strongly IB4ϩ, ϳ20% were weakly IB4ϩ and ϳ30% were IB4Ϫ. A␦ low-threshold mechanoreceptive (LTM) neurons were weakly IB4ϩ or IB4Ϫ. All 33 A-fiber nociceptors and all 44 A␣/␤-LTM neurons examined were IB4Ϫ. IB4ϩ compared with IB4Ϫ C-nociceptor-type neurons had longer somatic action potential durations and rise times, slower conduction velocities, more negative membrane potentials, and greater immunointensities for Nav1.9 but not Nav1.8. Immunointensities of IB4 binding in C-neurons were positively correlated with those of Nav1.9 but not Nav1.8. Of 23 C-neurons tested for both trkA and IB4, ϳ35% were trkAϩ/IB4ϩ but with negatively correlated immunointensities; 26% were IB4ϩ/trkAϪ, and 35% were IB4Ϫ/trkAϩ. We conclude that strongly IB4ϩ DRG neurons are exclusively C-nociceptor type and that high Nav1.9 expression may contribute to their distinct membrane properties.

Primary afferent neurones convey to the central nervous system information about the external and internal environments. They include low threshold neurones that respond to non-damaging, low intensity, stimuli and normally give rise to sensations other than pain, and high threshold or nociceptive neurones that respond to high intensity, potentially damaging stimuli and often result in a sensation of pain. This review will focus on somatic nociceptive afferent neurones, that is, those that project to the body wall (skin and muscle).Somatic nociceptive afferent neurones may be primarily exteroceptive or interoceptive. Exteroceptive nociceptive afferent neurones detect external noxious stimuli and provide the information necessary for withdrawal from that stimulus, while interoceptive nociceptive neurones detect noxious stimuli of internal origin, including changes that threaten homeostasis and/or result from damage to the tissues. Logically since the stimulus originates within the tissue, withdrawal makes no sense. However, the influence of interoceptive nociceptors on cardiovascular and other homeostatic control centres, does make sense.Both interoceptive and exteroceptive nociceptive afferent neurones are therefore important for the maintenance of internal homeostasis. There are, in addition, efferent effects of certain peptide-expressing primary afferent nociceptive neurones. In response to noxious stimuli these release peptides (substance P and/or calcitonin gene-related peptide (CGRP)) peripherally which have vasoactive effects, promoting inflammatory responses and thus healing of the tissues (Brain & Williams, 1985, 1988. These neurones are therefore thought to be important in the maintenance of tissue integrity, an important aspect of homeostasis. However, nociceptive neurones are more commonly considered in the context of the generation of pain sensation than in the context of their homeostatic function, perhaps because the unpleasant conscious sensation of pain demands our attention above other considerations.While discussing the homeostatic role of somatic primary afferent neurones, it may be useful to consider which types of unit may contribute more to exteroception and which to interoception. To some extent this is an unreal distinction, as the peripheral terminals of sensory nerve cells are