Correlation of cell body size, axon size, and signal conduction velocity for individually labelled dorsal root ganglion cells in the cat

Abstract

Measurements of cell body and peripheral and central axon sizes were made for primary sensory neurons outlined by the intracellular injection of HRP. Conduction velocities were also measured on the outlined processes.The sensory neurons were then subdivided into A and C cells on the basis of the conduction velocity of the impulses carried by the processes of these cells.Central processes of both A and C cells are smaller than the peripheral processes, but the size differential is greater for the C cells. For A cells there is a linear relation between the size of the peripheral axon and the conduction velocity of the impulses carried by these axons, but the confidence limits are wide. For C cells there is a linear relation between the size of the central process and conduction velocity of the impulses carried by the processes, but for the peripheral processes two aberrant processes resulted in no correlation between process size and conduction velocity. For A cells, the size of the central and peripheral processes and the conduction velocity of the impulses carried by the peripheral processes are linearly correlated with cell body size. By contrast no such correlations can be demonstrated for C cells. This presumably implies an important difference in that the size of the cell body is correlated with axon size and impulse conduction velocity for A cells but not for C cells.A widely accepted generalization is that large sensory cells give rise to myelinated axons and small sensory cells to unmyelinated axons. In this study, myelinated and unmyelinated are defined on the basis of impulse conduction velocity. For those cells that are clearly large (greater than 50 pm in diameter), the conduction velocity of the impulses carried by their processes is always greater than 2.5 m/s, and for those cells that are clearly small (less than 35 pm in diameter), the conduction velocity is always less than 2.5 d s . Thus for these cells the above generalization holds. For the intermediate-sized cells (35-50 pm), however, the size of the cell body bears no predictable relation to the conduction velocity of the impulses carried by those processes, and thus to whether the axons are myelinated or unmyelinated. Thus the above generalization does not hold for this intermediate group of cells, and since there are many cells in this size range, we feel that the generalization that large cells give rise to myelinated axons and small cells to unmyelinated axons is an oversimplification.

Morphometry of Dorsal Root Ganglion in Chronic Experimental Diabetic Neuropathy

“…3). The large-diameter population can be subdivided into intermediate-large and largest diameter groups (16). With this subdivision, the intermediate-large neurons (1,500 -2,100 m 2 ) were not reduced in diabetic animals, but the largest neurons (Ͼ2,200 m 2 ) were significantly reduced (by 43%).…”

Cell-Specific Alterations of T-Type Calcium Current in Painful Diabetic Neuropathy Enhance Excitability of Sensory Neurons

“…Medium-size cells with ADP display functional properties of nociceptors, with IB 4 -positive immunoreactivity in both control and diabetic groups but enhanced responses to capsaicin only in diabetic rats Previous in vivo recordings with sharp electrodes have shown that many medium-size DRG cells belong to slowly conducting A␦ sensory fibers, most of which are high-threshold mechanoreceptors capable of sensing painful stimuli (Harper and Lawson, 1985;Lee et al, 1986;Gerke and Plenderleith, 2001;Fang et al, 2005). Although acute enzymatic dissociation allows patch clamping and studies of ion channels, functional identification of dissociated DRG cells based on conduction velocity is not feasible.…”

The Endogenous Redox Agent L-Cysteine Induces T-Type Ca2+ Channel-Dependent Sensitization of a Novel Subpopulation of Rat Peripheral Nociceptors

“…However, expression of T-type Ca 2ϩ current is more heterogeneous between large (Ͼ45 m in diameter), medium (32-45 m), and small (20 -30 m) DRG cells (Scroggs and Fox, 1992). Based on studies using intact DRGperipheral nerve preparations, it is generally thought that most small and some medium DRG cells contribute to nociception (Lee et al, 1986). Thus, we restricted our experiments to cells Ͻ45 m in diameter (n ϭ 295).…”