The perikaryal projections of rabbit spinal ganglion neurons. A comparison of thin section reconstructions and scanning microscopy views

Basic anatomy and physiology of pain pathways

This article provides an integrated review of the basic anatomy and physiology of the pain processing pathways. The transmission and parcellation of noxious stimuli from the peripheral nervous system to the central nervous system is discussed. In addition, the inhibitory and excitatory systems that regulate pain along with the consequences of dysfunction are considered.

Perikaryal surface specializations of neurons in sensory ganglia

Slender projections, similar to microvilli, are the main specialization of the perikaryal surface of sensory ganglion neurons. The extent of these projections correlates closely with the volume of the corresponding nerve cell body. It is likely that the role of perikaryal projections of sensory ganglion neurons, which lack dendrites, is to maintain the surface-to-volume ratio of the nerve cell body above some critical level for adequate metabolic exchange. Satellite cells probably have the ability to promote, or provide a permissive environment for, the outgrowth of these projections. It is not yet known whether the effect of satellite cells is mediated by molecules associated with their plasma membrane or by diffusible factors. Furthermore, receptor molecules for numerous chemical agonists are located on the nerve cell body surface, but it is not known whether certain molecules are located exclusively on perikaryal projections or are also present on the smooth surface between these projections. Further study of the nerve cell body surface and of the influence that satellite cells exert on it will improve our understanding of the interactions between sensory ganglion neurons and satellite neuroglial cells.

Unexplained peculiarities of the dorsal root ganglion

The cell soma of primary afferent neurons in the dorsal root ganglion (DRG) is assigned by classical neurophysiology the role of a metabolic depot, charged with supporting the peripheral sensory ending, the conducting axon, and the central synaptic terminals. However, certain peculiarities of DRG morphology and physiology do not sit well with this being its only role. For example, why are DRG cell somata electrically excitable, why are some able to fire repetitively on sustained depolarization, and why does the DRG lack a blood-nerve barrier? Consideration of these and related questions leads to several intriguing hypotheses: (1) Electrical excitability of the soma may be required to insure the reliable propagation of impulses past the DRG T-junction and into the spinal cord. (2) Invasion of the afferent spike into the cell soma may provide an essential feedback signal necessary for the cell soma to regulate the excitability of the sensory ending. 3) The subpopulation of DRG neurons that have repetitive firing capability may be responsible for generating the background sensation that we feel as our body schema. Moreover, these neurons may be chemical sensors that provide essential information about our body's internal milieu.

Age-related decrease in the overall extent of perikaryal projections in rabbit spinal ganglion neurons

The overall extent of the perikaryal projections of sensory neurons from spinal ganglia of young adult and aged rabbits was estimated by a stereological method using the electron microscope. The extent of perikaryal projections was significantly smaller in the aged animals. This age-related decrease did not seem to depend on factors intrinsic to the neuron, but on the absence of a satellite cell covering over extensive portions of the nerve cell body surface. This decrease may influence the organization of the subplasmalemmal cytoskeleton, metabolic exchange between the nerve cell body and its environment and perhaps also neuronal metabolism.

Structural basis of neuron-to-neuron cross-excitation in dorsal root ganglia

Lanthanum was used as a tracer substance to determine whether small molecules in the bulk extracellular space in dorsal root ganglia have access to the narrow cleft that separates sensory neurons from their surrounding satellite cell sheath. Results showed that lanthanum is able to diffuse into this cleft, especially when the tissue is incubated with the tracer before fixation. Lanthanum gained access to the cleft at the seam where adjacent satellite cell processes meet. There appears to be preferential access in the axon hillock-initial segment region. Large diameter light neurons, which generally support fast conducting myelinated axons and carry information about non-nociceptive sensory events, proved more likely to admit lanthanum than small diameter dark neurons, which tend to have thin myelinated and unmyelinated axons and typically carry nociceptive information. Peripheral axotomy triggered a reduction in the access of lanthanum to the neuron-satellite cell cleft. These data bear on the mechanism of non-synaptic cell-to-cell cross-excitation within dorsal root ganglia, and in particular, lend support to the hypothesis that this interaction is mediated chemically rather than electrically.