Recordings of neuronal unitary discharges evoked by primary afferent input were made in the superficial part of the spinal cord's dorsal horn, the marginal zone and substantia gelatinosa (also known as laminae I and II), using fine micropipette electrodes filled with HRP. After physiological characterization with respect to primary afferent input, HRP was injected intracellularly iontophoretically into the recorded neuron. Following histochemical processing, the neurons so delineated were studied at the light and electron microscopic levels. No clear relationship between function and either general cellular configuration or synaptic ultrastructure appeared in these analyses, although the concentration of dendritic distribution could be related to the nature of primary afferent excitation. Nocireceptive cells had dendrites mostly branching and ending in lamina I and IIo, while the dendrites of innocuous mechanoreceptive cells arborized principally in lamina II and III. Glomerular synaptic complexes (large, complex arrays of axonic and dendritic profiles with synaptic interconnections) were found to contact a few neurons of both the nocireceptive and mechanoreceptive classes. All neurons received large numbers of simple axonic contacts (small axonic boutons with only 1 or 2 synaptic contacts with a single postsynaptic profile). A degree of specificity in the presynaptic articulations appeared to be reflected by the observations that (1) nocireceptive neurons were never found to receive synaptic contacts from boutons which resembled the known ultrastructure of peripheral innocuous mechanoreceptors, and (2) mechanoreceptive neurons were never seen to receive synaptic contacts from boutons which resembled the known ultrastructure of primary afferent nocireceptors. The axons of the labeled neurons of both nocireceptive and mechanoreceptive classes terminated in simple axonic synapses. All classes of neurons participated in dendrodendritic contacts; however, only some mechanoreceptive neurons had dendrites containing vesicles that were presynaptic to other profiles. No nocireceptive neurons, regardless of gross configuration, were found to have vesicles in their dendrites, but 3 nocireceptive neurons received synapses from presynaptic dendritic profiles.

The individual fine myelinated fibers of cutaneous mechanical nociceptors and "D-hair" receptors were identified by electrophysiological recording with micropipette electrodes in cats and monkeys. Their intraspinal terminations were labeled by iontophoresing horseradish peroxidase intracellularly and subsequent diaminobenzidine histochemistry. These terminations were examined with light and electron microscopy to determine the nature and organization of their synaptic contacts. Myelinated fibers of the mechanical nociceptors became unmyelinated before exhibiting many enlargements that made multiple synaptic contacts in the marginal zone (lamina I) and lamina V. Pre- or postsynaptic contacts were found only on enlargements. In the marginal zone of the cat, enlargements made simple axodendritic contacts or were scalloped, central terminals in glomeruli. In glomeruli, myelinated mechanical nociceptor enlargements were presynaptic to several dendritic appendages and postsynaptic to two different types of profiles. One type was interpreted as a presynaptic axon terminal, the other as a presynaptic, vesicle-containing, dendritic appendage. In lamina V of the cat the nociceptor synaptic complexes were similar, but simpler, and only axonal profiles were found to be presynaptic to them. In the monkey marginal zone and deep nucleus proprius, myelinated nociceptor terminations formed the central element of glomeruli, which consisted of postsynaptic dendritic appendages and presynaptic axon terminals. D-hair axons terminated in large numbers of enlargements in the nucleus proprius (laminae III and IV) and inner substantia gelatinosa (lamina IIi). Their large rounded enlargements formed the central terminals in glomeruli and were presynaptic to both ordinary and vesicle-containing dendritic appendages; the presynaptic dendritic profiles also often contacted each other. Profiles interpreted as axonal in origin were the only terminals presynaptic to the primary ending within the D-hair glomeruli. The results suggest that transfer of primary afferent information occurs only at enlargements of the primary fiber and that each primary fiber enters into more than one kind of synaptic arrangement. They also point out that synaptic glomeruli are common to functionally different types of primary afferent fibers and that the internal organization of glomeruli varies with the kind of primary fiber and the locus of the complex.

Thin fiber muscle afferents (group III/IV) signal the sensations of muscle ache and fatigue. Aging may influence the sensitivity of these afferents. However, little is known about mechanisms and receptors involved in this age-related impact. Seventeen young (27±3 years) and thirteen old (68±5 years) matched (sex, BMI, and activity) healthy adults participated in this study. During a first session, participants were familiarized with a twodimensional (2D) scale adapted from the Gracely Box Scale. In a second session, participants received 3 intramuscular (left abductor pollicis brevis) infusions (each 2 mL over 5 minutes) of combinations of metabolites: 1) pH=7.4, ATP=300nmol, lactate=1mmol -found in resting muscle (neutral concentration mixture), 2) pH=7.0, ATP=1μmol, lactate=15mmol -found during moderate exercise (low concentration mixture), and 3) pH=6.6, ATP=5μmol and lactate=50mmol -found during strenuous exercise (high concentration mixture). ATP activates P2X receptors, hydrogen ions (i.e., lowered pH) activate ASIC and TRPV1 receptors, and lactate enhances ASIC activation. These three metabolites act synergistically to activate group III/IV neurons. During the infusion, participants continuously rated their sensations of fatigue and ache on the 2D scale. Young participants reported significantly more fatigue and ache than the older adults with higher fatigue sensations during the infusion of both the low and high concentration mixture (both p<0.05) and more ache during the infusion of the high concentration mixture (p<0.05). In conclusion, although the combined activation of P2X, ASIC, and TRPV1 receptors causes sensations of muscle ache and fatigue in humans, healthy aging decreases the sensations signaled by these molecular receptors. The current findings cannot distinguish whether aging attenuates the excitability/sensitivity of these molecular receptors or alters the central processing of ache and fatigue. Interestingly, the present data contradict previous findings suggesting that sensitized muscle afferents underlie the higher prevalence of muscle fatigue and ache in older individuals.