STRENGTH-DURATION AND ACTIVITY-DEPENDENT EXCITABILITY PROPERTIES OF FROG AFFERENT AXONS AND THEIR INTRASPINAL PROJECTIONS

Excitability properties of afferent axons and terminal regions in frog dorsal roots (DR) and spinal cords in vitro were investigated by antidromic activation from three sites-the root, the entry zone (dorsal white matter or DW), and deep within the dorsal horn (DH)-while recordings were made from the DR. Two approaches were used to assess physiological differences between telodendria and trunk axons. Rheobases and strength-duration time constants (τ(sd)) of single DR fibers were measured by stimulation in the DH or in the DW. Conduction velocity was estimated on the basis of onset latencies of evoked spikes (the time from stimulation to action potential arrival at the recording electrodes). Population supernormality was evaluated on the basis of responses to conditioned and unconditioned submaximal stimuli delivered to the DH or to the proximal end of isolated DRs. Single-fiber action potentials occurred at longer latencies after DH stimulation than after DW stimulation. Estimated intraspinal conduction velocity was ≃0.6 m/s. Extraspinal conduction velocity in these fibers averaged 22.2 m/s. Average τ(sd) was longer in the DH than in the DW (670 μs vs. 204 μs). DH and DR test responses evoked 10-150 ms after a conditioning stimulus had increased areas relative to unconditioned test responses. Conditioning-associated changes in evoked responses were greater with the DH stimulation site than with the DR stimulation site, and these changes were not altered by treatment designed to block synaptic transmission. We conclude that membrane properties determining τ(sd) differ between large afferent axons and fine terminal regions of those axons. The characteristics we measured in terminal regions (decreased conduction velocity, prolonged τ(sd), and prominent supernormality) resemble central unmyelinated and thinly myelinated fibers or experimentally demyelinated fibers rather than large-diameter, myelinated trunk axons. The changes we observed in the progression from main axon to axon terminal are, to a great extent, consistent with predictions made on theoretical bases or from studies of small-diameter axon properties. These changes may have functional significance to the process of synaptic transmission. Conduction into terminal branches may be impeded by increased membrane capacitance, the branches may be more sensitive to altered ionic environment, and activity-dependent excitability changes may facilitate propagation and, hence, transmission.