Function and distribution of three types of rectifying channel in rat spinal root myelinated axons

1. The nature, distribution and function of rectifying channels in
rat spinal root myelinated axons has been assessed with selective
blocking agents and a variety of intracellular and extracellular
recording techniques. 2. The electrotonic responses of roots
poisoned with tetrodotoxin (TTX) to constant current pulses had
fast (rise time much less than 1 ms) and slow components, which
were interpreted in terms of Barrett & Barrett's (1982) revised
cable model for myelinated nerve. Depolarization evoked a rapid
outward rectification (time constant, tau approximately 0.5 ms),
selectively blocked by 4-aminopyridine (4AP, 1 mM), and a slow
outward rectification (tau approximately 15 ms), selectively
blocked by tetraethylammonium (TEA, 1 mM) or Ba2+ (0.5 mM).
Hyperpolarization evoked an even slower inward rectification,
selectively blocked by Cs+ (3 mM) but not by Ba2+. 3. From the
different effects of the blocking agents on the fast and slow
components of electrotonus, it was deduced (a) that the inward
rectification is a property of the internodal axon, (b) that the
slow outward rectifier is present at the nodes, and probably the
internodes as well, and (c) that the 4AP-sensitive channels have a
minor nodal and a major internodal representation. 4. TEA and Ba2+
reduced the accommodation of roots and fibres not poisoned with TTX
to long current pulses, whereas 4AP facilitated short bursts of
impulses in response to a single brief stimulus. 5. TEA and Ba2+
also abolished a late hyperpolarizing after-potential (peaking at
20-80 ms), while 4AP enhanced the depolarizing after-potential in
normal fibres, and abolished an early hyperpolarizing
after-potential (peaking at 1-3 ms) in depolarized fibres.
Corresponding to the later after-potentials were post-spike changes
in excitability and conduction velocity, which were affected
similarly by the blocking agents. Cs+ increased the post-tetanic
depression attributable to electrogenic hyperpolarization. 6. The
physiological roles of the three different rectifying conductances
are discussed. It is also argued that the prominent ohmic 'leak
conductance', usually ascribed to the nodal axon, must arise in an
extracellular pathway in series with the rectifying internodal
axon.