Separation of neuronal sites of driver potential and impulse generation by ligaturing in the cardiac ganglion of the lobster,Homarus americanus

Summary

This study documents the ability of individual neurons of theHomarus americanus cardiac ganglion to produce driver potentials (regenerative, 20 mV, 200 ms depolarizing responses to depolarization).

Partial block of impulse traffic, achieved by ligaturing, indicates that chemically mediated synaptic transmission is not essential to burst formation, initiation or coordination, but increases burst duration (Figs. 1, 9); electrotonic coupling suffices.

Ligatures placed within 1.2 mm of the soma of Cells 1 or 2 separate impulse supporting axon from a soma-proximal neurite region which does not support impulses, but generates a driver potential in response to depolarizing current (Figs. 2, 6, 10). Driver potentials persist with ligatures as close as 200 μm. Such a ligature excludes neuronal interaction via axon collaterals observable with lucifer yellow (Fig. 7), thus establishing that driver potentials are endogenous. Driver potentials are unaffected by TTX (Fig. 2, Table 1).

Driver potentials ofHomarus neurons isolated by ligaturing were compared with those ofPortunus isolated by TTX. The measured parameters are very similar (Table 1), as are effects of altered membrane holding potential (Figs. 3, 4) and drugs (Fig. 5). Driver potentials are reversibly blocked by Mn (4 mmol/l) or Cd (0.5 mmol/l). Tetraethylammonium chloride (5–50 mmol/l) reversibly augments the amplitude and duration of driver potentials; hyperpolarizing afterpotentials remain.

Driver potential responses during repetitive stimulation indicate a relatively refractory period and capability for graded responsiveness (Fig. 8).

Anterior neurons (Cells 1 or 2) isolated by ligaturing rarely exhibited spontaneity (Figs. 10, 11). Cell 3, isolated by 3 ligatures, consistently showed rhythmic burst generation (Fig. 11) arising from pacemaker depolarization.

The capability of driver potential generation in response to non-specific depolarization endows individual neurons with their pattern-forming ability.