Central control of the lower extrinsic protractor muscle in the buccal ganglia of Aplysia

Antagonistic responses of the radular protractor and retractor to the same putative neurotransmitters

1. Mechanisms of antagonistic responses of the radular protractor and the retractor to the same neurotransmitters, octopamine and serotonin, were investigated in a mollusc, Rapana thomasiana. 2. Hyperpolarization induced by octopamine or serotonin was found to depend mainly on the increase in Cl- conductance of the membrane in both the radular protractor and the retractor. 3. In the protractor, the enhancement of acetylcholine depolarization during the application of octopamine or serotonin was exhibited even when the membrane potential was varied by current application. Spontaneous firing was blocked by these amines, but larger spikes with higher frequency were elicited on giving a depolarizing current above threshold. 4. In the retractor, the inhibition of glutamate depolarization during the application of octopamine or serotonin was observed irrespective of membrane potential. Spontaneous spikes were stopped by these amines but reappeared on depolarization above threshold. Reappeared spikes during serotonin application were smaller than control spikes, while those during octopamine application were about the same as or larger than control ones. 5. The modes of actions of octopamine and serotonin on both muscles were considered.

Excitatory and inhibitory effects of acetylcholine on the anterior aorta muscle of Aplysia

The acetylcholine (ACh) responses of anterior aorta muscle cells of Aplysia kurodai were studied by intracellular recording and ACh iontophoresis. Three different types of membrane potential change were observed in response to ACh pulses: a rapid Na+-mediated depolarizing response, a slow, mainly Cl-, in part K+-mediated, hyperpolarizing response, and Na+ (for depolarizing component) and Cl- (for hyperpolarizing component)-mediated biphasic response. The depolarizing response and the depolarizing component of the biphasic response were effectively blocked by hexamethonium or atropine. The hyperpolarizing response and the hyperpolarizing component of the biphasic response were completely blocked by tubocurarine. Low concentrations of ACh inhibited and relaxed the anterior aorta muscle; high concentrations of ACh excited the muscle and initiated contraction. The biphasic responses to ACh are likely due to the presence of more than one ACh receptor type on the same cell membrane of the anterior aorta muscle fiber.

Role of pedal ganglia motor neurons in pedal wave generation in Aplysia

Presumptive pedal ganglia motor neurons involved in pedal wave generation in the foot of Aplysia were examined. Pedal motor neurons fired tonicly in the absence of pedal waves, but exhibited bursting with constant phase angles during pedal wave generation. Motor neurons which burst both in and 180 degrees out of phase with the pedal wave were recorded. The relative latency of neurons firing at large phase angles was more variable than that of neurons firing at small phase angles. Pedal neurons did not make monosynaptic connections among themselves. Their firing both during and in the absence of pedal waves was due to synaptic input, presumably from interneurons which generate the oscillations underlying the pedal wave. Motor neurons firing in phase had common synaptic input of the same sign, while some motor neurons firing out of phase had common synaptic input of opposite sign. There also appeared to be polysynaptic connections among pedal motor neurons. Evidence suggesting polysynaptic feedback from the motor neurons to the oscillator interneurons was found. The results are consistent with a multineuronal oscillator in which different elements drive different groups of motor neurons. The pedal wave is the result of cyclic activation of the motor neurons by the oscillator network. Pedal motor neurons evoked several different types of tension changes in the foot when intracellularly stimulated. Fast and slow tension increases were observed. Some caused decreased tension in the foot when stimulated suggesting that they might be inhibitory motor neurons. Other neurons appeared to be involved in the central control of foot tonus. Pedal motor neurons had conduction velocities ranging from 36 to 136 cm.sec-1 and were found to have axons in more than one foot nerve branch. This property may contribute to the longitudinal spread of the pedal wave.