This study by Katz and Miledi investigates synaptic transmission in the absence of nerve impulses, focusing on the giant synapse between an interneuron and a motor axon in the stellate ganglion of the squid. The authors use tetrodotoxin to block sodium currents, allowing them to study the graded response of the synapse to presynaptic membrane potential changes. Key findings include: 1. **Presynaptic Membrane Potential and Postsynaptic Potential**: The relationship between presynaptic and postsynaptic potential changes is graded, with no critical threshold required for synaptic response. This suggests that the release of transmitter substances is not dependent on the full action potential but on a depolarization of a certain magnitude. 2. **Calcium's Role**: Calcium is identified as a crucial ion for transmitter release. Removing calcium from the bath prevents transmission, while increasing calcium concentration facilitates it. This supports the hypothesis that calcium influx during depolarization facilitates the release of transmitter substances. 3. **ion Effects**: Magnesium ions reduce the slope of the input/output curve and the postsynaptic response to a given presynaptic depolarization. Barium ions have a similar effect to calcium. 4. **Pulse Duration and Electrode Position**: The duration of presynaptic depolarization and the position of the recording electrode significantly affect the synaptic response. Longer pulses and closer electrode placement to the synapse generally increase the postsynaptic response. 5. **Repetitive Pulses**: The synapse exhibits a refractory period and facilitation, similar to the neuromuscular junction. The refractory period is explained by the delayed rectification of the presynaptic membrane, which is not blocked by tetrodotoxin. 6. **Discussion**: The results are consistent with previous studies on the neuromuscular junction, supporting the hypothesis that calcium influx during depolarization facilitates the release of transmitter substances. This mechanism is likely involved in other forms of cellular secretion as well. Overall, the study provides detailed insights into the mechanisms of synaptic transmission in the absence of nerve impulses, highlighting the importance of calcium in this process.This study by Katz and Miledi investigates synaptic transmission in the absence of nerve impulses, focusing on the giant synapse between an interneuron and a motor axon in the stellate ganglion of the squid. The authors use tetrodotoxin to block sodium currents, allowing them to study the graded response of the synapse to presynaptic membrane potential changes. Key findings include: 1. **Presynaptic Membrane Potential and Postsynaptic Potential**: The relationship between presynaptic and postsynaptic potential changes is graded, with no critical threshold required for synaptic response. This suggests that the release of transmitter substances is not dependent on the full action potential but on a depolarization of a certain magnitude. 2. **Calcium's Role**: Calcium is identified as a crucial ion for transmitter release. Removing calcium from the bath prevents transmission, while increasing calcium concentration facilitates it. This supports the hypothesis that calcium influx during depolarization facilitates the release of transmitter substances. 3. **ion Effects**: Magnesium ions reduce the slope of the input/output curve and the postsynaptic response to a given presynaptic depolarization. Barium ions have a similar effect to calcium. 4. **Pulse Duration and Electrode Position**: The duration of presynaptic depolarization and the position of the recording electrode significantly affect the synaptic response. Longer pulses and closer electrode placement to the synapse generally increase the postsynaptic response. 5. **Repetitive Pulses**: The synapse exhibits a refractory period and facilitation, similar to the neuromuscular junction. The refractory period is explained by the delayed rectification of the presynaptic membrane, which is not blocked by tetrodotoxin. 6. **Discussion**: The results are consistent with previous studies on the neuromuscular junction, supporting the hypothesis that calcium influx during depolarization facilitates the release of transmitter substances. This mechanism is likely involved in other forms of cellular secretion as well. Overall, the study provides detailed insights into the mechanisms of synaptic transmission in the absence of nerve impulses, highlighting the importance of calcium in this process.