This is a return to a concept covered during week 5, but it’ll be useful to review this material.
The crayfish serves as a good example of the use of an electrical synapse, although it also contains very important chemical synapses as well. The crayfish system is an easy animal model, as a stimulation allows us to see a very apparent phenotypic change. In this case, the stimulation of its tail causes it to flick the posterior muscles in its tail (flexor muscles), to tumble the crayfish forward and get away from the stimulation.
To provide an overview of this behavior…
1. The crayfish is first jabbed in the tail.
2. This causes sensory neurons in the tail to send a chemical signal to the lateral giant interneuron. One of the reason this is a chemical synapse is because the sensory neurons are very small in comparison to the lateral giant interneuron. In order to amplify the signal, a chemical signal is necessary. If you used an electrical synapse, you would likely need a very large pre-synaptic sensory neuron to be able to obtain the same effect. One way to conceptually visualize this, is to consider a small smart car pushing another small smart car. The first smart car can probably move the second smart car. However, if you have a smart car pushing a giant 18 wheel semi-truck… that semi-truck is going nowhere, similar to a small neuron moving a large neuron.
To re-emphasize the point, depolarizations and hyperpolarizations are a means of communication. These “polarizations” are essentially voltage differences. To amplify the signal from the sensory neurons in the tail, which are relatively small, to the lateral giant interneuron, which is a very large interneuron, the signal needs to be amplified. Electrical synapses are a direct translation of a voltage change from the 1st neuron to the next neuron (discounting leakage and loss through membranes). Conversely, chemical synapses can amplify the original change in ion concentrations (voltage change), and through the release of chemical signals, can amplify the signal many-fold.
3. The LGI to MG is electrical, as the sizes are comparable. Further, it can maximize the speed of signaling by using an electrical synapse.
4. From this point, the MG stimulates flexor muscles in segments of the tail, causing forward movement. This stimulation is chemical.