Electrophysiological differences between specialized and multifunctional turtle spinal cord neurons involved in scratching and swimming motor patterns

Abstract

I examined the electrophysiology of both multifunctional and specialized neurons located in the turtle spinal cord, which are activated during swimming, scratching, and/or flexion reflex. My research analyzes these neurons to determine the electrophysiological differences between neurons that are activated during multiple motor patterns (multifunctional) and neurons that are activated only during a specific movement (scratch-specialized, flexion reflex-selective, and swim-specialized). The differences between multifunctional and specialized neurons could elucidate more about the organization of the central pattern generators (CPGs) located within the spinal cord. During analysis of intracellular recordings of multifunctional and scratch-specialized neurons, it was determined that most neurons preferred to fire during one phase of activity for both scratching and swimming. Furthermore, the multifunctional neurons could be divided into two groups (hip flexor-on phase preference and hip flexor-off phase preference) based on phase preference. Analysis of spiking activity in multifunctional and scratch-specialized neurons showed that multifunctional neurons tend to spike more rhythmically than scratch-specialized neurons. When analyzing the peak and trough phases of both multifunctional and specialized neurons it was determined that the multifunctional neurons are probably rhythmically modulated by inhibition more than excitation because the trough phases of activity for both scratching and swimming were significantly correlated for multifunctional neurons, while the peak phases were not. I hypothesize that alternation between the hip extensor and flexor, which are vital for successful scratching and swimming, are occurring due to these multifunctional CPG neurons. Furthermore, since the scratch-specialized neurons and flexion reflex-selective neurons were not as strongly rhythmically modulated, it is likely they are not part of the CPG and instead are responsible for starting scratching activity and flexion reflex, respectively. Also, this study showed the first swim-specialized neuron, which was tonically excited during swimming, but inactive during scratching. Similar to scratch-specialized neurons, this neuron was not strongly rhythmically modulated, so it is not likely to be part of the CPG. It was activated by each swim stimulation pulse at a relatively short latency. I hypothesize that similar to scratch-specialized neurons, the swim-specialized neuron is responsible for starting the motor pattern. Taken together, this study provides more insight into the organization of the spinal cord CPG for turtles.