Nervous system design for phototactic sea creature with two antagonistic tail muscles

[shanwilmo]

1. Create the nervous system of a simple fictional sea creature which is round with a single tail controlled by two antagonistic muscles. You may only use two light-sensitive
neurons, but as many excitatory and/or inhibitory neurons as you wish. Each neuron used
must be either entirely excitatory or entirely inhibitory.

The goal of the nervous system is to always have the sea creature swim toward light
regardless of the direction from which light is first encounter. Use the strategies
discussed in class such as collaterals and reciprocal inhibition make sure to explain any
complicated connections or functions and indicate whether a neuron is excitatory or
inhibitory.

Homework Equations

3. So due to the fact that this creature is so simple, I believe that I am over thinking it.

My creature would have Two light sensitive neurons. One on each side of its round body. Each of the light sensitive neurons would lead directly to an alpha motor neuron that creates the motor unit of each of the antagonistic muscle group. (the sensory neuron on its left side would be the alpha motor neuron for the muscle on the left side of its tail only. Visa versa for the other side) the Alpha motor neuron would be connected by an interneuron that would create reciprical inhibition between the two muscles, allowing them to be anatgonistic to each other.

The light sensitive neuron would have an inhibitory effect on the motor neuron when light is not present.

I may be just over thinking it, because I feel that my answer is not good at all and that it needs to be more complicated than that. I also think I maybe missing something in the question. Any help would be much appreciated!

 

[JHamm]

How does the creature know when the light is behind them instead of in front? Also from your description it seems as though sensing light will cause one of the muscles to contract and not seeing light will cause it to relax but this won't make your creature swim anywhere, if there is a light source on one side of the creature then its tail would just kink to one side indefinitely.

 

[shanwilmo]

Thanks so much for pointing that out! Yes you are right, rereading my answer I do need to be a little more thorough.

I would wire the animal to have an inhibitory neuron on the main motor neurons. I would arrange it so that the creatures tail would only be able to move in one direction, thus turning the creature towards the light. Once the sensory neuron inthe front would sense the light, the creature would resume firing both muscles in its tail, pushing the creature forward towards the light.

You do bring up a valid point that I did not address. The creature should be wired to just swim forward when no sensory neuron is stimulated, both motor neurons would fire, makingthe creature swim forward. If light is sensed in front of the creature, the rate of firing would increase, making the creature swim faster.

Does this make sense? Is there anything else I missed??

 

[shanwilmo]

Also, if light was sensed one side, the neuron on that side would fire in bursts (i left that part out, sorry). this would force the tail to contract to one side, relax and go back to neutral, then contract to the same side again, forcing the animal to turn towards the light.