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The Motor Programme

Neurons Important in Jumping

The metathoracic ganglion contains thousands of neurons, and probably many hundreds of these are involved in jumping. However, I am going to concentrate on just 4 neuron types (on one side - there are equivalent neurons on the other), which cover the most important bits of the motor programme.

  1. FETi. The excitatory Fast Extensor of the Tibiae motorneuron. This is the motorneuron shown in the picture on the previous page. It is a single neuron and causes the strong extensor muscle contraction that drives the whole behaviour.

  2. FlTi. The excitatory Flexor Tibiae motorneurons. These motorneurons cause the flexor muscle to contract. There are 7-8 of them, but they all do more-or-less the same thing in jumping.

  3. FI. The inhibitory Flexor Tibiae motorneurons. There are two of these motorneurons, and they cause the flexor muscle to relax rapidly.

  4. M. The multimodal interneuron. This inhibits the excitatory flexor motorneurons (FlTi).

What They Do in a Jump (actually, a kick)

The cartoon below shows the activity of these four neuron types in relation to the movement of the leg. The four traces are graphs of the membrane potential of the neurons (vertical axis), plotted against time (horizontal axis). The graphs show synaptic potentials (small lumps and bumps) and nerve impulses (the spike-like potentials).

The key points in interpreting these records are:

Left: A montage of intracellular recordings from neurons involved in the kick motor programme. (Note that all spikes are small due to the soma recording site, and in the FlTi they are barely visible.) Right upper: A cartoon showing the action of the leg. Right lower: The metathoracic ganglion showing the active neurons.

Science stuff: more on neurons and recording their activity

These records confirm the story I have been telling. The first thing that happens is that flexor excitor motorneurons are activated, causing the initial flexion. Next, the flexor and extensor excitor motorneurons are active together. The flexor muscle holds the tibia flexed using the mechanical system described previously, while tension builds up in the extensor muscle and energy is stored in the cuticle springs. This constitutes the co-contraction phase. During this, the M neuron is inhibited (dashed line). Finally, the M interneuron and the flexor inhibitor produce a burst of spikes. The M interneuron shuts down the flexor excitor motorneurons within the ganglion, so the flexor muscle is no longer activated, while the flexor inhibitor motorneurons speed up the relaxation of the flexor muscle. The end result is that the flexor muscle looses tension very rapidly and the leg extends with an explosive release of energy.


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