Neuron Details

1. Extensor motorneurons. The excitatory fast extensor tibiae (FETi) is the most important extensor motorneuron with respect to the kick and jump motor programme. The slow extensor (SETi) makes very little force contribution, and can be surgically eliminated from the programme without noticeable effect. The common inhibitor (CI1) has little effect on extensor muscle tension during the motor programme, since it does not innervate most of the muscle fibres. The dorsal unpaired median neuron (DUMETi) is active throughout the period in which extensor tension develops, but it is unclear what it's function is.
   
   
2. Flexor motorneurons. There are actually about 9 of excitatory flexor motorneurons (FlTi), and they come in various flavours (strong, intermediate, weak). However, they all do more or less the same thing in the motor programme.
   
   
3. Flexor inhibitor motorneurons. There are 2 of these, and their official names are CI2 and CI3, which stand for common inhibitor two and three. They are called common inhibitors because they go to several muscles, but the important target from our point of view is the flexor muscle. They have similar activity during the motor programme.
   
   
4. The M interneuron. Unlike the other neurons, this is an interneuron. That means that it is located entirely within the central nervous system. It is excited by several different modalities of sensory input, hence its name.

Programme Details

The activity shown in the cartoon is actually a montage, made up of recordings from different episodes of the motor programme. The FETi and FlTi activities were recorded simultaneously from one episode, but the FI and M activities were recorded separately from different animals. This is because of the technical difficult of recording from several neurons at once.

The recordings are made from grasshoppers performing hind-leg kicks, rather than grasshoppers actually jumping. This is for the obvious reason that it is impossible to keep microelectrodes in the animal if it jumps freely. However, there is good evidence that the motor programme driving the hind leg extension is the same in jumping and kicking (and, surprisingly, swimming).

Recording Details

(Very) Elementary Neuroscience

The recordings in the cartoon show the membrane potentials of the 4 neurons - i.e. the voltage difference between the inside and the outside of the cells, plotted against time. When they are not doing anything, neurons have a resting potential of about -70 mV (the inside is negative relative to the outside). Synaptic potentials arrive as a result of input connexions from other neurons, and briefly shift the membrane potential away from its resting state. If the synapse is excitatory, the membrane potential tends to shifts positive. If the synapse is inhibitory, the membrane potential tends to shift negative.

A neuron may receive synaptic input from many other neurons. It integrates the excitatory and inhibitory inputs, and if the resulting change in membrane membrane potential goes sufficiently positive (above threshold), the neuron itself produces a nerve impulse, or action potential (commonly called a spike). The action potential, unlike the synaptic potentials, travels down the axon of the neuron, and either causes a synaptic input to another neuron (if the spiking neuron is an interneuron), or a muscle contraction (if the spiking neuron is a motorneuron).

In the picture above the red interneuron neuron gets inhibitory (i) and excitatory (e) synaptic inputs from unknown (?) sources. The two excitatory inputs occur close together in time and add together (temporal summation), causing the red neuron to spike (s). The red neuron makes an excitatory synaptic connexion to the green neuron.

The key point is that neurons integrates the synaptic input they receive, and only produce output themselves if the summed input is above their spike threshold.

The Cartoon Records

The traces in the cartoon show intracellular recording made from the cell bodies of the motorneurons, and the neuropilar segment of M. The spikes are attenuated in amplitude, because the recordings are made from regions in the neuron which do not produce full action potentials. In each record the maximum vertical excursion is about 30 mV.