For the gamma motor neurons to function as error detectors though, the organism's brain must predict
how the length and velocity of the muscle will change during the movement. This information can be used with
knowledge of the inverse equations of the muscle to determine what stimuli will be required to maintain a constant
tension in the stretch receptors, and thus keep the Ia firing rate constant as well. The brain does all of this
automatically, but it is quite complex and its still not fully understood how it accomplishes this amazing task.
This stimulus essentially allows you to cheat and avoid these complexities. You can create a chart of the muscle length
and velocity during a movement. Then that data can be exported to a file, and you specify that data file as one of the
parameters of the stimulus. When the simulation starts the stimulus loads in the predicted movement file so it knows
what the muscle length and velocity should be for the desired movement.
Unlike other stimuli you do not specify a start and end time. These values are determined by the data file.
When it is loaded it finds the beginning and ending time for the data in the file and uses that to start and
stop the stimulus. Also, you need to make sure that when you create the data that you add only the muscle length
and velocity to the data chart, and that you add length first and velocity second. Otherwise, your data will be
swapped and it will be treating length data as velocity and vice-versa. Another important thing to keep in mind is
that you need to keep the Auto Collect Data Interval of the chart set to true. This will ensure that the data will
be collected using the same time step used by the physics engine. If you create a data file and then later change
the physics time step you will need to re-export a new file using the new time step. If you attempt to run the simulation
with a file that has a time step that is different from the physics time step an error will occur warning you that
you can not do this.
The stimulus uses the length and velocity data from the file in the inverse muscle equation to calculate the
amount of active tension that needs to be applied to maintain a steady tension. When the stimulus first starts
it takes a snapshot of the current tension level in the muscle and attempts to maintain that level. For more information
on the inverse equations that are used to perform these calculations please see this section on
Once you know the active tension that is required you can use the inverse of the stimulus-tension curve to calculate
the voltage stimulus that will produce that active tension.
However, the ultimate goal of this stimulus is to produce
a current that will be injected into a gamma motor neuron that is controlling the stretch receptor. In order to do that
the voltage stimulus needs to be converted into a current level. You do this by specifying a conductance and a base voltage
level. The current is then generated using ohm's law. The equations for this are shown below.
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