Student: Figure 11 in Proske & Gandevia (2012) is fixed. I used plurals and changed Motor Command to Motor Commands and Efference Copy to Efference Copy Signals!

Professor: Attaway. Nice quick thinking.

Student: And where do the Efference Copies go?

Professor: Actually, as many as four copies with one coming:

1)    from the spinal cord and going to an oculomotor nucleus,

2)    from the spinal cord and going via the spinocerebellar tract to the cerebellum,

3)    from the frontal cortex, going to the basal ganglia, then on to the cerebellum, and

4)    from the frontal cortex and going via the pontine nuclei to the cerebellum.

Professor: A more important question is, “What is the sensory signals?”
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Student: What a pile of articles! And what did you find on commanding movement?

Professor: It depends upon:

• What are the types of movements?
• What is the nature of the network(s) for motor command?

Student: Well, one way of looking at movements is to consider the muscle mass involved:

• Fine motor movements, like tapping on a button, or
• Gross motor movements, like doing deep knee bends.

Professor: Then another view of movement is how much learning is involved:

• None, or
• Over-learned involving:
  • Maintenance actions, like running smoothly at an easy pace,
  • Reflexive actions, like walking over tilted stones on a pathway, or
  • Voluntary actions, like switching to a different trail when running in the woods.

Professor: Now, what kind of motor command is involved with each?

Student: Interesting! Is this a trick question?

I thought that the only motor command is from Betz cells sending discharges down the corticospinal pathway.

Professor: Motor imagery (MI) studies have examined the role of the corticospinal pathway during imagined and actual movements. In a very comprehensive review, Guillot et al. (2012) found that:

“The contribution of the contralateral primary motor cortex (cM1) to imagined actions is […] controversial.”

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Student: Proske and Gandevia in 2012 described an exafferent component as the “afferent signals generated by stimuli of an external origin”.

Professor: Well said; almost an exact quote.

Student: For me, running is mostly automatic, overlearned, rhythmic movement. How come I have no Sense of Effort during long periods of slow, easy running?

Professor: The simple answer is there is NO mismatch according to the theory cited byProske & Gandevia.

• That is, no error exists between the efference copy and the signal from motor command at the ” ‘difference’  calculator “.
• Proske & Gandevia further down the legend state, “if the predicted and actual sensory feedback match, the anomalous situation arises where there is no perception at all”.

Student: They even wrote that this instance is an ‘anomalous’ situation! So,

• Are they referring to voluntary contractions, perhaps using fine motor control or learning a gross motor pattern?
• Then again, there are well over-learned, automatic movements like running! Is jogging through the forest in Germany, only noticing the holes dug out by WWII bombs, just an ‘anomalous’ situation?!

Professor: Very interesting and intriguing questions. Then the answer is a sense of effort is to have A Mismatch!

Student: And what is the “motor command”?

• What if during some or many walking or running or cycling strides, no motor command signal was ‘issued‘?
• Or is there always a command signal coming down the efferent nerves to the active motor pool?

Professor: Let’s do some digging into the History of Motor Command and Control of Muscle Recruitment. Maybe we can find the answers to those questions, or at least get the ‘dogma’ scientists state about command signals.

Reference

Proske U, Gandevia SC. (2012) The proprioceptive senses: their roles in signaling body shape, body position and movement, and muscle force. Physiol Rev 92: 1651–1697.

Student: Your post for October 15th, 2012 had the title, “What is meant by Effort or Exertion?” Have you gotten any clues?

Professor: Well, as a matter of fact I reread that Post, and I found a review article by Prostke and Gandevia (2012) that have provided some much needed insight.

Here is some of what I wrote back in 2012:

“… While jogging on the level at a slow pace for me, I was doing more thoughts about “Where is the next patch of thick grass for my sore feet?”  than “Push on each stride.” “What is my Rating of Perceived Exertion” was not anywhere in my self-talk. Coming to an upslope, however, to maintain my pace, at first I did have to focus a bit on “Push a little more with each stride”. Then once I noticed I was pushing “hard enough”, my self-talk shifted to focus upon running relaxed, and I comfortably crossed over the little hill.” [Emphases added]

Professor: Notice that effort for me was not conscious until I had to focus on my running. My research on effort had biased my ‘noticing’ to look for only a “motor command”.

Part way through their very informative review for an old-timer like me they wrote a subsection on “the Sense of Effort” which went like this:

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Student: Wow, was I ever slipping and sliding, just trying to walk outside. It was a street with patches of ice: step, step, slide, step, sliiiide, ….

Professor: My walking was off today, too. I was limping from sore knee like walking in a wooden leg: step, thump, step, thump, ….

Student: I noticed I was got ‘off balance’ but without thinking I never fell down.

Professor: Hmm. We could be correcting for perceived differences, i.e., errors, like from where we were supposed to be walking to actually where we were walking.

Malone, Bastian, & Torres-Oviendo (2012) recently studied walking errors in detail. Below is their Figure 1 that illustrates how this errors signal may happen and could be used to make a smooth adaptive transition from a sudden change, i.e., perturbation, while walking:

Fig. 1. Schematic of error signal and motor output. Shaded region represents the adaptation period. A: Parameters quantifying error are perturbed early in adaptation and decrease throughout adaptation. They also show the opposite perturbation in deadaptation. B: Motor outputs exhibit a smooth change from a set pattern A to a new value during adaptation (pattern A’), set by the environmental conditions. They also must be actively deadapted with a smooth transient from pattern A’ to pattern A when environmental conditions change back to the original state. [Emphases added.]

Student: How can we measure the error between what is expected and what is actual for walking?

Professor: What did Malone and her co-investigators utilize for measuring walking? They used gait parameters as shown below in their Figure 2:

Fig. 2. Definitions of parameters. A: Marker diagram for experiments 1 and 2 with limb angle convention shown. MT, 5th metatarsal head. By convention, positive limb angles represent when the ankle is in front of the hip (flexion) and negative angles when it is behind (extension). B: Schematic defining temporal parameters of locomotion during normal, symmetric walking. Time is represented along the horizontal axis, with time increasing from left to right. HS, time at heel-strike; TO, time at toe-off. Solid and dashed lines represent stance time periods (ST) for the slow (ST_s) and fast (ST_f) legs, respectively. White areas between these lines represent swing time periods (i.e., time intervals from TO to HS). Shaded areas indicate when both feet are on the ground, defined as double support periods (i.e., overlap in stance time for both legs); DS_s and DS_f are slow and fast double support periods, respectively. Slow and fast step timings (t_s and t_f) are defined as the time between consecutive heel-strikes. [Emphases added.]

Professor: The legend for their Fig. 2 shows these important parameters. The Stance Time for the ‘slow’ leg (ST_s) is from Heal Strike (HS) to Toe Off (TO) of the slow limb; vice versa for the ‘fast’ leg Stance Time (ST_f). Double Support for the slow leg (DS_s) begins at heel strike for the fast leg and lasts until toe-off for slow limb; vice versa for the fast leg. During Stride Time for the slow leg, its Step Time (t_s) lasts to the beginning of Double Support; vice versa for the fast leg (t_f). Stride Time (Tstride) for either leg was the difference in time from heel strike to the subsequent heel strike.
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