During a passive lengthening displacement, muscle spindle primary afferents activity increases, and if after the stretch, the muscle is passively moved back to its original position (shortening phase), the activity decreases.
But what happens when the muscle is shortened starting from a neutral position?
Would the Ia afferent remain the same or decrease?
Dear Francesco Budini,
1. Muscle spindle activity in man during shortening and lengthening contractions.
2. Force encoding in muscle spindles during stretch of passive muscle
https://dx.doi.org/10.1113%2Fjphysiol.1978.sp012265
Article Force encoding in muscle spindles during stretch of passive muscle
Thanks.
unfortunately
Ref.1 is about voluntary contractions
Ref.2 does not answer my question
Hi Francesco,
The answer to your question will in part depend on the 'state' of the prep/individual.
In completely relaxed/anaesthetised/de-efferented situation then would expect Ia activity to decrease with shortening, irrespective of initial starting length, as long as initial starting length is not too slack.
If there is some activity such as contraction or heightened state as in Jendrassik or increased mental activity, then Ia afferent may or may not decrease with shortening depending on the gamma motoneuron drive to the muscle spindles.
Thus 'state' is highly important.
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