From resting conditions:
Static fusimotor stimulation causes intrafusal contraction which imposes a significant stretch on the primary ending, which may extend by up to 20%.
Much of the external stretch is taken up at the polar regions, and the primary region may only extend by 5%.
When stretch is maintained there is no further mechanical change, however there is a reduction in afferent discharge due to electrical adaptation.
Static bag fibres provide a strong positive bias to primary endings whilst reducing sensitivity to length changes. Effects on secondaries are similar but smaller.
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Primary ending (group Ia) and secondary ending (group II) afferent discharge frequency during stretch alone (blue lines) and during stretch superimposed on tonic static gamma fusimotor stimulation (red lines) at 75Hz.
This shows graphically the effects of stretch alone (blue line) and stretch with simultaneous fusimotor stimulation (red line) in the standard test format for investigating spindle afferent ending responses.
The Y-axis represents the instantaneous firing frequency of the afferent neuron, with a scale from 0 to 100 impulses per sec. The Ia neuron responses are on the left, the group II neuron responses on the right. The ‘length’ trace shows when the stretch was applied; up = stretch. The fusimotor neuron in this case is the gamma dynamic (γd) type and the red dotted line shows the period when it was turned on.
Taking the primary ending as an example:
- initially there is only a low firing rate from this afferent ending
- stretch alone produces an instantaneous rise in firing rate to about 50/s which then sags a bit; as soon as the ramp phase of the stretch starts the firing rate jumps up again, but then falls back to about 40/s during the ‘hold’ phase of the stretch, ceases firing altogether during the release phase (as the muscle length decreases), and finally recovers to a low rate once the release has finished
- stimulating the gd alone provokes a large rise in the firing rate to about 100/s
- combining gd stimulation with the ramp stretch provokes a further small rise in firing to about 120/s, followed by a slow decline during the ‘hold’period of the stretch to about 80/s
- during the ‘release’ phase, there is now a rapid small fall in firing, then a ‘wobble’ as the release phase stops, and finally a sudden recovery down to the initial low firing rate
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