All 3 types of intrafusal fibre respond to fusimotor activity with graded contractions localised to the more polar regions. This causes the equatorial sensory region to be stretched. The consequences of this stretching vary according to the intrafusal fibre affected, and in turn this alters the firing rate of the afferent nerve terminals wrapped around the fibres.

Static bag 2

Local contraction occurs under the fusimotor end plates (nerve terminals), causing the convergence of the unstimulated regions either side. The more polar parts are braced by large elastic fibres, so most extension is in the central 2mm of the fibre. The primary ending is extended by 10-25%, with corresponding increase in afferent discharge up to 100Hz (Sb2 discharge acts as pacemaker for the primary sensory discharge of the whole spindle). Peak frequency discharge falls (adapts) during ‘hold’ phases despite the annulospiral nerve ending remaining at same level of extension.

Dynamic bag 1

These fibres are quite different. Local contractions occur under the motor plates, but contraction also occurs at the poles (extracapsular area) where there are no plates. The mechanism is not understood, but these fibres resemble ‘tonic’ fibres in which APs are not propagated, and there is a local junction potential which is coupled to contractile activity. The fibre is seen to move outwards at the end of capsular sleeve, but contraction is small and slow so the primary ending is extended by 2-8% only.  There is a small rise in Ia afferent discharge. Adaptation therefore less evident and the effect on secondaries is negligible.

Nuclear Chain Fibres

Most change in activity when driven by fusimotor stimulation at 50-150Hz. There is limited action potential propagation, so localised contractions occur in capsular sleeve region. These contractions are brief, so primaries are mechanically excited and follow the stimulation frequency 1:1 up to 75 Hz.  If stimulation frequency rises further, Ia discharge is driven 1:2 or 1:3 then becomes irregular (despite extension of chain fibre primaries by up to 20%).  There is powerful excitation of secondary endings, with pronounced adaptation during ‘hold’ phases.  Chain fibres thus powerfully contribute to primary and secondary response to fusimotor stimulation.

More: Effect of Stretch on spindle sensory discharges [Opens in new window]

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