Group II afferents from spindle secondaries also excite autogenic alpha motoneurones via mono & polysynaptic paths. Monosynaptic component involves about 50% of the motoneurones that are excited by Ia gamma motoneurones. highly responsive to elect stim of group II afferents (but not clear how much of this group II input is purely spindle in origin).
Classical stretch reflex 'the capacity of a muscle to resist extension' is sum of these spindle projections to muscle. The monosynaptic Ia component is responsible for the 'tendon jerk' . The 'tonic stretch reflex' is mainly disynaptic or polysynaptic.
Monosynaptic component thought to be unaffected by supraspinal inhibition (no interneurone for synaptic modification to occur), but presynaptic inhibition is possible. Reflex tension produced by maintained stretch depends on excitation from Ia and II afferents and autogenic inhibition from Ib tendon afferents. Ib are weakly autogenically inhibitory to gamma motoneurones, especially dynamic gamma motoneurones. Tonic stretch reflex also very dependent on supraspinal input to associated interneurones and so only operate when CNS 'wishes them to do so'.
Added to this must be a consideration of the mechanical properties of muscle.. length/tension relationship, although physiologically the muscle is always near optimum.
Spinal interneurones are an important target for supraspinal control and gating of reflexes, e.g. in walking cycle.
Renshaw cells may be an important stabilising factor on a and spinal interneurones as well as gamma motoneurones.
Jenddrassik manoeuvre. Tendon jerk is reinforced by clenching fists or jaw. Gamma pathway is centrally facilitated rendering spindle more sensitive to stretch.
Silent period. During a maintained voluntary contraction apply a maximal shock to a muscle nerve. Get a direct excitation in EMG followed by a silent period (80ms in human adductor pollicis). During the silent period the muscle is in tension above the voluntary level and the spindles are unloaded .. Ia discharge falls and reflex support of a motoneurones is lost. Longer latency human stretch reflexes.
Stretching relaxed or contracting muscles.
In a cat stretching a relaxed muscle produces a reflex development of tension with a large monosynaptic component. There is a background discharge in spindles at rest. In man this is less so and little reflex contraction in resting muscle. In contracting muscle however there is a powerful tonic stretch reflex. There is a monosynaptic component (M1) but functionally most of the response is at longer latency (M2). This could come from group Ia or group II inputs via polysynaptic spinal pathways (as shown for group II) or via higher centres, even cortex. Voluntary responses (M3) are at even longer latencies.
Gain of tonic stretch reflex in man varies with load (or rather force developed by muscle). Hence at strong voluntary contraction levels, large reflex tensions can also be developed.
Central Control of Dynamic & Static Fusimotor Neurones. Dynamic and Static fusimotor neurones are under separate central control. Fusimotor activity can be modified by stimulation of many areas of the CNS. Dynamic responsiveness of primaries greatly reduced by repetitive stimulation of anterior cerebellum.
There are separate corticospinal dynamic and static fusimotor pathways (differential respose to anaesthesia). Varying cortical stimulation strength recruits in order Sb2, chain, Db1 but all respond less than in normal function.
Basal ganglia damps down facilitatory reticular influence on gamma motoneurones.
Substantia nigra inhibits static gamma motoneurones when rostrally stimulated, and dynamic gamma motoneurones when caudally stimulated.
Red nucleus. Clear specific facilitation of dynamic fusimotors to flexors and extensors independent of a motoneurones. ('mesencephalic area for dynamic control').
Lateral Vestibular Nucleus: facilitates static gamma motoneurones in extensors. Also static gamma - alpha monosynaptic coactivation. (Sb2 or chain?).
Cerebellum complicated by the fact that it controls red nucleus, vestibular nucleus etc...
Thus there is clear evidence that dynamic and static gamma motoneurones are controlled independently by CNS and that coactivation involves static gamma motoneurones.