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The depolarization of the sensory terminals of muscle spindle primary endings is studied in terms of a simplified core conductor model of the system. The terminals branching from a group Ia afferent fibre have different geometrical structures, depending on whether they innervate the nuclear bag or the nuclear chain intrafusal fibres of a muscle spindle.

1. Tonic activation of muscular activity was observed during forced vibration of the gastrocnemius and soleus muscles in the decerebrate cat. Such tonic activity was not observed in the tibialis anterior muscle.2. Intervals of both unitary EMG and motoneuronal spikes were shown to occur according to the principle of integer multiplication of vibratory cyclic time.3. Gradual recruitment of tonic vibration reflex (vibratory facilitation) was found to survive after the cessation of vibratory stimulation (postvibratory facilitation).4. Nonsequential interspikes revealed that gradual tonic recriutment is effected by the spikes with a shorter firing interval.5. Muscle activities during TVR may be effected by PTP with some excitatory frequency associated with that of the applied vibration.6. Postvibratory facilitation did not correlate with the vibratory frequency. The firing of such phases may be determined by the intrinsic repetition rate of the individual motoneuron.

The frequency transfer characteristics of the tonic stretch reflex have been studied using homonymous Ia afferent stimulation as input to the soleus motoneurone. By varying additionally the frequency of inhibitory volleys from either the Ia component of antagonist muscle nerve or the homonymous Ib afferents, the suppressive effect on the excitatory synaptic activity has been investigated for a wide variety of different phase relations.

JANKOWSKA, E., S. LUND and A. LUNDBERG. The effect of DOPA on the spinal cord. 4. Depolarization evoked in the central terminals of contralateral Ia afferent terminals by volleys in the jlexor rejlex afferents. Acta physiol. scand. 1966. 68. 337-341. After injection of DOPA in unanaesthetized spinal cats volleys in the flexor reflex afferents (FRA) evoke a late long lasting contralateral dorsal root potential. Measurements of excitability of contralateral afferent terminals show a corresponding depolarization of Ia, but not of Ib or cutaneous afferent terminals. Results are also presented showing that transmission from the FRA to contralateral Ia afferent terminals can be inhibited from the FRA. In previous papers in this series it has been shown that after an i.v. injection of 1-3,4-dihydroxyphenylalanine (DOPA) volleys in the flexor reflex afferents evoke a late ipsilateral dorsal root potential (DRP) which is caused by a selective depolarization in the central terminals of Ia afferents (Andtn et al. 1966a, b). In the present paper it will be shown that after DOPA volleys in the FRA produce similar effects on the contralateral side. The experimental procedures have been given in the previous papers.

The distribution of Ia terminals to alpha-motoneurons has been investigated under conditions where a) the Ia fiber innervates a foreign muscle and b) the motor axon innervates a foreign muscle. This distribution has been assessed by examining which motoneurons develop an EPSP in response to stimulation of a single Ia afferent fiber. Cross-union of the medial gastrocnemius and lateral gastrocnemius-soleus nerves has been performed in 5--8 day old kittens and the effect of this procedure on Ia connections to alpha-motoneurons has been investigated in the adult about one year later. No rearrangement of synaptic connections has been observed under these conditions. The connections from self-unioned Ia afferents and to self-unioned motoneurons are also normal. The individual EPSP's in these motoneurons with regenerated axons have normal rise times. The findings of normal Ia connections as well as normal EPSP rise times in alpha-motoneurons with regenerated motor axons indicates reversibility of the changes seen in axotomized motoneurons.

THE monosynaptic reflex response evoked in the cat by a maximal volley in the group Ia afferent fibres from a synergic group of muscles has been measured as a fraction of total motoneurone pool of that muscle by Jefferson and Benson1 and found to range between 4.3 and 26.5 per cent. Apparently there has been no measurement of the activated fraction of the motoneurone pool with the analogous H-reflexes that are evoked in the gastrocnemius-soleus muscle of man by stimulation of the group Ia fibres in the popliteal nerve2–4.

A study has been made of the depression that occurs when a monosynaptic reflex pathway is subjected to repetitive stimulation. Reflex depression has a dual origin. High frequency or early depression is postsynaptic in origin and results from subnormality in the motoneurons. Low frequency, late, or enduring depression is presynaptic in origin. The conditioning volley-test volley technique and the frequency-mean monosynaptic reflex amplitude relation yield similar information concerning reflex depression. Each method has its advantages and for some purposes one or the other of the methods necessarily must be employed. The results of a variety of experiments are consistent with the proposition that reflex depression in the monosynaptic reflex pathway originates by action in the group IA afferent fibers of muscle origin that are responsible for monosynaptic reflex transmission. Depression is present at a frequency of 0.1 per second (6 per minute) and absent at a frequency of 0.05 per second (3 per minute). Thus it is impractical for most purposes to employ repetition rates that satisfy the requirement for designation as "single shock" stimulations. The temporal course of enduring depression has been determined. It is identical with that for a number of other phenomena observable in monosynaptic reflex pathways, which suggests a common origin. The mechanism of low frequency or enduring depression is discussed in the light of this suggestion.

THE aim of the present investigation has been to give, in cats, excess use to the synapses of some monosynaptic pathways through the spinal cord, keeping other monosynaptic pathways as controls. In an initial aseptic operation the nerves to some muscles of a synergic extensor group in one hind-limb have been severed and capped to prevent regeneration. Since the remaining muscles of such a synergic group have to substitute for the whole group in supporting the weight of the animal, it can be expected that there will be an excess of stress on them; consequently there will be an increased discharge from their stretch receptors along the group Ia afferent fibres, so giving an increased activation of the excitatory synapses on their motoneurones.