SUMMARY1. Longitudinal vibration was applied to the de-efferented soleus muscle of anaesthetized cats while recording the discharge of single afferent fibres from the proprioceptors within the muscle. Conditions were defined under which vibration can be used to excite selectively the primary endings of muscle spindles without exciting the secondary endings of muscle spindles or Golgi tendon organs.2. Frequencies of vibration of 100-500 c/s were used. The maximum amplitude of vibration which the vibrator could produce fell with increasing frequency; it was 250 ,t (peak to peak) for 100 c/s and 20 ,u for 500 c/s.3. Primary endings of muscle spindles were very sensitive to vibration. Most could be 'driven' to discharge one impulse for each cycle of vibration over the whole of the above range of frequencies, provided the initial tension was moderate (20-200 g wt.). The amplitude of vibration required to produce driving usually varied by less than a factor of two over the whole range of frequencies. The most sensitive endings could be driven by vibrations of below 10 t amplitude.4. Stimulation of single fusimotor fibres, whether static or dynamic fusimotor fibres, increased the sensitivity of primary endings to vibration. Contraction of the main muscle, produced by stimulating ac motor fibres, reduced the sensitivity of primary endings even when fusimotor fibres were also being stimulated.5. The secondary endings were very insensitive to longitudinal vibration and with the amplitudes available not one of twenty-five endings could be driven at 150 c/s or above; one ending could be driven at 100 c/s by vibration of 250 t amplitude. Stimulation of single fusimotor fibres, probably all of which were static fusimotor fibres, made them slightly more sensitive to vibration but none of them approached the sensitivity of the primary endings.6. The Golgi tendon organs were as insensitive as the secondary endings * Wellcome-Swedish travelling research fellow. Present address:

During walking in unrestrained cats the electromyographic activity in many hindlimb muscles has been correlated with the angular movement in the hip, knee and ankle joints. The activity is rather uniform in the extensors but individual in different functional groups of flexor muscles. Observations on the precise timing of the onset of the main extensor activity suggest that it is not a reflex effect produced by stimulation of receptors (from muscle or skin) in the limb. It is assumed that the basic activity is a centrally programmed alternating activation of extensors and flexors. A possible reflex regulation mainly from la afferents of this basic activity is discussed with special attention given to factors that may elucidate the difference in movement at the hip on one side and the knee and ankle on the other.

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SUMMARY1. The effect of electrical stimulation of the brain stem on reflex transmission has been investigated in decerebrate cats after partial transection of the spinal cord.2. Brain stem stimuli that do not evoke inhibitory post-synaptic potentials (IPSPs) in motoneurones or primary afferent depolarization may still effectively depress the excitatory and inhibitory synaptic actions evoked from the flexor reflex afferents (FRA) and from lb afferents. There is no effect on post-synaptic potentials from Ia afferents or on Renshaw IPSPs. The depression is not associated with any measurable change in conductance over the motoneuronal membrane.3. There is also inhibition from the brain stem of transmission from the FRA (but not from Ia and Ib afferents) to primary afferqnt terminals and to ascending spinal pathways. 4. It is concluded that this inhibition from the brain stem is exerted at an interneuronal level in spinal reflex paths.5. The inhibitory action is evoked from the region ofMagoun's inhibitory centres in the brain stem and is mediated by axons with a conduction velocity of at least 20 m/sec. The axons are distributed in the dorsal part of the lateral funicle.6. The pathway mediating the inhibition from the brain stem is named the dorsal reticulospinal system. Its possible role in maintaining the decerebrate control of reflexes is discussed and related to the problem of a selective control of some paths from a primary afferent system.

SUMMARY1. L-Noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholine (ACh) were administered micro-electrophoretically to feline lumbar neurones while recording their spike potentials extracellularly.2. There was no evidence to suggest that NA acts as an excitatory transmitter in the spinal cord.3. NA had potent inhibitory effects on some interneurones as revealed by a depression of spontaneous and synaptic firing and on the firing to a local application of an excitant amino acid. The effects on Renshaw cells and motoneurones were less marked.4. The depressant actions of 5-HT were less marked than those of NA. ACh and carbamylcholine had depressant effects on some NA-sensitive interneurones but were invariably far less potent and on other NA-sensitive cells were completely inactive.5. NA had no detectable effect on the normal spike amplitude but when the action potentials were reduced by excessive depolarization then both NA and synaptic inhibition increased the spike amplitude; this effect could be due to a hyperpolarization of the cell membrane.6. There was a correlation between the distribution of NA-sensitive cells and the relative densities of NA-containing terminals in various layers of the grey matter.7. It was postulated that NA acts as an inhibitory transmitter released from the terminals of descending pathways in the spinal cord. Other possible mechanisms were discussed but lacked experimental support.