A comparison of monosynaptic and polysynaptic reflex responses from individual flexor motoneurones

Abstract

Evidence from a number of types of experiments has shown that gradation of muscular contraction is accomplished by changes both in the number of motoneurones active and in the frequency of discharge of individual motoneurones (Creed, Denny-Brown, Eccles, Liddell & Sherrington, 1932). It is also well known that different spinal reflexes have different abilities to excite fractions of the motoneurone pool supplying a particular muscle. On the other hand, there appears to be little information on one important feature of the functional organization of motoneurones particularly relevant to quantitative control of skeletal muscle contraction, namely, the degree of influence various reflex pathways have on a particular motoneurone in comparison to other motoneurones of the same pool.The present study examines the effectiveness of the monosynaptic and several polysynaptic reflex arcs in discharging individual knee flexor motoneurones. These experiments should be useful in indicating whether a given motoneurone responds equally well to different reflex pathways or whether variation in excitation throughout a particular pool is dependent upon the nature of the reflex process. Information on such a point would be valuable in helping to ascertain the manner in which different mechanisms combine to determine the participation of motoneurones in activity. For instance, it has been postulated that a significant control of motoneurone excitability is exerted via the fusimotor (gamma) effect upon the afferent limb on the monosynaptic reflex, the Group I fibres from the muscle spindle (Eldred, Granit & Merton, 1953). Fitting the motor control of the spindle into the framework of mechanisms concerned with the functional organization of motoneurones would require knowledge of how the effectiveness of the monosynaptic drive varies among members of a pool in comparison to other excitatory pathways.The motoneurones studied were taken from the pool monosynaptically excited by Group I volleys from the peripheral nerves supplying the

Observations on the discharge of flexor motoneurones

“…The occurrence of background activity was better correlated with high polysynaptic responsiveness than with other measures available in these experiments. For the majority of units this also meant a positive correlation to monosynaptic responsiveness (Perl, 1962). Only one unit was observed which had a significant amount of background activity but which was relatively unresponsive to reflex tests.…”

“…The designations of peripheral nerves used as afferent sources follow the convention used in the preceding paper; BST for the selected combination of nerves to biceps posterior and semitendinosus; S for sural; and TS for the combined branches to gastrocnemius and soleus (triceps surae). The relationship between stimulus intensity and volley composition is also discussed there (Perl, 1962).…”

“…The experimental procedure is detailed in the preceding paper, which describes the responsiveness to reflex tests of the motoneurones upon which the observations which follow were made (Perl, 1962). Notes or photographic records on background activity were kept for the majority of this population.…”

“…Attention was paid only to those instances of repetitive discharge occurring within the usual observation period, 50 msec after a volley. The majority of neurones which had high firing indices to polysynaptic tests (Perl, 1962) showed at least occasional repetitive firing to single volleys of maximal size from BST nerve or sural nerve. This usually took.…”

“…The present report is concerned with the discharge of single knee flexor motoneurones as recorded from fine ventral root filaments of an unanaesthetized spinal preparation. The cells studied represent a population with widely differing reflex responsiveness (Perl, 1962). Descriptions are given of both background discharge and evoked responses, particular attention being paid to the latency of monosynaptically and polysynaptically evoked discharge.…”