RESPONSES OF MOTONEURONS UNDERGOING CHROMATOLYSIS

The delayed and asynchronous firing of chromatolytic motoneurons in response to group I afferent volleys is shown to be evoked monosynaptically, there being an abnormally long and variable delay between onset of monosynaptic action and generation of impulse discharge. Intensity of monosynaptic excitatory action is reduced, and considerable variability in the form of successively evoked postsynaptic potentials is often observed. No evidence has been found for the development of excitatory group I polysynaptic pathways. Reduction in responsiveness of finer dendrites is indicated by the feeble "d" response evoked by an antidromic volley in a chromatolytic motor nucleus. Antidromic impulses appear to invade the cell bodies and coarse dendrites, but die out at points short of the normal extent of dendritic invasion. Vigorous firing of Renshaw cells can be elicited by antidromic volleys. Chromatolytic motoneurons appear to maintain reasonably normal resting membrane potentials, but are more susceptible to damage than are normal cells. Action potentials are large and usually overshoot the resting potential level. Post spike potentials are similar to those of normal cells except for a less prominent, or absent, early phase of depolarisation. In contrast with the reduced responsiveness of peripheral dendrites, there is a lowered threshold for antidromic and segmental reflex synaptic activation of the more central regions, probably the cell bodies and nearby coarse dendrites, of motoneurons undergoing chromatolysis.

Studies on the flexor reflex.-V. General conclusions

The experiments described in the preceding four papers bear on various problems presented by reflex activity. Their results confirm some of the inferences already drawn elsewhere from other experimental work, and they allow certain further inferences. A brief prefatory statement of all these inferences and of the experimental evidence which allows them will advantageously introduce the description of the processes set up in the ipselateral flexor centres of the spinal cord by a single centripetal volley and by a single antidromic volley. Then, finally, discussion of the theories of reflex excitation can be undertaken in the light of the present experimental observations. the statement treats of the subject in its present phase only; the references to relevant papers are therefore restricted in the main to the more recent ones. II. Inferences from Experimental Observations. 1. The convergence of Different Afferent Paths on the same Motoneurones The following evidence shows that this occurs:- (a) Histological .-Each motoneurone receives its “ boutons terminaux ” from many individual afferent terminals (Cajal, 1903). (b) Physiological .-Centripetal volleys set up in different afferent nerves excite the same motoneurones (Camis, 1909; Cooper, Denny-Brown, and Sherrington, 1926; 1927; Sherrington, 1929; Cooper and Denny-Brown, 1929 ; Eccles and Sherrington, 1930 ; 1931, a ; 1931, b ).

Studies on the flexor reflex.-IV. After-discharge

In many preparations the flexor reflex elicited by the application of a moderately strong break-shock to an ipselateral afferent nerve has an after-discharge following the initial reflex discharge, even when the strength of the break-shock it such that it sets up no more than a single centripetal volley (Sherrington, 1921, a ; 1921, b ; Adrian and Forbes, 1922). The prolonged excitatory condition which must occur at some part of the central reflex pathway clearly has some affinity to the persistence of the c. e. s. which forms the basis of facilitation (Eccles and Sherrington, 1930). It is of interest therefore to investigate the effect of an antidromic volley on after-discharge. It must, however, be remembered that an antidromic volley set up during an after-discharge will be prevented from reaching some motoneurones by meeting centrifugal (reflex) impulses. Denny-Brown (1929, p. 273) observed that an antidromic volley set up during the after-discharge of either a flexor or extensor reflex (in response to a tetanic stimulation) was followed by a period of quiescence owing to a temporary lapse of the after-discharge. The duration of this period seemed to be too long for a central refractory period set up by the antidromic volley, so he suggested that there might be a temporary exhaustion of the central exciting agent (c. e. s.). II. Method. The general technique is as described previously (Eccles and Sherrington, 1931, a ). In all cases the muscle (tibialis anticus) has been completely deafferented. Tetanic stimuli have been provided by a neon-tube oscillator.

Studies on the flexor reflex.-III. The central effects produced by an antidromic volley

In 1822 Magendie demonstrated that no reflex activities were evoked by impulses passing into the spinal cord by a ventral root. This result has been confirmed and extended by other investigators who showed that no action currents can be detected in other nerves when the central end of a cut ventral root is stimulated (Mislawski, 1895; Bernstein, 1898). Attempts have been made to account for the irreversibility of conduction in the reflex arc by postulating a “dynamical polarisation” of the nerve cell so that the conduction would be solely from dendrite to axon, never the reverse (cajal, 1891; van Gehuchten, 1900). The antidromic impulses “back-fired” into a motoneurone might, however, be blocked at the synapse (sherrington, 1900, p. 798). It seems unlikely that the conduction of nerve impulses in the cell body and dendrites of a motoneurone would differ fundamentally from the conduction in peripheral nerve fibres, e. g ., that impulses passing along the dendrites would suffer an irreciprocal decrement. In the present paper it has been assumed that antidromic impulses in motor nerve fibres are blocked at the synapses of the motoneurones.