Timothy C. Cope

Selectivity in synaptic changes caudal to acute spinal cord transection

The amplitude of monosynaptic EPSPs produced by the action of single semitendinosus (ST) Ia afferent fibers in ST motoneurons display little or no increase in amplitude within hours of a spinal cord transection performed at the thirteenth thoracic segment. This finding is in marked contrast to results obtained at the medial gastrocnemius (MG) Ia fiber-motoneuron synapse, where EPSPs have been shown to be enlarged, even in preparations with normal ST EPSPs. The increase in EPSP amplitude reported previously at the MG Ia-motoneurons synapse is selective and therefore is probably not a direct consequence of injury to the isolated segment of the spinal cord.

Functional Synaptic Changes Caudal to Spinal Cord Transection

Studies on the response of the central nervous system to injury generally require the use of two populations of animals — control and experimental. This places a premium on the choice of an experimental system which is sufficiently well defined to permit unequivocal conclusions that any changes are the result of the experimental manipulation rather than the natural variability of the system.

Fluctuations in Group Ia EPSPs: Consequences for Mechanisms of Transmitter Release and Measured Parameters of Averaged EPSPs

Publisher Summary This chapter explains the consequences for mechanisms of transmitter release and measured parameters of averaged excitatory postsynaptic potentials (EPSPs). The connection between Ia fibers and α -motoneurons has proven to be useful in studies of central synaptic transmission. A major advantage for modern physiological studies has been the ability to analyze the averaged EPSPs evoked by the action of single Ia fibers. This has allowed examination of well resolved EPSPs whose shapes are uncontaminated by dispersion in the arriving afferent input which is characteristic of the volley evoked by electrical stimulation of a nerve bundle. At single Ia-motoneuron connections, the latency from the Ia fiber trigger impulse to onset of the EPSP in single trials has been measured. It is found that latency fluctuates from trial to trial. The distribution of latencies is unimodal with a range as high as 650 μs. It is found that if one averages all EPSPs at the low end of the latency distribution and compares them to those at the high end of the same distribution, one always finds that short latency EPSPs are larger on the average than those generated at long latencies.