. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Nervous Systems 801 one-to-one relay conduction of giant neurone systems is a relatively evolution- ary development. When an interneurone or motoneurone is stimulated by two incoming vol- leys separated by different times, the second volley is usually found to be most effective at two periods after the first; these are periods of early and of delayed facilitation. 1. Early facilitation (spatially determined). Two afferent volleys converg- ing over different axons on one motoneurone are most effective when syn- c
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Nervous Systems 801 one-to-one relay conduction of giant neurone systems is a relatively evolution- ary development. When an interneurone or motoneurone is stimulated by two incoming vol- leys separated by different times, the second volley is usually found to be most effective at two periods after the first; these are periods of early and of delayed facilitation. 1. Early facilitation (spatially determined). Two afferent volleys converg- ing over different axons on one motoneurone are most effective when syn- chronous. One afferent volley alone may not be able to elicit a motor re- sponse, but together they summate. If one volley is delayed after the first, it becomes less effective as the excitatory process from the first decays. Vary- ing the interval, then, between spatially converging subliminal impulses is useful in showing the time course of the excitatory process. Temporal sum- mation at one synapse (bouton) is impossible because of refractoriness. Spa- tial summation is generally required for synaptic excitation. This is well shown in the tail of the crayfish, where flexion of one sensory hair on a uro- pod sends one impulse into the sixth abdominal ganglion; stimulation of sev- eral, usually four, adjacent hairs is necessary to elicit one efferent (post syn- aptic) impulse.^^"* In vertebrates it is not probable that a single impulse in one afferent axon ever elicits a reflex response. In the superior cervical sympathetic ganglia of the cat two submaximal volleys in different preganglionic branches sum maximally at synchrony, the summation declines to zero at msec.^-^' ^^"^ In the oculomotor nucleus of the rabbit summation between two converging volleys declines to a minimum. at\ cT .25 .5 .75 1 2 3 4 5 6 7 £ Fig. 301. Facilitation curve of motoneurones of oculomotor nucleus of rabbit. Motor response in mv. to a test shock to presynaptic (floor) neurones at
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