Neuron Patterns Controlling Transmission of Ipsilateral Hind Limb Reflexes in Cat

Abstract

THE SEGMENTAL REFLEX discharge (7,31,21) must be considered of anatomical rather than functional significance in that it contains, in unnatural combination, those elements which constitute the several distinct ipsilateral reflexes. In the present paper are the results of experiments designed to resolve the segmental reflex into its functional components. The observation that a major division of the segmental reflex into its direct (two-neuron-arc) and indirect (multineuron-arc) components followed segregation of muscle afferent and cutaneous afferent fibers for afferent stimulation (21) provides the point of departure for the experiments to be described. Some of the present observations have been mentioned briefly in a preliminary note (23). A general discussion of these and other results will be found in another paper (25). The afferent fibers of the A group (14) exhibit a range of diameters extending from 20~ to 1.5~ (36). In a dorsal root the whole range of fibers is present, but in the peripheral nerves significant segregation .s are found (36, 8, 29, 14) which permit a degree of selective stimulation of the various components (21). For the purposes of the present discussion the afferent fibers will be classified into groups, each group being marked by a peak in the fiber distribution plots of one or another of the several peripheral nerves. Group I consists of the largest afferent fibers, which are to be found only among the afferent fibers arising from muscle. Approximately these fibers range from 20~ to 12~ in diameter (8, 29), with a distribution peak at 15 to 16~. Group II contains fibers of approximately 12~ to 6~ in diameter, with a mode at 8 to 9p. These fibers form a prominent peak in the fiber distribution plots of cutaneous nerves (8, 30, 14), but they are poorly represented among the muscles afferent fibers (8, 29). Group III consists of fibers gathered about a peak at 3 to 4~ (the delta pile). These last are to be found in both muscle and cutaneous nerves. Another category, to consist of the C fibers, the afferent and reflex function of which is proven (3, 2), should be included as group IV. These fibers have not been studied during the course of the present experiments. Since group I and group II fibers are the lowest threshold fibers in muscle and cutaneous nerves respectively, they may be excited in isolation by the simple expedient of selecting the appropriate nerves for stimulation (21). There is no means at present of stimulating group III fibers in isolation but their contribution to reflex action, on stimulation, is easily recognizable as addition to the reflex discharges caused by stimulation of the larger, lower threshold fibers [after-discharge?.] (45)

Types of neurone in and around the intermediate nucleus of the lumbosacral cord

“…The response to strong stimulation of the muscle and cutaneous nerves was a repetitive discharge beginning after a long latency for all but flexor longus digitorum (D), and this exceptional discharge was probably spontaneous (cf. H), for it was not seen with weaker stimulation of this 1*07i (Lloyd, 1943;Eccles & Lundberg, 1959b varying the strength of the stimulus applied to the biceps-semitendinosus nerve at 1 15 msec before a second testing stimulus, which was at a constant strength supramaximal for Group I (F). As is shown in A, the testing stimulus was unable to excite any Group I fibres that had been excited by the first stimulus.…”

Direct and indirect sensory input pathways to the motor cortex; Its structure and function in relation to learning of motor skills.

“…We now know that there are many afferent fibers from a muscle which send impulses not only to the spinal cord, but also to the higher centers, the function of which is yet to be fully understood. However, modern study of the spinal cord was started by simply classifying afferent fibers into groups Ia, Ib, and IT [48], ignoring all other complications. Needless to say, this classification simplified the study of the spinal cord and greatly contributed to the subsequent study of the central nervous system.…”

A brief historical note on the classification of nerve fibers

“…Besides this classification, lloyd in 1943 22 , used a slightly different classification of nerve fibers, studying reflex activities in animals, which also acquired widespread use. he classified the fibers in the following way: type I-fibers with diameters from 12 to 20 μm found only in afferent muscle nerves (equivalent to the Aα fibers of the previous classification); type II-fibers with diameters from 6 to 12 μm, prominently represented in cutaneous nerves and having a poor representation in muscle nerves (equivalent to the Aβ fibers of the previous classification); type III-fibers with diameters between 3 and 4 μm, representing the Aδ from erlanger and Gasser classification; and type Iv-fibers represented by the unmyelinated ones(equivalent to the c fibers of the previous classification).…”