Classification of human peripheral nerve fibre groups by conduction velocity and nerve fibre diameter is preserved following spinal cord lesion

Abstract (1) Single nerve fibre action potentials (APs) of lower sacral nerve roots were recorded extracellularly with two pairs of wire electrodes during an operation in which an anterior root stimulator for bladder control was implanted in 9 humans with a spinal cord lesion and dyssynergia of the urinary bladder. Roots that were not saved and that were used to record from were later used for morphometry. (2) Nerve fibre groups were identified by conduction velocity distribution histograms of single afferent and efferent fibres and partly by nerve fibre diameter distribution histograms, and correlation analysis was performed. Group conduction velocity values were obtained additionally from compound action potentials (CAPs) evoked by electrical stimulation of nerve roots and the urinary bladder. (3) The group conduction velocities and group nerve fibre diameters had the following pair-values at 35.5°C: Spindle afferents: SP1 (65 m/s/13.1 μm), SP2 (51/12.1); touch afferents: T1 (47/11.1), T2 (39/10.1), T3 (27/9.1), T4 (19/8.1); urinary bladder afferents: S1 ( 41 m/s/−), ST (35/−); α-motoneurons: α 13 (−/14.4), α 12 (65m/s/13.1 μm), α 11 (60?/12.1)(FF), α 2 (51/10.3)(FR), α 3 (41/8.2)(S); γ-motoneurons: γ β (27/7.1), γ 1 (21/6.6), γ 21 (16/5.8), γ 22 (14/5.1); preganglionic parasympathetic motoneurons: (10 m/s/3.7 μm). (4) The values of group conduction velocity and group nerve fibre diameter measured in the paraplegics were very similar to those obtained earlier from brain-dead humans and patients with no spinal cord lesions. Also, the number and the density of myelinated fibres were preserved in the roots. Thus, the classification and identification of nerve fibre groups remained preserved following spinal cord lesion. A direct comparison can thus be made of natural impulse patterns of afferent and efferent nerve fibres between paraplegics (pathologic) and brain-dead humans (supraspinal destroyed CNS, in many respects physiologic). (5) When changing the root temperature from 32°C to 35.5°C, the group conduction velocities changed in the following way in one case: SP2: 40 m/s (32°C) to 50 m/s (35.5°C), S1:31.3 to 40, ST: 25 to 33.8, M: 12.5 to 13.8; α 2 : 40 to 50, α 3 : 33 to 40. The group conduction velocities showed different temperature dependence apart from SP2 fibres and α 2 -motoneurons. (6) Upon retrograde bladder filling the urinary bladder stretch (S1) and tension receptor afferent (ST) activity levels were undulating and increased. As compared to activity levels detected in a brain-dead human, S1 (designates afferents, not cord segment) and ST afferents fired, even when the bladder was empty, with an activity level similar to that observed in a brain-dead human with the bladder half filled. Two reasons are brought forward for a too small storage volume of the urinary bladder in paraplegics: too high afferent activity of the bladder due to changed receptor field signal transduction mechanisms and too low compliance.