1. The synaptic linkage between single, identified sensory fibres associated with Pacinian corpuscle (P.c.) receptors and central neurones of the dorsal column nuclei was examined in decerebrate or anaesthetized cats. Paired recordings were made from individual neurones in the gracile division of the dorsal column nuclei and from the hind‐limb interosseous nerve in which it is possible to identify and monitor the activity of each P.c. fibre activated when recording from the intact nerve with a platinum hook electrode. Individual P.c. fibres were activated by vibration delivered with an 0.2 mm diameter probe to the interosseous P.c. receptors. 2. Thirty‐five P.c. fibre‐gracile neurone pairs were isolated in which activity in the single, identified P.c. fibre evoked suprathreshold responses (mean latency +/‐ S.D., 10.3 +/‐ 1.5 ms) in the gracile neurone. A single impulse arriving over one P.c. fibre could generate pairs or triplets of output spikes from several target neurones thus revealing a potent synaptic organization within the dorsal column nuclei for the transmission and amplification of weak sensory signals. 3. The potency of the linkage for some pairs resulted in post‐synaptic response levels of up to 400 impulses s‐1 when a single input fibre was discharging one impulse on each vibration cycle at 200‐400 Hz. 4. Gracile neurones driven by single P.c. fibres had phase‐locked responses to vibration at frequencies of up to 400‐500 Hz. However, the responses displayed much greater phase dispersion than those of P.c. fibres, indicating that a major component of phase dispersion in the vibration‐induced responses of dorsal column nuclei neurones is attributable to the properties of the synaptic linkage between an individual fibre and the target neurone. 5. The potent actions of single, identified P.c. fibres on their target neurones are consistent with the hypothesis that phase‐locked responses in dorsal column nuclei neurones to vibration at 100‐400 Hz may reflect the functional domination of the target neurone's output by one or a few of its converging fibres.
1. Peripheral and central neural contributions to vibrotactile adaptation were investigated in decerebrate or anesthetized cats by recording from sensory nerve fibers associated with Pacinian corpuscle (PC) receptors and from central neurons of the dorsal column nuclei that receive their input from vibration-sensitive receptors of the forelimb footpads. Responsiveness of units was assessed using 1-s duration, test vibration stimuli delivered with 1- to 2-mm-diam probes at different times following adapting trains of vibration (usually 300 Hz) that lasted from less than 1 min up to 50 min. 2. Cuneate neuron responsiveness underwent marked depression following prior vibration. The extent of the depression and the time course of recovery in responsiveness were dependent on the intensity and duration of the adapting vibratory stimulus. The recovery time course (often several minutes) was approximately exponential and resembled the reported time course of subjective vibrotactile adaptation obtained in psychophysical experiments. 3. Response depression in PC fibers was only seen at low amplitudes of the test vibration and displayed a brief time course of recovery in comparison with that seen in cuneate neurons. It is therefore unlikely to account for the adaptation time course either in cuneate neurons or at a subjective level. Furthermore, as the adaptation seen in PC fiber responses had a similar time course in both cutaneous and mesenteric PC fibers it is unlikely that mechanical changes in the skin contribute significantly to the adaptation in PC fiber responses to vibration. 4. The time course of afferent-induced inhibition following long periods of prior vibration was too brief to account for the response adaptation in cuneate neurons. 5. As the long-term response depression in cuneate neurons following their prior activation was seen for inputs from unconditioned sites within the neuron's excitatory receptive field, as well as from the conditioned site, it appears that the response adaptation is attributable to changes in the central neuron or in synaptic processes associated with the central neuron. It is proposed that this adaptation may be due to an increase in extracellular potassium ion concentration that alters the responsiveness of the central neurons.
SUMMARY1. In decerebrate or anaesthetized cats, the vibration-induced responses of dorsal column nuclei neurones were examined, first, when their input came from simultaneously recorded pairs or other combinations of identified Pacinian corpuscle (P.c.) afferent fibres of the interosseous nerve, and secondly, when different convergent sets of P.c. fibres were engaged by footpad vibration.2. Suprathreshold actions were observed on individual dorsal column nuclei neurones from two or more identified P.c. fibres. Recruitment of these convergent fibres usually led to summation in the dorsal column nuclei neurone as reflected in higher response levels compared with those evoked by single-fibre inputs.3. When the input was increased from one to two or more identified P.c. fibres the dorsal column nuclei neurones could retain a single, dominant phase of response to high-frequency (> 100 Hz) vibration even though these fibres, in isolation, evoked responses in the target neurone at substantially different latencies. However, on average, phase locking was significantly tighter in response to single-fibre input than to multiple P.c.-fibre input.4. Dorsal column nuclei neurones were also able to retain phase-locked responses to high-frequency vibration when phase differences between different convergent inputs were systematically introduced to alter the degree of synchrony in the activity arriving over convergent, identified P.c. fibres.5. When the input to dorsal column nuclei neurones came from the skin it was found that with the recruitment of two converging sets of P.c. fibres the dorsal column nuclei neurones were able to retain phase-locked responses to high-frequency vibration even when phase shifts were introduced between the two sets of P.c. inputs.6. In conclusion, the observed integrative processing by dorsal column nuclei neurones of vibration-induced inputs arriving over identified, convergent P.c. fibres, or sets of P.c. fibres, is consistent with our hypothesis that the retention of phase-locked responses to vibration at frequencies > 100 Hz may reflect the functional domination of the target neurone by just one or a few of its convergent input fibres.
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