Signal2

Pairs of intramuscular fine‐wire electrodes (Teflon‐coated stainless steel wires, 75‐um diameter, 1 mm Teflon removed, tips bent back ~1 and ~2 mm to form the hooks) were used to record electromyography (EMG) from the back muscles on the most painful side or on the dominant side, when pain was symmetrical. Electrodes were inserted with guidance of real‐time ultrasound imaging at the level of L4/L5 lamina into the short, deep fibres of the multifidus muscle (DM), superficial fibres of multifidus (SM) and lumbar erector spinae (LES). Pairs of surface EMG electrodes (Ag/AgCl) were placed over the erector spinae muscle 2–3 cm lateral to T9 spinal process (TES). The ground electrode was positioned over the anterior superior iliac spine. EMG data were pre‐amplified 2000 times, band‐pass filtered (20–1000 Hz) and sampled at 2000 Hz using a Power 1401 Data Acquisition System with Signal2 software (Cambridge Electronic Design, CED, UK). Data were exported and analysed with Matlab 6.5 (The MathWorks, USA).

The slices were placed in the recording chamber of an interface type and were super-fused at 2.5 mL/min with warm (32 ± 0.5 °C), modified ACSF (see above). A concentric bipolar stimulating electrode (FHC, Inc., Bowdoin, ME, USA) was placed in cortical layer V. Stimuli of 0.033 Hz frequency and duration of 0.2 ms were applied using a constant-current stimulus isolation unit (WPI). Glass micropipettes filled with ACSF (2–5 MΩ) were used to record field potentials. Recording microelectrodes were placed in cortical layer II/III. The responses were amplified (EXT 10-2F amplifier, NPI), filtered (1 Hz–1 kHz), A/D converted (10 kHz sampling rate), and stored on PC using the Micro1401 interface and Signal 2 software (Cambridge Electronic Design Ltd., Cambridge, UK).
A stimulus-response (input–output) curve was made for each slice. To obtain the curve, stimulation intensity was gradually increased stepwise (15 steps; 5–100 μA). One response was recorded at each stimulation intensity. The recording was performed in standard ACSF with stimulation intensity adjusted to evoke a response of 30% of the maximum amplitude. LTP was induced by theta-burst stimulation (TBS). TBS consisted of ten trains of stimuli at 5 Hz, repeated 5 times every 15 s. Each train was composed of five pulses at 100 Hz. During TBS pulse duration was increased to 0.3 ms.