The inhibitory mechanism in the nervous system does not always block the transmitted signal. Lateral inhibition in the information processing of the sense is deeply involved in the mechanism of contrast enhancement. The first report of lateral inhibition was Hartline's study of Limulus lateral eyes. In the human retina, lateral inhibition caused by reciprocal synapses between the photoreceptor and horizontal cells plays a role in reducing the receptive fields of the bipolar cells. In the pain system, a similar mechanism can be used to pinpoint where you feel pain. Periaqueductal gray (PAG), which has reciprocal connections with extensive areas of the brain, is involved in sleep, wakefulness, and regulation of the autonomic nervous system, including the circulatory, respiratory, and thermoregulatory systems. The dorsolateral PAG is involved in the noradrenaline-mediated, and the ventrolateral PAG is involved in the serotonin-mediated descending pain modulation. Both pain and descending pain modulation circuits may be part of the nocifensive response for maintaining homeostasis.

Although the site of pain in knee osteoarthritis is generally assumed to be within the joint, soft tissues outside the joint may also be involved. The patient was a 73-year-old woman who visited our clinic because intra-articular injection had no effect on her knee pain. We diagnosed medial collateral ligament edema via ultrasound and early osteoarthritis of the knee and gastrocnemius lateral head bursitis via MRI. Ultrasound identified the gastrocnemius lateral head bursitis, and the pain improved after injection of a local anesthetic. Extra-articular lesions may cause pain in osteoarthritis of the knee, and ultrasound may be useful in their identification and treatment.

Sarcoidosis is a multi-organ disease of unknown cause, resulting in damage to diverse organs. We report a case in which sub-perception high-frequency spinal cord stimulation (SCS) was remarkably effective in improving lumbar pain due to neurosarcoidosis lesions in the spinal cord, which had not resolved with conventional conservative treatment. A man in his 70s developed sudden left lower back and lower limb pain, muscle weakness, and spasticity. Steroid therapy was effective in temporarily relieving symptoms, but medication and nerve blocks were not effective in treating severe pain relapse. Therefore, SCS was performed as the next step in pain relief. High-frequency SCS was particularly effective in alleviating symptoms, and activities of daily living significantly improved.