Neurobiology of pain

Abstract

Pain is a sensory experience evoked by stimuli that injure or threaten to destroy tissue. It is the main reason most patients seek the advice of physician. Pain pathway includes: i) Transduction which is a process of changing noxious information into electrical signals that activate nerve endings. ii) Transmission refers to relay function by which message is carried from site of tissue injury to the brain. iii) Modulation: the body tries constantly all along the transmission to reduce the noxious and painful information. iv) Perception is a subjective awareness produced by noxious sensory signals. Quality of pain perceived will depend on provoking stimulus, site of stimulation and nature of fibers transmitting. There is a distinction between sharp & chronic pain. Immediate pain is transmitted by A δ fibers while prolonged unpleasant burning pain by C fibers. Neurons in the dorsal horn give rise to fibres which cross within the white commissure of spinal cord and ascend as lateral spinothalamic tract becoming spinal lemniscus through the brainstem. Relay in the ventral posterolateral and non-specific centrolateral intralaminar nuclei of the thalamus and via superior thalamic radiation travel to the sensory cerebral cortex. The ascending pathway pursue a polysynaptic way to the sensory cerebral cortex giving collaterals all along for modulation. Hypothalamus can evoke emotional responses and reactions involving the autonomic nervous system. The complexity of connections in the brain and the fact that the signal does not go only to the sensory cortex explains why a patient under general anesthesia who experiences no pain at all still experiences an increased heart rate, blood pressure and renal suppression. The body tries constantly all along the nervous system to modulate noxious and painful information and is called the Endogenous Neuromodulation System.