General chronic pain

Chronic pain is a long-lasting pain, as opposed to acute pain, which happens when you hit your finger with a hammer (see Figure 1). Examples of chronic pain include pain from rheumatoid arthritis and cancer pain, and it can be caused by a variety of sources. In a review of scientific studies, researchers found that taking cannabis was generally associated with improvements in chronic pain measures.¹ The conditions that they reviewed were chronic pain that was related to: neuropathy, cancer pain, multiple sclerosis, rheumatoid arthritis, musculoskeletal issues, and chemotherapy-induced pain.¹ The relation of these types of pain to the endocannabinoid system, and how cannabis exactly acts on them, is dependent on the condition. Figure 1: Different kinds of pain. The three steps in pain perception are illustrated in Figure 2: The first step is the activation of nociceptor and an action potential is generated and conducted towards the central nervous system by afferent neurons. There are two types of neurons: the Að-neurons and the C-neurons. The former are myelinated neurons which measure 1-5μm in diameter; they have a quick, sharp pain (fast fiber pain). The latter are unmyelinated neurons with a small diameter (o.3μm) and have a relatively slow conductance of action potentials (slow fiber pain). C-neurons react to mechanic, thermal and chemical stimuli and are responsible for deep, arching and poorly localized (slow fiber) pain. Cell bodies of nociceptive neurons are localised in the spinal ganglia. The second step in pain perception is the transmission of information from the nociceptors to the spinal cord. The afferent nociceptive neurons connect in the dorsal horn of the spinal cord to a complex of other neurons. When the pain signal is transported further, the signal will first pass the anterior commissure to go to the brain on the contralateral side. The third step is the onward transmission of pain information to higher brain centers. The signal can enter the brain via the spinothalamic tract. This tract projects via the medulla and midbrain to the thalamus, which projects further to the somatosensory cortex and limbic system. Pharmacological treatment of pain is focused on decrease of pain stimulators at the level of nociceptors, inhibition of transmission of action potentials (e.g. local anaesthetic) and central modulation (e.g. opioids). Figure 2: The process of pain perception (caption and image taken with permission from TRC Pharmacology Database). References:

1. Whiting, Penny F.; Wolff, Robert F.; Deshpande, Sohan; Di Nisio, Marcello; Duffy, Steven; Hernandez, Adrian V.; Keurentjes, J. Christiaan; Lang, Shona; Misso, Kate; Ryder, Steve; Schmidlkofer, Simone; Westwood, Marie; Kleijnen, Jos (2015). Cannabinoids for Medical Use. Jama, 313(24), 2456.