Pain and pleasure

Some philosophers, such as Jeremy Bentham, Baruch Spinoza, and Descartes, have hypothesized that the feelings of pain (or suffering) and pleasure are part of a continuum. Some philosophers, such as Jeremy Bentham, Baruch Spinoza, and Descartes, have hypothesized that the feelings of pain (or suffering) and pleasure are part of a continuum. There is strong evidence of biological connections between the neurochemical pathways used for the perception of both pain and pleasure, as well as other psychological rewards. From a stimulus-response perspective, the perception of physical pain starts with the nociceptors, a type of physiological receptor that transmits neural signals to the brain when activated. These receptors are commonly found in the skin, membranes, deep fascias, mucosa, connective tissues of visceral organs, ligaments and articular capsules, muscles, tendons, periosteum, and arterial vessels. Once stimuli are received, the various afferent action potentials are triggered and pass along various fibers and axons of these nociceptive nerve cells into the dorsal horn of the spinal cord through the dorsal roots. A neuroanatomical review of the pain pathway, 'Afferent pain pathways' by Almeida, describes various specific nociceptive pathways of the spinal cord: spinothalamic tract, spinoreticular tract, spinomesencephalic tract, spinoparabrachial tract, spinohypothalamic tract, spinocervical tract, postsynaptic pathway of the spinal column. Activity in many parts of the brain is associated with pain perception. Some of the known parts for the ascending pathway include the thalamus, hypothalamus, midbrain, lentiform nucleus, somatosensory cortices, insular, prefrontal, anterior and parietal cingulum. Then, there are also the descending pathways for the modulation of pain sensation. One of the brainstem regions responsible for this is the periaqueductal gray of the midbrain, which both relieves pain by behavior as well as inhibits the activity of the nociceptive neurons in the dorsal horn of the spinal cord. Other brainstem sites, such as the parabrachial nucleus, the dorsal raphe, locus coeruleus, and the medullary reticular formation also mediate pain relief and use many different neurotransmitters to either facilitate or inhibit activity of the neurons in the dorsal horn. These neurotransmitters include noradrenaline, serotonin, dopamine, histamine, and acetylcholine. Pleasure can be considered from many different perspectives, from physiological (such as the hedonic hotspots that are activated during the experience) to psychological (such as the study of behavioral responses towards reward). Pleasure has also often been compared to, or even defined by many neuroscientists as, a form of alleviation of pain. Pleasure has been studied in the systems of taste, olfaction, auditory (musical), visual (art), and sexual activity. Well known hedonic hotspots involved in the processing of pleasure include the nucleus accumbens, posterior ventral pallidum, amygdala, other cortical and subcortical regions. The prefrontal and limbic regions of the neocortex, particularly the orbitofrontal region of the prefrontal cortex, anterior cingulate cortex, and the insular cortex have all been suggested to be pleasure causing substrates in the brain. One approach to evaluating the relationship between pain and pleasure is to consider these two systems as a reward-punishment based system. When pleasure is perceived, one associates it with reward. When pain is perceived, one associates with punishment. Evolutionarily, this makes sense, because often, actions that result in pleasure or chemicals that induce pleasure work towards restoring homeostasis in the body. For example, when the body is hungry, the pleasure of rewarding food to one-self restores the body back to a balanced state of replenished energy. Like so, this can also be applied to pain, because the ability to perceive pain enhances both avoidance and defensive mechanisms that were, and still are, necessary for survival. The neural systems to be explored when trying to look for a neurochemical relationship between pain and pleasure are the opioid and dopamine systems. The opioid system is responsible for the actual experience of the sensation, whereas the dopamine system is responsible for the anticipation or expectation of the experience. Opioids work in the modulation of pleasure or pain relief by either blocking neurotransmitter release or by hyperpolarizing neurons by opening up a potassium channel which effectively temporarily blocks the neuron. It has been suggested as early as 4th century BC that pain and pleasure occurs on a continuum. Aristotle claims this antagonistic relationship in his Rhetoric: