The somatosensory system
is a part of the sensory nervous system. The somatosensory system is a complex system of sensory
neurons and neural pathways that responds to changes at the surface or inside the body. The axons (as
afferent nerve fibers) of sensory neurons connect with, or respond to, various receptor cells. These
sensory receptor cells are activated by different stimuli such as heat and nociception, giving a functional
name to the responding sensory neuron, such as a thermoreceptor which carries information about
temperature changes. Other types include mechanoreceptors, chemoreceptors, and nociceptors which
send signals along a sensory nerve to the spinal cord where they may be processed by other sensory
neurons and then relayed to the brain for further processing. Sensory receptors are found all over the
body including the skin, epithelial tissues, muscles, bones and joints, internal organs, and the
cardiovascular system.
Ancient Greek philosophers identified five forms of love: essentially, familial love (in Greek, Storge),
friendly love or platonic love (Philia), romantic love (Eros), guest love (Xenia) and divine love (Agape).
Modern authors have distinguished further varieties of love: unrequited love, empty love, companionate
love, consummate love, infatuated love, self-love, and courtly love. Numerous cultures have also
distinguished Ren, Yuanfen, Mamihlapinatapai, Cafuné, Kama, Bhakti, Mettā, Ishq, Chesed, Amore,
Charity, Saudade (and other variants or symbioses of these states), as culturally unique words,
definitions, or expressions of love in regards to a specified "moments" currently lacking in the English
language.[9][10][11]
Scientific research on emotion has increased significantly over the past two decades. The color wheel
theory of love defines three primary, three secondary and nine tertiary love styles, describing them in
terms of the traditional color wheel. The triangular theory of love suggests "intimacy, passion and
commitment" are core components of love. Love has additional religious or spiritual meaning. This
diversity of uses and meanings combined with the complexity of the feelings involved makes love
unusually difficult to consistently define, compared to other emotional states.