味覺（酸、甜、苦、咸、鮮）是通過作用于味覺細(xì)胞（taste cell）上的受體蛋白，激活味覺細(xì)胞以及相連的神經(jīng)通路。而辣的感覺是通過辣椒素（capsaicin）等作用于舌頭中的痛覺纖維上的受體蛋白而產(chǎn)生的。這條通路同時也是痛覺的傳導(dǎo)通路。因此從神經(jīng)科學(xué)的角度來說辣更類似于痛覺。
不同的化學(xué)物質(zhì)激活受體蛋白，使味覺細(xì)胞去極化，之后釋放神經(jīng)遞質(zhì)激活下游的神經(jīng)通路，從而產(chǎn)生相應(yīng)的味覺（氫離子-酸，葡萄糖；蔗糖、果糖等-甜；奎寧-苦；鈉離子-咸；氨基酸-鮮）。
其他答案已經(jīng)提到了辣椒素受體TRPV1，我就不再贅述了。另外的一種在痛覺中起到重要作用的受體是TRPA1。TRPV1能被辣椒素等化學(xué)物質(zhì)以及有害的熱刺激激活；TRPA1能被芥末等化學(xué)物質(zhì)以及有害的冷刺激激活。（此處的激活，主要離子通道在靜息狀態(tài)下更容易開放。）當(dāng)這些TRP通道開放時，陽離子進(jìn)入細(xì)胞，細(xì)胞去極化產(chǎn)生動作電位進(jìn)而激活痛覺神經(jīng)通路。
不同的食物，產(chǎn)生辣味的化學(xué)物質(zhì)不同，但主要是作用于TRPV1和TRPA1受體。
生姜 – ‘Gingerol, or sometimes [6]-gingerol, is the active constituent of fresh ginger. Chemically, gingerol is a relative of capsaicin and piperine, the compounds which give chilli peppers and black pepper their respective spicyness.’ 辣椒素類似物，應(yīng)該是作用于TRPV1
生大蒜 – allicin，主要作用于TRPA1，也作用于TRPV1
芥末，山葵，小蘿卜等 - Allyl isothiocyanate，作用于TRPA1
引用from neuron to brain的原文來回答辣是味覺還是痛覺的問題：主要在于如何定義味覺。
‘It is important to note that taste, as a percept, results from sensations in addition to those reported by the taste buds. The aroma detected by olfaction, as well as by texture and temperature, which are sensed by somatosensory neurons, all contribute to the ultimate identification and appreciation of food. The hot taste of chili peppers particularly reinforces this concept. Hot chilies are not sensed by taste cells per se, but rather by pain fibers in the tongue that are activated by the compound capsaicin. A capsaicin receptor has been cloned and shown to be a calcium-selective cation channel. The hot receptor, known as transient receptor potential (subfamily V, member 1), or TRP-V1, is also found in small diameter sensory fibers (C fibers) responding to noxious temperatures (see following section). Thus, nature has provided chili peppers with a chemical targeted to this receptor, possibly to discourage herbivores by activating pain fibers - a not entirely successful strategy in the case of humans with a preference for spicy foods.’
(From neuron to brain, fifth edition p.p. 402-403)
結(jié)論：
辣和痛覺的神經(jīng)通路一致，而與酸、甜、苦、咸、鮮等味覺神經(jīng)傳導(dǎo)通路不同。
不同的辣味由不同的化學(xué)物質(zhì)引起，但其激活的受體蛋白大致相同。
參考文獻(xiàn)：
From neuron to brain (fifth edition) by John G. Nicholls, A. Robert Martin, Paul A. Fuchs, David A. Brown, Mathew E. Diamond, David Weisblat p.p. 402-403
Macpherson, Lindsey J, Bernhard H Geierstanger, Veena Viswanath, Michael Bandell, Samer R Eid, SunWook Hwang, and Ardem Patapoutian. “The Pungency of Garlic: Activation of TRPA1 and TRPV1 in Response to Allicin.” Current Biology?: CB 15, no. 10 (May 24, 2005): 929–34. doi:10.1016/j.cub.2005.04.018.
Ferrandiz-Huertas, Clotilde, Sakthikumar Mathivanan, Christoph Jakob Wolf, Isabel Devesa, and Antonio Ferrer-Montiel. “Trafficking of ThermoTRP Channels.” Membranes 4, no. 3 (January 19, 2014): 525–64. doi:10.3390/membranes4030525.
