Body and Breath - Yoga Shop Talk

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Yoga shoptalk, December, 2002

Question: I'm introducing the nervous system to a class of hatha yoga teachers-in-training, and in preparing for the class I ran into some terminology that you do not use in your book, specifically a classification of sensory receptors into three groups, namely exteroceptors, proprioceptors, and interoceptors. Would you please explain these terms, and share why you did not introduce them?

Answer: This classification of sensory receptors was introduced by Sherrington about a century ago. Briefly (and simplistically), exteroceptors sense modalities such as touch and pain on the surface of the body, proprioceptors are sensitive to stimuli associated with joints and muscles, and interoceptors are receptors in the viscera (internal organs). I did not introduce these terms for two reasons. First, in order to keep the text comprehensible to non-scientists, I did not use any more terminology than seemed absolutely necessary, and it appeared to me that we could take a pass on these three. Second, and more importantly, I always tried to avoid terms that have been used in different ways at different times, or that for one reason or another are confusing. Exteroceptors and interoceptors do not present serious problems in that regard, but the term proprioceptors can create problems, because the latter term is too easily confused with the term proprioception, which has conflicting meanings that have caused confusion for many generations of health science students.

The problem: First there is "conscious proprioception," or "head proprioception," which in a restricted sense refers only to the vestibular sense, described in chapter one. That's simple enough. Second there is "unconscious proprioception," which is applied again in a restricted sense only to the input to the central nervous system from muscle spindles and Golgi tendon organs, input as it happens that we do not consciously sense (thus we append the term "unconscious" to this one). Again that is a reasonably simple concept. Third, however, there is the term kinesthesis, or joint sense, knowing by feel, for example, that your elbows are extended and your hands are stretched fully overhead, or that your hip joints are abducted and knees are straight. Many authors consider this to be a form of conscious proprioception, but others argue that even though kinesthesis undeniably makes use of proprioceptors (apologies for even more confusion), the term proprioception is already burdened with two meanings and should not include a third, namely the conscious sense of kinesthesis. I avoided all of this confusion by commenting in chapter one only on the vestibular sense (head proprioception is not mentioned), kinesthesis (conscious proprioception is not mentioned), and reflexes associated with the muscle spindles and Golgi tendon organs (unconscious proprioception is not mentioned). My suggestion is not to bother with such arcane details except in private conversation. Concentrate instead on clarifying terminology that is precise but potentially confusing, such as prostate/prostrate, or perineum/peritoneal/peroneal/perianal. Getting these wrong will diminish your credibility in a hurry among health professionals, not unlike verbalizing (in the presence of an English teacher) libary instead of library, nuqular instead of nuclear, probally instead of probably, or confortable instead of comfortable.