Like muscle afferents, cutaneous afferents are not homogeneous, but the reflex pathways fed by the different types of cutaneous afferent have been documented less well than those fed by muscle afferents. Cutaneous afferents are responsible for a wide range of sensations, but most are also capable of modulating motor behaviour through spinal, supraspinal and transcortical pathways. There is a tendency for clinicians to group all cutaneous afferents together, and this creates confusion, leads to the usage of different terms for the same function and the same term for different functions, and makes the systems appear more complex than necessary. There is heterogeneity in: (i) the type of receptor (e.g. mechanoreceptor, thermoreceptor, nociceptor), (ii) the peripheral afferents (which range from large myelinated Aβ afferents to slow unmyelinated C afferents), (iii) the spinal pathways fed by the afferents (few or many interneurones), (iv) their central projection (spinal, supraspinal, transcortical), (v) the importance of the cutaneous contribution (predominantly ‘private’ vs. shared pathways), and (vi) their functional role (contributing to the normal usage of the limb or responsible for withdrawal from a noxious agent).

This heterogeneity is reflected in the terminology: ‘flexor’ or ‘withdrawal’ reflexes are considered nociceptive responses (though mechanoreceptors may play a role in their generation), whereas ‘cutaneomuscular’ reflexes refer to responses involved in the control of normal movement. A thesis of this book, addressed in many chapters, is that cutaneous afferents may have a proprioceptive role, perhaps as important for some movements as that played by muscle spindle afferents, whether that influence is mediated by a primarily ‘private’ pathway or by modulating the activity of some other system.