Receptors of the somesthesia
Receptors can really be ordered in many ways; one almost loses one's head. I will introduce them through different perspectives.
First, the type of fibre: in fact, the central nervous system (CNS) has several somatotopic maps (1) and the information that reach them depends on the type of fibre which is associated with the receptor, each following a specific path. This information will make its way from the periphery to the CNS through second- and third-order neurons to specific regions of the brain, some of which have somatotopic representations of the body.
What does that mean? This means that, for example, information transmitted by a mechanoreceptor such as a Ruffini corpuscle, which is carried by an A-beta fibre, will not have the same neural path as the information transmitted by a temperature-sensitive C fibre and their targets in the CNS are not the same.
In the category 'fibres', a distinction can be made between fast fibres, slower fibres and very slow fibres. Fast fibres are completely myelinated (they have a myelin sheath that acts a bit like a "turbocharger" on the conduction speed), slower fibres have a thin myelin sheath, not as important as that of fast fibres. Slow fibres do not have a myelin sheath, they are the slowest. The conduction (or information transmission) speed can vary from 120 m/s in fast fibres to 0.5 m/s in slow fibres. That makes quite a difference.
Proprioceptors use the fastest fibres, those that are widely myelinated. These are the receptors found in Golgi's muscle spindles and tendon organs. I am very happy that it is so, I can now go down the ski slope with peace of mind, my receptors and their fibres will have all the time they need to do their work so that my brain, my muscles and my whole body will adapt to every turn, every bump. These fibres are A-alpha fibres, type Ia and Ib.
Next come the mechanoreceptors which are the skin receptors (Ruffini, Pacini, Merkel, Meissner), then the receptors at the base of the hair and some receptors sensitive to muscle stretching, deep pressure and acute pain. These are A-beta, A-delta fibres from group II and group III. Some of the A-delta fibres are sensitive to heat and cold. A-beta fibres are myelinated, and A-delta fibres are slightly myelinated. The latter are therefore slower.
Then finally type C fibres. They are most of the time called nociceptors or themoreceptors but this is a mistake : they do not only carry pain and temperature. Besides beeing sensitive to heat, cold and pain, they react to pleasant touch, tickling, itching, sexual sensations and they are sensitive to energy expenditure and chemicals. C-type fibres are not myelinated, they are the slowest. They form the Group IV. You notice that some A-delta fibres are sensitive to acute pain and that C fibres are sensitive to pain, but they are slower. This is how we can explain the sharp pain felt at the time of an injury and the more diffuse pain that follows.
Let's talk a bit now about the "Groups" category. I've already named them, the groups range from I to IV.
Groups I and II refer to highly specialised receptors (neuromuscular spindles and Golgi tendon organs) or encapsulated receptors (Merkel discs, Ruffini corpuscles, Meissner corpuscles, Pacini corpuscles and Krause corpuscles). These receptors are associated with large, fast, myelinated fibres. They are sensitive to mechanical deformations.
Groups III and IV refer to free-nerve endings that are associated with small, slow, poorly or not myelinated fibres. Some of these receptors are sensitive to light mechanical deformations, and others will be sensitive to mechanical pressure from light to pinch, temperature and metabolites such as ATP (adenosine triphosphate, the energy molecule), lactic acid and pH acidity (chemoreception). (Craig, p. 91)
It will be useful to differentiate these groups when we discuss the chapter on the pathway of afferent neurons in the posterior horn of the spinal cord. Each group will have a well-defined territory.
(1) "Somatotopy is a point-by-point neuronal mapping of the body's sensory input. It is a topologically well organised "homotopy" from the topological point of view, which means that neighbourhood relations are maintained (although breaks must occur in the translation of a three-dimensional surface into a two-dimensional surface)." Craig, "How do you feel?", p. 306