Pain, temperature, crude touch 7 • Fig. 7.1 Ascending somatosensory pathways from the body. A, First-, second-, and third-order neurons are shown for the two main pathways conveying cutaneous information from the body to the cerebral cortex: the dorsal column/medial lemniscal and the spinothalamic pathways. Note that the axon of the second-order neuron crosses the midline in both cases, so sensory information from one side of the body is transmitted to the opposite side of the brain, but the levels in the neuraxis at which this takes place are distinct for each pathway. Homologous central pathways for the head originate in the trigeminal nucleus and are described in text, but they are not illustrated for clarity. B, Major spinocerebellar pathways carrying tactile and proprioceptive information to the cerebellum from the upper and lower parts of the body. Again, pathways from the head originate in the trigeminal nuclei but are not shown for clarity. A midsagittal view of the nervous system shows the levels of the spinal and brainstem cross sections in panels A and B.
with them and that underlie conscious and unconscious proprioceptive functions will be covered in this chapter. However, because these receptors also initiate pathways that are intimately involved in the control of movement, they will be discussed again in . Finally, the enteroceptive division has receptors for monitoring the internal state of the body and includes mechanoreceptors that detect distention of the gut or fullness of the bladder. Aspects of enteroceptive division are also covered in as they are related to autonomic functions.
The somatosensory pathways can also be classified by the type of information they carry. Two broad functional categories are recognized, each of which subsumes several somatosensory submodalities. Fine discriminatory touch sensations include light touch, pressure, vibration, flutter (low-frequency vibration), and stretch or tension. The second major functional group of sensations is that of pain and temperature. Submodalities here include both noxious and innocuous cold and warm sensations and mechanical and chemical pain. Itch is also closely related to pain and appears to be carried by particular fibers associated with the pain system.
Thesensoryfunctionsofvariouscutaneoussensoryreceptorshavebeenstudiedinhumansubjectswithatechniqueknownasmicroneurography, inwhichafinemetalmicroelectrodeisinsertedintoanervetrunkinthearmorlegtorecordtheactionpotentialsfromsinglesensoryaxons.Whenarecordingcanbemadefromasinglesensoryaxon,thereceptivefieldofthefiberismapped.Mostofthevarioustypesofsensoryreceptorsthathavebeenstudiedinexperimentalanimalshavealsobeenfoundinhumanswiththistechnique.
Afterthereceptivefieldofasensoryaxonhasbeencharacterized,theelectrodecanbeusedtostimulatethesamesensoryaxon.Intheseexperimentsthesubjectisaskedtolocatetheperceivedreceptivefieldofthesensoryaxon,whichturnsouttobeidenticaltothemappedreceptivefield.