see also:

• paraesthesiae or numbness
• pain
• neurology

• light touch is the sensation arising from activation of touch-sensing low-threshold mechanoreceptors (LTMRs) nerve cells via activation of various classes of mechanoreceptors detecting stretch, skin movement, compression, vibration or hair fiber deflection
  • in addition:
    • Merkel cells (release noradrenaline and serotonin) and keratinocytes (release ATP) within the skin epidermis, play a role in touch perception and can also be classified as mechanoreceptors as these then stimulate the afferent neurons
    • the outer root sheath (ORS) at the base of hair follicles have a a higher percentage of touch-sensitive receptors than equivalent cells in the skin¹⁾
      • these cells release serotonin and histamine via vesicles to signal to surrounding cells that touch has occurred - interacting specifically with low-threshold mechanoreceptors (LTMRs) in addition to a simple mechanical response, capable of feeling gentle touches
      • in contrast, skin cells reacting to light touch release mainly histamine and very little serotonin
• NB. high-threshold mechanoreceptors (HTMRs) neurons respond to harmful mechanical stimuli - see pain
• cutaneous sensory neurons can be classified as either Aβ, Aδ, or C based on their cell body sizes, axon diameter, degree of myelination and axonal conduction velocities²⁾
  • Aδ and Aβ sensory neurons have medium and large cell body sizes with lightly and heavily myelinated processes, thereby exhibiting intermediate (5–30m/s) and rapid conduction velocities (16–100m/s), respectively.
    • most Aβ neurons are LTMRs
    • most Aδ neurons respond to noxious stimuli - pain, temp but some are LTMRs
  • C-type sensory neurons are the smallest and most abundant, with unmyelinated axons and the slowest conduction velocities (ranging from 0.2–2m/s)
    • most C-type neurons respond to noxious stimuli - pain, temp
    • some are are LTMRs
• cold
  • a TRPM8 protein channel on the membranes of sensory neurons which innervate the skin, oral cavity, and eyes and responds to cold temperatures (between 8°C and 28°C) by opening up and allowing ions to flow into the cell, which triggers a nerve signal to the brain.
    • TRPM8 also the reason menthol, eucalyptus, and certain other compounds produce that characteristic cooling sensation.
    • cold primarily triggers changes in the pore region, while menthol binds a different part of the protein and induces shape changes that propagate to the pore
    • when cold is combined with menthol, the response is enhanced synergistically
    • TRPM8 also has a “cold spot” - a specific region of the protein that is uniquely important for sensing temperature and helps prevent the channel from becoming desensitized during prolonged cold exposure
    • dysfunction in TRPM8 has been linked to chronic pain, migraines, dry eye and certain cancers
    • acoltremon, a menthol analogue drug that activates TRPM8, is an FDA-approved eye drop for dry eye disease
  • if hands or feet sense cold, this is likely to trigger a shiver response to reduce risk of hypothermia
• itch
  • TRPV4 is expressed in Aβ-LTMRs, as well as in subsets of sensory neurons linked to itch and pain pathways, including those expressing TRPV1
  • in addition to producing itch, TRPV4 also acts as a negative feedback telling the brain when scratching is sufficient - blocking TRPV4 appears to reduce frequency of itching but prolongs the scratch response

• 1st order neurons
  • light touch sensations are transmitted in 1st order peripheral sensory nerves to the spinal cord where they may activate reflex responses as well as sending signals to the brain stem and brain
  • hairy skin also contains C-LTMRs, a specific type of tactile neuron that is not present on glabrous sites and is involved in processing affective touch response
  • cell bodies for these nerves reside within dorsal root ganglia (DRG) and cranial sensory ganglia (in parts of the head or neck not covered by the cervical nerves, the first-order neuron will be the trigeminal nerve ganglia or the ganglia of other sensory cranial nerves)
• 2nd order neurons send signals to the thalamus
  • cell body is either in the spinal cord or in the brainstem
  • ascending signals in the spinal cord ascend via the dorsal column-medial lemniscus pathway via either:
    • gracilis (T7 and below)
    • cuneatus (T6 and above)
  • all fibers decussate in the medulla
• 3rd order neurons
  • cell body in the ventral posterior nucleus of the thalamus and ends in the postcentral gyrus of the parietal lobe in the primary somatosensory cortex (S1).
  • social affective touch which elicits a pleasant emotive reaction is also processed in the anterior cingulate cortex and the prefrontal cortex without requiring processing in S1
    • in the hypothalamus they induce the release of oxytocin, a hormone that decreases stress and anxiety and increases social bonding

• ascending spinal cord pathway is in fibres are carried in the spinothalamic tract