Somatic senses

• Somatosensory receptors are neurons.
• Receptors for somatic sensation below the chin have their cell bodies in the dorsal root ganglia, receptors for the head have their cell bodies in the brain.
• Parts of the neurons that transduce touch, pressure, etc into electrical signals are in their nerve endings, which are located at the tips of fibers in the skin and viscera.
• There is a variety of receptors in the skin, including free nerve endings that detect mechanical stimuli, temperature, and chemicals, and Merkel receptors which are mechanoreceptor nerve endings in contact with specialized epithelial cells called Merkel cells.
• Encapsulated receptors, such as Meissner and Pacinian corpuscles, are mechanoreceptors sheathed in connective tissue.
• At the bottom of the epidermis are saucer-shaped Merkel disks, which are responsible for signal contact and are very sensitive to deformation of the skin, more tonic than phasic.
• Most mechanoreceptors are phasic, given a sustained constant stimulus, the nerve ending’s membrane depolarizes but then returns to baseline.
• Meissner corpuscles, located at the top of the dermis, are mainly found in the tongue and hairless skin, and are erogenous zones, palms and fingertips.
• Pacinian corpuscles, located deep in the dermis, are nerve endings sheathed in many layers, can sense tiny displacements if motion is quick, and are phasic, responding strongly to vibration and other fast changing stimuli.
• Receptors are not uniformly distributed over the body surface, with palms, fingertips and lips having more densely packed receptors and higher acuity, like the fovea.
• Thermoreceptors are free nerve endings that respond to temperature changes.
• Cold receptors respond maximally at around 30 degrees, while warm receptors respond maximally at around 45 degrees.
• Pain receptors are activated above 45 degrees.
• Nociceptors are free nerve endings that respond to noxious stimuli, some respond to damaging mechanical stimuli, others to damaging heat or chemicals.
• Somatosensory afferent fibers fall into two groups, small and large, with small fibers coming mainly from free nerve endings, and large fibers coming from Merkel disks or encapsulated mechanoreceptors such as Meissner or Pacinian corpuscles.
• Small fibers, C and A-delta, come from free nerve endings, with C fibers being unmyelinated and conducting spikes at speeds up to 2m, and A-delta being thicker and myelinated, conducting up to 30m.
• Large fibers, A-beta, come from Merkel disks or encapsulated mechanoreceptors such as Meissner or Pacinian corpuscles, are myelinated and conduct 70m/s.
• Large fibers turn upward on reaching the spinal cord, run ipsilaterally up to the medulla in tracts called the dorsal columns, and synapse at the medulla on cells who axons cross the midline.
• Nociceptive signals evoke responses from CNS, triggering withdrawal.
• Primary somatosensory cortex is somatotopic, with neighbouring areas of skin projecting to neighbouring cells in cortex, so S1 is a map of the contralateral body surface.
• Fast pain is carried by A-delta fibers, slow carried by C fibers.
• Large fibers provide feedback to the brain, especially to motor cortex as it manipulates objects.
• Pain from different organs are referred to different regions on body surface.
• Pain in an internal organ is often felt in surface, referred pain.
• Long fiber information has to travel a long way up to the brain quickly.
• Analgesics work by various mechanisms, including aspirin which inhibits prostaglandins and inflammation, and slows transmission of pain signals; opioids decrease transmitter release from primary sensory neurons and post-synaptically inhibit secondary sensory neurons.
• Lateral inhibition among somatosensory fibers enhances spatial differences, creating the somatosensory version of the Chevreul illusion.
• Nociceptive signals report damage or danger, evoke pain or itch.
• Many can be handled in the spinal cord without immediate input from the brain.
• Signals from the spinal cord travel via the ventroposterolateral (VPL) nucleus of the thalamus.
• Different sized fibers have different functions.
• Small fibers synapse directly or via interneurons on motoneurons or on dorsal horn neurons whose axons cross the midline and run in the spinothalamic tracts, in the lateral part of the cord, between the dorsal and ventral horns.
• Nociceptors from different locations converge on a single descending tract, and when the tract sends signals to the brain, the brain can’t tell where stimulus came from.
• Signals from the head travel via the ventroposteromedial (VPM) nucleus.
• Descending pathways through the thalamus can block nociceptive cells in the spinal cord.
• There are two types of pain: fast and slow.
• Nociceptive signals can trigger emotional distress, nausea, vomiting and sweating.
• The midline is different in the two systems.
• Information for both systems is contralateral, with info from the right side ending up in the left side of the brain.
• Both pass the primary somatosensory cortex, S1, in parietal lobe.