PSL300

• Small fibers evoke simple responses to specific stimuli: withdrawing from pain, brushing away a bug, thermoregulatory and sexual responses.
• Signals from the spinal cord travel via the ventroposterolateral (VPL) nucleus of the thalamus.
• Cold receptors respond maximally at ~30°C body temperature.
• Some nociceptors respond to damaging mechanical stimuli, others to damaging heat or chemicals.
• Many of these tasks can be handled in the spinal cord, without immediate input from the brain.
• They are myelinated, and conduct at 70 m/s.
• Thermoreceptors are free nerve endings.
• Large fibers provide feedback to the brain, especially to motor cortex, as it manipulates objects.
• Cold fibers also respond briefly to temperatures > 45°C, causing paradoxical cold: a hot object, touched briefly, may feel cold.
• Some nociceptors respond to chemicals released from damaged cells (K, histamine, prostaglandins) or to serotonin released by platelets in response to injury.
• Nociceptive signals evoke responses from the CNS, for example, nociceptive signals trigger withdrawal, such as pulling your hand back from a hot stove.
• The reason for the 2 types of pain is likely that pain evokes 2 distinct responses: quick withdrawal (to get away from the painful thing) and prolonged immobilization (to promote healing).
• Many nociceptors (and also thermoreceptors) have ion channels belonging to a family called transient receptor potential (TRP) channels.
• Touch, injury, and injury gating can block or dampen pain signals, for example, if you rub a sore shoulder, it feels better.
• Pain in an internal organ is often felt on the body surface, referred pain.
• Lateral inhibition enhances spatial differences, as in the visual system.
• We have 2 types of pain: fast and slow, for example, when you stub your toe, you feel an immediate sharp pain, followed by a duller sensation.