Temperature is sensed through thermoreceptors in the peripheral nervous system. There are "warm" and "cold" subtypes of thermoreceptors; "warm" types are unmyelinated C fibers, and "cold" types are slightly more myelinated A-delta fibers. The presence of myelin makes the velocity of signal propagation from the "cold" types faster than that of the "warm" types. "Warm" thermoreceptors fire more action potentials in response to warmer stimuli and fire less action potentials for cold stimuli; the reverse is true for "cold" thermoreceptors. These axons synapse at the gray matter of the dorsal horn of the spinal cord, then the spinal cord axons decussate as they ascend the spinothalamic tract and synapse finally at the ventral posterior lateral nucleus of the thalamus, where integrative processes and reactions begin to occur. Hope this helps!