Low threshold, NPY-negative neurons are secretomotor neurons, innervating salivary glands. Sympathetic fibers innervating the eye separate from the carotid plexus within the cavernous sinus. Upon reaching it, they form a loose, unmyelinated covering over the innervating axons. This nerve usually joins with the phrenic nerve before innervating the diaphragm. High threshold, NPY-negative neurons are vasomotor neurons, innervating blood vessels. This is termed muscle tone and is maintained by the motor neurons innervating the muscle. Tibial nerve (innervating the plantar surface of the foot)3. Instead they involve one or a few of the smaller neurons innervating the tail. The plantar nerves are a pair of nerves innervating the sole of the foot. Sural nerve (innervating the lateral portion of the foot)4. Damage to the fibers innervating the teres minor is clinically significant.