Neural mechanisms and pathways in craniofacial pain

Many free nerve endings of small-diameter afferents (A-delta or C nerve fib res) respond to craniofacial noxious stimuli and a number of neurochemicals are involved in their activation or sensitization. The small-diameter noci ceptive afferents project to the trigeminal (V) brainstem complex where the y can excite nociceptive neurones that have been categorized as either noci ceptive-specific (NS) or wide dynamic range (WDR). These neurones project t o other brainstem regions or to the contralateral thalamus. The lateral and medial thalamus contain NS and WDR neurones which have properties and conn ections with the overlying cerebral cortex or other thalamic regions indica tive of a role for most of them in the sensory-discriminative, affective or other dimensions of pain. Some of the V brainstem NS and WDR neurones resp ond exclusively to cutaneous sensory inputs and have features indicating th eir involvement in acute superficial craniofacial pain, Many of the neurone s, however, receive convergent inputs from afferents supplying other cranio facial tissues (e.g. cerebrovascular, muscle) as well as skin, and are like ly involved in deep pain, as well as spread and referral that is typically seen in headache and several craniofacial pain conditions involving deep ti ssues. Convergence may also be an important factor underlying the neuroplas tic changes in V neuronal properties that may occur with peripheral injury or inflammation. These changes include a prolonged enhancement of the cutan eous as well as deep afferent inputs to most NS and WDR neurones and expans ion of their cutaneous or deep mechanoreceptive field and increased EMG act ivity in the jaw musculature. They involve NMDA, non-NMDA and opioid neuroc hemical mechanisms within peripheral tissues as well as within the CNS. Suc h modulatory effects on brainstem neuronal properties reflect the functiona l plasticity of the central V system, and may be involved in the developmen t of headache and other conditions that manifest craniofacial pain.