THE SPINOMESENCEPHALIC TRACT--ANATOMY AND PHYSIOLOGY

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The spinomesencephalic tract (SMT) is made up of several components each with varying sites of origin, physiological properties, spinal trajectories and midbrain termination sites. Midbrain regions associated with the termination of the SMT may be involved in sensory, motor and autonomic responses to different modalities of stimulation, including pain and temperature. Because of this it continues to be the long range objective of research related to the SMT to gain a better understanding of the different components of this projection system in order to better understand their role in the spinal and supraspinal processing of sensory information derived from cutaneous, muscle and visceral structures. The specific aims of the present research plan include evaluation of: (a) the origin and termination of SMT cells in the upper cervical cord; (b) trigeminal and spinal inputs to cervical SMT cells, including convergence from dural, skeletal muscle and cardiac afferent fibers; and (c) the contribution of propriospinal pathways to the functional properties of cervical SMT cells. Anterograde and retrograde tracing techniques will be used to study the termination and origin of SMT axons. These studies will include a quantitative evaluation of the terminal projection from different regions of the upper cervical cord to structures throughout the rostrocaudal extent of the midbrain. A quantitative evaluation of the laminar and segmental distributions of cells projecting to specific midbrain targets will also be carried out. The physiology of identified SMT cells will be studied with single-unit recording techniques similar to those used in previous studies. Trigeminal and spinal afferents will be activated by electrical, chemical and natural stimuli. Propriospinal pathways from the lumbosacral cord will be excited and/or inhibited using natural and electrical stimulation. The research plan addresses several important questions related to the organization of SMT components at different levels of the cord. The results of these studies should extend the basic understanding of spinal and supraspinal regions involved in the central processing of sensory, motor and visceral information in general, and specifically that related to craniofacial, muscle and cardiac pain. The results of the proposed studies should also contribute to the understanding of central pathways that may be involved in the multidimensional affective and motivational aspects of acute and chronic pain.