Functional properties of spinomesencephalic tract (SMT) cells in the upper cervical spinal cord of the cat

Response and receptive field properties were evaluated for 62 spinomesencephalic tract cells in the upper cervical spinal cord (C1-C3) of cats anesthetized with sodium pentobarbital and alpha-chloralose. Recordings were made from cells in laminae I-VIII and X contralateral to antidromic stimulating electrodes positioned in the rostral, caudal and intercollicular region of the midbrain. The mean antidromic threshold for all cells was 185 +/- 132 microA, and conduction velocities ranged from 2.3 to 38.6 m/sec. Twelve cells were backfired from both midbrain and diencephalic stimulation sites. Receptive fields ranged from simple, i.e., ipsilateral forelimb or face, to complex, i.e., excitatory and/or inhibitory responses from large portions of the body. Peripheral receptive fields included muscles, joints, cornea, dura, forelimbs, hind limbs, tail, and/or testicles. Five functional classes of cells were observed: (a) wide dynamic range (14 cells); (b) high threshold (2 cells): (c) low threshold (4 cells); (d) deep/tap (11 cells); and (e) non-responsive (31 cells). Eight cells were evaluated for responses to different doses (5-150 micrograms) of intravenous (i.v.) serotonin. Two of the 8 cells exhibited excitatory effects, whereas 2 cells classified as deep/tap and 4 cells classified as non-responsive were not affected. The results of this study have shown the upper cervical component of the spinomesencephalic tract is made up of a heterogenous population of cells involved in the integration of varied inputs from large portions of the body. It is proposed that the large population of SMT cells in the upper cervical spinal cord may be involved in pain mechanisms, especially those related to the affective consequences of acute and chronic pain.