Introduction

As discussed in Chapter 2, several of the sensory pathways that ascend from the spinal cord or brainstem to higher levels of the monkey central nervous system have a nociceptive component and thus may contribute to pain sensation. Spinal cord projections with nociceptive components that ascend to the brain in the anterolateral quadrant of the spinal cord include the spinothalamic, spinoreticular, spinomesencephalic and spinohypothalamic tracts; nociceptive projections that ascend in the dorsolateral or dorsal funiculus are the spinocervical tract and the postsynaptic dorsal column pathway (see Willis and Coggeshall, 2004). Brainstem projections include the trigeminothalamic tract (Price et al., 1976).

To investigate the physiology of an individual spinal cord or brainstem neuron that belongs to one of the ascending nociceptive pathways, it is important to “identify” the neuron by showing that the axon of the individual neuron under investigation actually projects to the appropriate target (Willis and Coggeshall, 2004). Recordings from a neuron unidentified in terms of its projection can be misleading, since many unidentified neurons are likely to be interneurons, and these could be excitatory or inhibitory and might or might not influence the activity of sensory projection neurons. For instance, many spinal cord interneurons belong to neural circuits that function to control motor output (Jankowska et al., 1981; Rudomin et al., 1987).

Identification of a projection neuron is typically accomplished by demonstrating that the neuron can be activated antidromically in response to electrical stimulation in a region in which the axon of that projection neuron synapses (Trevino et al., 1973; Bryan et al., 1974; Haber et al., 1982; see Willis and Coggeshall, 2004).