, 1976, Brown et al., 1980b, Bryan et al., 1973, Craig, 1976, Enevoldson and Gordon, 1989b, Hongo et al., 1968, Lundberg, 1964 and Taub and Bishop, 1965). On the basis of fiber and cell body counts, there are an estimated 4,000–6,000 SCT

neurons in the cat, with a much more even spread along the rostrocaudal extent in comparison to PSDC neurons, which seem to be concentrated Rapamycin cell line in cervical and lumbar enlargements. Most SCT neurons are located within lamina IV and have dorsally directed dendrites that terminate abruptly at the lamina II/III border. The majority have cone-shaped dendritic trees, with a few displaying more prominent ventral dendritic arborizations (Figure 4C). Like PSDC neurons, SCT neurons have axon collaterals that extend several segmental levels and may have local actions in spinal reflex pathways (Brown, 1981b). The neural components of the dorsal horn, which include presynaptic sensory inputs, locally projecting interneurons, descending modulatory inputs, and long-range projection neurons, are linked by a highly complex set of synaptic connections. Dorsal horn neurons not only receive synaptic input from primary afferents but also from neighboring excitatory

and inhibitory neurons, each with relative input strengths that most likely differ among modules of neuronal connections. Though our knowledge of dorsal horn circuit organization is still in its selleck compound infancy, recently old gained genetic access

to both pre- and postsynaptic neurons will allow for modality-specific dissection of dorsal horn circuits. As with all primary afferents, LTMRs use glutamate as their principal fast transmitter; therefore, all LTMR subtypes have an excitatory action on their postsynaptic targets of the dorsal horn (Brumovsky et al., 2007 and Todd et al., 2003). However, synaptic arrangements between LTMR subtypes and their postsynaptic targets can be quite complex, often forming synaptic glomeruli, structures that not only include primary afferent axonal boutons and postsynaptic dendrites but also synaptic contacts with axons of neighboring interneurons. The presence of synaptic glomeruli allows for input modulation at the very first synapse within the dorsal horn and is thus thought to be the anatomical substrate for primary afferent presynaptic modulation. Within the dorsal horn, two main types of synaptic glomeruli have been described. Type I glomeruli are present largely in lamina II, have dark primary afferent axons, are thought to arise from unmeylinated fibers and axonal contacts that are GABA reactive, and are thought to arise from purely GABAergic interneurons.