Interoception refers to the representation of the internal world, and includes the processes by which an organism senses, interprets, integrates, and regulates signals from within itself.The brain communicates with internal organs via the peripheral nervous system and non-neuronal systems.

Thalamic relay cells fire in two distinct modes, burst or tonic, and the operative mode is dictated by the inactivation state of low-threshold, voltage-gated, transient (or T-type) Ca 2ϩ channels. Tonic firing is seen when the T channels are inactivated via membrane depolarization, and burst firing is seen when the T channels are activated from a hyperpolarized state. These response modes have very different effects on the relay of information to the cortex. It had been thought that only tonic firing is seen in the awake, alert animal, but recent evidence from several species suggests that bursting may also occur. We have begun to explore this issue in macaque monkeys by recording from thalamic relay cells of unanesthetized, behaving animals. In the lateral geniculate nucleus, the thalamic relay for retinal information, we found that tonic mode dominated responses both during alert behavior as well as during sleep. We nonetheless found burst firing present during the vigilant, waking state. There was, however, considerably more burst mode firing during sleep than wakefulness. Surprisingly, we did not find the bursting during sleep to be rhythmic. We also recorded from relay cells of the somatosensory thalamus. Interestingly, not only did these somatosensory neurons exhibit much more burst mode activity than did geniculate cells, but bursting during sleep was highly rhythmic. It thus appears that the level and nature of relay cell bursting may not be constant across all thalamic nuclei.

There is a strong correlation between the behavior of an animal and the firing mode (burst or tonic) of thalamic relay neurons. Certain differences between first-and higher-order thalamic relays (which relay peripheral information to the cortex versus information from one cortical area to another, respectively) suggest that more bursting might occur in the higher-order relays. Accordingly, we recorded bursting behavior in single cells from awake, behaving rhesus monkeys in first-order (the lateral geniculate nucleus, the ventral posterior nucleus, and the ventral portion of the medial geniculate nucleus) and higher-order (pulvinar and the medial dorsal nucleus) thalamic relays. We found that the extent of bursting was dramatically greater in the higher-order than in the first-order relays, and this increased bursting correlated with lower spontaneous activity in the higher-order relays. If bursting effectively signals the introduction of new information to a cortical area, as suggested, this increased bursting may be more important in corticocortical transmission than in transmission of primary information to cortex. pulvinar ͉ lateral geniculate nucleus ͉ medial dorsal nucleus ͉ medial geniculate nucleus ͉ ventral posterior nucleus A key feature of the thalamus is the ability of its relay cells to fire in two distinct modes (called tonic and burst), and the firing mode is determined by the inactivation state of voltagegated, T-type Ca 2ϩ channels in the membranes of the soma and dendrites (1-3). This firing mode strongly affects the nature of the signal that is relayed to the cortex (4). For example, compared with tonic mode, burst mode produces much more nonlinear distortion in the relay of information, but the information relayed has greater detectability because of a greater signal-to-noise ratio and stronger activation of postsynaptic cortical targets (4-9). From these properties, the hypothesis has been forwarded that burst firing, with its greater detectability and cortical activation, serves as a ''wake-up call'' to the cortex that there has been a change in the outside world (e.g., a novel stimulus within the receptive field of a relay cell for one of the sensory thalamic nuclei); tonic mode, with its more linear relay of information, is then better suited for a more faithful analysis of the relayed information (4, 7). Thus, burst mode would be more effective for cells dealing with information that is not fully attended to, and the burst would help to redirect attention to the novel stimulus and, ultimately, lead to a shift in firing to tonic mode.Evidence of burst and tonic firing has been reported in various species during waking behavior, including cats (10-12), rats (13-16), guinea pigs (17, 18), rabbits (5, 6), monkeys (19), and humans (20)(21)(22). In general, bursting is relatively rare during full wakefulness and more common during periods of inattention or drowsiness (6). However, nearly all of these data were obtained from cells in first-order thalamic relays, namely, the lateral geniculate nu...