Single-unit recordings from the ventroposterior complex of nuclei have shown that the responses of many of the neurons in these nuclei to stimuli resemble those of firstand second-order neurons in the ascending tracts. The receptive fields of thalamic cells are small but somewhat larger than those of primary afferent fibers. Moreover, the responses may be dominated by a particular type of sensory receptor. For example, VPL and VPM nuclei have cells whose receptive fields typically reflect input either from one type of cutaneous receptor (FA or SA) or from proprioceptive receptors, as expected from their medial lemniscal input. In contrast, cells of the VPI and posterior nuclei show responses to activation of nociceptors, the main input to the spinothalamic pathway.
Thalamic neurons often have inhibitory as well as excitatory receptive fields. The inhibition may actually take place in the dorsal column nuclei or in the dorsal horn of the spinal cord. However, inhibitory circuits are also situated within the thalamus. For example, the VPL and VPM nuclei contain GABAergic inhibitory interneurons (in primates but not in rodents), and GABAergic inhibitory interneurons in the reticular nucleus of the thalamus project into the VPL and VPM nuclei. (The reticular nucleus of the thalamus is a thin shell of neurons that surrounds much of the thalamus. Thalamocortical and corticothalamic neurons both send axon collaterals to the reticular nucleus, whose neurons then respectively complete feedback and/ or feed-forward inhibitory circuits with neurons in other thalamic nuclei.)
One difference between neurons in the VPL and VPM nuclei and sensory neurons at lower levels of the somatosensory system is that thalamic neuron excitability depends on the stage of the sleep/wake cycle and on the presence or absence of anesthesia. During a state of drowsiness or during barbiturate anesthesia, thalamic neurons tend to undergo an alternating sequence of excitatory and inhibitory postsynaptic potentials. The alternating bursts of discharges in turn intermittently excite neurons in the cerebral cortex. Such patterns of excitation and inhibition result in an α rhythm or in spindling on the electroencephalogram. This alternation of excitatory and inhibitory postsynaptic potentials during these two states may reflect the level of excitation of thalamic neurons by excitatory amino acids that act at NMDA and non-NMDA receptors. It may also reflect inhibition of the thalamic neurons by recurrent pathways through the reticular nucleus.
Thalamic neuron receptive fields are on the side of the body contralateral to the neuron, and the receptive field locations vary systematically across the ventroposterior nuclear complex. That is, the VPL and VPM nuclei are somatotopically organized such that the lower limb is represented most laterally and the upper limb most medially in the VPL nucleus, and the head is represented even more medially in the VPM nucleus. Moreover, the fact that thalamic neurons often receive input from only one class of receptor suggests that there are multiple somatotopic maps laid out across the ventroposterior nuclear complex. That is, there appear to be separate somatotopic maps for SA, FA, and proprioceptive and pain sensations laid out across the ventroposterior nuclear complex.
These maps are not randomly interspersed. As already mentioned, pain sensation is largely mapped across the VPI nucleus. In addition, the cutaneous receptors appear to drive cells located in a central “core” region of the VPL-VPM complex, whereas proprioceptive information is directed to cells that form a “shell” (VPS) around this core. This parallel flow of information into thalamus and then onto the cortex is diagramed in
Fig. 7.5