The type of input and output connections of the various tha- lamic nuclei described above allows them to be grouped into two broad functional categories: relay nuclei and association nuclei (Table 6.1). Not all nuclei fit into this categorization, and there are also non-specific thalamic nuclei, which include the intralaminar and midline nuclei and the reticular nucleus. The input and output connections are also illustrated dia- grammatically inFigs. 6.3and 6.4.Figure 6.3shows subcortical inputs to various thalamic nuclei, whileFig. 6.4shows outputs of the various thalamic nuclei to the cortex. It must be recalled that there are almost always feedback loops from the cortex to the thalamus, i.e., if a thalamic nucleus outputs to a specific
region of the cerebral cortex, it also receives input from that region as well.
Relay nuclei
Relay nuclei are characterized by having a specific well- defined input bundle, and by projecting to a specific functional area of the cerebral cortex. The sensory, motor, and limbic systems all have thalamic relay nuclei. The input–output con- nections of the relay nuclei are shown inFigs. 6.3and 6.4. Sensory system
Somatic sensation utilizes the VPM and VPL nuclei, with the body represented in the VPL and the face represented in the VPM. The medial lemniscus body fibers, carrying light touch, vibration, and proprioception information from the arms, legs, and trunk project onto the VPL. The spinothalamic tract, carrying light touch, pain, and temperature sensation from the body also projects to the VPL. The facial portion of the medial lemniscus, as well as the trigeminothalamic tract (equivalent of the spinothalamic tract for the head), project to the VPM.
Special sensory systems
The medial geniculate nucleus receives auditory inputs from the brachium of the inferior colliculus and outputs to the auditory cortex in the superior temporal lobe. The lateral geniculate nucleus receives inputs from the optic tract, and outputs to the visual cortex in the occipital lobe.
The motor system
The VA and VL nuclei comprise the motor areas of the thala- mus. They receive inputs from the cerebellum via the denta- tothalamic tract (predominantly the VL nucleus) and from the basal ganglia via the thalamic fasciculus (predominantly the VA nucleus). The VA and VL project to the motor and premotor cortex. The VA projects more to the premotor cortical regions while the VL projects more prominently to the motor cortex.
Table 6.1. Thalamic nuclei and their connections
Type Name of nucleus Major subcortical input Major output
Specific relay Lateral geniculate Medial geniculate Ventral posterolateral Ventral posteromedial Ventral lateral, ventral
anterior (VL/VA) Anterior
Optic tract (retinal ganglion cells) Inferior brachium (inferior colliculus) Medial lemniscus, spinothalamic tract
Trigeminothalamic tracts (chief sensory/spinal nucleus) Cerebellum, basal ganglia
Mammillothalamic tract (mammillary body)
Viscual cortex Auditory cortex Somatosensory cortex Somatosensory cortex Motor/premotor cortex Cingulate gyrus Association Pulvinar Lateral posterior Lateral dorsal Dorsomedial
Retinal ganglion cells, superior colliculus Superior colliculus
Mixed
Amygdala, septal area, olfactory cortex
Parietal-occipital-temporal Association cortices Parietal association cortex Cingulate gyrus
Prefrontal cortex Non-specific Part of VA
Intralaminar
Various thalamic nuclei
Reticular formation, basal ganglia, cerebellum, somatosensory systems
Prefrontal cortex Collaterals to widespread
cortical areas
The limbic system
The relay nuclei for the limbic system are the anterior nucleus and the lateral dorsal nucleus. The anterior nucleus receives inputs from the mammillothalamic tract (coming from the hippocampus), and outputs to the cingulate gyrus. The lateral dorsal nucleus does not receive a well-defined input bundle, but also outputs to the cingulate gyrus, and is considered part of the limbic thalamus. These nuclei are thus involved in emotional behavior and memory and new learning.
Association nuclei
There are two main association nuclei in the thalamus, the dorsomedial (DM) nucleus, and the pulvinar. The lateral posterior nucleus is connected to the pulvinar, and appears functionally linked to it, and so these two nuclei are often referred to as the pulvinar/LT complex. The main subcortical inputs are shown in Fig. 6.3and the main cortical connections are shown inFig. 6.4.
In contrast to relay nuclei, association nuclei do not receive input bundles from modality-specific long tracts. Rather, they receive inputs from broad regions of cortex, some subcortical structures and other thalamic nuclei. Likewise, they do not output to a specific functional cortical area, such as the motor cortex or visual cortex, but rather to the broad regions of cortex known as the association cortex.
The two main regions of association cortex in the human brain are the prefrontal cortex, located in the frontal lobes ante- rior to the motor regions, and the parietal–occipital–temporal association cortex, adjacent to the sensory, visual, and auditory cortices. Each of these association areas is connected with a thalamic association nucleus.
The dorsomedial nucleus is connected with the prefrontal association cortex. It receives inputs both from the prefrontal cortex itself, as well as from the limbic system, including the
amygdala. It is involved in high-level cognitive function, emo- tional behavior and memory. As a side note, the surgical opera- tion called prefrontal lobotomy interrupted the pathway from the frontal association cortex to the dorsomedial nucleus.
The pulvinar/LP complex is connected with the parietal– occipital–temporal association cortex and receives reciprocal inputs from it. It also receives inputs from the lateral genicu- late and the superior colliculus. Thus, it is involved in visuos- patial processing. Interestingly, and not immediately clear from its pattern of neuronal connections, the pulvinar also seems to have a role in speech. Lesions in the dominant pulvi- nar have been associated with anomia.
Non-specific nuclei
The intralaminar and midline nuclei seem to be important in the functioning of the basal ganglia and the limbic system, but the details here are far from known. They have a very broad range of inputs, including the reticular activating system, the limbic system, and the basal ganglia, and they project back to these same areas as well as to the cerebral cortex. These broad, non-specific connections suggest that the intralaminar and midline nuclei may help regulate the level of cortical excitabil- ity. The centromedian nucleus, though, probably functions a little more specifically with the basal ganglia and the motor system.
The reticular nucleus covers the outer surface of the thala- mus. Inputs and outputs connecting the thalamus to various areas of cortex give off collaterals to the overlying portion of the reticular nucleus. The reticular nucleus, in turn, projects back to those areas of the thalamus. For example, the area of the reticular nucleus overlying the VPL receives collaterals from the VPL and the sensory cortex, and projects back to the VPL. Thus, the reticular nucleus is the one thalamic nucleus Fig. 6.3. Non-cortical inputs into the thalamic nuclei. Anterior nucleus (AN), reticular nucleus (R), ventral anterior nucleus (VA), ventral lateral nucleus (VLo/VLc), lateral dorsal nucleus (LD), dorsal medial nucleus (DM), lateral posterior nucleus (LP), ventral posterolateral nucleus (VPL), ventral posteromedial nucleus (VPM), centromedian nucleus (CM), intralaminar nuclei (Int).
which has no outputs going to the cortex. It probably helps to control thalamic output, and seems important in regulating and modulating the output of the thalamus to the cortex dur- ing different phases of sleep.