6.07 - The Somatosensory Thalamus and Associated Pathways

Abstract

This chapter focuses on the early stages of the large part of the somatosensory system that is concerned with tactile sensations and perception. To a great extent, these abilities depend on afferent pathways from low-threshold mechanoreceptors in the skin for information about skin contact and vibration, and receptors in muscles and joints (and probably skin) for information about the relative positions of body parts. Branches of these afferent pathways reach nuclei in the dorsal column–trigeminal complex in the cervical spinal cord and lower brainstem to relay to the contralateral ventroposterior complex in the thalamus. Neurons activated by receptors in the mouth and on parts of the face project to the ipsilateral as well as the contralateral thalamus. The ventroposterior complex of primates includes a ventroposterior proper nucleus, composed of ventroposterior medial division and ventroposterior lateral division. Ventroposterior nucleus (VP) of monkeys is activated by slowly adapting and rapidly adapting classes of cutaneous mechanoreceptors. This information is relayed to areas 3b and 1, and, to a limited extent, area 2 of the anterior parietal cortex. Area 3b corresponds to S1 of nonprimates such as rats and cats, where both S1 and S2 receive inputs from VP. Ventroposterior superior nucleus (VPS), which has only been identified as such in primates, receives inputs relayed from muscle spindle and probably joint receptors, and projects to areas 3a and 2 of the anterior parietal cortex. A group of cells along the dorsorostral margin of VP responds to muscle spindle receptor stimulation in other mammals such as cats and rats, with this group of cells variously recognized as a distinct nucleus or a part of the VP. These neurons project to a strip of the cortex along the rostral border of S1 variously described as area 3a or the dysgranular cortex. Ventroposterior inferior nucleus (VPI) of primates receives inputs from the spinothalamic subsystem that includes a relay of wide dynamic range afferent information, and this information is relayed most densely to second somatosensory (S2) and parietal ventral area (PV). VPI also projects less densely to superficial cortical layers in other somatosensory areas (3a, 3b, 1, and 2). VPI is seldom recognized in nonprimate mammals, as it is often poorly developed, but VPI has been identified in cats and raccoons, where it also projects densely to S2. A VPI is apparent in squirrels, where a projection to PV has been described, but VPI is at best a thin layer of cells on the ventral surface of VP in rats. VPI projections appear to provide a modulating influence on the responses of cortical neurons that are activated by the thalamic relay of information from cutaneous and deep receptors in VP and VPS. Information from cutaneous receptors about skin indentations and from deep receptors about position sense is combined in higher-order somatosensory areas to allow objects to be identified by shape and texture during active touch.