The principal pathways to the cerebral cortex for touch, vibratory sense, and proprioception are shown in Figure 1. Fibers mediating these sensations ascend ipsilaterally in the dorsal columns of the spinal cord to the medulla, where they synapse in the gracilus and cuneate nuclei. The second-order neurons from these nuclei cross the midline and ascend in the medial lemniscus to end in the contralateral ventral posterior lateral (VPL) nucleus and related specific sensory relay nuclei of the thalamus. This ascending system is called the dorsal column or medial lemniscal system. The fibers within the dorsal column pathway are joined in the brainstem by fibers mediating sensation from the head. Touch and proprioception from the head are relayed mostly via the main sensory and mesencephalic nuclei of the trigeminal nerve.
Fig1. Ascending tracts carrying sensory information from peripheral receptors to the cerebral cortex. A) Dorsal column pathway mediates touch, vibratory sense, and proprioception. Sensory fibers ascend ipsilaterally via the spinal dorsal columns to medullary gracilus and cuneate nuclei; from there the fibers cross the midline and ascend in the medial lemniscus to the contralateral thalamic ventral posterior lateral (VPL) and then to the primary somatosensory cortex. B) Ventrolateral spinothalamic tract mediates pain and temperature. These sensory fibers terminate in the dorsal horn and projections from there cross the midline and ascend in the ventrolateral quadrant of the spinal cord to the VPL and then to the primary somatosensory cortex.
Somatotopic Organization
Within the dorsal columns, fibers arising from different levels of the cord are somatotopically organized (Figure 1). Specifically, fibers from the sacral cord are positioned most medially and those from the cervical cord are positioned most laterally. This arrangement continues in the medulla with lower body (eg, foot) representation in the gracilus nucleus and upper body (eg, finger) representation in cuneate nucleus. The medial lemniscus is organized dorsal to ventral representing from neck to foot.
Somatotopic organization continues through the thalamus and cortex. The VPL thalamic neurons carrying sensory information project in a highly specific way to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe (Figure2). The arrangement of projections to this region is such that the parts of the body are represented in order along the postcentral gyrus, with the legs on top and the head at the foot of the gyrus. Not only is there detailed localization of the fibers from the various parts of the body in the postcentral gyrus, but also the size of the cortical receiving area for impulses from a particular part of the body is proportional to the use of the part. The relative sizes of the cortical receiving areas are shown dramatically in Figure3, in which the pro portions of the homunculus have been distorted to correspond to the size of the cortical receiving areas for each. Note that the cortical areas for sensation from the trunk and back are small, whereas very large areas are concerned with impulses from the hand and the parts of the mouth concerned with speech.
Fig2. A lateral view of the left hemisphere showing some principal cortical areas and their functional correlates in the human brain. The primary somatosensory area is in the postcentral gyrus of the parietal lobe, and the primary motor cortex is in the precentral gyrus. (Reproduced with permission from Waxman SG: Clinical Neuroanatomy, 26th ed. New York, NY: McGraw-Hill; 2010.)
Fig3. Sensory homunculus, drawn overlying a coronal section through the postcentral gyrus. The parts of the body are represented in order along the postcentral gyrus, with the legs on top and the head at the foot of the gyrus. The size of the cortical receiving area for impulses from a particular part of the body is proportionate to the use of the part. Gen., genitalia.
Studies of the sensory receiving area emphasize the very discrete nature of the point-for-point localization of peripheral areas in the cortex and provide further evidence for the general validity of the law of specific nerve energies. Stimulation of the various parts of the postcentral gyrus gives rise to sensations projected to appropriate parts of the body. The sensations produced are usually numbness, tingling, or a sense of movement, but with fine enough electrodes it has been possible to produce relatively pure sensations of touch, warmth, and cold. The cells in the postcentral gyrus are organized in vertical columns. The cells in a given column are all activated by afferents from a given part of the body, and all respond to the same sensory modality.
In addition to the primary somatosensory cortex, there are two other cortical regions that contribute to the integration of sensory information. The sensory association area is located in the parietal cortex and the secondary somatosensory cortex is located in the wall of the lateral fissure (also called sylvian fissure) that separates the temporal from the frontal and parietal lobes. These regions receive input from the primary somatosensory cortex.
Conscious awareness of the positions of the various parts of the body in space depends in part on impulses from sensory receptors in and around the joints. Impulses from these receptors, from touch receptors in the skin and other tissues, and from muscle spindles are synthesized in the cortex into a conscious picture of the position of the body in space.
