The spinothalamic tract also projects to other thalamic regions, including the posterior nucleus and the central lateral nucleus of the intralaminar complex of the thalamus. The intralaminar nuclei of the thalamus are not somatotopically organized, and they project diffusely to the cerebral cortex as well as to the basal ganglia (see ). The projection of the central lateral nucleus to the S-I cortex may be involved in arousal of this part of the cortex and in selective attention.
Third-order sensory neurons in the thalamus project to the somatosensory cortex. The details of this projection pattern are shown in
Fig. 7.5 . The main somatosensory receiving
CHAPTER 7 The Somatosensory System areas of the cortex are called the S-I and S-II areas. The S-I cortex (or primary somatosensory cortex) is located on the postcentral gyrus, and the S-II cortex (secondary somatosensory cortex) is in the superior bank of the lateral fissure (see
Fig. 7.5
As previously discussed, the S-I cortex, like the somatosensory thalamus, has a somatotopic organization. The S-II cortex also contains a somatotopic map, as do several other less understood areas of the cortex. In the S-I cortex the face is represented in the lateral part of the postcentral gyrus, above the lateral fissure. The hand and the rest of the upper extremity are represented in the dorsolateral part of the postcentral gyrus, and the lower extremity on the medial surface of the hemisphere. A map of the surface of the body and face of a human on the postcentral gyrus is called a sensory homunculus. The map is distorted because the volume of neural tissue devoted to a body region is proportional to the density of its innervation. Thus in humans, the perioral area, the thumb, and other digits take up a disproportionately large expanse of cortex relative to their size.
The sensory homunculus is an expression of place coding of somatosensory information. A locus in the S-I cortex encodes the location of a somatosensory stimulus on the surface of the body or face. For example, the brain knows that a certain part of the body has been stimulated because certain neurons in the postcentral gyrus are activated.
The S-I cortex has several morphological and functional subdivisions, and each subdivision has a somatotopic map. These subdivisions were originally described by Brodmann, and they were based on the arrangements of neurons in the various layers of the cortex, as seen in Nissl-stained preparations. The subdivisions are therefore known as Brodmann areas 3a, 3b, 1, and 2 (see ). Cutaneous input dominates in areas 3b and 1, whereas muscle and joint input (proprioceptive) dominates in areas 3a and 2. Thus separate cortical zones are specialized for processing tactile and proprioceptive information (see
Fig. 7.5
Within any particular area of the S-I cortex, all the neurons along a line perpendicular to the cortical surface have similar response properties and receptive fields. The S-I cortex is thus said to have a columnar organization. A comparable columnar organization has also been demonstrated for other primary sensory receiving areas, including the primary visual and auditory cortices (see ). Nearby cortical columns in the S-I cortex may process information for different sensory modalities. For example, the cutaneous information that reaches one cortical column in area 3b may come from FA mechanoreceptors, whereas the information that reaches a neighboring column might originate from SA mechanoreceptors.
Besides being responsible for the initial processing of somatosensory information, the S-I cortex also begins higher-order processing such as feature extraction. For example, certain neurons in area 1 respond preferentially to a stimulus that moves in one direction across the receptive field but not in the opposite direction (Fig. 7.6 ). Such neurons presumably contribute to the perceptual ability to recognize the direction of an applied stimulus and could help detect slippage of an object being grasped by the hand.