The posterior nucleus and central lateral nucleus of the intralaminar complex of the thalamus are also targeted by the spinothalamic tract. These intralaminar nuclei do not possess a somatotopic organization, and they project diffusely to both the cerebral cortex and the basal ganglia. The central lateral nucleus's projection to the S-I cortex is thought to play a role in arousing this region of the cortex and in selective attention.

Third-order sensory neurons in the thalamus project to the somatosensory cortex, specifically the S-I and S-II areas. The S-I cortex, also known as the primary somatosensory cortex, is located on the postcentral gyrus, while the S-II cortex, or secondary somatosensory cortex, can be found in the superior bank of the lateral fissure. Both these areas, similar to the somatosensory thalamus, exhibit a somatotopic organization. The face is represented in the lateral part of the postcentral gyrus in the S-I cortex, whereas the dorsolateral part of the postcentral gyrus represents the hand and the rest of the upper extremity. The lower extremity is represented on the medial surface of the hemisphere. This representation of the body and face on the postcentral gyrus is known as a sensory homunculus and is distorted due to the varying densities of innervation. For instance, the perioral area, thumb, and other digits occupy a disproportionately large area of the cortex relative to their size.

The S-I cortex utilizes place coding to determine the location of a somatosensory stimulus on the body or face. Activation of specific neurons in the postcentral gyrus signifies stimulation of a particular body part. The S-I cortex consists of different subdivisions, referred to as Brodmann areas 3a, 3b, 1, and 2, based on the arrangement of neurons in the cortex layers. Areas 3b and 1 primarily process cutaneous input, while areas 3a and 2 specialize in muscle and joint input (proprioception). Hence, separate cortical zones are dedicated to processing tactile and proprioceptive information. The S-I cortex exhibits a columnar organization, wherein neurons along a perpendicular line to the cortical surface possess similar response properties and receptive fields. Similar columnar organization has been observed in other primary sensory receiving areas such as the primary visual and auditory cortices. Adjacent cortical columns within the S-I cortex may process information from different sensory modalities. For example, one column in area 3b may receive cutaneous information from FA mechanoreceptors, while a neighboring column could receive information from SA mechanoreceptors.

Apart from initial processing of somatosensory information, the S-I cortex also engages in higher-order processing like feature extraction. Certain neurons in area 1 respond preferentially to stimuli moving in one direction across the receptive field but not in the opposite direction. These neurons likely contribute to the ability to perceive the direction of a stimulus and may aid in detecting slippage of an object grasped by the hand.
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.