Overview
This lecture covers the organization and function of the primary and secondary somatosensory cortices, their roles in processing sensory input, and the importance of somatotopic maps in perception and localization.
Primary Somatosensory Cortex (S1)
- Areas 1, 2, and 3 are considered the primary somatosensory cortex (S1).
- S1 receives sensory input from the thalamus via the VPN and VPL nuclei.
- Touch and pain sensations cross the midline and ascend to the thalamus before reaching S1.
- S1 localizes sensations (touch, pain, pressure) to specific body regions.
- S1 neurons are sensitive to light touch; areas 1 and 2 process more complex features like limb position and object shape.
- Electrical stimulation of S1 causes tingling or numbness in the contralateral body part, rarely pain.
- Lesions in S1 impair fine sensory discrimination and position sense but do not abolish touch or pain sensation.
- S1 is crucial for localizing sensory input, but the emotional quality of pain involves other brain areas.
Secondary Somatosensory Cortex (S2)
- S2 is mainly located in the parietal operculum (lid over the Sylvian fissure).
- S2 also contains a somatotopic map, but the sequence runs from face to arm to leg (lateral to medial).
- S2 receptive fields tend to be bilateral, activating with touch to similar spots on both sides of the body.
- S2 is involved in stereognosis (recognizing objects by touch).
- Lesions to S2 can cause astereognosis, an inability to identify objects by touch.
Somatotopic Organization and Homunculus
- Both S1 and S2 have a somatotopic (body-mapped) organization called the homunculus.
- In S1, lower limb representation is medial, and the face is lateral; S2 reverses this order.
Key Terms & Definitions
- Primary Somatosensory Cortex (S1) — brain area (areas 1-3) processing and localizing touch and pain.
- Secondary Somatosensory Cortex (S2) — area involved in more complex touch processing and bilateral sensory integration.
- Somatotopic Map (Homunculus) — body map in the brain where specific regions correspond to body parts.
- Stereognosis — ability to identify objects by touch.
- Astereognosis — loss of the ability to identify objects by touch.
Action Items / Next Steps
- Practice identifying coins or objects by touch to experience stereognosis.
- Review the somatotopic organization of the primary and secondary somatosensory cortices.