Notes

• Two major areas in primates
  • SI - on posterior bank of central sulcus and crown of post-central gyrus.
  • SII - on lip and upper bank of lateral fissure.
• SI contains 4 distinct areas (3a, 3b, 1, and 2) with different characteristics and functions:
  • Areas 3a and 3b receive most of the direct projections from the thalamus and in turn project to areas 1 and 2.
  • Areas 3b and 1 receive input from skin receptors.
  • Areas 3a and 2 receive input from joint and muscle receptors.
• Effects of cortical lesions:
  • Large lesions of all of SI produce deficits in position sense and in discrimination of size, texture and shape. They also produce motor deficits in hand function. (Link between somatosensory information and motor-commands).
  • Area 3b lesions result in severe deficits in tactile discrimination of texture, size and shape.
  • Area 1 lesions affect texture but not size discrimination.
  • Area 2 lesions affect size and shape but not texture discrimination.

Cortical Receptive Fields

• Receptive fields in area 3b, like those in the thalamus, are larger than those of DRG neurons and typically have excitatory centre - inhibitory surround structure.
• Receptive fields in "higher" cortical areas are larger than those in 3b, and many are much more complex (e.g., sensitive to the orientation of an edge (cf. visual cortex), the direction of movement across the skin, or the surface curvature of objects).
• Some neurons in area 2 are selective for the 3-dimensional shape of objects.

Cortical Somatotopy

• Human somatotopy - classic map ("homunculus") based on cortical stimulation (produced by Penfield).
• Distorted nature of homunculus.
• Differences between species (enlarged representation of face and facial vibrissae in rodents (the "musunculus")).
• Neurophysiological evidence in monkeys indicates that each of the 4 areas comprising SI is somatotopically organized (i.e., a "full homunculus" for each one of these regions).
• Note that multiple representations of the receptor surface are a common feature of all sensory cortices.
• Note that representations are commonly "mirror-reversed" across field boundaries.

Columnar Organization

• Neurons in a "column" running orthogonal to the cortical surface and through the cortical layers have very similar RFs and modality properties (including adaptation rate).
• A cortical column is approx. 300 - 600 μm in diameter.
• Columnar organization was first described in detail by Mountcastle and his colleagues in somatosensory cortex and is now recognized as one of the fundamental principles of the organization of sensory cortex.