Thalamic Projections To Subcortical Regions And Somatosensory Cortex

Two anatomically smaller but nonetheless important elements of the ascending sensory system do not project to cortical regions and therefore do not contribute to conscious perception. First, many fibers carrying pro-prioceptive information do not travel with the dorsal column-medial lemniscal pathway but rather project in the lateral columns directly to the ipsilateral cerebellum. The cerebellum uses proprioceptive information to modulate movement of muscles in the extremities.

Another important ascending system forms the afferent arm of protective reflex pathways that operate on the subconscious level. As they ascend to the thalamus, all spinal and cranial sensory tracts send collateral branches to the brain stem reticular formation, a diffuse network of neurons within the core of the brain stem. Various nuclear groups within the reticular formation function as coordinating centers that extract relevant sensory information and in turn activate appropriate muscle groups through their motor axons. Different regions of the reticular formation have been identified that elicit the cough reflex, blink reflex, and gag reflex. Respiratory and cardiovascular centers within the reticular formation regulate breathing rate and heart rate.

Sensations are not perceived until sensory information reaches the primary sensory cortex. The post-central gyrus (area SI) acts as the primary receiving area for somatosensory projections from the thalamus, and, like most cortical regions, it has an elaborate organizational framework with several major components:

1. Areas that receive input from one submodality are arranged in four succeeding cortical strips that run parallel to the central sulcus: area 3a (most anterior), muscle stretch receptors; area 3b, cutaneous receptors; area 1, rapidly adapting cutaneous receptors; area 2 (most posterior), deep pressure receptors.

2. Information from different parts of the body project to different parts of each cortical strip. Thus, each strip contains an orderly representation of the body called a homunculus (Latin for "little man'') that is derived from the location of the receptive fields of the input neurons. The homunculus is not true to scale, however, and certain areas, such as lips and fingertips, are grossly overrepresented. The high density of sensory receptors in these locations requires proportionally large cortical receiving areas. Experiments in primates have recently demonstrated that the homunculus is not invariant, and its boundaries shift slightly in response to the level of activity within each set of sensory fibers. Concert pianists might be expected to have disproportionately large fingertip receiving areas.

3. Each homuncular area is subdivided further into functional modules or columns that lie perpendicular to the cortical surface. All the cells within a vertical column have similar function; for example, columns of slowly adapting cells are separated from columns of rapidly adapting cells.

4. The cortex is layered like a cake, with each layer containing functional aggregates of cells and fibers. Input fibers from the thalamus synapse on cells in layer 4. Layer 6 contains fibers that feed back to the thalamus. Fibers projecting to other subcortical structures are present in layer 5. More superficial layers contain fibers that project to other association areas of the cortex.

The modular organization of the primary somatosen-sory cortex exemplifies one of the major organizing principles of the brain (Fig. 6). Neurons that have similar functions occupy similar positions. Principles of modular organization apply to most sensory association areas, including secondary somatosensory cortex (SII) located at the lateral tip of SI and even higher association areas found in the lower part of the parietal lobe. However, functional attributes of the cells within the modules change in succeeding stages of information processing. Through convergence of input from SI cells, the neurons in SII display larger receptive fields and more complex response properties. Input from several different modalities also converges on a single module at higher levels, so that separate aspects of a sensory stimulus can come together as a unified perception of an object.

Suggested Readings

Dubner R, Bennett GJ. Spinal and trigeminal mechanisms of nociception. Annu Rev Neurosci 1983; 6:381-418.

Garner EP, Hamalainen HA, Palmer CI, Warren S. Touching the outside world: representation of the motion and direction within primary somatosensory cortex, In: Lund JS, Ed., Sensory processing in the mammalian brain: neuron substrates and experimental strategies. New York: Oxford University Press, 1989, pp. 49-66.

Willis WD, Coggeshall R. Sensory mechanisms of the spinal cord, 2nd ed., New York: Plenum Press, 1991.

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