Answer
The nervous system has many more sensory fibers and sensory pathways than motor fibers. This stems from the importance of properly understanding the environment prior to responding. These ascending tracts provide the bridge between the inputs from the environment and the organizing centers of the brain that provide the complexity of our responses. Somatosensory systems include the receptors and pathways for transmission of sensory information from the soma to the portions of the brain that need to integrate it and act upon it. While much of this is conscious, there are also ascending systems that convey unconscious information involved in coordination (proprioception) and brain stem reflexes.
There are several different modalities that fall under the broad topic of "exteroception". These sensations include well-localized touch 2-point discrimination), light touch, pain, temperature and vibration sense. These sensations can be tested clinically. Proprioception is the ability to detect the position of the body in space. This may be consciously perceived, such as with joint position sense, or it may be a sensation that is not perceived consciously, such as from muscles and ligaments.
Conscious sensation
We will first discuss the pathways for conscious perception before considering unconscious sensation. These pathways for conscious somatic sensation, at their simplest, require three neurons (and two relay sites) from the periphery all the way to termination the cerebral cortex. These steps are often described as first, second and third order neurons in the sensory pathway. The signal can be modified at each of the relay points (nuclei).
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