ABSTRACT
The ascending pathways convey conscious (cortical) and unconscious (subcortical) sensory information to the higher levels of the central nervous system. These pathways are concerned with transmission of a variety of sensory modalities, regulation of muscle tone and mediation of intersegmental reflexes. They may exhibit monosynaptic connections or utilize an extensive network of neurons. The modalities of the general somatic sensations include pain, temperature, tactile, joint, vibration, and pressure sensations. These modalities are categorized into epicritic (discriminative) and protopathic sensations. The epicritic (discriminative) modalities include sensations such as fine touch, two-point discrimination (the ability to distinguish two blunt points from one another), joint sensation, and vibratory sense. These sensations are received by the encapsulated receptors and transmitted by the thickly myelinated and fast conducting fibers. The protopathic sensations, which include pain, temperature, and crude touch, are received by the free nerve endings (uncapsulated receptors) and conveyed to the spinal cord by small, thinly myelinated and/or unmyelinated fibers. These fibers represent the peripheral processes of the pseudounipolar neurons of the dorsal root ganglia (DRG), which are contained in the peripheral nerves. Visceral pain also projects to the cerebral cortex; however, its peripheral transmission is maintained by fibers that predominantly accompany the sympathetic fibers. The central transmission of this modality is identical to the somatic sensations. Subcortical sensations emanate from Golgi tendon organs, muscle spindle, and other exteroceptive receptors, conveying information to the cerebellum, inferior olivary nucleus, and tectum via the spinocerebellar, trigeminocerebellar, spino-olivary, and spinotectal tracts.
