History And Definitions

The direct relation of somatic sensation to the nervous system was first put forth by Herophilus of Chalcedon in the third century bc. Through careful dissections he identified nerves as structures directly connected to the brain and spinal cord and attributed sensation and motor function to them. y The concept of the subdivision of somatic sensation into modalities was solidified by the specificity theory of Muller, who proposed that the stimulation of certain primary afferent fibers yielded a specific modality of sensation and that these specific nerve fibers were tuned to respond to only certain stimuli. y

The specificity theory's greatest challenge came from the experimental findings of Lele and Weddell, who noted that free nerve endings in the cornea produced sensations of more than one modality. y Subsequently, many subtypes of mechanoreceptors were shown to respond to various types of stimuli. [4 However, the elegant intraneural microstimulation studies of Ochoa and Torebjork have demonstrated that an elementary sensation can be reliably elicited by stimulation of a single mechanoreceptive unit.y Most likely, modality specificity exists at the most fundamental levels of the sensory unit at threshold levels of stimulation, but suprathreshold levels of stimulation activate receptors nonspecifically.

Because the sensory modalities of proprioception, vibration, and touch share neurophysiology, clinical concurrence, and some anatomy beyond the level of the mechanoreceptors, these sensations are considered together.

Proprioception is any postural, positional, or kinetic information provided to the central nervous system by sensory receptors in muscles, tendons, joints, or skin; touch is the sensory modality concerned with the ability to perceive superficial, non-noxious stimulation of the skin; and vibration, which is sometimes referred to as pallesthesia, is the ability to perceive sinusoidal rhythmic stimulation on both a superficial and deep level.

Alterations in sensation are often spoken of in terms of hyperesthesia, hypesthesia, and paresthesia. Hyperesthesia is an exaggeration of any sensory modality response. Hyperesthesia is distinct from hyperpilaphesie, which represents augmentation of tactile faculties in response to other sensory deprivation (e.g., touch in the blind). Hypesthesia represents a diminution of any sensory modality; however, it is most frequently used in the discussion of painful and tactile stimulation. Paresthesia reflects a perversion of sensation, producing a perception that is abnormal in

character (e.g., tingling on tactile stimulation) or abnormal sensations that an individual experiences in the absence of stimulation, including feelings of tingling, burning, prickling, crawling, and so on. Paresthesia has also been applied in a general sense to refer to sensations of a purely subjective nature. y

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