There are three elements of proprioception with separate peripheral receptor representation: (1) the perception of limb movement is mediated by muscle spindle receptors, cutaneous mechanoreceptors, and joint receptors; (2) the perception of limb position is mediated by muscle spindle receptors and cutaneous mechanoreceptors; and (3) the perception of force of muscular contraction is mediated by corollary discharges and tendon organ receptors. y Specific mechanoreceptors that have been implicated in proprioception include SA II cutaneous afferents that respond to planar stretch of the skin and muscle spindle endings that respond to muscle stretch. Neither of these receptors evokes a coherent sensation when stimulated in isolation. This suggests that spatial summation among various receptors is critical for the perception of position sense. [iei
Proprioception is transmitted through heavily myelinated nerve fibers. The cell body of the first-order neuron is in the DRG and goes without synapsing into the ipsilateral funiculus gracilis or cuneatus to synapse in the nucleus gracilis or cuneatus within the medulla. As these fibers travel in the dorsal columns they are topographically localized between the fibers transmitting vibration and those transmitting discriminative touch in the intermediate region of the dorsal columns. The second-order neurons are located within the gracilis and cuneatus nuclei. The third-order neurons are located in the VPL nucleus of the thalamus following the course of the dorsal column nuclei to the somatosensory cortex, as previously mentioned. The sensory cortex gets very precise information on the position and movements of the joints.
Pacinian corpuscles (PC) are the mechanoreceptors responsible for the sensory transduction of vibratory sensation. These receptors are excited by very rapid changes in tissue distortion such as that produced by sinusoidal vibration. y When the skin is locally anesthetized by cocaine, there is a manyfold elevation in threshold for frequencies in the range of 5 to 40 Hz whereas that for higher frequencies is scarcely unchanged. This suggests that the perception of vibration depends on two separate primary afferents: one innervating the skin and the other innervating the deep tissues. y Detection of vibration of the frequency range of 5 to 40 Hz probably depends more on RA transduction, whereas PC units account for detection at higher frequencies of vibration. y As stimulus intensity is increased, PC fibers may initially discharge irregularly or at some submultiple of the stimulus frequency. With slight increments, however, the point of phase locking between stimulus and firing of afferent nerve fiber is quickly reached. [8
Vibratory stimuli are transmitted through large myelinated, group Abeta fibers through the dorsal columns to the
somatosensory cortex. Like proprioceptive fibers, the first-order neurons are in the DRG, second-order neurons in the cuneatus and gracilis nuclei, and third-order neurons in the VPL nucleus of the thalamus. With regard to the cortical representation of vibratory sensation, many years ago Holmes stated that the "appreciation of vibration is possible through the thalamus alone." y Although the thalamus may play a significant role in the appreciation of vibratory sensation, Mountcastle has clearly demonstrated activity in cortical neurons in response to vibratory stimulation and has localized the function of frequency discrimination to a cortical level. y
Discriminative or complex touch includes two-point discrimination, touch localization, direction of movement of an object drawn on the skin, stereognosis, and graphesthesia. The finest levels of surface structure are transduced by RA and PC afferent fibers. y It is most probable that these complex sensations result from a simultaneous stimulation of several types of receptors. The nerve fibers transmitting discriminative modalities have their first-order neuron origin in the DRG. These fibers enter the DRG through myelinated fibers in the medial division of the dorsal root. They follow an identical course to that of proprioceptive fibers to the somatosensory cortex by means of the dorsal columns. Within the dorsal columns these fibers occupy the most posterior region. y
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