meissner, merkel, or pacini unit may produce a distinct and differential percept in the mind of the subject indicating an absolute specificity within the tactile system. it has even been demonstrated that a single impulse in a meissner afferent may produce a percept. in contrast, no percept is reported when a single ruffini afferent is stimulated which might indicate that spatial summation is required. consistent with the perceptive findings, neural responses in the somatosensory cortex have been recorded on micro-stimulation of single afferents connected to meissner, merkel, pacini endings but not with single ruffini afferents. on the basis of collateral studies in man and monkey a very tight match has been claimed between magnitude estimation of sensation of skin deformation, on the one hand, and response of merkel afferents in the monkey, on the other. in man, deviations from such a linear relation was found in combined psychophysical and microneurography recordings. in the hairy skin meissner units are lacking altogether. instead there are hair follicle and field afferents which have large receptive fields while merkel, pacini, and ruffini are present. cutaneous ruffini units in the hairy skin are important for position sense and kinesthesia as pointed out in another section. a caveat is justified with regard to end organ morphology. the four kinds of units considered above were physiologically identified in man (fa/ra and sa units, i.e. fast and slowly adapting type i and type ii, ) whereas end organ morphology has been inferred on the basis of animal studies. particularly, it seems likely that saii afferents may be connected to other morphological structures than the classical ruffini ending. affective touch light touch is coded not only in large myelinated afferents but in small unmyelinated afferents as well. tactile c-afferents (ct) were described long ago in non-human species but did not attract much interest until it was shown that they are numerous in human hairy skin. in contrast, they are lacking altogether in glabrous skin. a number of findings from both normal subjects and from unique patients lacking large tactile afferents indicate that ct afferents are essential for the pleasurable aspect of friendly touch. particularly, ct afferents respond vigorously to slow caressing movements, and, importantly, the size of the afferent response matches the sense of pleasure reported by the subject. fmri studies of brain activity indicate that ct activate the insular cortex but not the primary or secondary somatosensory cortex consistent with the hypothesis that ct may play a role in emotional, behavioral, and hormonal responses to pleasant skin-to-skin contact between individuals. tactile afferents in motor control it has been shown that tactile afferents from the glabrous skin of the hand exert profound effects on hand and finger muscles in the subconscious control of grip force whenever we lift and manipulate objects. the friction between skin and object surface is extracted as soon as your fingers close around the object and contraction force of the muscles gripping the object is adjusted accordingly. moreover, any tendency to slipping is monitored by tactile afferents and gives rise to swift reflexes resulting in subconscious adjustments of motor output. many forms of dexterous handling of objects include successive phases of different motor activity. it has been shown that tactile sense organs in the glabrous skin are involved in timely linking the separated phases to a purposeful motor act. pain related afferents afferents responding to noxious stimuli are known as nociceptors. there are 2 main groups, unmyelinated c afferents and small myelinated aδ fibers. most studies are focused on c nociceptors. the nociceptive c-fibers constitute a very large proportion of somatic afferent nerve fibers. there are two main groups: mechano-sensitive and mechano-insensitive c nociceptors. mechano-sensitive c nociceptors, also known as polymodal c nociceptors are activated by several kinds of stimuli, i.e. mechanical, thermal, and chemical. the mechano-insensitive c nociceptors, also known as silent nociceptors, differ from polymodal afferents in other respects as well, e.g. they do not respond to heat or they have very high heat thresholds, receptive fields on the skin are larger, conduction velocity is slower, and activity-dependent slowing of conduction velocity of the axon is more pronounced. the mechano-insensitive nociceptors may be sensitized particularly by inflammatory mediators to render them mechano-responsive, a process that may account for the tenderness we experience following a physical injury. moreover, electrical activation of c-mechano-insensitive fibers demonstrates that they have a role in neurogenic vasodilation which has not been found with polymodal nociceptors. it is suspected that the inflammatory mediators bind to protein receptors on mechano-insensitive nociceptors, but sensitization may also be caused by changes in gene expression that affect expression of transduction proteins. in either case, the sensitization of mechano-insensitive nociceptors has been observed to result in hyperalgesia, chronic pain. about ten percent of the afferents classified as mechano-insensitive nociceptors seem to constitute a group of “itch specific” units because they respond to pruritogen substances including histamine with an activity that corresponds to the sensation of itch. temperature sensibility thermoreceptors can be separated into two groups for warmth and cold detection. a subset of unmyelinated fibers are responsible for warmth detection. they are mechano-insensitive, low in number, and innervate small receptive fields. aδ fibers are responsible for cold detection. however, there seems to be a subset of c-fibers that may function as cold-receptors along with a-fibers. remarkably, these c-cold fibers seem to produce a sensation of unpleasant heat when there is no input from a-fibers. altogether thermoreceptive afferents have not been studied as much as other systems. autonomic efferent activity spinal nerve sympathetic ganglion multilingual microneurography exploration of sympathetic efferent system is unique from technical point of view as multiunit recordings have been very prosperous whereas single unit recording is essential with most other systems. soon after microneurography was launched it was demonstrated that sympathetic activity is much different in muscle and skin nerves. instantaneous sympathetic activity in muscle nerves (msa / msna) is heavily controlled by baroreflex mechanisms, resulting in a characteristic cardiac rhythmicity as well as a close and inverse relation to the small variations of blood pressure that normally occur continuously in phase with respiration. in contrast, the sympathetic activity in skin nerves (ssa/ssna) lacks a tight relation to cardiac and respiratory events. on the other hand, sympathetic activity in skin nerves is dependent on a number of other mechanisms because changes are easily evoked e.g. by arousal, emotions, and ambient temperature changes, which stimuli are not effective with efferents in muscle nerves. these and other findings demonstrate that sympathetic efferent activity is highly differentiated, as individual effectors are governed by their own control systems and specific reflexes. the amount of muscular sympathetic activity, measured as number of bursts per 100 heart beats, varies considerably between subjects but, on the other hand, it is highly reproducible over time within the individual subject. however, there is some increase with age. counter-intuitively, there seems to be only a weak and barely significant correlation between sympathetic efferent activity and hypertension as found in group studies. in 1998, microneurography recordings were performed for the first time on a spaceflight aboard the space shuttle columbia with the purpose to explore the effect of microgravity on the human sympathetic nerve system. two astronauts measured msna from peroneal nerves of their fellow astronauts. the findings support earlier observations that weightlessness results in a decrease of msna activity through a baroreflex mechanism. strengths and limitations the microneurography technique allows the recording of impulse activity of individual nerve fibers with absolute resolution in attending human subjects. hence the subject is able to cooperate in various kinds of tests while the exact and complete information carried by the individual nerve fiber is monitored and offered for analysis of correlations between neural activity and physical or mental events. on the other hand, the particular physical conditions involving a microelectrode freely floating in the tissue preclude brisk and large movements because the exact electrode position is easily jeopardized. the experiment can be time-consuming when targeting deeply located nerves because the search procedure can be particularly demanding. however, accessing superficial subcutaneous nerves such as the superficial peroneal nerve on the dorsum of the foot is very rapid. this has permitted that the technique that was initially felt to be unsuited as a diagnostic test can now be applied for clinical purposes. microneurography strength is in its unique power for exploration of normal neural mechanisms as well as pathophysiological conditions of various neurological disorders. microneurography records intact axons in vivo and is minimally invasive. there have been no reports of persistent nerve damage. as a result, repeated recordings with the same subject are possible and longitudinal observations can be made. the technique has recently been used in clinical trials of new anti-neuropathic pain agents. during the recording, it is important to create an atmosphere of psychological confidence and to observe carefully the subject´s reactions so that the procedure can be adjusted accordingly. the technique requires training and skill and it is highly recommended to take up the method in an experienced laboratory where the method is regularly used.