Sensory integration in the perception of movements at the human metacarpophalangeal joint

Amplitude of muscle stretch modulates corticomotor gain during passive movement

Previous studies have shown that the excitability of corticomotor projections to forearm muscles exhibit phasic modulation during passive movement (flexion-extension) about the wrist joint. We examined the stimulus-response properties of flexor carpi radialis (FCR) and extensor carpi radialis (ECR) to transcranial magnetic stimulation (TMS) applied over the contralateral motor cortex while the wrist was moved passively at two different sinusoidal frequency-amplitude relationships. Movement velocity (and therefore, the rate of change in muscle length) at the time of stimulation was held constant. Motor evoked potential (MEP) amplitudes were facilitated during passive muscle shortening and suppressed during passive muscle lengthening with suppression being more evident at higher stimulation intensities. For both FCR and ECR, during the shortening phase, responses were facilitated during the large amplitude movement relative to the small amplitude movement. It is suggested that the altered gain may be related to the thixotropic properties of muscle.

Cutaneous stimulation from patella tape causes a differential increase in vasti muscle activity in people with patellofemoral pain

Patella taping reduces pain in individuals with patellofemoral pain (PFP), although the mechanism remains unclear. One possibility is that patella taping modifies vasti muscle activity via stimulation of cutaneous afferents. The aim of this study was to investigate the effect of stretching the skin over the patella on vasti muscle activity in people with PFP. Electromyographic activity (EMG) of individual motor units in vastus medialis obliquus (VMO) was recorded via a needle electrode and from surface electrodes placed over VMO and vastus lateralis (VL). A tape was applied to the skin directly over the patella and stretch was applied via the tape in three directions, while subjects maintained a gentle isometric knee extension effort at constant force. Recordings were made from five separate motor units in each direction. Stretch applied to the skin over the patella increased VMO surface EMG and was greatest with lateral stretch. There was no change in VL surface EMG activity. While there was no net increase in motor unit firing rate, it was increased in the majority of motor units during lateral stretch. Application of stretch to the skin over VMO via the tape can increase VMO activity, suggesting that cutaneous stimulation may be one mechanism by which patella taping produces a clinical effect.

Coding of position by simultaneously recorded sensory neurones in the cat dorsal root ganglion

Muscle, cutaneous and joint afferents continuously signal information about the position and movement of individual joints. How does the nervous system extract more global information, for example about the position of the foot in space? To study this question we used microelectrode arrays to record impulses simultaneously from up to 100 discriminable nerve cells in the L6 and L7 dorsal root ganglia (DRG) of the anaesthetized cat. When the hindlimb was displaced passively with a random trajectory, the firing rate of the neurones could be predicted from a linear sum of positions and velocities in Cartesian (x, y), polar or joint angular coordinates. The process could also be reversed to predict the kinematics of the limb from the firing rates of the neurones with an accuracy of 1-2 cm. Predictions of position and velocity could be combined to give an improved fit to limb position. Decoders trained using random movements successfully predicted cyclic movements and movements in which the limb was displaced from a central point to various positions in the periphery. A small number of highly informative neurones (6-8) could account for over 80% of the variance in position and a similar result was obtained in a realistic limb model. In conclusion, this work illustrates how populations of sensory receptors may encode a sense of limb position and how the firing of even a small number of neurones can be used to decode the position of the limb in space.

Tactile feedback contributes to consistency of finger movements during typing

Touch typing movements are typically too brief to use on-line feedback. Yet, previous studies have shown that blocking tactile feedback of the fingertip of typists leads to an increase in typing errors. To determine the contribution of tactile information to rapid fine motor skills, we analyzed kinematics of the right index finger during typing with and without tactile feedback. Twelve expert touch typists copy-typed sentences on a computer keyboard without vision of their hands or the computer screen. Following control trials, their right index fingertip was anesthetized, and sentences were typed again. The movements of the finger were recorded with an instrumented glove and electromagnetic position sensor. During anesthesia, typing errors of that finger increased sevenfold. While the inter-keypress timing and average kinematics were unaffected, there was an increase in variability of all measures. Regression analysis showed that endpoint variability was largely accounted for by start location variability. The results suggest that tactile cues provide information about the start location of the finger, which is necessary to perform typing movements accurately.

The detection of human finger movement is not facilitated by input from receptors in adjacent digits

These experiments were designed to determine whether cutaneous input from a digit provides a general facilitation of the detection of movements applied to an adjacent digit. The ability to detect passive movements at the proximal interphalangeal joint of the right index finger was measured when cutaneous (and joint) input was removed (using local anaesthesia) from the tip of one or both digits adjacent to the test finger (16 subjects). The same parameter was also measured when input was artificially increased by stimulation of the adjacent digits at three intensities: below, above and at perceptual threshold (PT; 15 subjects). Detection of flexion or extension movements was not altered by anaesthesia of one or both adjacent digits. Since it was possible that too few tonically active afferents in the hand had been blocked to reveal an effect, the median nerve was blocked, with movements applied to the little finger, causing no measurable impairment in acuity (three subjects). Simultaneous electrical stimulation of the tips of the adjacent digits at intensities above PT impaired movement detection, but had no effect when delivered at or below PT. To test whether the effect of detectable electrical stimuli was due to a specific interaction between the artificial input and the input evoked by moving the digit, or due to mental distraction, stimuli were delivered above PT to either the left or right little finger, or the test index finger during movement of the index finger. Electrical stimulation of the index finger significantly reduced detection by approximately 50%, but stimulation of the remote little fingers did not. Electrical stimulation is a non-natural stimulus, so a "natural" stimulus was applied by continuously stroking the tips of the adjacent digits with a brush (10 subjects). The natural stimulus also significantly reduced movement detection by approximately 50%. Together, these findings suggest that tonic inputs from digital nerve afferents adjacent to, or more remote from the passively moved finger do not facilitate movement detection. However, the reduced detection during stimulation of the adjacent digits shows that there is nevertheless some interaction between the various proprioceptive inputs from the digits.

Proprioception: peripheral inputs and perceptual interactions

Much emphasis has been placed on the specific role of specific inputs from muscle, joint and cutaneous afferents in the detection of movement. However, particularly for the hand, multiple inputs from the moving part are likely to be important. This chapter reviews some recent studies which examine the co-operative interaction between the various proprioceptive channels. Proprioceptive control of movement must also take account of the length of the various limb segments, a variable which is independent of muscle lengths and joint angles. Evidence is presented that body image can be affected by the tonic discharge of non-muscle receptors.

Neck muscle vibration and spatial orientation during stepping in place in humans

Unilateral long-lasting vibration was applied to the sternomastoid muscle to assess the influence of asymmetric neck proprioceptive input on body orientation during stepping-in-place. Blindfolded subjects performed 3 sequences of 3 trials, each lasting 60 s: control, vibration applied during stepping (VDS), and vibration applied before stepping (VBS). VDS caused clear-cut whole body rotation toward the side opposite to vibration. The body rotated around a vertical axis placed at about arm's length from the body. The rotation did not begin immediately on switching on the vibrator. The delay varied from subject to subject from a few seconds to about 10 s. Once initiated, the angular velocity of rotation was remarkably constant (about 1 degrees /s). In VBS, at the beginning of stepping, subjects rotated for a while as if their neck were still vibrated. At a variable delay, the direction of rotation reversed, and the effects were opposite to those observed during VDS. Under no condition did head rotation, head roll, or lateral body tilt accompany rotation. The results confirm and extend the notion that the neck proprioceptive input plays a major role in body orientation during locomotion. The body rotation does not seem to depend on the same mechanisms that modify the erect posture; rather, the asymmetric neck input would seem to modify the egocentric body-centered coordinate system.

Modality-related scalp responses after electrical stimulation of cutaneous and muscular upper limb afferents in humans

To elucidate whether the selective electrical stimulation of muscle as well as cutaneous afferents evokes modality-specific responses in somatosensory evoked potentials (SEPs) recorded on the scalp of humans, we compared scalp SEPs to electrical stimuli applied to the median nerve and to the abductor pollicis brevis (APB) motor point. In three subjects, we also recorded SEPs after stimulation of the distal phalanx of the thumb, which selectively involved cutaneous afferents. Motor point and median nerve SEPs showed the same scalp distribution; moreover, very similar dipole models, showing the same dipolar time courses, explained well the SEPs after both types of stimulation. Since the non-natural stimulation of muscle afferents evokes responses also in areas specifically devoted to cutaneous input processing, it is conceivable that, in physiological conditions, muscle afferents are differentially gated in somatosensory cortex. The frontocentral N30 response was absent after purely cutaneous stimulation; by contrast, it was relatively more represented in motor point rather than in mixed nerve SEPs. These data suggest that the N30 response is specifically evoked by proprioceptive inputs.

Phasic modulation of corticomotor excitability during passive movement of the upper limb: effects of movement frequency and muscle specificity

Modulations in the excitability of spinal reflex pathways during passive rhythmic movements of the lower limb have been demonstrated by a number of previous studies [4]. Less emphasis has been placed on the role of supraspinal pathways during passive movement, and on tasks involving the upper limb. In the present study, transcranial magnetic stimulation (TMS) was delivered to subjects while undergoing passive flexion-extension movements of the contralateral wrist. Motor evoked potentials (MEPs) of flexor carpi radialis (FCR) and abductor pollicus brevis (APB) muscles were recorded. Stimuli were delivered in eight phases of the movement cycle during three different frequencies of movement. Evidence of marked modulations in pathway excitability was found in the MEP amplitudes of the FCR muscle, with responses inhibited and facilitated from static values in the extension and flexion phases, respectively. The results indicated that at higher frequencies of movement there was greater modulation in pathway excitability. Paired-pulse TMS (sub-threshold conditioning) at short interstimulus intervals revealed modulations in the extent of inhibition in MEP amplitude at high movement frequencies. In the APB muscle, there was some evidence of phasic modulations of response amplitude, although the effects were less marked than those observed in FCR. It is speculated that these modulatory effects are mediated via Ia afferent pathways and arise as a consequence of the induced forearm muscle shortening and lengthening. Although the level at which this input influences the corticomotoneuronal pathway is difficult to discern, a contribution from cortical regions is suggested.